In many applications it is important in the construction of piezo electric accelerometers to keep production costs as low as possible whilst at the same time endeavouring to attain as high as possible electrical output in relation to the acceleration induced into the piezo electric elements involved.

In practice, in relation to the construction of piezo electric accelerometers for use at low frequencies it is also necessary to maximise the source capacity since it has been found that the provision of high source capacitance facilitates simplification of the design of systems, installations or the like in which such accelerometer are utilised.

In known arrangements for the production of acceptable accelerometers using piezo electric material in the so-called radial mode of strain application, the piezo electric material is subjected to compressive or tensile forces along directions radially directed of the mounting thereof from a support.

With such known arrangements it is a common practice, to deposit a layer of piezo electric material onto a support element of the requisite mechanical properties.

This support element can, if desired, be formed by a further piezo electric material.

To maximise the sensitivity of the device it is conventional practice to arrange for the line of zero strain of the resulting accelerometer to lie along the junction between the piezo electric element(s) and the associated support.

It is convenient to note that, in general, piezo electric materials are operationally reversible in that an applied mechanical strain will produce an electrical output and in that any electrical signal input will produce a related mechanical strain effect in the piezo electric material. In addition, piezo electric materials are pyroelectric since any change in the temperature of the piezo electric material will produce a corresponding electrical output.

In this connection it has been found that relatively small changes in the ambient temperature cause production of electrical outputs related to the temperature change and which are sufficiently large enough to cause spurious readings from the accelerometer-.

It is an object of the present invention to provide an accelerometer which is particularly suitable for use at low frequencies, and in which the significance of ambient temperature changes upon the output of an accelerometer is reduced.

The frequency range for accelerometer applications

contemplated for the present invention is from 0.1 Hz to 500Hz. Within this range it is desirable that the lower limit should be selectively variable at least within the range 0.1Hz to 20 Hz. In the case of the

5 upper range limit such selective adjustment should be at least possible between 10Hz and 500Hz. Also it is required that the source impedance should be less than 1000 Ohms, and that any associated circuitry should be able rapidl to recover from overloads of up to, for

1Q example, in applications of accelerometers to the monitoring of the performance of a motor vehicle suspension system, 3g for accelerometers mounted to the body of a vehicle and up to 50g in applications where the accelerometers are mounted to the wheels of a

-J5 vehicle. In so far as the question of ambient temperature conditions is concerned the accelerometer needs to be capable of operating over a temperature range of -55 to +125 degrees Celsius.

A further object of the invention is to provide an

20 accelerometer in which the effects of the inherent pyroelectric characteristics of the piezo electric material are eliminated or are at least reduced. It is a still further object of the present invention to provide a band pass filtering and amplifying circuit

25 which is able to operate with a low voltage supply of, for example, 5 volts, which is suitable for use with low frequency piezo electric accelerometers and which is able to produce from the output of the accelerometer a high level low impedance signal for subsequent 0 processing and/or use.

It is a common practice to use a combined filtering and

amplifying circuit with accelerometers and it has been found that there are two severe problems associated with piezo electric accelerometers which lead to serious difficulties in the design of filtering and amplifier circuits. The first arises as a result of the inherent pyroelectric response of piezo eiectric materials to changes in temperature which creates spurious output signals. This output is an amount of electrical charge per degree of temperature change. In practice, if a resistive load is connected across a piezo electric element then the maximum voltage produced across any such load due to pyroelectric effect will be proportional t-o the rate of change of the temperature and inversely proportional to the magnitude of the resistive load. The minimum value for the resistive load is determined by the lower limit of the frequency response required. The pyroelectric effect output signal may be reduced by lengthening the thermal time constant of the accelerometer system and by using high frequency pass filters to prevent the low frequency pyroelectric noise signals from appearing at the output of the accelerometer system. In a practical arrangement the pyroelectric signals are so large as compared with the desired signal output of the accelerometer, i.e., that due to an acceleration variation, that no amplification of the signal can be allowed in advance of an initial high pass filtering so as to avoid any amplification stage from being over-loaded by the low frequency pyroelectric signals. Furthermore, this problem is aggrevated by the use of a low power supply voltage.

The second problem is concerned with preventing

signals at frequencies above the required pass band frequency range from appearing at the output of the system. The difficulties arising from the fact that the piezo electric accelerometer will exhibit resonant behavior at frequencies determined by the precise method of mounting and by the geometry and properties of the various materials employed in the construction of the accelerometer. This situation causes an increase in the sensitivity of the accelerometer at higher frequencies which need to be eliminated by a complementary reduction in the gain of an associated amplifier and filter circuit. In a practical accelerometer an increase of sensitivity by a factor of 20 is possible at frequencies as low as 2kHz compared with the sensitivity at 20Hz. This situation requires that the low band pass section of the circuit should precede any amplification stage.

According to a first aspect of the invention there is provided an accelerometer unit characterised by first and second piezo electric elements mounted upon opposed surfaces of an electrically conductive sheet, plate or the like, and in that the sheet, plate or the like is adapted to be so centrally supportable that the first and second piezo electric elements are subjectable to similar forces upon subjecting the piezo electric elements to acceleration forces.

Preferably, the first and second piezo electric elements are connected electrically in parallel, the arrangement being such that any electical output arising from pyroelectric effects of one piezo electric element are caused to negate electrical output arising

from pyroelectric effects of the other piezo electric element.

In a preferred construction the first and second piezo electric elements are of annular form, and the sheet, plate or the like and the support means such has a bores whereby electrical connections to at least one of the first and second piezo electric elements can .pass through both .elements , the sheet, plate or the like and the support to enable electical connection to at least one of the piezo electric elements.

Preferably, the first and second piezo electric elements are so secured to the sheet, plate or the like as to provide an integral composite element.

Conveniently the piezo electric elements are secured the sheet plate or the like by adhesive.

In accordance with a further aspect of the invention there is provided in connection with an accelerometer unit a combined band pass filter and amplification unit in which the source capacitance of the accelerometer unit is utilised as at least a part of the capacitance of the initial high frequency level pass filter of the circuit.

According to a further aspect of the invention there is provided a combined band pass filter and amplification circuit for use with an accelerometer, wherein the source capacity of the accelerometer is utilised as at least a part of the capacitance of the initial high frequency level pass filter of the

circuit .

Preferably, the source capacitance provides the total capacitance for the filter.

For a better understanding of the invention and to show how to carry the same into effect reference will now be made to the accompanying drawings in which;

Figure 1 is a view in section of an accelerometer incorporating the principles of the invention;

Figure 2 is an elevation of Figure 1.

Figure 3 illustrates a known filter and amplifier circuit ;

Figure 4 illustrates an embodiment of a filter and amplification circuit in accordance with the invention;

Figure 5 is a circuit diagram of a more detailed version of the circuit of Figure 2.

Referring to Figure 1 an annular support element has annular surfaces 2 and 3 and a central bore 4. First and second disc-like annular piezo electric elements 5 and 6 having the same external diameter of, for example, 20-40 millimetres are respectively bonded to or deposited upon the surfaces 2 and 3. The elements 5 and 6 have central bores 7 and 8 respectively and are so positioned upon the support that these bores 7 and 8 are co-axial with the bore 4 of the support 1.

The sandwich comprising the support 1 and the two piezo electric elements is supported from a main support plate or the like 10 by way of an electrically conductive post 11, including a main body 12, a first smaller diameter portion 13 and a second smaller diameter portion 14. The provision of the two smaller diameter portions 13 and 14 leads to the provision of annular abutment surfaces 15 and 16 respectively at the opposite ends of the main body 12. The post 11 has an axial through bore 17.

The sandwich comprising the support 1 and the two piezo electric elements is supported from a main support plate or the like 10 by way of an electrically conductive post 11, including a main body 12, a first smaller diameter portion 13 and a second smaller diameter portion 14. The provision of the two smaller diameter portions 13 and 14 leads to the provision of annular abutment surfaces 15 and 16 respectively at the opposite ends of the main body 12. The post 11 has an axial through bore 17.

The smaller diameter portion 13 engages in a through bore 18 in the main support plate 10. To ensure that the post is firmly connected to the plate 10 the free end region of the portion 13 is swaged so as to overlie the adjacent edge portion of the rim of 'the bore 18 thereby cramping the plate 10 between the swaged part and the abutment surface 15 thereby firmly connecting the post 11 to the plate 10.

An electrically insulating sleeve 19 is engaged over the smaller diameter portion 12. The outer diameter of

the sleeve is such that it is a clearance fit into the bores 7 and 8 of the piezo electric elements 5 and 6 and the bore 4 of the support 1.

In addition, it will be noted that the inner peripheral region of the outermost surface of the element 6 is in electrical contact with the main body 12 of the post 11.

An electrically conductive washer 20 is engaged over the portion 14 to overlie the inner peripheral region of the outermost surface of the .piezo electic element 5 and makes electrical contact therewith.

The assembly of the washer and sandwich on the post is held in place by a nut 21 engaging with a threaded part of the portion 14.

It should be noted that the thickness of the washer 20 is chosen such that the outermost annular surface thereof is sufficiently proud of the outermost annular surface of the sleeve 19 as to ensure that when the nut 21 is tightened electrical contact is ensured between the piezo electric element 5, the washer 20, the nut 21, and the post 11 and thus between the outermost surfaces of both of the piezo electric elements 5 and 6.

Electrical connection is automatically made between the internal faces (those adjacent to the surfaces 2 and 3 of the support 1) of the piezo electric elements 5 and 6 since the support 11 is itself electrically conductive.. Hence the internal faces thereof are

effectively connected electrically in parallel to each other. The support 1 is electrically coupled to a desired location of the main support 10 by a wire 22 which at one end 23 is connected to the support 1 by way of a connection tag 24 provided on the periphery of the support 1 and whose other end 25 connects with the main support 10.

For convenience, the wire 22 passes to the underside of the support 10 by way of the central bore 17 in the post 11 so that the other end 25 of the wire 22 can be connected to a desired loction of the support 10. It will be appreciated that the location of connection of the wire end 25 to the support 10 will be electrically isolated from the swaged over parts of the post portion 13. To offset unacceptable vibration of the wire 22 the latter can be bonded to the adjacent element 5 intermediate of the attachment to the tag 24 and the engagement of the wire with the post.

In practice, the support 10 is constituted by a printed circuit board. With this arrangement it will be appreciated that the post 11 can be effectively regarded as an electrical component so that on mounting the post the latter will automatically connect with the correct part of any printed circuit provided upon the board.

The two piezo electric elements 5 and 6 can be mounted to the support 1 in such manner as to form an integral composite unit. Conveniently, the two elements 5 and 6 are attached to the support by adhesive. Furthermore, the connection leads to the piezo electric

elements can be connected by adhesive to the surface of the associated element.

In use, if the accelerometer constuction is accelerated longitudinally of the post 11 the required force being applied to the accelerometer by way of the support 10 then the inertia of the periphery of the s ^' andwich comprising the support 1 and the piezo electric elements 5 and 6 will cause strain to be produced in the two piezo electric elements 5 and 6 of a magnitude proportional to the acceleration, thereby producing an electrical ^' output signal which is proportional to the applied acceleration.

It will be appreciated that with the above described accelerometer the pyroelectric outputs of the two piezo electric elements 5 and 6 will cancel each other.

With the above construction it has been found that with a combined source capacity (piezo electric elements capacitance) of substantially 50nF the accelerometer has an output of 30 millivolts/g.

In addition, it will be noted that the construction of the accelerometer of the invention has the added advantage that the source capacity of the parallel connected piezo electric elements is effectively increased by a factor of four as compared with the source capacity of a conventional bimorph piezo electric element of the same physical dimensions as either of those included in the accelerometer of the invention.

particular application of the described

accelerometers is to the measurement of motor vehicle wheel and body acceleration with a view to providing information upon the instantaneous conditions prevailing in the suspension system of the vehicle which information can be used as input to a control system for enabling adjustment of the operational characteristics of the suspension system with a view to at least enhancing passenger comforts.

Referring to Figure 3 which illustrates a high frequency pass filter circuit known as a Salen and Key Filter. The circuit essentially incorporates an amplifier stage including an amplifier 25 having positive and negative voltage inputs 26 and 27 and an output 28. The input 26 is connected by way of a resistor a 29 to a common ground line 30, and by way of serially coupled capacitors 31 and 32 to the signal input terminal 33 of the circuit.

The negative input 27 of the amplifier connects with the line 30 by way of a resistor 34. The output 28 is connected through a resistor 35 to the input 27 and thus effectively in series with the resistor 34, and by way of a further resistor 36 to the junction of the capacitors 31 and 32. The amplifier output 28 provides the output of ^* the filter and amplifier circuit.

In use, the capacitors 31 and 32 and the resistors 29 and 36 serve as the low pass filter for the circuit whilst the combination of the resistances 35 and 36 control the pass band gain of the complete circuit.

The circuit of Figure 3 has certain inherent deficencies, when used as a low pass filtering and

amplifier circuit for use with piezo electric transducers as a result of the consequences of piezo electric material being pyroelectric and also exhibiting a non-linear frequency response leading to greater sensitivity at higher frequencies.

The input high pass filter is modified by incorporating as at least a part of the high pass filter the actual capacity of the piezo electric transducer. This is shown in Figure 4 in which the capacitor 32 is replaced by the source capacitance 37 of the piezo electric transducer 38. In addition, with a view to controlling (irrespective of any voltage gain at the amplifier stage) the magnitude of any high frequency input to the amplifier below a level at which input signals would be clipped an additional low pass input filter is introduced into the circuit. This additional low pass filter includes the resistor 39 and the shunt capacitor 40.

The significance of the modifications to the basic circuit may be briefly considered as follows:

The introduction of the transducer source capacity as part of the initial high pass filter of the circuit facilitates close control over the level of the low frequency cut-off frequency of the circuit and any phase shifts involved.

With the introduction of the shunt capacitor 40 and the series resistor 39, it is necessary to ensure that the impedance of the piezo electric transducer capacity is much greater than that of the resistor 39 so that the introduction of the latter will not adversely affect

the effectiveness on the high pass frequency performance of the overall circuit. However, the low pass filter combination of the resistance 39 and the capacitor 40 is effective in reducing the overall circuit gain at the higher frequncies.

In practice, provided that the high frequency cut-off level is set to be at least a factor of fifty times the frequency of the low pass filter cut-off frequency corner any interaction between the two filters will not affect the performance of the high pass filter to any significant extent.

It is convenient to note that the capacitor 40 in so far as the high pass ^* filter is concerned is effectively coupled in parallel with the piezo electric transducer source capacity 37. This has the additional effect of causing a reduction in the output voltage of the circuit of Figure 4. In practical terms this reduction can be reduced as far as possible by selecting the value of the capacitor 40 to be much less than that of the transducer source capacitance.

In practice, the Q of the filter circuit is determined by the ratio of the values of the resistors 34 and 35, which additionally determine the band pass gain of the filter circuit.

If required, it is possible to increase the band pass gain of the circuit and at the same time introduce a second low pass section to the filter circuit. This is achieved by introducing the capacitor 41 and the resistor 42 at the output side of the amplifier 25.

As so far discussed the circuit of Figure 4 has been found to provide a satisfactory performance in the filtering of the output from the piezo electric transducer 38 and in controlling the effects of peak voltages arising from, for example, overload conditions and which are in excess of supply voltage levels.

However, for certain applications of accelerometers, for instance, their application to the control of suspension systems of motor vehicles the rate of recovery from large overloads which have created excessive peak voltage level signals lying within acceptable frequency bands for the filter circuit may well be regarded as being too slow for such an application for the accelerometers.

In order to control this overload situation it is necessary to restrict the overall gain of the circuit thereby to prevent overload conditions from arising during normal conditions of use of a transducer. To deal with this particular problem it is proposed to provide the circuit of Figure 4with an additional amplification stage, thereby to enable the requisite extra gain for the circuit, together with further band pass filtering and output buffering. Figure 5 is a circuit diagram of a circuit incorporating the circuit of Figure 4 together with the additional stages mentioned above.

The circuit of Figure 5 incorporates a second amplifier 43 with inputs 44, and 45 and an output 46. The output 28 of the first amplifier 25 is coupled to the input 45 of the second amplifier 43 by way of a capacitor 47 which in conjunction with a resistor 48 provides a

further high pass filter.

It will be noted that apart from the capacitor 47 the second amplification stage is direct coupled to the output 28 of the first amplifier in order that the g second amplification stage will exhibit the required fast overload recovery capability required from the overall circuit.

As a result of the particular amplifiers used (units known under the reference identification LM208) input

10 protection diodes are provided, which diodes are arranged to conduct if the differential input voltage exceeds approximately 0.5 volts. If the output should be large enough to cause this conduction of the input protection diodes the capacitor 47 will charge or

15 discharge whereafter a relatively long time period will be required for their output to reach equilibrium after an input signal is removed. Thus the gain of the first amplification stage must be set so that the peak AC signal at the output 28 from the first amplifaction

20 stage cannot exceed 0.5 volts for any normal input signals to the circuit.

The output 46 of the second amplifier 43 is buffered and inverted, by an NPN transistor 49 with a view to obtaining a larger output voltage swing than can be

25 attained from an amplifier. Resistors 50 and 51 are provided in the transistor base circuit to ensure that the output voltage of the second amplifier 43 is about 2.5 volts for an output voltage of 2.5 volts at an output terminal 52. The gain of the transistor stage

30 is controlled by the resistors 53 and 54 . These resistors also determine the lowest output voltage that

can be obtained. A typical ratio is 1:20.

A resistor 55 is included to limit the maximum output voltage to a value slightly below that of the input power supply in order that any associated control circuitry involved can distinquish the difference between a large signal and a partially disconnected transducer.

A resistor 56 provides negative feedback whilst the combination of this resistor 56 and a capacitor 57 in parallel therewith provides a further stage of low pass filtering .

The input 44 of the amplifier 43 connects by way of resistors 58, 59, and a variable resistance 60 to the voltage line 30, the latter being connected to a common ground line 61 by way of a resistor 62 and a second variable resistor 63.

In practice, the ratio of the values of the resistor 59 and the variable resistor 60 to the value of resistor 56 determines the gain of the second amplifier stage. As mentioned above the second amplifier stage is DC coupled to allow for the required fast overload recovery. This condition imposes the requirement that the source resistances to the two amplifiers 25 and 43 need to be balanced to reduce any errors caused by input bias currents.

In the further filter arrangement including the resistor 47 and the capacitor 48, the value of the resistor 47 is of necessity of a large value to reduce the capacitance of capacitor 48, a value of 10 Megohms

being typical. The resistor 59 has to be of a value much less than that of resistor 48 whereupon the resistor 58 is required as a DC balance resistance. The resistor 58 is shunted by a capacitor 64 to prevent the introduction of unwanted phase shifts at the input 44 to the second amplifier 43 due to its input capacity acting with the resistor 58.

The variable resistor 63 is provided to adjust for the 2.5 volts bias voltage required on the voltage line 30 and to enable fine adjustment of DC offset at the circuit output 52.

A high frequency filter incorportating a resistor 65 provided in the positive voltage supply line 66 and a capacitor 67 connecting the supply line 66 to the common line 61 is provided to remove any high frequency noise components which may be present on the 5 volts supply line.

To provide for compensation for a fall in the output of the transducer as a result of an increase in the temperature of the transducer a negative temperature coefficient thermistor 68 and an associated load resistor 69 are provided between the voltage line 30 and the filte.r circuit 41,42.