JPH06167351A - Control method of nonlinearity of change device - Google Patents

Abstract

PURPOSE: To obtain the high sensitivity and low linearity without increasing the additional cost or complexity by suitably selecting the thickness of the diaphragm of a converter and the separate distance between a clamped diaphragm and a fixed capacitor. CONSTITUTION: A silicon wafer 10 sandwiched between glass boards 12 and 14 has the desired thickness of a silicon diaphragm 24. A step recess for partitioning a gap 22 is formed between the diaphragm 24 and a capacitor plate 16 formed at the boards 12, 14. A differential capacitance of the capacitance C1 between the diaphragm 24 and the plate 16 and the capacitance C2 between the diaphragm 24 and the plate is generated by the movement of the diaphragm 24. The nonlinearity can be introduced to a converter in a range for canceling the intrinsic nonlinearity of the opposite direction by excess capacitance by regulating the thickness of the diaphragm 24.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates generally to transducers and, more particularly, to the design and construction of transducer systems for optimum performance with respect to linearity and sensitivity.

[0002]

BACKGROUND OF THE INVENTION Many types of transducers have been proposed which convert various physical effects (acceleration, force, pressure, etc.) into corresponding movements of the transducer member relative to one or more other fixed members. There is. The relative position of the transducer motion members is a measure of physical effect. One common method of determining the position of a motion transducer member is to measure the electrical capacitance between the transducer member and one or more fixed members.

FIG. 1 is a cross-sectional view of an idealized transducer in which a movable plate is located between a pair of fixed plates. Each plate has a flat, electrically conductive, equal area and is symmetrical at the other points. The capacitance between the movable plate and either fixed plate is inversely proportional to the separation distance. That is, the position of the movable plate can be determined by measuring the capacitance.

The performance characteristics of such a converter can be represented by its capacitance. However, practical converter systems include some form of electronic device to convert the capacitance change into a more useful form of information, such as voltage. The conversion itself also affects the performance characteristics of this combination.

The total "sensitivity" of the transducer system used in this case means the change in electrical output caused by the change in physical input. The change in output is linearly related to the change in input. Scaling factors such as gain (scal
It is important that the introduction of the ing factor) into the system does not affect the linearity.

The sensitivity of a capacitive transducer system is considered to be the product of three unrelated factors. That is, FIG.
Is the change (displacement) of the position of the movable member with respect to the change of the physical input. FIG. 2 shows changes in capacitance with respect to changes in position (displacement). FIG. 3 shows a change in electric output with respect to a change in capacitance.

The first two factors relate to the characteristics of the converter and the third relates to the characteristics of the electronic device.

It is desirable to increase the sensitivity of the transducer and minimize the effects of mechanical and electrical inaccuracies. This is
Factor 1 by increasing the displacement of the movable member for a given change in physical input (eg by reducing the internal force that constrains the displacement) or Factor 2 by increasing the change in capacitance for a given displacement ( For example, by reducing the separation between the movable member and each fixed plate), or both. Due to each of these actions, the displacement becomes a portion where the separation distance is relatively large for a given change in the physical input. That is, it is desirable to maximize the amount (displacement / separation distance).

The effect of each of the three factors can be evaluated by referring again to the ideal converter of FIG. It is assumed that the displacement of the movable member from the center position, that is, the zero position is proportional to the physical input (factor 1). Distance (D1
-D2) is proportional to this displacement. By definition, the capacitance between two parallel plates is inversely proportional to the separation of these plates, so capacitance C1 is inversely proportional to D1 and capacitance C2 is inversely proportional to D2 (factor 2). If the change in electrical output is proportional to (1 / C1-1 / C2) (factor 3), then this change is proportional to the physical input.
That is, the electrical output is zero when the physical input is zero (no displacement) and increases in direct proportion to the increase in the physical input. The sensitivity, ie the change in the electrical output with respect to the change in the physical input, is constant and independent of the value of the physical input. That is, the non-linearity is zero. This sensitivity can be increased by increasing the amount (displacement / separation) without introducing non-linearity. In this idealized system, each of the three factors that influence the linearity of the system is itself linear.

The ideal case of high sensitivity and zero nonlinearity is not available in practical transducer systems. There are basic limitations that reduce sensitivity and increase nonlinearity. Moreover, sensitivity and non-linearity are interrelated, with improvements in one causing degradation in the other.

Each of the above three factors affecting sensitivity and linearity is affected by different limitations. Each example in this case will be described below.

Factor 2, the limitation affecting the change in capacitance with respect to position change, is introduced by the unavoidable external capacitance between the movable member and the fixed capacitor plate. This occurs both within the transducer itself and within the external connections to the measuring electronics. The external capacitance is in parallel with the transducer capacitances C1 and C2,
It produces an effective transducer capacitance equal to the sum of these two capacitances. Electronic devices cannot react to this effective capacitance and isolate the effects of transducer capacitance and external capacitance. Since the external capacitance does not change in response to the position of the movable member, the relationship between the effective transducer capacitance and position of the movable member and the corresponding physical input is changed.

The effect of external capacitance on overall system sensitivity and linearity depends on the electronics algorithm used for Factor 3. Even if the change in electrical output for a change in capacitance is proportional to (1 / C1-1 / C2) as the best possible choice for the least nonlinearity,
The external capacitance still causes reduced sensitivity and increased nonlinearity. When non-linearity is detected, it becomes more sensitive to increasing inputs as shown in FIG. If the sensitivity is further increased by increasing the amount (displacement / separation distance),
Increases non-linearity.

This effect of the external capacitance becomes even more important when the capacitance of the transducer is reduced due to miniaturization as the external capacitance increases with respect to the transducer capacitance.

The external capacitance acts as part of the transducer capacitance that does not respond to physical input. In general, limits on ideal performance are always present when not all parts of the transducer capacitance respond equally to the physical input.

This limitation can affect Factor 1 ie the change (displacement) of the position of the movable member with respect to the change of the physical input when the displacement of the entire area of the movable member is not the same for the change of the physical input. . This non-uniform displacement makes the sensitivity non-linear, and this non-linear direction also increases the sensitivity with increasing input.

An example of this limitation is the introduction of suspension members which mechanically position the movable member relative to the fixed member. The displacement of the suspension member varies from total displacement at the connection to the movable member to no displacement at the connection to the fixed member. To the extent that the suspension members are included as part of the capacitance of the sensor, the effects add to the non-linearity. This non-linear direction is the direction in which the sensitivity increases with increasing input.

A more complicated example of this limitation is when the movable member and its suspension member are merged into a clamped diaphragm as shown in FIG. The circular flexible diaphragm is mounted around its periphery in a fixed structure. Two capacitor plates are located on each side of the diaphragm relative to the fixed structure at equal distances for each purpose. A pressure differential is applied before and after the diaphragm to bend it toward one fixed capacitor plate and away from the other plate.

Even if the displacement of each region of the diaphragm is directly proportional to the pressure input, the entire region of the diaphragm does not have the same deflection. The curved diaphragm itself exhibits non-uniform displacement resulting in non-linearity. Since the entire region operates in this way, it is practically impossible to exclude the unfavorable region of the diaphragm from the transducer capacitance. That is, the sensitivity is non-linear, and the direction of this non-linearity also tends to increase as the input increases.

As in the case of the previous example, the sensitivity is the amount (displacement /
Increasing by increasing the separation distance increases the non-linearity.

Factor 3, the limit affecting the change in electrical output for changes in capacitance, is introduced by the choice of the algorithm that defines the conversion of the transducer capacitance into an electrical output. As described above, output (1 / C1-1 / C2)
Proportional to yields a linear output for an ideal converter and is thus the best choice. For example, making the output of an ideal transducer system proportional to (C1-C2) makes the sensitivity very non-linear for significant displacements, approaching infinity as the movable plate approaches one of the fixed plates. That is, the choice of electronics conversion algorithm has a significant effect on nonlinearity.
As in the previous case, this non-linear direction is the direction in which the sensitivity increases with increasing input. Also, as in the case above, the sensitivity is increased by increasing the amount (displacement / separation distance), which increases the non-linearity.

All these practical limits on the performance of practical capacitance converter systems work to reduce sensitivity and increase nonlinearity. The direction of non-linearity is always the direction of increasing sensitivity with increasing input.

The conventional method of reducing this non-linearity is to use a relatively low amount (displacement / separation), ie to limit the displacement of the movable member to a small fraction of the separation between each member.
However, also in this case, the sensitivity is lowered. The nonlinearity is
An acceptable compromise between non-linearity and sensitivity must be obtained because the displacement approaches zero only when it approaches zero. With decreasing sensitivity, mechanical stability and accuracy become increasingly critical as the displacement for a given input decreases. Also, the stability and noise of electronic devices become increasingly critical as the signal level for a given physical input decreases. These problems are even more critical as the size of the transducer is reduced.

Mainly the linearity is improved by compensating for the non-linearity in factor 3, ie by converting the change in capacitance into a change in electrical output. However, a linear homogenous function of the transducer capacitance, or a ratio of such functions, does not reduce the sensitivity to increasing inputs and cancels out the non-linearities mentioned above. This imposes the use of non-linear functions, which adds cost and complexity. The required circuit complexity increases significantly with increasing correction accuracy, and this variant is only practical for medium-performance or high-cost converters.

The specifications of US Pat. Nos. 4,542,436 and 4,858,097 exemplify means for reducing the non-linearity of capacitive sensors. U.S. Pat. Nos. 4,054,833, 4,295,376 and 4,386,312 generally reduce the non-linearities inherent in sensor circuits or occur within the circuits themselves. A circuit design method for compensating the nonlinearity has been proposed.

As is apparent from the above, there is a need for a solution that produces high sensitivity and low non-linearity without adding additional cost or complexity.

There is also a need for a method that results in substantially zero nonlinearity over the transducer performance range without unduly compromising the degree of sensitivity.

What is also needed is a method that results in substantially zero nonlinearity when using a non-ideal electronic device conversion algorithm.

What is further needed is a method that can increase the amount (displacement / separation) without unduly compromising the degree of non-linearity.

In addition, one direction of non-linearity is introduced into the fabrication of the transducer device so that the opposite direction inherent non-linearity due to the effects of external capacitance, unequal displacement and non-ideal electronic device conversion algorithms. There is a need for a method that can offset

[0031]

SUMMARY OF THE INVENTION In accordance with the present invention, a method and corresponding apparatus for constructing a transducer and cooperating system that allows nonlinearity to be substantially independent of sensitivity will be described. In practice, the parameters that accomplish this can be varied to increase or adjust the sensitivity of the system while still retaining the non-linear control conditions introduced into the system.

In accordance with the present invention, the well-known increase in sensitivity with increasing input is eliminated by introducing equally opposite non-linearities to cancel the conventional upward curve shown in FIG. You can By using the present invention,
There is no need to sacrifice sensitivity or use complex electronic corrections to improve the linearity of the transducer system. Rather, the sensitivity can be increased or the linearity can be reduced or adjusted by changing the correction parameter, which shows the characteristic that the sensitivity decreases as the input increases, as shown by the downward curve in FIG. FIG. 4 shows the required correction that results in a decrease in sensitivity with increasing input. This effect can be tailored to match the transducer's normally inherent non-linear sensitivity. With correct correction, the sensitivity of this system is practically increased and remains substantially zero nonlinearity. The method of obtaining this result will be described later.

Stretching the material, and in a preferred embodiment the diaphragm material, causes stiffness.
s) or even more resistance to displacement is well known. As the displacement increases, the increase rate of the displacement with respect to the increase of the given rate of the applied force decreases. This non-linearity of the transformation of the input into displacement is negligible for small displacements relative to the material thickness, but such non-linearity increases rapidly with increasing displacement over thickness.

In this region of motion of the material, the displacement of the material anywhere is a constant part of the maximum displacement. That is, the "shape" of a material in the mechanical sense does not change substantially with changes in displacement. In the preferred embodiment, the material at each location on the diaphragm is displaced to some extent proportionally at each other location.

As the displacement increases further, additional effects are introduced and the "shape" of the material changes. It is believed that the advantages of the present invention are less significant in this region of material operation.

If the material controlling the displacement of the movable member of the transducer is manipulated to introduce such non-linearity during the conversion of the input into displacement, the effect on the sensitivity of the transducer is that it is sensitive to increasing input. , Which is exactly the desired offsetting effect. A non-linear parameter is included in the suspension system for the movable member, or this parameter cooperates with all movable members, as in the case of a clamped diaphragm, for example.

In a preferred embodiment of the present invention, the method of adjusting this displacement non-linearity to compensate for the normally non-linear performance of the transducer system includes the selection of diaphragm thickness and clamping diaphragms. Includes a choice between the separation distance between the fixed capacitor plates.

The effect of these two types of parameters is to increase the amount (displacement / separation distance) to increase the nonlinearity in the direction of increasing sensitivity to an increase in input, and to increase the amount (displacement / diaphragm thickness). It is clear that increasing increases non-linearity in the opposite direction. The displacement is common to both of these quantities. For a given displacement, the separation distance and diaphragm thickness in this case control the non-linearities in opposite directions, and these two parameters are adjusted to produce zero non-linearities regardless of the degree of sensitivity. can do.

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

An experimental way of implementing the principles and concepts of the present invention is to construct multiple transducers with various parameter values and evaluate the results. Due to the large number of variables involved in the actual transducer and the complexity of the effect, this experimental method is very time consuming and expensive and generally has little knowledge of adjusting the parameters to obtain the desired result.

The preferred solution produces a mathematical model of the transducer system and quantifies the resulting effect. The parameters of this model are then adjusted to obtain the desired performance. Parameters that particularly affect the non-linear displacement of the movable member are adjusted to compensate for the normally non-linear sensitivity of the transducer system. The construction and testing of several actual transducers can be used to confirm model prognosis.

The model must accurately predict the relationship between electrical output and physical input. The model consists of a set of sub-models, each describing a particular contribution to the result. The preferred solution is as follows. A) Define a sub-model to define the position of the moving parts of the transducer according to the physical input. B) Define another submodel to define the combined capacitance of the transducer according to the position of the movable member. C) Still define another sub-model to define the combined electrical output according to the combined capacitance.

The given model is the product of these sub-models.

[0044]

[Equation 1]

This model is used to predict the performance of the proposed converter structure. The physical inputs are incremented to find the sensitivity and linearity of the structure and the appropriate parameters adjusted to obtain the desired performance. A particularly suitable parameter is that non-linearities introduced by factors such as extra capacitance and non-uniform displacement are compensated by non-linearities introduced by the non-linear displacement effects of the material controlling the displacement of the movable member. To adjust.

[0046]

EXAMPLE A detailed description of the model is given in the case of a pressure transducer having a circular silicon diaphragm nominally centered between two circular capacitor plates of uniform thickness as shown in FIG. It will be described below. This case is an extreme example of non-uniform displacement. Those skilled in the art given this description can easily develop other models for other types of transducer systems.

The first step A) defines a sub-model which represents the displacement of the diaphragm according to the applied pressure. The need to define diaphragm displacement is common in many fields,
Many models have been developed. The model used in the present invention is the Scientific Transls published in 1966.
Israeli Program (Isr)
L. Andreev translated from Russian by ael Program
a) Author's paper “Elastomeric member of equipment (Elasti
c Elements of Instrument
s) ”.

The displacement of the diaphragm is determined by first determining the maximum (center) displacement of the diaphragm caused by the applied pressure and then by determining the displacement of the diaphragm and other regions for the same applied pressure.

The maximum (center) displacement Wp is derived from the following equation.

[0050]

[Equation 2]

In this equation, the coefficients a, b and c are as follows.

[0052]

[Equation 3]

The parameters and the corresponding measurement units are as follows.

[0054]

[Equation 4]

The pressure / displacement equation described above represents the displacement of the center of the diaphragm as a function of the applied pressure and the diaphragm method.

The displacement of each of the other regions of the diaphragm is defined by representing the displacement of each of a series of optionally narrow concentric regions or rings of the diaphragm. This displacement is part of the center displacement which varies from 1 at the center to zero at the clamping edge. That is, the displacement Wr at a certain radius is as follows.

[0057]

[Equation 5]

In this equation, SF is a "shape factor" that changes with radius. The shape factor is derived from the following equation.

[0059]

[Equation 6]

With this formula

[0061]

[Equation 7]

"Z" is an experimentally determined coefficient.
For the possible working areas of the material in this case, the "shape" does not change substantially due to the change in displacement. Therefore, "Z" is constant. A value of about Z = 3 has been found to be suitable for thin silicon materials.

The above-mentioned shape factor formula represents the displacement of each region of the diaphragm as a relation of the central displacement. That is, the displacement of each region of the diaphragm is known as a function of the applied pressure and the size of the diaphragm.

The second step B) defines a sub-model for defining the combined capacitance of the transducer as a function of the displacement of the diaphragm as described above.

The general formula for the capacitance between two parallel plates is:

[0066]

[Equation 8]

With this formula

[0068]

[Equation 9]

In this case the calculation is made by considering the diaphragm as a series of arbitrarily narrow concentric rings, each parallel to a capacitor plate, as shown in FIG. The distance between each ring and the capacitor plate is the distance without displacement described above minus the displacement of the ring. The total capacitance is the sum of the capacitances for all rings that contribute to this capacitance. This sum is bounded at the center by the holes in the capacitor plate with radius R1 and outside by the capacitor plate with radius Ro. Ie

[0070]

[Equation 10]

With this formula

[0072]

[Equation 11]

This equation is applicable to both capacitances C1 and C2 of appropriate use with algebraic signs of displacement.

The extra capacitance in parallel with the transducer capacitance adds these values to get the total effective transducer capacitance.

The third step C) defines the submodel so as to define the combined electrical output as a function of the combined capacitance.

This submodel is simply an expression of the chosen algorithm that defines the conversion of capacitance to electrical output. For example, in the above example of an ideal transducer, the submodel would be

[0077]

[Equation 12]

With this formula

[0079]

[Equation 13]

The combination of each said sub-model represents the desired model of the transducer system.

This model is one example of how it can be applied to a particular transducer system structure. Other models of this structure are possible, and different transducer system structures require different models.

The model includes the property of producing bidirectional non-linearities that balance to obtain zero non-linearities for the system.

Solving the equation relating the center displacement of the diaphragm to the pressure to determine the displacement, the displacement depends on the pressure which rises to the first power and represents the expected linear relationship:
It also depends on the pressure rising to a higher power, which represents the non-linear displacement caused by the increasing stiffness with increasing input.

The effect of uneven displacement of the diaphragm is introduced by the "shape factor" equation. The effect of extra capacitance is introduced by including in the conversion of diaphragm position into capacitance. Also, the effect of non-ideal electronic conversion on the output of capacitance is introduced in the electronic device submodel. These three coefficients introduce higher order terms relating output to pressure input. These terms give rise to non-linearities in the opposite direction introduced by the non-linear displacements described above.

It has been found that the non-linearity of the transducer system caused by unequal displacement, the extra capacitance and the non-ideal electronic conversion algorithm are mainly caused by the third order term. The coefficients of these terms can be adjusted to provide near perfect cancellation. That is, non-linearity is eliminated by the selection of transducer structural members corresponding to terms in each equation that define non-linearity in the opposite direction to that caused by unequal displacement, excess capacitance and non-ideal electronic conversion algorithms. You can

Depending on the complexity of the model, the preferred solution for the application of the model is that all transducer system parameters can be input into the model, which determines the electrical output for the input pressure increment and the resulting sensitivity and nonlinearity. To generate a computer program that reports sex. This is repeated for a range of parameter values and optimum values that are determined for the desired performance.

A number of parameters are included. The way the data is organized in a meaningful way helps to get real results quickly. The preferred solution is to change the two parameters while keeping all other parameters constant. The two variable parameters are chosen so that they have a strong influence on the two non-linearities that must eventually be balanced and that they can be changed relatively easily during the production of the actual transducer.

It is clear from the above pressure / displacement equation that the displacement and the non-linearity of displacement strongly depend on the thickness, diameter and back strain of the diaphragm. Any and all of these parameters can be adjusted to affect the non-linearity. However, in typical constructions the diameter is relatively limited by factors such as cost and ease of manufacture. Also, many components of the transducer are affected by changes in diameter, making diameter changes difficult to implement quickly. The reverse strain value is also limited by factors such as assembly technology and material properties.

However, the thickness of the diaphragm is easily adjusted during manufacture and does not affect other parameters. Further, the above-mentioned non-linearity of displacement can be controlled relatively large. Therefore, this is the preferred variable used to adjust the non-linearity of the displacement.

A preferred parameter for controlling the non-linearity caused by excess capacitance and uneven displacement is the separation between the non-displaceable diaphragm and the fixed capacitor plate. Since the separation distance and the diaphragm thickness are both controlled by the size of one member of the transducer,
Diaphragm structure, sensitivity and nonlinearity can be controlled by adjusting the parameters of a single transducer member.

The preferred solution is to define a range of performance results for a given transducer structure using diaphragm thickness and separation as design variables. These results dictate the value of one of the parameters relative to the given values of the other parameters to obtain the desired level of nonlinearity and composite sensitivity. It has been found that sensitivity increases while maintaining zero nonlinearity. However, the required accuracy of the dimensions obtained becomes more stringent and limits the sensitivity that can be obtained with practical structures.

The preferred construction of the present invention produces a clamped silicon diaphragm. A silicon wafer 10 having a thinned area is formed, and supporting glass substrates 12 facing each other, on which capacitor plates 16 and 18 are arranged or formed, respectively.
It is sandwiched between 14. FIG. 6 shows such a converter structure.
The diaphragm is made of silicon using a silicon processing method that allows both diaphragm thickness and capacitor clearance parameters to be independently determined in a single set of processing steps. In particular, the transducer structure shown in FIG. 6 is shown with the layers separated from each other for clarity.
A silicon wafer 10 sandwiched between 2 and a lower glass substrate 14 is provided. The silicon wafer 10 is etched on both sides to form a step-like recess that partitions a gap 22 between the thin silicon diaphragm 24 and the capacitor plates 16 and 18 formed on the upper and lower glass substrates together with the diaphragm thickness shown as a dimension 20. To do. The semiconductor material itself is mixed with impurities, and both fixed capacitor plates 16 and 18 are fixed.
Have sufficient conductivity to form a flexible conductive capacitor plate common to all. In this way, the movement of the silicon diaphragm 24 causes the differential capacitance between the first capacitance C1 between the silicon diaphragm 24 and the upper capacitor plate 16 and the second capacitance C2 between the silicon diaphragm 24 and the lower capacitor plate 18. To generate. When pressure is applied to the thin silicon diaphragm plate 24, the diaphragm 24 flexes to one or the other according to the differential pressure across it, increasing the capacitance of one and decreasing the capacitance of the other. In such a structure, the total capacitance C1 + C2 remains substantially constant.

The upper and lower glass support substrates 12 and 14 are covered on both sides by performing a metal vapor deposition process so as to form the conductive biases 26 and 28 which perform two functions together with the capacitor plates 16 and 18. It is a structure. Firstly, the bias fluids 26, 28 act the external fluid on the diaphragm 24 to create a differential pressure before and after it to deflect it. Second, the conductive bias 26, 28
Form an electrical path from each capacitor plate 16, 18 to the conductors 30, 32 on the opposite sides of the glass substrate.
An electrical connection is formed in the silicon material 10 with both conductors 30, 32 to detect changes in capacitance in response to changes in pressure applied to the transducer. Multiple such transducers are made simultaneously by using large diameter silicon wafers and corresponding large diameter glass substrates. The glass and silicon materials are made as shown and then electrically sealed to one another so that such layers are attached to each other. With such a seal, the glass supporting substrates 13 and 14 are sealed in the circumferential direction around each diaphragm 24.

With respect to the processing of silicon 10, the wafer covers each side thereof and defines a plurality of certified circular areas each having a diameter corresponding to the diameter of the diaphragm. Then, silicon is etched on both sides of the wafer to etch the silicon silicon material to a desired depth corresponding to the desired capacitor gap 22. Many etchants are used for etching coated silicon. Glass
To further elaborate on the construction of the silicon-glass capacitive transducer, reference is made to the above-referenced U.S. patent application "High Sensitive Compact Pressure Transducer". The entire unmasked silicon wafer is removed again, except for the etching mask, and then the silicon is removed evenly on both sides of the wafer until the desired diaphragm thickness is reached. The silicon material is removed in known proportions and the wafer is time etched to obtain the desired diaphragm thickness 20 without affecting the stepped recess dimension 22. Practically a diaphragm thickness of 2 to 3 microns 20
Is possible. One skilled in the art with reference to the prior art can readily envision other methods of making diaphragms with suitable diaphragm thickness and offset dimensions.

It is clear that the capacitor gap 22 is related to the output capacitance of the converter as the output capacitance is larger with smaller gaps and diaphragm deflections in the range considered. The reason is that the spacing between the fixed capacitor plate and the diaphragm is smaller, increasing the capacitance. Also, by adjusting the diaphragm thickness, the non-linearity can be introduced into the transducer in a range that cancels out the inherent non-linearity in opposite directions due to extra capacitance or the like. That is, as the deflection of the diaphragm increases with increasing input fluid, the diaphragm material itself creates a pair of non-linearities of increased resistance to deflection.

Radial reverse strain of the silicon diaphragm also affects the deflection, and this parameter can also be used to linearize its output. Radial counter-distortion of the diaphragm can occur during the electrical sealing of the silicon diaphragm material to the glass support substrates 12,14. By appropriately selecting the thermal expansion coefficient of the glass substrates 12 and 14 relative to the thermal expansion coefficient of the silicon wafer 10, a desired reverse strain can be introduced into the diaphragm 24. Bulky glass substrates 12, 14
Has a thermal expansion less than that of the thin silicon material 10 at hermetically sealed concentration. That is, composite sandwich glass
When the silicon-glass structure is heated to a predetermined sealing temperature, fixed to each other by electrosealing and then cooled, the diaphragm 24 retains reverse strain. This means that when fixed to the glass support substrates 12, 14 at high temperature, the silicon diaphragm 24 shrinks more than glass during the cooling process. Different back strains can be produced in the diaphragm 24 by the difference in the coefficient of thermal expansion time and the selection of the temperature at which the electrical sealing occurs.

The output of the capacitive displacement transducer contains a capacitance that changes in response to a physical input such as pressure.
To produce an output that can be used in a typical control system,
It converts the capacitance into a corresponding electrical signal.
Many circuits are possible to make the electrical conversion to this capacitance. However, such circuits generally introduce additional non-linearities into the system or require extremely complex circuitry to produce adequate linearity, adding substantial cost to the system. FIG. 7 shows an electrical circuit which is well suited for use in the converter of the present invention and which is capable of performing electrical conversion of capacitance while maintaining linearity at low or reasonable cost.

In order to maintain the full linearity of the transducer system, the measurement of the displacement of the movable member must be carried out using the reciprocal capacitance difference, ie 1 / C1-1 / C2. As will be described in more detail below, the transducer capacitances C1 and C2 switch to the inputs of the differentiator to produce an output representation of the reciprocal of the capacitance. A difference circuit is used to produce the difference in capacitance indication, but feedback of the sum of the reciprocal capacitances is sent to the ramp generator driving the converter capacitance. The effect of extra capacitance is
It is eliminated by allowing the supply current to flow only through the transducer capacitance and allowing the measured voltage to be based solely on the supply current.

Direct measurement of the reciprocal capacitance can be easily performed, but circuit simplification can be done by measuring the capacitance itself rather than the reciprocal value. This is clear from the following relationship.

[0100]

[Equation 14]

FIGS. 7 and 8 exemplify circuit diagrams and waveforms of the conversion circuit of the preferred embodiment of the present invention. The converter capacitances C1 and C2 are alternately switched to the differentiating circuit 40 and are alternatively driven by the ramp generator 42. The differentiating circuit 40 produces an output signal having an amplitude proportional to the transducer capacitance, and the demodulating circuit 44 shifts this signal to ground reference. The difference circuit 50 induces the sum (C1 + C2).
This sum is held by the holding circuit 52. This sum is held constant by the integrating circuit 54, eliminating the need for actual division. Alternatively, a division method may be used in which the ratio of the difference and the sum is generated by a dividing circuit.

The ramp generator 42 is of ordinary construction,
The output at the connection point 56 has a triangular waveform 58 with alternating positive and negative voltage gradients. 2 complete measurement cycles
It consists of two alternating half cycles. The switches 60, 62 actuated by the waveform 64 during the first half cycle are in the positions shown. Such a switch connects plate 16 of converter capacitance C1 to the output of the ramp generator and plate 18 of capacitance C2 to ground. Differentiating circuit 40 includes converter capacitances C1 and C2 and a resistor 6
6, a capacitor 68, and a differential amplifier 70.
The non-inverting input of amplifier 70 is connected to ground, and feedback through resistor 66 adds the inverting input at node 72 at the actual ground potential. Common conductor of converter 7
4 connects the movable member 24 to the connection point 72.

During phase 1 of the first part of the first half cycle, the output of the ramp generator, ie, part 76 of waveform 58, maintains a constant positive slope. The resulting constant rate of voltage change across C1 induces a constant current in the output of amplifier 70 through C1 and thus through resistor 66. After a short settling time, the output of amplifier 70 at node 78 settles to a negative voltage level which is representative of the value of converter capacitance C1 as shown by portion 80 of waveform 82. Amplifier 7
Zero frequency compensation is required for large values of resistor 66 or converter capacitance and is produced by capacitor 68.

The capacitance between the input connection 72 and the circuit ground is the stray capacitance associated with the transducer movable member 24, the capacitance associated with the circuitry itself, and the transducer capacitance C2 grounded by the switch 62. Consists of. Since this connection point is actually at ground potential, the voltage before and after this capacitance cannot change, and substantially no current can flow through this connection point, thus converting the lamp signal current. To prevent conversion from the device capacitance C1. In addition, the current required to deliver the extra capacitance to ground associated with plate 16 of C1 does not affect the signal current produced by the output of ramp generator 42 and through converter capacitor C1.

That is, the output with waveform 82 of differentiator circuit 40 is unaffected by the extra capacitance from the transducer elements to the environment and circuit input capacitance.

During Phase 2 of the first half cycle, the ramp generator output gradient is reversed, as shown in portion 84 of waveform 58. In this way, the polarity of the output of the differentiating circuit 40 is reversed as shown by the portion 86 of the waveform 82.

Ideally, the level of the portions 80, 86 of the waveform 82 that have adequate headroom for polarity is proportional to C1 during both Phase 1 and Phase 2 of the first half cycle. However, in reality, the input current of the amplifier 70 cannot be ignored as compared with the signal current generated through C1. This is in fact the case for the extremely small values of transducer capacitance associated with small transformer structures. However, the amplifier input currents flow through resistor 66 during phase 1 and phase 2 equally and in the same direction. As an effect in this case, the differential circuit output waveform 82
Easy to add voltage offset to. The peak-to-peak amplitude of this signal is unaffected by this amplifier input current and is an accurate indication of C1. Peak The additional advantage of using the peak value is that the signal level effectively doubles the value obtained by only opening phase 1 or phase 2. Further, as an effect of unequal ramp generator gradient, waveform 58 is unimportant during phase 1 and phase 2 as this effect does not affect peak-to-peak output.

The peak-peak output voltage value generated for C1 by the differentiating circuit 40 is converted to a ground standard value by the demodulating circuit 44 including the coupling capacitor 88, the switch 90 and the differential amplifier 92. During the latter part of phase 1, switch 90 is closed by waveform 94 and waveform 82 on capacitor 88.
Charging to the negative voltage level of the portion 80 of FIG. Switch 90
Is open to the rest of the first half cycle. During this time period, the input to amplifier 92 is simply waveform 82 which is dropped by the negative voltage across capacitor 88. This total effect is additive. The waveform 96 of this signal appears at the output of amplifier 92, which functions as a unity gain buffer. The level of this signal during the latter part of phase 1, ie part 98
Is approximately 0V. During the latter part of phase 2 of the first half cycle, the level of this signal, ie 100, is
0 peak to peak output and converter capacitance C1
And is called ground potential in this case. This level is sampled by difference circuit 46 and sum circuit 50 as shown by waveform 102.

A comparator (not shown) within the switch control circuit 104 ends the first half cycle when the ramp generator output waveform 58 is approximately zero volts. In this case zero charge is stored in C1 and prevents circuit transitions when switching C1 and C2 for the next half cycle.

Switches 60, 62 during the second half cycle
Reverses the position shown as shown by waveform 64 and connects converter capacitance C2 to the ramp generator output C1.
Ground. This measurement process is repeated and during the latter part of phase 2 the level of the part 106 of the waveform 96 is proportional to the converter capacitance C2. This level is sampled by difference circuit 46 and sum circuit 50 as shown by waveform 108. In this way, the measurement cycle is completed. The output of the system is characterized by the following relationships:

[0111]

[Equation 15]

That is, this output is the desired ratio of the difference and sum of C1 and C2 times the reference voltage. Normalization of circuit gain and converter capacitance does not substantially affect the output. Changes in non-critical parameters, such as the dielectric constant of the transducer capacitance, change the output of the feedback integrator circuit 54, resulting in different ramp gradients that return the output of the system to the value it had prior to the change.
Changes in insignificant parameters are thus ignored, ie the output of the stem is, in the preferred embodiment, the transducer member 24.
Make the movement of the robot respond to the parameters in question.

FIG. 9 shows a diagram of the linear performance of a converter using the concepts and principles of the present invention as compared to previously known converters. The vertical axis of this diagram shows the% of non-linearity that includes both positive and negative values. The horizontal axis of this diagram shows the relative gap size between the movable member or diaphragm 24 and the fixed capacitor plate. The lower curved line 120 shows the non-linearity for generally known transducers. When extended, this line becomes an asymptote for both the horizontal and vertical methods. That is, when the gap is reduced as in the general case, the non-linearity increases rapidly to an unacceptable level. Also, the linearity improves as the gap increases. By increasing the gap spacing, the non-linearity is somewhat improved until the converter output capacitance is significantly reduced. This is apparent because the capacitance is inversely proportional to the gap spacing. Conventional converter manufacturers try to improve the linearity to the extent that wide gap spacings are acceptable, and then complex and expensive electronic circuitry Was further improved.

The upper line 122 in FIG. 9 shows the linearity obtained when the diaphragm stiffness is used as a non-linear parameter to offset other inherent non-linearities such as extra capacitance. As shown, these lines pass through a non-linearity of zero, indicating that optimum linearity is obtained utilizing the proper choice of transducer structural parameters. Although these lines are characteristic of a transducer with a defined diaphragm thickness, a group of all such zero-passing lines is available for transducers with different diaphragm thicknesses. The gradient of these lines also corresponds to the output of the converter system. Thus, a higher output of the converter system and a non-linearity of zero are obtained, but the constraint on the tolerance of the capacitor gap is correspondingly limited.

FIG. 10 shows the predicted non-linearity (vertical axis) versus the gap dimension micron (horizontal axis) for the indicated diaphragm thickness T. The predicted relative output is numerically shown to the right of each curve.

For each diaphragm thickness, there is a range of gaps that results in a range of non-linearities that cross through zero. The desired value of non-linearity can be selected by selecting a gap of appropriate value. The output amplitude can be increased by choosing a thinner diaphragm and maintaining the non-linearity at the desired value. In this case, the range of the gap that satisfies the nonlinearity requirement becomes smaller.

This selection of parameters to be adjusted and the method of providing this result are merely exemplary. The power increase and non-linearity correction made available by this method can be implemented in various other ways using various transducer structures.

It should be noted that the operation results of the clamped diaphragm transducer are shown diagrammatically, particularly with the introduction of the non-linearity caused by the stiffness of the diaphragm 24. This diagram shows the performance of a typical transducer with a diameter of about 0.250 in and an inverse strain of about 40 microstrain. This transducer was designed for a pressure input of about 3 in head. 3 shows a group of curves in which diaphragms 24 of different thickness cooperate. The output V of the full scale converter system is shown numerically adjacent to each line. Importantly, each line of this group passes through a non-linearity of zero, where the corresponding capacitance gap and diaphragm thickness values are within the practical range of use and manufacture of the transducer device. By constructing a silicon diaphragm thickness of, for example, T = 6 microns, zero non-linearity is obtained with an offset or gap of approximately 12.5 microns. In particular, the transducer system remains linear over the entire operating range rather than within a band. A converter system output of about 2.94V is available for such a converter structure. The top line 124 has a much steeper slope than the other lines and contains a corresponding larger output of 6.11V. A transducer that produces such an output and has zero nonlinearity has a transducer diaphragm thickness of about 3 microns and a gap of about 12.8 microns. Obviously, thinner diaphragms will produce more deflection for a given pressure resulting in increased output capacitance and thus higher system output voltage.
Each line between the lines described above defines a transducer having an intermediate diaphragm thickness and producing an intermediate output voltage.

Importantly, the output voltages associated with the lines of the diagram are relative and independent of the particular type of circuit utilized to convert the capacitance to a voltage output. As can be seen from this diagram, the output can be doubled by simply halving the diaphragm thickness, with zero or minimal non-linearity. Nothing has changed in the circuit to get this result. An important technical advantage of the invention is thus obtained, in which the linearity and the output of the converter system can be influenced without modifying or changing the electrical converter circuitry. Figure 1
The circuit used to produce the 0 plot characteristics was of the type described above with respect to FIG.

When making some transducer diaphragms from wafers of silicon material, it is often difficult to obtain uniform dimensional characteristics in the diaphragm over the entire wafer area. For example, the etching rate changes from batch to batch of silicon wafer, and from the central area to the peripheral edge. Therefore, not all diaphragms are exactly the same thickness and do not have the same offset or capacitive gap. That is, it is practical to have a transducer that defines an acceptable range of non-linearity and falls generally within such range.

The two broken lines 126 and 128 shown in FIG.
Indicates a range of acceptable non-linearity of ± 0.2%. Such a range is optional and depends on the particular application of the transducer. For example, with a diaphragm thickness of 6 microns, the gap spacing varies from about 11 microns to about 15 microns and is still within ± 0.2% nonlinearity. Thus, a substantial tolerance of gap spacing is obtained without sacrificing substantial linearity. Also, when higher power is desired with such a relatively thin diaphragm, the gap spacing tolerance is reduced. Also, as shown in FIG. 10, for a diaphragm thickness of 3 microns, the gap spacing should be maintained at about 12.8 ± 0.03 microns. The same solution is performed within the diaphragm tolerance range, resulting in transducer operation that is within a specified nonlinearity range.

Many other diagrams and groups of lines demonstrating the operation of transducers with different diaphragm diameters are induced and other sets of diagrams have different diaphragm back distortions. First
For a group of curves, the pressure / deflection equation described above can be used to define various diaphragm deflections for a given pressure and a given diaphragm thickness.
Different groups of curves can be used utilizing the same iterative method for different parameter values of diaphragm thickness. Other iterative methods can be performed by varying the total scale pressure with back strain. Having found the deflection of the diaphragm for each example, the corresponding capacitance range can be calculated using the equations above as well. The output voltage using the feedback-using circuit can in this case likewise be derived using the algorithm described above. Finally, the linearity of the circuit can be ascertained by defining what the electrical output is under ideal conditions compared to a real system constructed according to the principles and concepts of the present invention.

The table attached as a notation shows a computer printout after several iterations of the calculation which determines the linearity obtained on the basis of the changes in the transducer parameters determined in advance. This data shows a typical transducer in which the full scale deflection of the diaphragm responds to pressure equivalent to 3 in head. The condenser diaphragm diameter is 200 mils, but its fluid passage holes are coma mils in diameter. The stray capacitance associated with such a structure is about 1.7 pf. Importantly, unlike the textual representation, the Young's modulus value for silicon is 14 for thin diaphragms. This is 26.2284 × 10 for thick silin
Contrast with the reported value of ⁶ psi. The displayed performance gives the output voltage with feedback (Vowfb) and the% linearity of the system using circuit feedback (Lwfb).

[0124]

[Table 1]

As can be seen by this exemplary iteration, the diaphragm thickness is held constant at 10 microns and the gap is increased from 12 microns to 28 microns for three different iterations of diaphragm back strain. The increased strain value increases upwards from 60 microstrains to 100 microstrains in 20 microstrain increments. The deflection decreases as the radial microstrain increases, as expected. The first two lines in the table show the display performance for a transducer with 60 diaphragm backstrains and are shown as invalid data because the deflection is substantially equal to the gap.
Although not shown, the 2 micron deposition of material on the glass substrate forms a fixed capacitor plate and reduces the number of gap parameters shown by a corresponding amount. A non-linear range of ± 0.2% for the same portion of this table creates a very strict requirement for the capacitor gap. For example, a gap change of 18 to 19 microns goes through zero to -0.5.
A non-linear change of 9 to +0.43 or a total non-linearity of 1.02% is produced. Since this is well outside the 0.2% range, we must keep very close tolerances on the gap parameters. However, if such a tolerance can be maintained, 0.584 to 0.5
A voltage output of 39V is obtained. By withholding such a large voltage output, the restrictions on the capacitance gap tolerances are relaxed, yet operation can be kept within a specified non-linear range. Reverse strain 1
By increasing to 00 microstrain, the non-linearity passes through zero with a gap tolerance of 19 to 20 microns. The non-linear range at this operating limit is about 0.2%. However, the voltage output decreases, but is still well within the range of outputting an electrical indication of changes in input pressure. In this example, the output voltage is 0.336 to 0.315 for a full-scale diaphragm deflection of about 8.12 microns. This example shows how to vary the reverse strain to obtain acceptable non-linearity and output voltage, but perform similar analysis for other similar calculation results for different diaphragm thickness while keeping the reverse strain constant. it can.

As described above, the calculation result is calculated from the above equation, and an easy method for confirming various parameters of the converter to obtain a desired output is obtained. In practice, the calculated figures are highly relevant to the actual converter device. By actually selecting various structural parameters of the converter, the total linearity of the converter system can be corrected even if circuit feedback is not normally used. Thus, it will be apparent to those skilled in the art how to improve the operation and interaction of a capacitive displacement transducer and its linearity using the present invention.

Although described above in detail with clamped diaphragm type transducers, the principles and concepts of the present invention are applicable to many other types of transducers that utilize movable members in response to physical input stimuli. As an example, a flat diaphragm is used and is fixed to a support structure by a suspension member. The flat member itself does not deform in response to pressure, but rather moves due to flexure or deformation of the suspension member. Suspension member transducers are made using models other than those described above, taking into account material stiffness, and introducing non-linearities in the desired direction and quantity will introduce other non-linearities caused by external capacitance, etc. cancel.

Thus far, a converter system has been described which produces a linear output without resorting to unusual, complex or highly accurate circuits. An important technical advantage of the present invention is that a more linear output is obtained by selecting various structural parameters of the converter. A more particular technical advantage of the present invention is that the inherent non-linearity, such as due to external capacitance, can be offset by introducing non-linearity in another direction.
A significant technical advantage of the present invention is that the third order non-linearity caused by the inherent stray capacitance can be offset in exactly the same way by introducing third order non-linearity in different directions. A cooperating technical advantage provided by the present invention is that it is possible to improve the overall linearity of the transducer device by using a stiffness factor that allows the movable transducer member to flex. Yet another technical advantage of the present invention is that based on the foregoing, various structural parameters of the transducer can be mechanically selected to produce ideal results. Another technical advantage provided by the present invention is that ideal linearity, or at least an acceptable range of non-linearities, can be obtained while maximizing the converter output. Yet another technical advantage of the present invention is that the entire transducer system including the electrical circuit along with the sensor itself yields optimum performance.

[0129]

[Related Application] The present invention is directed to U.S. Patent Application No. 304,344, "Highly Sensitive Compact Pressure Transducer," dated January 30, 1989.
And U.S. Patent Application No. 304,35 dated January 30, 1989.
No. 9, “Precision converter circuit and linearization method” and 1990
U.S. Patent Application No. 462,448, dated January 18, 2014, which is a continuation of the prior U.S. patent application specification referred to as the "Precisive Capacitive Transducer", and each of these patent application specifications is incorporated herein Referenced.

Although the preferred embodiment of the present invention has been described with respect to a special converter device and circuit structure, it goes without saying that the present invention can be modified in various ways without departing from the spirit thereof.

[Brief description of drawings]

1 is a cross-sectional view of a capacitive displacement transducer.

FIG. 2 is a diagram of ideal versus actual transducer characteristics for sensitivity.

FIG. 3 is a cross-sectional view of a clamp diaphragm transducer having a diaphragm that displaces in response to an input stimulus.

FIG. 4 is a diagram showing an inherent non-linearity that increases the sensitivity as the input increases, and a non-linearity in the opposite direction that the sensitivity decreases as the input increases.

FIG. 5 is an enlarged cross-sectional view showing a displaced transducer diaphragm and a method of analyzing displacement in various areas of the diaphragm.

FIG. 6 is a cross-sectional view of a glass-silicon-glass converter constructed according to the present invention.

FIG. 7 is a block diagram of a circuit for converting transducer capacitance values into corresponding electrical signals.

FIG. 8 is a series of waveform charts showing the operation of the circuit of FIG.

FIG. 9 is a diagram showing the linearity obtained by using the principles and concepts of the present invention as compared to conventional methods.

FIG. 10 is a diagram showing 3% linearity versus diaphragm capacitor plate separation for various parameter values of diaphragm thickness, all passing through a 0% non-linear area.

[Explanation of symbols]

 10 Constraint Support (Silicon Wafer) 12, 14 Glass Substrate 16, 18 Capacitor Plate 24 Movable Member (Silicon Diaphragm)

Claims (22)

[Claims] 1. A first member having a movable member which responds to an input stimulus.
In the method of controlling the non-linearity of a converter, which reduces the converter performance due to the non-linearity of the second type, a second type of non-linearity is defined in the converter, which is defined by the stiffness that is changed by the input during its manufacture. Upon introduction, control to introduce the non-linearity into a restraint member attached to the movable member to offset the first type of linearity to control the total non-linearity of the transducer. Law. 2. The thickness of the constraining member is selected so that the first and second types of non-linearity are substantially offset. The control method according to claim 1, wherein the control method is equilibrated. 3. The control method according to claim 2, wherein the output of the transducer is determined by selecting a gap size between the movable member and the capacitive plate. 4. The control method according to claim 3, wherein a predetermined reverse strain is applied to the restraining member to adjust the total non-linearity of the transducer. 5. The desired non-linearity of a capacitive transducer having a capacitance between the plate and the movable member is reduced by selecting the desired capacitive plate for the movable member gap spacing and the thickness of the restraining member. The control method according to claim 1. 6. The control method according to claim 5, wherein the total non-linearity is made substantially zero by selecting a specific gap interval and limiting a member thickness. 7. The control method of claim 5 wherein a gap spacing is selected to define the first type of non-linearity. 8. A converter made by the control method of claim 1. 9. A first member having a movable member that responds to an input stimulus.
In the method for controlling the non-linearity of a transducer, which reduces the transducer performance due to the non-linearity of the type, the movable member is attached to a fixed frame by a restraint support, A second type in which the constrained support is made of a material having a predetermined thickness such that the change causes a change in resistance to the movement, the change in resistance cancels out the first type of non-linearity. A control method comprising controlling the total non-linearity by defining the non-linearity of. 10. The control method according to claim 9, wherein the movable member and the restraint support are configured as a tightening diaphragm. 11. The method of claim 10, wherein the diaphragm is configured to produce a capacitance change in response to a change in input stimulus to the transducer. 12. The control method according to claim 11, wherein a diaphragm thickness and a capacitive gap parameter are selected to reduce the total non-linearity. 13. The non-linearity of the first type changes with a changing input stimulus, and further, the constraining support and the movable member are configured as a tightening diaphragm,
10. The method of claim 9 wherein the diaphragm is formed to a thickness that introduces a second type of non-linearity in the opposite direction that substantially tracks the first type of non-linearity with a varying input. 14. The method of claim 13 further comprising configuring the diaphragm as a thin silicon diaphragm. 15. A converter made by the control method of claim 9. 16. A method of controlling non-linearity of a transducer having a movable member that is movable in response to an input stimulus, the sensitivity of which increases with an increase in the input stimulus, wherein total non-linearity can be controlled. As described above, a parameter of the transducer that reduces the sensitivity with an increase in input is selected, and this parameter is selected as a change in the resistance of the motion of the movable member with a change in the input stimulus, and the selected resistance of the movable member is A control method consisting of creating a transducer with and controlling total non-linearity. 17. The control method according to claim 16, further comprising forming the connecting portion of the movable member so as to increase the deflection of the movable member so that the resistance to the deflection is increased. 18. The control method of claim 16 further comprising the transducer having a thin clamped silicon diaphragm. 19. The control method according to claim 16, wherein the third-order nonlinearity in one direction is balanced by selecting transducer parameters that produce third-order nonlinearity in different directions. 20. The capacitive gap spacing of the transducer is further selected to determine non-linearity in the one direction and the stiffness of the movable member is selected to determine non-linearity in different directions. 19 control methods. 21. The control method according to claim 19, further comprising selecting gap spacing and stiffness parameters to balance the non-linearities in the one direction and the different directions with each other. 22. A converter constructed according to the control method of claim 16.