CN101408409A - High precision digital type linear displacement transducer - Google Patents

Abstract

The invention discloses a high-precision digital linear displacement sensor, which comprises a motor-driven grating disc, a light-transmitting scribe line is arranged on the grating disc, a linear track is arranged above the grating disc, and a reflective infrared sensor is respectively fixed at both ends of the linear track. A photoelectric sensor, another reflective infrared sensor is connected with the moving object on the linear track, and the side of the grating disc facing the reflective infrared sensor is a bright surface. The invention has the advantages of large measurement range, simple structure, convenient installation, low cost, high measurement accuracy and resolution, no need for zero adjustment, and direct direction judgment through software.

Description

High precision digital linear displacement sensor

Technical field:

The invention relates to a linear displacement sensor, in particular to a digital linear displacement sensor with high precision.

Background technique:

In industrial production, especially in aerospace, automatic ordnance equipment and other fields, linear displacement detection is required to have higher accuracy and resolution, which greatly enhances the requirements for linear displacement sensors. There are many types of sensors currently used for linear displacement measurement, but the existing sensors have more or less problems in practical applications, some of which are complex and costly, some have high environmental requirements, and some have low precision. The linear range is small, and some have complex structures and high process requirements. For example, capacitive sensors have simple structure and fast dynamic response, but they are easily affected by parasitic capacitance and external interference. The inductive sensor has simple structure, large output power, small output impedance and strong anti-interference ability, but it has slow dynamic response and is easily disturbed by magnetic field. The magnetic grid sensor is easy to install and use, low in cost, low in single precision, needs to be shielded when used, and has poor anti-interference ability. The grating ruler has high measurement accuracy and good controllability, and is widely used. Such as Germany HEIDENHAIN, Japan MITUTOYO and other companies produce linear displacement grating scale accuracy and resolution is very high. However, the grating ruler has high requirements on the use environment, requiring the grating to run smoothly, without sudden changes and relatively low speed, and not to be polluted by dust, oil and water vapor on the industrial site. Domestic grating sensors are heavy, have low precision, and unstable performance, while imported high-precision grating scales are expensive. In addition, both the optical fiber linear displacement sensor and the laser sensor have the problem of high cost.

Invention content:

The invention provides a high-precision digital linear displacement sensor with simple structure, convenient installation, low cost, high measurement accuracy and resolution.

The technical solution adopted by the present invention to solve its technical problems is:

A high-precision digital linear displacement sensor is characterized in that it includes a motor-driven grating disc, a light-transmitting scribe line is arranged on the grating disc, a linear track is arranged above the grating disc, and a reflective infrared sensor is fixed at both ends of the linear track. A photoelectric sensor, another reflective infrared sensor is connected with the moving object on the linear track, and the side of the grating disc facing the reflective infrared sensor is a bright surface.

The beneficial effects of the high-precision digital linear displacement sensor of the present invention are mainly manifested in:

1. The structure is simple, and the grating disk is easy to manufacture. It only needs to open a light-transmitting scribe line, and the line width does not need to be very narrow, as long as the reflective photoelectric sensor used can work. Easy to install and realize digital non-contact detection.

2. Low cost and large measurement range.

3. High precision and resolution. Under the premise of keeping the motor speed stable, high-frequency pulse filling method can be used to obtain high precision.

4. There is no need for zero adjustment before measurement, and no direction judgment circuit, and the direction can be directly judged by software, which facilitates the detection process.

Description of drawings:

The present invention will be further described below in conjunction with drawings and embodiments.

Fig. 1 is a structural diagram of a high-precision digital linear displacement sensor of the present invention;

Fig. 2 is a schematic top view 1 of the measurement principle of the linear displacement sensor of the present invention;

Fig. 3 is the timing diagram 1 of the measurement principle of the linear displacement sensor according to the present invention;

Fig. 4 is a schematic top view 2 of the measuring principle of the linear displacement sensor according to the present invention;

Fig. 5 is a top view schematic diagram 3 of the measuring principle of the linear displacement sensor according to the present invention;

Fig. 6 is the timing diagram 2 of the measuring principle of the linear displacement sensor according to the present invention.

Fig. 7 is a schematic top view 4 of the measurement principle of the linear displacement sensor according to the present invention;

Fig. 8 is a structural block diagram of the sensor interface circuit of the present invention;

Fig. 9 is a schematic diagram of a high-frequency pulse counting circuit when the sensor interface circuit of the present invention uses 74HC4040 as a counter.

Detailed ways:

A high-precision digital linear displacement sensor, including a grating disk 5 driven by a motor 6, a light-transmitting scribe line 7 is arranged on the grating disk, a linear track 4 is arranged above the grating disk, and a reflective infrared sensor is fixed at both ends of the linear track. Photoelectric sensors 1 and 2 (static sensors), and another reflective infrared sensor 3 (moving sensor) are connected with the moving object on the linear track, and the side of the grating disk facing the reflective infrared sensor is a bright surface.

The circular grating used has only one light-transmitting engraved line, and the grating disk is made of materials such as matt aluminum with good reflective performance, and the side facing the reflective photoelectric sensor is a bright surface. When the light-transmitting line passes the reflective photoelectric sensor, the sensor outputs a low-level pulse. When the engraved line does not pass through the reflective photoelectric sensor, the light emitted by the infrared emitting tube of the sensor is reflected by the grating disk, and the receiving end outputs a high level. 4 is the straight track (track) of the object, the measured object runs straight along the track (track), the reflective photoelectric sensors 1 and 2 are fixed at both ends of the track, and the reflective photoelectric sensor 3 follows the movement of the measured object. For the convenience of analysis, the trans-type photoelectric sensors 1 and 2 are referred to as "static sensors" hereinafter (the photoelectric sensor 1 is called the working static sensor, and the photoelectric sensor 2 is called the auxiliary static sensor), and the reflective photoelectric sensor 3 is referred to as the "dynamic sensor". ", the speed of the micro synchronous motor is called "synchronous speed" for short.

1. The moving direction of the measured object is far away from the working static sensor

The top view of the measurement principle is shown in Figure 2. The synchronous steering is clockwise, n is the synchronous speed, and the initial position of the measured object is at A0. After the static sensor 1, the dynamic sensor, and the static sensor 2, three pulse signals will be output accordingly (assuming that they are all negative pulse signals after processing).

The timing diagram of the sensor measurement principle is shown in Figure 3. Before the measurement, after the synchronous motor rotates, the output pulse signals of static sensor 1, dynamic sensor and static sensor 2 are shown as pulse A, B and D in Fig. 3 respectively. The initial phase difference between the static sensor 1 and the dynamic sensor is θ ₁ . At this time, we fill the high-frequency pulse between the two pulses, start counting the high-frequency pulse at the falling edge of the static sensor 1 pulse, and stop counting at the falling edge of the moving sensor (as shown by pulse C in Figure 3), then the counted high-frequency pulse The number of pulses N ₁ is proportional to θ ₁ . Similarly, high-frequency pulses are filled between the output pulses of two static sensors, assuming θ=θ ₁ +θ ₂ +θ ₃ +θ ₄ , then the number of high-frequency pulses N ₂ is proportional to θ. When the grating reticle continues to rotate and contacts the static sensor 1 again, the number N ₈ of high-frequency pulses counted by the synchronous motor rotating 360° can be obtained. Available:

${\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="A20081023632800161.png,A20081023632800171.png"\; state="\{\{state\}\}">L</figure-callout>}}_1=\frac{{\mathrm{<figure-callout\; id="5"\; label="L"\; filenames="A20081023632800161.png,A20081023632800201.png"\; state="\{\{state\}\}">L</figure-callout>}}_5\sin {\mathrm{\&theta;}}_1}{\sin (180-{\mathrm{\&theta;}}_1-{\mathrm{\&theta;}}_5)}$ (Assume that θ ₅ and L ₅ are known) (2)

When the measured object has a linear displacement, it drives the dynamic sensor to move synchronously, assuming that it moves from position A ₀ to position A ₁ . At this time, the output pulse of the static sensor remains unchanged, and the output pulse of the dynamic sensor is shown as F in Figure 3. At this time, we fill the high-frequency pulses between the two pulses as shown in Figure G, then the number of pulses H ₅ is proportional to θ ₁ + θ ₂ , and N ₄ = N ₅ -N ₁ is proportional to θ ₂ . Available:

${\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="L"\; filenames="A20081023632800201.png"\; state="\{\{state\}\}">L</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="5"\; label="L"\; filenames="A20081023632800161.png,A20081023632800201.png"\; state="\{\{state\}\}">L</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 5</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 180</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 5</span>}}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

Similarly, when the object under test moves from position A ₁ to position A ₂ with A ₁ as the initial point, the output pulse waveform of the motion sensor is shown in Figure 3 H, by counting the high frequency between the falling edges of the two pulses

The number of pulses is N ₇ (as shown in Figure 1). At this time, the number of pulses represented by the new angular displacement is N ₆ =N ₇ -N ₅ . Then the angular displacement θ ₃ of the object to be measured is:

Then the linear displacement of the object motion is:

${\mathrm{<span\; class="notranslate">\; <figure-callout\; id="3"\; label="L"\; filenames="A20081023632800161.png,A20081023632800201.png"\; state="\{\{state\}\}">L</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="5"\; label="L"\; filenames="A20081023632800161.png,A20081023632800201.png"\; state="\{\{state\}\}">L</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 5</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 180</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 5</span>}}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 6</span>}<span\; class="notranslate">\; )</span>$

The general formula for linear displacement measurement can be obtained by the above method:

$\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; G</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&theta;</span>}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 180</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&theta;</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; G</span>}}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; Q</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 7</span>}<span\; class="notranslate">\; )</span>$

In the formula, L-displacement of the measured line; L _G -the distance between the _motor shaft center and the center of the static sensor 1; θ _C -the initial moment phase difference between the dynamic and static sensors; The angle between the connecting line and the straight track; θ - the phase angle generated by the linear movement of the moving sensor; L _Q - the distance between the static and moving sensors at the initial moment; N _B - the number of high-frequency pulses between the static and moving sensors at this moment; N _C - The number of high-frequency pulses between static and dynamic sensors at the initial moment; the number of high-frequency pulses in the range of N-360 degrees.

It should be explained here that the "number of high-frequency pulses between static and dynamic sensors at the initial moment" is not a fixed number. If the initial moment is defined at A ₀ , then N ₁ is "the number of high-frequency pulses between static and dynamic sensors at the initial moment"; if the initial moment is defined at A ₁ , then N ₄ is "the number of high-frequency pulses between static and dynamic sensors at the initial moment" . It depends on the measurement needs, and different functions can be realized by software in the interface circuit program.

The dynamic measurement method is used here, that is, the synchronous motor keeps rotating, and the controller continuously records the number of high-frequency pulses between the static and dynamic sensors, and constantly updates and displays the calculation results according to the formula. Through the display, the linear displacement of the object can be measured regularly and the dynamic process of the linear movement of the measured object can be observed.

The accuracy and resolution of measurement results depend on the frequency of high-frequency pulses and the degree of uniform rotation of the synchronous motor. As long as the frequency of high-frequency pulses is high enough and the synchronous motor rotates at a uniform speed, theoretically high measurement accuracy and resolution can be obtained.

Here we can also see that the role of the static sensor 2 is redundant, and only one static sensor (working static sensor) and one dynamic sensor are needed to complete the linear displacement detection. In our design, we still use two static sensors, which can facilitate users to use different detection methods to complete more functions.

And the position of the linear track and the grating disc will not affect the measurement, as long as the linear track does not pass through the center of the circle and is within the range of the disc, it can be detected. As shown in Figure 4, the linear track (trajectory) is detected at any position by a single-reflection infrared sensor, and formulas 7 and 8 are also applicable.

2. The moving direction of the measured object is close to the working static sensor

When the moving direction of the measured object gradually approaches the working static sensor, as shown in Figure 5. The initial position of the dynamic sensor is at A _0. After the transparent reticle passes through the static sensor and then the dynamic sensor, two pulses will be output, as shown in A and B in Figure 6. The phase difference between the two pulses is the initial phase difference between the two sensors θ=θ ₁ +θ ₂ +θ ₃ . At this time, high-frequency pulse counting is started at the falling edge of the static sensor pulse, and counting is stopped at the falling edge of the moving sensor (as shown by pulse C in Figure 6), and the counted high-frequency pulse number _N1 is proportional to θ. It is assumed that the initial distance between static and dynamic sensors is L=L ₁ +L ₂ +L ₃ .

When the measured object produces a linear displacement, it will drive the dynamic sensor to move synchronously, assuming that it moves from position A ₀ to position A ₁ . At this time, the output pulse of the static sensor remains unchanged, and the output pulse of the dynamic sensor is shown as D in Figure 6. At this time, we fill the high-frequency pulses between the two pulses as shown in Figure E, and the number N ₃ of high-frequency pulses is calculated to be proportional to (θ ₂ +θ ₃ ).

N ₂ =N ₁ -N ₃ can be obtained through subtraction, as shown by C in the figure. _N2 is proportional to the angular displacement _θ1 . And the number N ₆ of high-frequency pulses counted when the synchronous motor turns 360° can be obtained. Then the calculation formula is as follows:

L ₁ =LL ₂ -L ₃ (13)

Similarly, when the object under test moves from position A ₁ to position A ₂ with A ₁ as the initial position, the output pulse waveforms of the static sensor and the dynamic sensor are shown in D and F in the figure, and the two pulses drop by counting The number of high-frequency pulses between edges is N ₅ (shown as G in the figure). Available:

L ₂ =L ₂ +L ₃ -L ₃ (17)

The general formula for linear displacement measurement can be obtained by the above method:

$\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; Q</span>}}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; G</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; B</span>}}}{\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 180</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; G</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; B</span>}}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 18</span>}<span\; class="notranslate">\; )</span>$

L-displacement of the measured line; L _G -the distance between the motor shaft center and the static sensor; θ _G -the angle between the line formed by the center of the static sensor and the motor shaft and the straight track; L _Q -the distance between the static and dynamic sensors at the initial moment N _B - the number of high-frequency pulses between the static and dynamic sensors at this moment; N _G - the number of high-frequency pulses between the static and dynamic sensors at the initial moment; _N - the number of high-frequency pulses in the range of 360 degrees; Phase difference between sensors.

3. The moving direction of the measured object is unknown or uncertain

The synchronous steering is provided by the synchronous motor in the sensor, and the direction is fixed (assuming it is fixed in clockwise direction). However, the moving direction of the measured object is unknown or uncertain in many cases. In this case, the moving direction of the object must be judged first. The method of discrimination is very simple: when the grating line passes through the static and dynamic sensors for the N-1 time, the number of high-frequency pulses between them is obtained, and it is recorded in the interface circuit software as "the last high-frequency pulse between static and dynamic sensors". number". Then record the number of high-frequency pulses obtained after the grating reticle passes through the static and dynamic sensors for the Nth time as "the number of high-frequency pulses between the static and dynamic sensors at this moment". Subtract the "high-frequency pulse number between static and dynamic sensors last time" from "the number of high-frequency pulses between static and dynamic sensors at this moment". If it is positive, it means that the measured object is moving away from the working static sensor. At this time, use 7, 8 to calculate the line displacement. If it is negative, it means that the object moves towards the direction close to the working static sensor. At this time, formulas 18 and 19 are used to calculate the linear displacement. When the grating reticle passes through the static sensor for the N+1th time, the Nth "number of high-frequency pulses between static sensors at this moment" is taken as the "Number of high-frequency pulses between static sensors last time" for the N+1th time. ” to calculate and judge. If you only need to calculate the linear displacement and do not need to know the direction of movement, there is no need to be so cumbersome, just always use formulas 7, 8 or 18, 19, and take the absolute value of the result.

In a more general situation, the synchronous speed is not fixed, and the moving direction of the object is not fixed, and the situation shown in Figure 7 will appear: the initial angle between the static and dynamic sensors is greater than 180°. To be able to use equations 7, 8 or 18, 19 for calculations, the synchronous steering should be reversed. Therefore, the inventive sensor usage process can be obtained:

① Make the miniature synchronous motor rotate before detection, and identify it through the program in the microcontroller of the interface circuit, so that the pulse obtained by the grating line passing through the static sensor for the first time is used as the initial pulse, and the output pulse of the dynamic sensor always lags behind the output of the static sensor pulse. And calculate the initial distance between the static and dynamic sensors and the corresponding angle.

②If the initial angle between the static and dynamic sensors is less than 180°, no processing will be done; if it is greater than 180°, the steering of the synchronous motor will be changed, and the work of step "①" will be repeated.

③ Determine the moving direction of the object. If the object is moving towards the approach of the working static sensor, the linear displacement is calculated using Equations 18 and 19. If the object moves away from the working static sensor, formula 7 and 8 are used to calculate the linear displacement.

When measuring, the number of high-frequency pulses between two static sensors can also be used as a reference instead of the number N of high-frequency pulses in the 360-degree range.

4. Analysis of sensor performance and advantages and disadvantages

4.1 Sensor precision, accuracy and resolution analysis

For linear displacement sensors, measurement precision, accuracy and resolution are the most important technical indicators. Measurement precision refers to the degree of agreement (coincidence) between the measured values after repeated measurements of the measured value under the same conditions. From the perspective of measurement error, precision reflects the random error of the measured value. In this system, the measurement precision mainly depends on the uniformity of the synchronous rotational speed.

该角度所对应的线位移也是非常小的，因此线位移测量分辨率和精度都很高。而且电机转速越慢，系统分辨率越高，精度也越高。但电机转动速度越小，系统响应越慢。为了提高系统响应速度，应使电机快速旋转，此时可以通过增加高频脉冲频率来提高系统精度和分辨率。Measurement precision (accuracy) refers to how close the measured value of a measurand is to its "true value". From the perspective of measurement error, the accuracy reflects the systematic error of the measured value. At this point, let’s calculate the theoretical resolution and accuracy that the system can achieve. Assuming that the synchronous motor speed in the sensor is 200r/min (3.3r/s), the number of high-frequency pulses that the motor drives the grating reticle to rotate 360 degrees is : $\frac{1/3.3}{1/(25\&times;{10}^6)}\&ap;7575757$ indivual. Then the theoretical minimum resolvable angle is

The linear displacement corresponding to this angle is also very small, so the resolution and precision of linear displacement measurement are very high. And the slower the motor speed, the higher the system resolution and the higher the accuracy. But the smaller the motor rotation speed, the slower the system response. In order to improve the response speed of the system, the motor should be rotated rapidly. At this time, the system accuracy and resolution can be improved by increasing the high-frequency pulse frequency.

4.2 Analysis of the advantages and disadvantages of the new linear displacement sensor system

The advantages of the new linear displacement sensor are as follows:

(1) The measurement range is large. It is only necessary to ensure that the moving range of the object is within a circle with the grating line as the radius. Increasing the length of the grating line can increase the range.

(2) Simple structure, convenient installation and low cost.

(3) The measurement accuracy and resolution are high.

(4) There is no need for zero adjustment before measurement.

(5) No additional direction judgment circuit is needed, and the direction is directly judged by software.

There are also shortcomings and improvement methods in this system design:

(1) There is a certain systematic error

Mainly electrical error and synchronous speed error. The electrical error is composed of two parts. The controller used in the system enters the interrupt after receiving the signal from the moving sensor, and at the same time sends out the latch signal to read the number of high-frequency pulses. It takes a certain time. This time will cause the number of high-frequency pulses read to be more than The actual number of pulses forms the error of the measurement system, which can be reduced by improving the controller (such as selecting CPLD, FPGA or DSP as the controller). In addition, when the controller performs sine calculation, there will be certain errors when using the look-up table method, and the error can be reduced by improving the algorithm and using a high-end controller. The synchronous speed error is mainly due to the high-frequency pulse count error caused by the uneven synchronous speed, and the error can be reduced by selecting a motor with high speed uniformity.

(2) Slow response speed

When the measured object produces a linear displacement, the sensor cannot respond immediately, but can only respond when the reticle rotates to the new displacement. The response speed can be improved by increasing the synchronous speed. Or use the sensor for timing measurements.

(3) Not suitable for measuring the displacement of high-speed objects

5. Sensor interface circuit design

5.1 Overall block diagram of hardware design

In the design, the combination of AT89S51 single-chip microcomputer and high-speed counter 74HC4040 or CPLD can effectively reduce the system cost and enhance the scalability of the system. The overall block diagram of the interface circuit is shown in Figure 8.

5.2 Signal processing module

If a reflective infrared sensor is used as a position sensor, a comparator such as LM339 can be used to shape the output signal of the sensor.

5.3 High frequency counting module

The working frequency of 74HC4040 can reach 50MHZ. The high-frequency pulse counting circuit designed in this subject is shown in Figure 9. The counting unit uses a 25M active crystal oscillator as the high-frequency pulse input, and triggers the 74HC4040 to start counting through the falling edge of the pulse. Since the 74HC4040 is a 12-bit binary counter, the system adopts two cascade connections, and connects the highest bit of the first chip to the CLK terminal of the second chip, thus using the high four bits of the first chip and the low The four bits together form a 16-bit binary counter. However, if only 16-bit counting is used, the system requirements cannot be met, because the time required for the 16-bit counter to be full is $\frac{1}{25\&times;{10}^6}\&times;2^{16}=0.002621$ seconds, which means that the time interval between the sensor grating reticle passing through the moving sensor and the static sensor must be less than 0.002621 seconds, and the synchronous motor cannot reach such a high speed. So we connect the Q4 of the second 74HC4040 to the T0 terminal of the single-chip microcomputer. After counting 2 ¹⁶ numbers, Q4 has a falling edge, which triggers T0 counting. Since T0 itself is a 16-bit binary counter, the above structure can form a 32-bit binary counter. If the time required to complete is $\frac{1}{25\&times;{10}^6}\&times;2^{32}=171.8$ seconds, can meet the requirements. At the same time, the 25M clock is divided by 2 ¹⁶ , and the counting frequency of the T0 terminal is 381HZ, which can also meet the requirements.

In addition, CPLD can also be used as a high-frequency counting module. CPLD has the characteristics of high frequency, high integration, strong anti-interference ability, good reliability, and strong maintainability. Although the use of CPLD increases the system cost, it improves the system performance. The present invention selects EPM7032 produced by ALTERA Company as the CPLD module in the hardware design. In the experiment, the CPLD is used to complete the counting function, and the single-chip microcomputer is used to complete the calculation and display storage functions.

5.4 Display module and data storage module

The digital tube dynamic display method is adopted, and the 8255 expands the I/O port of the single-chip microcomputer. Among them, PA port outputs segment code, PB port and PC port output bit code. A reverse driver 75452 is added to the bit scanning port to provide sufficient driving current, and the segment data port is connected to each segment of the LED through the same direction driver 7407. The historical data of linear displacement measured in this system should be saved so that users can inquire. For this reason, non-volatile memory 24C04 is used in the hardware system to ^save these information. read and write.

Claims (1)

1. A high-precision digital linear displacement sensor, which is characterized in that it includes a motor-driven grating disc, a light-transmitting scribe line is arranged on the grating disc, a linear track is arranged above the grating disc, and a reflector is fixed at both ends of the linear track. Infrared photoelectric sensor, another reflective infrared sensor is connected with the moving object on the linear track, and the side of the grating disc facing the reflective infrared sensor is a bright surface.

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