WO2008132419A1

WO2008132419A1 - High-frequency sensor system

Info

Publication number: WO2008132419A1
Application number: PCT/GB2007/001620
Authority: WO
Inventors: Roger Darraba
Original assignee: Autoliv Development Ab; Beattie, Alex, T., S.
Priority date: 2007-05-01
Filing date: 2007-05-01
Publication date: 2008-11-06

Abstract

A vibration sensor system (1), for sensing vibrations in a frequency range shaving a lower limit of at least 2 kHz, the sensor comprising: a piezoelectric element (2); and an amplifier (4), the piezoelectric element being connected to an input of the amplifier, the amplifier having a lower cut-off frequency and a resistor (5) that principally determines the lower cut-off frequency, the resistor having a resistance of 200 kΩ or less.

Description

"High-Frequency Sensor System"

Description of Invention

THE PRESENT INVENTION relates to a high-frequency sensor system, and in particular concerns a sensor which is useful in detecting whether a vehicle is involved in a crash situation.

Known vehicle-mounted crash detectors commonly output frequencies below 400 Hz when a vehicle is involved in a crash situation, with the output signals corresponding to vibrations arising from the crash. Signals of this type may be integrated to arrive at a measurement of the change in speed of the vehicle, which in turn may provide an indication of the severity of the crash.

Different types of accelerometer are used, for instance those measuring the movement of a mass element by a strain gauge, or measuring a change in capacitance.

A relatively cheap alternative with low accuracy is a piezoelectric element. As will be understood by those skilled in the art, a piezoelectric element gives rise to a potential difference when subject to an appropriate mechanical strain caused by acceleration, and conversely a piezoelectric element will change its physical dimension when subjected to an appropriate potential difference.

Piezoelectric elements are associated, however, with high impedance, and therefore processing of signals arising from such elements requires relatively expensive high impedance amplifiers which are sensitive to electrostatic discharges. The high impedance amplifiers require insulation from leakage of current, protection against humidity and shielding against electromagnetic disturbances, and inevitably this gives rise to a high cost, thus negating many of the advantages of using a piezoelectric element. It is an object of the present invention to provide a high-frequency piezoelectric sensor suitable for use as a vehicle crash detector.

Accordingly, one aspect of the present invention provides a vibration sensor system for sensing vibrations in a frequency range having a lower limit of at least 2 kHz, the sensor comprising: a piezoelectric element; and an amplifier, the piezoelectric element being connected to an input of the amplifier, the amplifier having a lower cut-off frequency and a resistor that principally determines the lower cut-off frequency, the resistor having a resistance of 200 kΩ or less.

Advantageously, the amplifier is a voltage amplifier.

Preferably, the resistor that principally determines the lower cut-off frequency comprises the input resistance of the amplifier, and has resistance of less than 100 kΩ.

Conveniently, the amplifier is a charge amplifier.

Advantageously, the resistor that principally determines the lower cut-off frequency of the amplifier is a feedback resistor of the amplifier.

Conveniently, the feedback resistor has a resistance of 100 kΩ or less.

Preferably, the lower limit of the frequency range is at least 5 kHz.

Conveniently, the upper limit of the frequency range is less than 100 kHz.

Advantageously, the upper limit of the frequency range is less than 50 kHz. Preferably, the sensor system further comprises a band pass filter through which signals from the piezoelectric element pass after amplification.

Conveniently, the output from the amplifier passes through a plurality of parallel band pass filters with respective frequency ranges.

Advantageously, the outputs from the plurality of band pass filters are analysed to determine properties of the movement of the sensor.

Preferably, the amplifier provides the band pass filter function.

Conveniently, signals from the or each band pass filter pass through a respective envelope circuit to provide an envelope function of these signals.

Advantageously, the signal output by the or each envelope circuit is analysed by a microcontroller.

Another aspect of the present invention provides a method of sensing vibrations in a frequency range having a lower limit of at least 2 kHz, comprising the steps of: providing a piezoelectric element; and connecting the piezoelectric element to an input of the amplifier, wherein the amplifier has a lower cut-off frequency and a resistor that principally determines the lower cut-off frequency, the resistor having a resistance of 200 kΩ or less.

In order that the present invention may be more readily understood, embodiments thereof will now be described, by way of example, with reference to the accompanying drawings in which:

FIGURE 1 is a circuit diagram showing elements of a system for use with a sensor system embodying the present invention; FIGURES 2 and 3 show alternative amplifiers for use with the present invention; and

FIGURE 4 is a circuit diagram showing elements of a system suitable for use with alternative sensor systems embodying the present invention.

With reference to figure 1 , a sensor 1 embodying the present invention comprises a piezoelectric element 2. In preferred embodiments, the cross- sectional area of the piezoelectric element 2 is less than 10mm². The piezoelectric element 2 is connected by wires 3 to the input of an amplifier 4. The amplifier 4 has an input impedance, which is represented in figure 1 by a resistor 5 connected across the input terminals of the amplifier 4, the resistor 5 having resistance R,_π. It will be appreciated that the input impedance of the amplifier may arise from the presence of components other than resistors, however.

A band pass filter 10 is also provided, which allows only signals having certain frequencies to pass. Preferably, the lower limit of the band pass filter 10 is 5 KHz or more. The upper limit of the band pass filter 10 may be 20 KHz or more, and in embodiments of the invention may be 50 KHz or 100 KHz. The band pass filter 10 may be integrated with the amplifier 4 or may be a separate component, and may be positioned either before or after the amplifier 4 in the circuit.

Figure 2 shows one example of a combined amplifier and band pass filter 11 , in which the amplifier is a voltage amplifier. An input resistor R_jn is connected is parallel with the input terminals, with a lower cut-off frequency of the amplifier being principally determined by the value of this resistor R_1n. As will be understood amplifiers are primarily effective within a predetermined frequency range having upper and lower cut-off frequencies. The amplification of the amplifier is therefore significantly higher above the lower cut-off frequency than below it. Amplification is provided by an operation amplifier OA, with one input of the amplifier being connected to the positive terminal of the operational amplifier OA, and a feedback circuit comprising two further resistors R₁, R₂ being connected from the output of the operational amplifier OA to its negative terminal.

Figure 3 shows an alternative amplifier and band pass filter 12, where the amplifier is a charge amplifier. In this case, the input impedance of the amplifier is approximately equal to zero. The input terminals of the amplifier are connected to the negative and positive terminals respectively of an operational amplifier OA, with the positive terminal of the operational amplifier OA further being connected to ground. A feedback circuit, connected between the output of the operational amplifier OA and its negative input, comprises a feedback resistor R_a and a feedback capacitor C_a, which are connected and parallel with one another. For reasonable amplifications, the capacitance of the feedback capacitor C_amust be approximately equal to that of the piezoelectric element 2. The value of the feedback resistor R_a principally determines the lower cut-off frequency of the amplifier.

The output from the amplifier 4 is passed to an envelope circuit 5. As will be understood by a person of skill in the art, the envelope circuit 5 effectively acts as an envelope detector and extracts the mean amplitude of the signal output by the amplifier 4.

The signal from the envelope circuit 5 is passed to a microcontroller 6, which is configured to interpret these signals. Due to the presence of the envelope circuit 5, the microcontroller 6 receives a relatively low-frequency signal.

In use, it is envisaged that the piezoelectric element 2 of the sensor 1 will be embedded in, or abutted firmly against, part of the chassis of a vehicle. If the vehicle is involved in a crash situation, acoustic waves will travel through the chassis of a vehicle. As will be understood by a person of skill in the art, these waves will comprise longitudinal compression waves and also transverse waves. The speed of these waves will be determined by the strength of the interatomic bonds within the material making up the vehicle chassis. These waves will cause the piezoelectric element 2 to accelerate back and forth, at a frequency equal to the frequency of the acoustic waves.

Vehicle chassis are typically formed from, or principally from, steel which has a speed of sound of 5950 m/s for longitudinal waves and 3230 m/s for transverse waves. Acoustic waves initiated by a crash may have a frequency of 0 to several MH₂, but lower frequencies are subsequently generated when substantial deformation occurs and the higher frequency waves are damped. The frequencies of greatest interest are generally around 5 KHz to 20 KHz. An advantage of using these waves to detect whether the vehicle is involved in a crash situation is that these waves will travel very swiftly through the vehicle chassis (in view of the strong interatomic bonds within steel), and are generated early in the crash, thus giving an early indication if the vehicle is involved in a crash.

As mechanical strain is repeatedly exerted on the piezoelectric element 2 by its acceleration, a potential difference will repeatedly arise across the piezoelectric element 2. The piezoelectric element 2 will therefore behave as a capacitor, and it will be understood that the impedance of the piezoelectric element 2 will be:

Z = 1/(C2πf)

It will be appreciated, therefore, that the impedance is inversely proportional to the frequency of the acoustic waves, and therefore that the piezoelectric element 2 will have a relatively low impedance at high frequencies. By way of example, a sensor adapted to respond to a frequency range of 5 kHz to 20 kHz, with a piezoelectric element having a capacity of 1 nF (corresponding to a cross-section of around 5 mm²) will result in an impedance of 31 kΩ, with a lower impedance at higher frequencies.

As will be understood by a skilled person in the art, in the case of a voltage amplifier as discussed above, the input impedance of the amplifier 4 should be substantially matched to the impedance of the piezoelectric element 2. A suitable input impedance for the amplifier 4 would therefore be 30 kΩ. The lower cut-off frequency for the amplifier 4 is then f_c = 1/(2πCR) = 5 kHz.

Voltage amplifiers with an input impedance of 100 kΩ or below are normally regarded as low-impedance amplifiers, and it will be seen that when providing a sensor which is adapted to respond only to higher frequencies, in the range that will encountered when detecting acoustic waves travelling through a vehicle chassis, a low impedance amplifier may be used. It will be appreciated, therefore, that the problems discussed above associated with high-impedance amplifiers may be avoided, and that the overall cost of the sensor may be reduced.

In preferred embodiments, the input impedance of the amplifier 4 is less than 100 kΩ.

In the case of a charge amplifier, as discussed above, while the lower cut-off frequency f_c of the amplifier may be affected by several factors, it is principally determined by the feedback resistor R_a, since f_c = 1/2πC_aR_a and so the value of R_a should be chosen appropriately (R_a = 1/2Uf₀C₃), and may be around 200 kΩ.

It will be appreciated that the properties of the amplifier 4 will effectively provide a band pass filtering function to allow only signals within a certain frequency range to be effectively amplified. It may, however, be desired to concentrate on a portion of the frequency spectrum which is narrower than that accommodated by the amplifier 4, and therefore a separate band pass filter may be provided. Signals may pass through the band pass filter before or after amplification by the amplifier 4.

With reference to figure 4, an alternative sensor 7 is shown. The alternative sensor 7 comprises a piezoelectric element 2 and amplifier 4 as discussed above, but in this embodiment the output from the amplifier 4 is fed into four individual band pass filters 8a,8b,8c,8d. These band pass filters 8a,8b,8c,8d each have different band pass ranges, and in the typical example a first band pass filter 8a has a range of 5-7 kHz, a second band pass filter 8b has a range of 7-10 kHz, a third band pass filter 8c has a range of 10-14 kHz, and a fourth band pass filter 8d has a range of 14-20 kHz.

The outputs of each of the band pass filters are fed into separate respective envelope circuits 9a,9b,9c,9d, with the outputs from the envelope circuits 9a,9b,9c,9d being fed into the microcontroller 6.

It will be appreciated that the alternative sensor 7 will be able to distinguish between vibrations having different frequencies, since different combinations of frequencies will be passed or blocked by the band pass filters 8a,8b,8c,8d. Indeed, if an incoming signal comprises a superposition of different vibrations, the alternative sensor 7 will be able to analyse the component frequencies, as long as they fall within appropriate frequency ranges.

It is envisaged that this will allow the alternative sensor 7 to distinguish between crashes of different types, i.e. which different locations and directions of impact are involved, since the resonant frequencies of different parts which may suffer an impact are different, as are the damping profiles of the various parts of the chassis through which the vibrations may travel. A look-up table may be used to relate various combinations of frequency to different types of crash, or the signals from the band pass filters 8a, 8b, 8c, 8d may be analysed to determine properties of the crash. As will be understood, the type of crash may determine the manner in which certain safety devices, such as air-bags, are activated.

It will be appreciated that the present invention provides a relatively inexpensive sensor, which will be particularly responsive to acoustic waves travelling through a vehicle chassis, thus allowing swift determination of whether the vehicle is involved in a crash situation.

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

Claims

1. A vibration sensor system (1) for sensing vibrations in a frequency range having a lower limit of at least 2 kHz, the sensor comprising: a piezoelectric element (2); and an amplifier (4), the piezoelectric element (2) being connected to an input of the amplifier (4), the amplifier having a lower cut-off frequency and a resistor (R_in, R₃) that principally determines the lower cut-off frequency, the resistor (R_|n, R₃) having a resistance of 200 kΩ or less.

2. A sensor system (1) according to claim 1 , wherein the amplifier (4) is a voltage amplifier.

3. A sensor system (1) according to Claim 2, wherein the resistor (R_1n) that principally determines the lower cut-off frequency comprises the input resistance of the amplifier (4), and has resistance of less than 100 kΩ.

4. A sensor system (1) according to Claim 1 , wherein the amplifier (4) is a charge amplifier.

5. A sensor system according to claim 4, wherein the resistor that principally determines the lower cut-off frequency of the resistor is a feedback resistor (R₃) of the amplifier (4).

6. A sensor system according to claim 5, wherein the feedback resistor (R₃) has a resistance of 100 kΩ or less.

7. A sensor system (1) according to any preceding claim, wherein the lower limit of the frequency range is at least 5 kHz.

8. A sensor system (1) according to any preceding claim, wherein the upper limit of the frequency range is less than 100 kHz.

9. A sensor system (1) according to any preceding claim, wherein the upper limit of the frequency range is less than 50 kHz.

10. A sensor system (1) according to any preceding claim, further comprising a band pass filter through which signals from the piezoelectric element (2) pass after amplification.

11. A sensor system (1) according to any preceding claim, wherein the output from the amplifier (4) passes through a plurality of parallel band pass filters (8a, 8b, 8c, 8d) with respective frequency ranges.

12. A sensor system (1) according to Claim 11 , wherein the outputs from the plurality of band pass filters (8a, 8b, 8c, 8d) are analysed to determine properties of the movement of the sensor (1).

13. A sensor system (1) according to any one of claims 10 to 12, wherein the amplifier (4) provides the band pass filter function.

14. A sensor system (1) according to any one of Claims 10 to 13, wherein signals from the or each band pass filter (8a, 8b, 8c, 8d) pass through a respective envelope circuit (5, 9a, 9b, 9c, 9d) to provide an envelope function of these signals.

15. A sensor system (1) according to claim 14, wherein the signal output by the or each envelope circuit (5, 9a, 9b, 9c, 9d) is analysed by a microcontroller (6).

16. A method of sensing vibrations in a frequency range having a lower limit of at least 2 kHz, comprising the steps of: providing a piezoelectric element (2); and connecting the piezoelectric element (2) to an input of ane amplifier (4), wherein the amplifier (4) has a lower cut-off frequency and a resistor (R_in, R₃) that principally determines the lower cut-off frequency, the resistor (R_1n, R₃) having a resistance of 200 kΩ or less.