GB2326719A

GB2326719A - Force sensitive devices

Info

Publication number: GB2326719A
Application number: GB9813171A
Authority: GB
Inventors: John Karl Atkinson; Andrew William James Cranny; Russell Paul Sion
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-06-19
Filing date: 1998-06-19
Publication date: 1998-12-30
Anticipated expiration: 2018-06-19
Also published as: GB2326719B; GB9712835D0; GB9813171D0

Description

Improvements in or relating to force sensitive devices Introduction This patent describes improvements made to a device which enables the force experienced by a simple force sensor to be converted into an electrical signal which is then capable of being used as a measure of the degree of force experienced by an object to which the sensor is attached. For example the invention may be used to realise more easily a force sensitive washer which can be mounted between a retaining nut and an internal combustion engine cylinder head block in such a manner as to enable the pressure pulses caused by the engine to be measured.

Background of the Invention The electrical measurement of force is particularly useful in certain application areas where for example it is desirable to implement some degree of control over a process or operation that produces the force. One such example is the internal combustion engine where the measurement of the force experienced at certain strategic locations on the engine block or cylinder head, for example, can be used to determine the pressures exerted by the engine's firing or combustion cycle.

The exact timing of the point of maximum pressure is particularly useful information which can be gainflilly employed in an engine management system such as might be realised by a controlling microcomputer. Hence a continuous electrical signal which is proportional to the engine pressures at all times is of particular use since this signal enables the precise timings of the various levels of engine pressure to be accurately measured.

Another attractive option in the control of internal combustion engines is the ability to control air/fuel mixture ratios to the point of extreme settings in order to obtain optimum performance.

Very often this strategy results in unwanted side effects such as the occurrence of engine-knock. A sensor capable of measuring engine pressures can be used in this context to enable the early detection of the onset of engine-knock and hence facilitate the so-called "lean burn" mode of operation of the engine that is popular among engine manufacturers.

There is a considerable amount of prior art in the area of the electrical measurement of force, particularly with regard to load sensing. Much of the prior art is public domain and methods for measuring force utilising piezoelectric materials such as quartz or lead zirconium titanate are well established. Another popular technique for measuring force is by utilising the piezoresistive effect exhibited by certain materials and the use of electrical resistance strain gauges in this context is very well known.

Of the piezoelectric devices, most of the devices employed in the prior art are inherently expensive due to the need for complex manufacture to render them suitable for mechanical mounting. In addition the associated electrical circuits needed to measure their output signal, which takes the form of released charge, are also complex and expensive. Further disadvantages to the use of piezoelectric devices are their poor response at low frequencies and their high susceptibility to the unwanted effects oftemperature variations.

Piezoresistive devices generally use comparatively inexpensive electrical circuits to measure the changes in resistance which result from the strain experienced due to applied force. A major problem in many applications however is the difficulty in mounting resistive strain gauges in a suitable mechanical arrangement. This latter consideration stems from the fact that it is often not possible to locate a resistive strain gauge at a point where it will experience sufficient strain to enable an accurate measurement to be made. Also the use of adhesives which is often necessary in the mounting of resistive strain gauges makes them particularly susceptible to the problem of creep between the gauge and the supporting structure. In particular metal foil resistive strain gauges, which are popular sensing elements in commercial force sensors, also suffer from a relatively low sensitivity.

The characteristics of piezoresistive devices used in the measurement of force can be significantly improved if the force sensing elements are fabricated as thick film resistors. For example it is possible to print force-sensing resistors onto an electrically insulating substrate and then sandwich them between this substrate and an electrically insulating material in such a way as to form a force sensing washer. By measuring the change in resistance of the force sensing resistors it is then possible to determine the degree of force applied to the washer.

In order to measure the change in resistance of the force sensing resistors it is usually necessary to connect them to an electrical circuit such as a Wheatstone's bridge for example. In this arrangement the change in resistance of the force sensing resistors relative to fixed valued resistors included in the bridge results in an out-of-balance condition in the bridge which produces an output voltage change proportional to the degree of applied force.

The exact choice of the fixed value resistors largely determines the characteristics of the resulting force sensor. For example a sensor with good thermal stability can be achieved if the fixed value resistors and the force sensing resistors share the same temperature characteristic and are subjected to the same thermal excursions. This can be achieved if the fixed value resistors are fabricated identically to the force sensing resistors and are located on the same substrate material but at a point of constant force regardless of the degree of force applied to the force sensing resistors.

Similarly for a full Wheatstone bridge configuration the optimum output signal change for any given applied force will be obtained when the resistance values of the fixed resistors are identical to the resistance values of the force sensing resistors with zero applied force. This condition can be conveniently achieved when for example the force sensing resistors and the fixed value resistors are simultaneously fabricated as thick film resistors where both types of resistor can be simultaneously printed with the same thick film material and to a common geometry and, in particular, thickness.

The present invention also overcomes some of the difficulties associated with achieving the conditions referred to above by making it possible to fabricate a force sensitive device in such a way that the fixed value resistors and the force sensing resistors can be co-located onto a common substrate in such a way that the force sensing resistors can be subjected to the applied force whilst the fixed value resistors are not. In this way it becomes possible to fabricate both the force sensing and fixed value resistors as almost identically matching devices located in such a way as to have a shared thermal environment.

Summary of the Invention According to the present invention a method is described for implementing a force sensitive device that can be realised as an electrically insulating substrate material, such as a ceramic or a metal coated with an electrically insulating layer of glass ceramic or other suitable material, fabricated, for example, in the shape of a washer, or any other suitable shape, onto which is deposited one or more thick film force sensitive resistors.

When the force sensitive resistors are subjected to an applied force, their resistance will change due to the changes in geometry experienced and any piezoresistive effects on the resistivity of the material comprising the resistors.

The relationship between the strain experienced (E) and the change in resistance (6R) of an electrical resistor of resistance R ohms, when the strain is applied in the same plane as the electrical current flow through the resistor, is given by the following equation: 8RiR = E (1 + 2v) + 8p/p (Equation 1 } Where v is Poisson's ratio for the material comprising the planar electrical resistor and 5p/p is the unit change in the resistivity of the material comprising the resistor due to the piezoresistive effect.

A preferred embodiment of the sensor described here is one where the material comprising the force sensing resistors is a thick film resistor paste. This confers several advantages including a strain sensitivity higher than that of most commercially available resistive strain gauges due to a higher value for the Sp/p term shown in Equation 1. Additionally it is possible to fabricate the sensor in such a way as to produce very small surface areas for the electrical resistors onto which the applied force can then be concentrated. This in turn concentrates the stress due to the applied force, and consequently the strain experienced by the force-sensing resistor for any given modulus of elasticity.

A preferred embodiment of the invention is also one where the force sensitive resistors are printed onto the electrically insulating substrate simultaneously with fixed value resistors in an interconnected arrangement that forms a Wheatstone's bridge for example, or some other suitable measurement circuit.

As previously mentioned, the optimum thermal characteristics and output signal change from a Wheatstone's bridge arrangement of force sensing resistors and fixed value resistors are obtained when the resistance value of the unstrained force sensing resistors is equal to the resistance value of the fixed value resistors. This can be explained with reference to figure 1 where an electrical connection of a force sensitive resistor R and a fixed value resistor RL is shown.

If an applied force produces a small change 8R in the resistance of the force sensitive resistor R, then for an applied voltage V, the output voltage Vo will experience a small change SVO as determined by the following expression:

(Equation 2) Setting X = RL, this expression can be re-written as follows:

(Equation 3) Which is maximised when the function f (X) = X + X + 2 (Equation4) x is at a minimum value.

Differentiating equation 4 with respect to X and setting equal to zero gives: 1 (EquationS) 1 ~ X2 = {Equation 5} Which is true when X = 1 and hence RL = R.

2 Differentiating equation 5 with respect to X gives X 3 which for X = 1 is positive hence showing that the function shown in equation 4 is in fact minimised when RL = R Hence the output signal, as defined in equation 2, is maximised when RL = R and the change in the output voltage is given by: (Equation 6) Vs 4R Figure 2 shows a plan view and a cross-section along the line BB of an embodiment of this invention where the electrically insulated substrate consists of a steel washer 1 onto which is printed and fired an electrically insulating layer of glass ceramic 2. A series of force sensitive resistors a, b, c, d, e, and f are printed onto the surface of the electrically insulated washer and interconnected electrically to form the Wheatstone's bridge as indicated schematically in Figure 3. In this particular example six separate force sensitive resistors are shown although other numbers may be used of course. Fixed value resistors g and h are also shown printed onto the electrically insulated washer to complete the Wheatstone's bridge. Again for the purposes of this example only two fixed value resistors are shown although other numbers may be used.

As previously explained, a preferred embodiment of the invention is one where the fixed value resistors have the same value of electrical resistance as the force sensing resistors connected in series when no force is applied to them. This then results in the arms of the Wheatstone's bridge having equal values of electrical resistance and the bridge being balanced giving zero output signal for zero applied force. In the embodiment illustrated in Figure 2 this is achieved by a suitable design ofthe relative geometries ofthe fixed value resistors and the force sensing resistors.

In this embodiment, an electrically insulating washer 3, made from a ceramic or a metal coated with an electrically insulating layer of glass ceramic or other suitable material, can be placed in contact with the washer carrying the force sensing resistors and fixed value resistors in order to act as a force transmitting component to form a force sensitive device.

Figure 4 shows a plan view, and a cross-section along the line RR, of a typical mounting arrangement for one embodiment of the force sensor here described. An electrically insulating washer consisting of a steel washer 1 onto which is printed and fired an electrically insulating layer of glass ceramic 2 which serves as a supporting substrate for the force sensing resistors a and b. In the illustration of figure 4, the washer is shown mounted over a securing stud 5, which might be screwed into an engine block 6 for example. An electrically insulating force transmitting component 3, in the shape of a washer for example, is secured in contact with the force sensing resistors by a retaining nut 7.

With the arrangement here described, any forces experienced by the retaining nut, due to pressure pulses in the engine block for example, will result in a force being experienced by the force sensing resistors mounted on the washer. This force will result in a change in the resistance of the force sensing resistors which can then be measured using a suitable electrical circuit such as the Wheatstone's bridge for example.

A preferred embodiment of this invention is where the electrically insulated force transmitting component is shaped in such a way as to allow the applied force to be transmitted to the force sensitive resistors but not to the fixed value resistors. This can be achieved by forming the force transmitting component with cut away portions which overlay the fixed valued resistors on the force sensitive washer, for example as shown in figure 2, where the cut away portions are realised as holes 8 and 9 directly above the fixed value resistors and h.

Another example is illustrated in figure 5 where a chord is cut away from the force transmitting component 3 such that a fixed value resistor RL can be located on the force sensing washer I under the cut away portion so that no force is applied to the fixed value resistor by the force transmitting component. In this example force sensing resistors R1 R2 and R3 are shown in dashed outline beneath the force transmitting component 3. The force sensing resistors and the fixed value resistor can be interconnected via the solder connection pads 4 to give the circuit configuration shown in figure 6 for example.

A further advantage of the use of cut away portions in the force transmitting component is that it facilitates the making of electrical connections to the force sensing and fixed value resistors by the soldering of wires for example. This is illustrated in figure 2 where a circular cut away section 10 in the force transmitting component 3 is shown located above the solder connection pads 4. In this way interconnecting wires can be soldered to the connection points without encumbrance from the force-transmitting component which might otherwise mechanically or electrically impede the wires.

There is also some merit in placing the force sensing resistors approximately equidistant around the circumference of a washer, such as the arrangement shown in figure 2 for example, in that approximately equal strains are experienced in the resistors due to the roughly equal distribution of any applied force when the washer is subjected to force. This arrangement has a tendency to decrease any errors arising from load eccentricity and results from the principle of a tripod which is always able to balance even on a less than fiat surface.

A preferred embodiment of the invention is also one where the force sensitive resistors, such as the arrangements illustrated in figures 2 and 4 for example, are electrically connected in series. Such an arrangement further decreases any errors arising from load eccentricity. In this embodiment changes in the combined resistance of the force sensing resistors will indicate the total strain that is experienced by the resistors regardless of how it is distributed between the individual devices. Thus if there were any tendency for the stud and washer assembly to rock sideways any imbalance in the components of strain so produced would be averaged out by the mounting and electrical interconnection arrangement of the force sensitive devices. In a situation where it is not feasible to interconnect the force sensing resistors in this way it is of course possible to measure the individual resistance changes and then sum or average them mathematically to achieve the same effect.

A preferred embodiment of the force sensing resistors is also one where the force applied and the electrical current flowing through the force sensing resistors are both normal to the p]ane of the supporting substrate. This mode of operation results in an optimum level of sensitivity due to the fact that the piezoresistive coefficient, and hence sensitivity, is maximised in this configuration whilst the effect of any temperature coefficient of expansion mismatch between the substrate and the materials comprising the force sensing resistor is minimised.

Figure 7 shows a typical embodiment of a piezoresistive force sensor where a highly conducting layer of material 71 is deposited onto a suitable insulating substrate 72. A resistive material 73 is then deposited on top of the bottom layer conductor 71 and a further layer of high conductivity material 74 is deposited on top ofthe resistive material 73. The conducting layers 71 and 74 may be deposits of a suitably processed thick film paste containing a conductive material such as gold, although other materials may be used. A suitable supporting insulating substrate 72 may be made of aluminium oxide although other materials may also be used including metal onto which has been deposited an electrically insulating layer. The resistive layer 73 may be a suitably processed thick film resistor paste or other strain sensitive material.

The cross-section along the line AA in Figure 7 shows the overlapping nature of the layers which are arranged so as to leave an area of resistive material sandwiched between the two conducting layers. This area of overlap 75 (shown hatched in figure 7) can be designed to be as large or as small as is necessary for the particular application for which it is intended. This is in order to define precisely the amount of stress, and hence strain, experienced by the resistor for any given applied force such as to optimise device sensitivity.

Figure 8 shows a further embodiment wherein the geometry of the overlapping layers is arranged to be circular in order to minimise excessive localised stress concentrations such as could occur with rectangular geometries for example. The embodiment illustrated in figure 8 shows a plan view of a circular arrangement of a bottom conducting layer 81 onto which the resistive layer 83 and a top conducting layer 84 have been deposited so as to form an area of overlap 85 (shown hatched in figure 8) which forms the active area of the sensor. Other stress concentration avoiding geometries are of course possible particularly ones where square corners or sharp changes in direction of the boundaries ofthe patterns defining the active area ofthe sensor are avoided.

A further embodiment of the invention is illustrated in Figure 9 wherein the force sensitive resistors 93, 94 are fabricated onto a supporting substrate 91. The supporting substrate 91 may be constructed from an electrically insulating material or from a non-electrically insulating material, such as a metal, covered with an electrically insulating material 92. In this embodiment the force sensing resistors 93, 94 are covered with a layer of electrically insulating material 96 such as glass ceramic for example. This arrangement then obviates the need for an electrically insulating force transmitting component and the force to be sensed can be applied to the force sensing devices using a force transmitting component made from any suitable material regardless of its electrical conductivity, such as a metal for example.

An additional embodiment of the invention is illustrated in Figure 10 where a force transmitting component 107 is bonded to a force sensing device comprising a supporting substrate 101 and force sensitive resistors 103, 104 using a bonding material 108 such as a room temperature vuicanising compound or some other suitable material for example. A preferred embodiment is where the bonding material possesses a low modulus of elasticity compared with that of the force sensing resistors. This arrangement has the advantage of resulting in a one-piece assembly for the force sensing device and the electrically insulating force transmitting component as well as helping to maintain a repeatable mechanical positioning of the two components during handling. Obviously if this embodiment is used with a force sensing device wherein an electrically insulating layer has been deposited onto the force sensing resistors then the force transmitting component itself does not need to be electrically insulating.

A further embodiment of the invention is illustrated in Figure 11, where an area of the force sensing device's supporting substrate 110 which is covered by a cut away section of an overlaying force transmitting component 111 is used to locate a compensating component 109 such as a surface mount variable resistor for example. This component may then be used to balance a Wheatstone's bridge by interconnecting the component with force sensing resistors 113, 114 and fixed resistors 115 in the manner illustrated by the circuit diagram shown in Figure 12, for example, where component Rv indicates an adjustable compensating component 109. In this way individual force sensors may be adjusted using such a compensating component to optimise their operating characteristics.

Some preferred embodiments of this invention have been described as being shaped as washers although other shapes may be used. Indeed for certain applications the use of other shapes will be mandatory both for the supporting substrate of the force sensitive devices and for the force transmitting component. Similarly any cut away sections of the force transmitting component can be implemented as circles, chords or indeed any other suitable shape.

Claims

1. A means for implementing an improved device for sensing force which utilises one or more force sensing resistors fabricated as thick film resistors consisting of overlapping layers of thick film materials which are printed and then fired onto an electrically insulating supporting substrate which by its construction may be used to form a force sensing device and where the force is applied to the force sensing resistors via a force transmitting component which is shaped so as to only apply force to the force sensing resistors and not to one or more fixed value resistors co-located on the force sensing device's supporting substrate.

2. A device as described in claim 1 above where the electrically insulating supporting substrate consists of a metal washer with a deposited electrically insulating layer onto which the force sensing resistors and the fixed value resistors can be subsequently deposited.

3. A device as described in claim 1 where the force sensing resistors are arranged as a number of individual force sensitive resistors that are electrically connected in series for the purpose of measuring their change of resistance.

4. A method of individually measuring the change in resistance values of force sensitive resistors in a device as claimed in claim 1 and then summing or averaging these changes in resistance values in order to determine the total amount of force experienced by the device.

5. A device as described in claim 1 where the force sensing resistors are fabricated using patterns designed to minimise localised stress concentrations.

6. A device as in claim 1 where an electrically insulating layer is deposited over the force sensing resistors so as to electrically insulate the force sensing resistors from a force transmitting component.

7. A device as in claim 1 where a force transmitting component is glued or otherwise fixed to the force sensitive device.

8. A device as in claim 1 where a compensating component is co-located on the force sensitive device.

9. A device as in claim 1 where electrical connections are made to the force sensing resistors and fixed value resistors in areas that are not subjected to any applied force by virtue of the shape ofthe force transmitting component.