GB2394289A

GB2394289A - Force sensitive devices for measuring bolt forces

Info

Publication number: GB2394289A
Application number: GB0223788A
Authority: GB
Inventors: John Karl Atkinson; Russell Paul Sion
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-10-14
Filing date: 2002-10-14
Publication date: 2004-04-21
Anticipated expiration: 2022-10-14
Also published as: GB2394289B; GB0223788D0

Abstract

A method for designing force sensors, such as those suitable for measuring bolt forces, which overcomes some of the difficulties usually associated with such designs by ensuring that the force applied to the sensitive layer can be made to be only a fraction of the total force applied to the sensor. In this way the sensor can be used to measure forces that would normally result in damage or offset shift if the whole force were to be applied to the sensing element. This is achieved by the suitable location of a force sensitive material sandwiched between force transmitting elements such that the transmitted force is shared between the force sensitive material and adjacent layers of material having a similar thickness and similar mechanical properties.

Description

An improved method for designing force sensitive devices Introduction

5 In designing sensors for the measurement of force, it is usually important to ensure that any applied force does not cause the material comprising the sensor to be stressed beyond its elastic limit. This is particularly the case with most piezoelectric and piezoresistive sensors since the sensor is likely to be damaged or to undergo an irreversible change in its operating point (also referred to as a change in offset) as a result of the over stressing.

At the same time, for any given full-scale force, it is usually desirable for the sensor material to be subjected to the maximum stress that it can withstand in order to maximise the sensitivity of the device. These constraints generally mean that the dimensions of the sensor must change as a function of the designed full-scale force value to be measured.

This invention describes a means whereby the dimensions and the placement of force sensitive materials used in the construction of a force sensor can be optimised within the constraints imposed by the need to maximise sensitivity and minimise damage or offset changes.

Background

Certain types of force sensor involve the use of layers of piezoelectric or piezoresistive materials in their construction. These force sensitive layers are often deposited onto a 25 supporting substrate using a variety of different methods such as screen-printing, spinning, dipping, gluing, vapour deposition etc. The supporting substrate can take the form of a force-

transmitting member such that the force sensitive material can be sandwiched between this and another force-transmitting member to enable force to be applied to the force sensitive material. A typical example of this arrangement is one where the force transmitting members

( take the form of washers suitable for use with a bolt such that the force exerted by tightening a nut onto the bolt can be measured.

It is also often the case that the force sensitive material can be arranged as a layer such that 5 any changes in the electrical properties of the force sensitive layer can be monitored by a measuring circuit. The measured electrical property may be electrical resistance in the case of a piezoresistive material or released charge in the case of a piezoelectric material for example.

The connection of the force sensitive layer to the measuring circuit is usually effected using conductive electrode materials that are also deposited onto the supporting substrate.

The electrically connected force sensitive layer may take the form of a sandwich structure as shown in Fig. 1 whereby the force sensitive material 13 is sandwiched between two conductive electrodes 11 and 12 and deposited onto a supporting substrate 14. The measured force may then be applied to the sandwich structure as illustrated by the arrow shown in Fig. 1 through 15 the use of a suitable force-transmitting member brought into contact with the sandwich structure. The measuring circuit to which the force sensitive material is connected can then be used to determine changes in the electrical properties of the force sensitive material and hence enable the applied force to be measured.

20 Fig.2 illustrates a different arrangement whereby the force sensitive material 23 is connected to the measuring circuit by conductive electrodes 21 and 22 that are also deposited onto a supporting substrate 24. In a similar manner to that described above, the measuring circuit may then be used to determine any modification of the force sensitive material's electrical properties brought about by an applied force.

In both the cases illustrated in Fig. 1 and Fig.2, it is also possible to subject the force sensitive material to stress by, for example, bending the supporting substrate and hence inducing a measurable change in the electrical properties of the material such that the bending force can be measured. Although widely employed in force measurement techniques this method is not 30 particularly relevant to the invention described here.

( The examples described above illustrate that several different configurations can exist for the orientation of the plane of the applied force relative to the plane of the measured electrical property of the force sensitive material. In each instance different levels of sensitivity can be obtained depending on the type of the force sensitive material employed. It is also apparent 5 that force can be applied directly to the force sensitive material to induce changes in electrical properties as opposed to methods whereby the force sensitive material is subjected to stress as a result of a bending force for example.

For the method whereby force is applied directly to the force sensitive material, the stress 10 experienced by the sensor material is related to the applied force by the expression: cr = F Eqn 1.

where is the stress (in Pascals), F the applied force (in Newtons) and A the area (in square 15 metros) over which the force is applied. Consequently for a given maximum value of force the area term must be chosen to ensure that the maximum allowable value of stress is not exceeded. In practice a safety factor is usually employed and the area is designed to be slightly larger than that calculated from the maximum allowable stress value. Also the maximum value of force used for the calculation is generally chosen to be a factor higher than 20 the fullscale maximum value to afford some degree of overload protection for the force sensor. The exact factor used often depends on the application, with larger factors being used in safety-critical applications, but typically may lie between one and two times the full-scale maximum force value.

25 Consequently for the measurement of very large forces, such as those typically encountered with nut and bolt fasteners, it is often necessary to employ large surface area sensors if avoidance of sensor damage and offset changes is to be achieved. This can pose difficulties with the design of the sensor.

( If the sensor was a sandwich type piezoresistor, for example, the requirement for a large surface area could result in low values of resistance being obtained since the resistance is given by: R = P Eqn.2 s where p is the resistivity of the material, I is the length of the resistor (i.e. thickness of the sandwiched material) and A is the crosssectional area.

Low values of resistance can give problems arising from excessive power dissipation, which 10 is inversely proportional to resistance. This problem is exacerbated due to the fact that, for reasons of maximising sensitivity, it is desirable to use as large an excitation voltage as possible. Hence lower values of resistance will place a lower ceiling on the maximum usable excitation voltage (to which power dissipation is directly proportional) and consequently limit the sensitivity of the sensor.

Among other reasons such as efficiency and cost, another important reason for limiting the power consumption is avoidance of the undesirable phenomenon of self-heating, whereby the sensor's resistance is altered by the dissipated power thus causing erroneous measurements.

20 Further problems can arise if the required surface area of the force sensitive layer is so large as to make the resulting sensor impractical in use. This is also the case for piezoelectric devices where an additional problem due to excessive charge leakage over large areas can also present difficulties.

25 In the case of bolt force sensors a further constraint is placed upon the designer in that the sensor must be dimensioned to fit standard bolt sizes. For example, a sufficiently large inner radius to accommodate the bolt must be incorporated whilst the outer radius must not be impractically large thereby restricting applicability. These constraints consequently limit the

available area of supporting substrate to that of a relatively narrow annulus.

( Bolt force sensors employing some of the methods described above have been successfully realised to fit standard bolts (UK Patent No.2310288) but are limited in the range of forces they can withstand due to the problems described.

Claims

( Description of the invention The present invention describes a method for designing force sensors, such as those suitable for measuring bolt forces, which overcomes some of the difficulties usually associated with 5 such designs. Since the effective area of a force sensitive layer over which the force is applied determines the stress experienced by the layer for any given force, then if the force applied to the sensitive layer can be made to be a fraction of the total force applied to the sensor the sensor can be used to measure forces that would normally result in damage or offset shift if the whole force were to be applied to the sensing element. This can be achieved by the suitable location of a force sensitive material that is sandwiched between force transmitting elements such that the transmitted force is shared between the force sensitive material and adjacent layers of material having similar thickness and modulus of elasticity. If the relative stress profile across the interface of the force transmitting elements 15 and the force sensitive material and adjacent layers is known then it is possible to design the relative areas of the force sensitive material and adjacent layers so as to result in a suitable level of stress being experienced by the force sensitive material for any given applied force. This method is illustrated in the embodiment described below. 20 Fig.3 shows a cross-sectional view of the construction of a typical bolt force sensor wherein two force-transmitting members 31 and 32 are arranged so as to sandwich a layer of force sensitive material 33 such that a portion of any force produced by tightening a nut 34 onto a bolt 35 will be transmitted to the force sensitive layer and hence cause changes in the electrical characteristics of the force sensitive layer. These changes in the electrical 25 characteristics of the force sensitive layer, such as resistance, capacitance, inductance or developed charge for example, can be measured by an electrical circuit to which the force sensitive layer is connected by electrodes 36 and 37. For clarity, the electrical interconnection between the electrodes and a measuring circuit has been omitted from Fig.3. ( Additional layers of a material 38, preferably having similar mechanical properties, and in particular their modulus of elasticity, to those of the force sensitive material 33, are arranged to fill the space between the force-transmitting members 31 and 32 that is not occupied by the force sensitive material. In this way the layer between the two forcetransmitting members has 5 approximately equal width over the area of their interface. Fig.4 shows an embodiment whereby a force sensitive layer 43 forms an annulus bordered by an inner and an outer annulus of a spacer material 48 designed to have similar mechanical properties to the material used for the force sensitive layer. Preferably the force sensitive 10 material can be used to form both the force sensitive layer 43 and the spacing layer 48 by employing the technique of simultaneously depositing both layers during the fabrication of the sensor. The force sensitive layers can be differentiated from the spacing layers by leaving a small gap 15 between the adjacently deposited layers. This arrangement has the added benefit of reducing the possibility of crack propagation in the force sensitive layer since any such crack propagating from an area of high stress, in an inner annulus of spacing material for example, to an area of lower stress is likely to terminate at the gap. The gaps should ideally be as small as possible however so as not to compromise the optimization of the sensor design through the 20 introduction of any significant discontinuities in the force distribution profile across the surface of the force sensitive layer 43 and the spacing layer 48. The fabrication of a small gap can also be facilitated by the simultaneous fabrication of the sensing layer 43 and spacing layers 48 in that any potential dfflculties of alignment can then 25 be avoided if both sets of layers are simultaneously deposited using a masking arrangement to fabricate the gaps. It is also possible to omit the gaps and differentiate the force sensitive layer from the surrounding adjacent layers simply by suitable patterning of electrodes that make the ( connections between the force sensitive layer and an electrical measuring circuit onto a continuous layer of force sensitive material. Fig.5 shows a typical plot of relative stress patterns developed along a radius at the interface 5 of the two force-transmitting elements in a bolt force sensor designed as described above, when it is subjected to a force by the action of tightening a nut onto the bolt. The plot was derived from a finite element analysis of the forces. It can be seen that the stress appearing at the interface of two force-transmitting elements rises to a maximum close to the central axis of the bolt and then diminishes with distance away from this axis. This is further illustrated in Fig.6 where the stress (o) along the interface of two force transmitting members 61 and 62 is plotted as a function of distance from the central axis of a bolt 65 and superimposed onto a drawing of a bolt force sensor. It can be seen that if the force sensitive element is located within the area 63 bounded by the dashed lines XX and YY as 15 shown, then it will experience an effective overall stress Os that is approximately the average of stresses o and (S2. Thus it is possible to locate the force sensitive element at a position that results in it being subjected to an optimum level of stress for any given applied force resulting from the tightening of a nut 64 onto the bolt 65. Claims

1. A method for designing mechanical force sensors such that the stress experienced by a material having electrical characteristics that are force sensitive can be optimised and 5 characterised in that the placement and dimensions of the force sensitive material comprising the sensor are determined by the stress profile developed along the interface of two opposing force transmitting members and wherein a layer of the force sensitive material is sandwiched between the opposing force transmitting members in an arrangement such that an active section of the material experiences a fraction of the 10 total stress experienced by the whole of the layers located between the opposing force transmitting members.

2. A device designed according to the method of claim 1 wherein the force sensitive material is a piezoresistive material comprising one or more force sensitive resistors.

3. A device designed according to the method of claim 1 wherein the force sensitive material is a piezoelectric material.

4. A device designed according to the method of claim 1 wherein the two opposing force 20 transmitting members are formed as washers suitable for use on a bolt.

5. A device designed according to the method of claim I wherein the force sensitive material is a piezoresistive material deposited onto a supporting substrate using a layer deposition technology, such as thick film screen printing, thin film deposition or other 25 methods.

6. A device designed according to the method of claim I wherein the force sensitive material is a piezoresistive material deposited onto a supporting substrate in the form of a washer using a layer deposition technology, such as thick film screen printing, 30 thin film deposition or other methods.

7. A device designed according to the method of claim 1 wherein the force sensitive material takes the form of a patterned layer so as to produce an area of material that can be used for sensing purposes and that is immediately adjacent to one or more areas 5 of similar thickness.

8. A device designed according to the method of claim 1 wherein the force sensitive material takes the form of a patterned layer so as to produce an area of material that can be used for sensing purposes and is immediately adjacent to one or more areas of 10 similar thickness and separated from them by a small gap.

9. A device designed according to the method of claim 1 wherein the force sensitive material takes the form of a patterned layer so as to produce an area of material that can be used for sensing purposes wherein the sensitive layer is defined by the 15 deposition of an electrode pattern onto a part or parts of a continuous layer of material.