GB2317707A

GB2317707A - Cable testing arrangement

Info

Publication number: GB2317707A
Application number: GB9720257A
Authority: GB
Inventors: Bert Bergner
Original assignee: Whitaker LLC
Current assignee: Whitaker LLC
Priority date: 1996-09-24
Filing date: 1997-09-24
Publication date: 1998-04-01
Also published as: GB9619892D0; GB9720257D0

Abstract

A cable fault detecting arrangement comprises an oscillator 11 connected to an electrode 6 of a cable 4 such that the cable forms an impedance (inductance or capacitance) element of an oscillating circuit. The oscillating circuit produces a test signal (F Test) at a frequency which is dependent upon the condition of the cable and from this frequency it is detected if the cable is faulty. The oscillating circuit may include one or more termination impedance elements CT connected across an end of the cable and may be arranged such that the fault detecting arrangement operates within a bandwidth outside that of the sensor operation. By filtering signals the sensor and fault detection activities may operate simultaneously without interfering with one another. A direct current offset voltage may be applied to the cable to detect cable leakage currents between electrodes 6, 8. The arrangement may be used on a piezoelectric cable for sensing the presence of someone within the seat of a vehicle.

Description

MEANS FOR SELF-TESTING PIEZOELECTRIC CABLE AND A METHOD THEREFOR This invention relates to a means for testing whether a piezoelectric cable sensor is undamaged and operating correctly.

In many instances sensors require a self-test or selfdiagnosis function to ensure correct functioning. For example in the automotive industry, sensor devices that trigger safety functions such as airbags need to be particularly reliable and fail-safe. In U.S. patent 5,404,128 a presence detecting sensor positioned in a seat of a vehicle to detect the presence of a human being is disclosed. The sensor is interconnected to control means that control functioning of the vehicle. The sensor is a piezoelectric element for detecting vibrations and is provided with a self diagnosis function. One of these self diagnosis means is to position a termination resistance between electrodes, and pass an electric current through the sensor. If the sensor is severed, this can be detected as the circuit is open no current f-lows. If a short circuit occurs along the sensor, this is also detected by the zero voltage drop thereacross.

Another self test means proposed is the provision of a vibration generation means in the form of another portion of piezoelectric material, whereby the response of the sensor is detected. The latter self-test is inappropriate for piezoelectric coaxial cable sensors as implemented in U.S.

patent 5,164,709., which describes the use of such cable as a presence detecting sensor in an automobile seat. The former self-test means could be implemented in a piezoelectric coaxial cable, however there are certain configurations of damage to the cable that would not be appropriately detected by provision of a termination resistance only. For example, reduced insulation resistance between outer and inner electrodes due to ingress of humidity may not be correctly diagnosed by a sensor cable with termination resistance.

It is therefore an object of this invention to provide a reliable means for self-diagnosis of a piezoelectric sensor cable.

It would be advantageous if self-diagnosis could occur in parallel to normal operation of the sensor cable.

It would be advantageous to provide a self-testing means that can be very simply and easily integrated into electronic processing means provided for airbag activation.

Further objects and ' advantageous features will be apparent from the description, drawings and claims.

Objects of this invention have been achieved by providing the cable self-diagnosis means according to claim 1. In particular, the objects of this invention have been achieved by providing a sensor device comprising a sensor cable having electrodes separated by a dielectric, and selfdiagnosis means, the self-diagnosis means comprising an oscillator circuit connected to an electrode and adapted to oscillate, thereby producing a test signal (Ftest) at a frequency determined by the integrity of the sensor cable for detection of damage thereto, the cable providing inductance or capacitance (ACK, CT, LT) forming part of the oscillator circuit.

Advantageously, a self-testing means for piezoelectric cable is provided, whereby damage such as severing of the cable is detected in a simple and reliable manner.

It is also advantageous that the self-testing means can be operated in parallel to normal operation of the sensor without interfering therewith. This is particularly important in certain applications where the detecting means must operate extremely rapidly such as in automotive airbag applications. The latter can be achieved by operating the self-diagnosis circuit at a frequency that is outside the operational band width of the sensor, whereby interference is avoided by the provision of appropriate filtering.

A further advantageous feature could be the provision of a direct current (DC) offset voltage between electrodes of the sensor cable, such that a reduction of dielectric resistance can be detected. In a further advantageous embodiment, provision of a termination capacitor would ensure that the sensor cable is intact over its whole length in an easily detectable manner.

Further advantageous aspects of this invention will be apparent from the following description, drawings and claims.

Embodiments of this invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic representation of a sensor cable with a self-diagnosis electrical circuit, the sensor being provided with a termination resistance; Figure 2 is a block diagram representing the selftesting function of an embodiment according to this invention; Figure 3 is a schematic circuit representation of a sensor with self-diagnosis means according to this invention; Figure 4 is a schematic representation of another embodiment; Figure 5 is a schematic representation of yet another embodiment; Figure 6 is an equivalent circuit diagram of a sensor, for example as represented in figures 3 or 4; Figure 7 is a circuit diagram similar to figure 6, but with provision of an offset direct current voltage means; Figure 8 is a circuit diagram of the sensor and analyzing means.

Referring to figure 1, a sensor 2' is shown comprising a piezoelectric coaxial cable 4 having an outer electrode or conductor 6 and an inner electrode or conductor 8 separated therefrom by a dielectric 7. At a first end 9 of the cable, the inner electrode 6 is connected to a reference voltage Vref through a resistance Rv, and the outer electrode is connected to earth (ground). An electrical current(i) flows through the cable which is provided at the other (second) end 12 with a termination resistance RT connected between the outer and inner electrodes. A potential difference at the first end 9 can be measured between the inner electrode 8 at point 10 and ground, such voltage being the test voltage VT . If the cable is severed, no current flows and VT=Vref. In normal operation, if R=Rvt then VT will simply be Vref divided by 2. A severed cable can thus be easily detected. A partially damaged cable may however not be detected. For example when the outer electrode is partially damaged or the dielectric resistance between outer and inner electrodes is affected by humidity a current may nevertheless still flow through the circuit.

Furthermore, the capacitance of the cable (Ck) and the resistance Rv, Rt act as a 'high pass' RC filter. For short cable lengths, the capacitance of the cable is low, which increases the cut-off frequency of the 'high pass' filter.

When operational frequencies generated by the piezo cable are of a low frequency, it is necessary to provide very large resistance values for Rt,Rv in order to decrease the cut-off frequencies such that the operational signals can be detected.

The high value of such resistances is impractical and may induce an unacceptable margin of error.

Figure 2 is a block diagram showing the principle of operation of a sensor 2 comprising a piezoelectric coaxial cable 4 as shown in figure 1, interconnected to an oscillator 11 and an impedance transformer 13, whereby the oscillator provides an alternating signal Ftest at a frequency out of the range of frequency (i.e. bandwidth) of the operating signal Foperation - Referring to figure 3, the cable is shown interconnected to an oscillator by a coupling capacitor CO to the cable inner conductor 8. The cable has a length Lk having a capacitance Ck that is a function of this length. The capacitance Ck of the cable and coupling capacitor CO determine, with the circuitry of the oscillator 11, the frequency of oscillation Ftest. If the cable 4 is damaged, the capacitance Ck changes and the oscillator frequency is modified. For example if the cable is severed and therefore shorter, the capacitance Ck of the cable is smaller and the frequency of the oscillator increases. This deviation of the frequency Ftest can be detected by an appropriate electronic analysis circuit of the sensor. A particularly simple selftesting means is thus provided. The embodiment of figure 3, however, has a disadvantage in that severance of a small piece of the cable proximate the end 12 may not sufficiently change the capacitance Ck SO as to vary the frequency of oscillation in a manner that can be reliably detected. This can be overcome by providing a termination capacitor CT between outer and inner electrodes 6,8 at the second end 12 of the cable 4 as shown in figure 4. The inductances and capacitors L0 and CO could be tuned appropriately with the cable capacitance Ck and termination capacitor CT to provide a resonating frequency out of the range of the operating frequency generated during use of the sensor. Such use may for example measure vibrations of a mass on a car seat. Selftesting can thus occur simultaneously to operation without interference.

Referring to figure 5, the embodiment of figure 4 can be modified by adding in series to the termination capacitor CT, a termination inductance LT. In this embodiment, the cable 4 merely acts as a conductor between the oscillator and series capacitor and inductance circuit CT,LT. By appropriate dimensioning of the oscillator to resonate with the termination circuit, damage to the cable would prevent resonance. Detection of damage would thus be simplified to detecting the state of resonance (normal operation) or no resonance (damage).

Referring to figure 6, the sensor cable is represented as an electrical circuit 4 for connection to an operational amplifier with input resistance RA. The cable generates a voltage during operation Piezo and has a capacitance Ck (which may include the termination capacitance CT) and a resistance RK that represents the resistance of the dielectric 7 and other "leakage" resistances in parallel between the outer and inner conductors 8,6 respectively. An undamaged cable, has a dielectric with a resistance in the Giga-Ohm range such that the current flowing through the resistance RK is substantially 0. Damage or humidity may lower the resistance RK such that a leakage current (ik) flows therethrough, thereby affecting the value of the operational voltage Vpiezo.

In other words the operational signal may be influenced leading to defective sensing. In order to detect a leakage current iK as shown in figure 7, a voltage Vref provides a DC offset voltage between outer and inner conductors that drives a leakage current iK through the resistance RK. If the resistance RK is very high, as it should be with an undamaged cable, iK is almost 0. The DC potential difference at the poles of the resistance Ra is therefore almost zero in the latter case. If the dielectric resistance RK is reduced, the flow of leakage current iK modifies the direct current offset voltage, enabling detection of the impaired dielectric or other leakage between outer and inner conductors.

Referring to figure 8, a circuit diagram of the sensor 2 is shown comprising various circuits that are indicated by the dotted lines as: the cable circuit 20; oscillator circuit 22; operational amplifier circuit 24; and impedance transformer and filter circuits 26,32. The oscillator circuit 22 comprises a differential stage oscillator with a frequency depending on the values of inductances L,, CO and the capacitance Ck of the cable (which depends on the capacitance of the cable alone and any termination capacitors). The frequency of the oscillator may also depend on provision of a termination inductance according to the embodiment of figure 5. In the latter case, the oscillator circuit needs to be modified accordingly (e.g. by removing inductance Leo) The oscillator circuit 22 outputs a signal Ftest that can be processed by the analyzing circuits of the sensor. The operational signal of the sensor cable (at point 10) is fed to an operational amplifier circuit 24 that comprises an operational amplifier Ao with a low pass filter 33 comprised of the circuit R2,C1 and R1. The oscillator circuit 24 in the present example operates as a charge amplifier, however it is also possible to consider providing a voltage amplifier. A capacitor C2 couples the amplifier output signal 30 to the output signal Fozration whilst eliminating the DC signal. The operational output signal Fowration is then fed to the sensor analyzing circuit. The low pass filter filters the oscillator 11 frequencies such that the operational output signal Fozration is not influenced by the oscillation frequency (which operates out of the bandwidth of the operational signals generated by mechanical forces on the sensor cable).

The plus pole input 29 of the operational amplifier Ao is connected to a voltage source Vref that creates a direct current offset voltage at the output 30 of the amplifier, which is connected to an impedance transformer 26. The impedance transformer comprises an amplifier A3 having a gain of 1, the impedance transformer acting to decouple the operational signal from the DC output 35. After the impedance transformer, is a low pass filter 32. The low pass filter 32 substantially removes the operational frequency of the output 30 and provides the offset DC signal. The value of the DC signal is effected by leakage current (ik) flowing through the resistance RK representing the cable dielectric, and should have the value of Ve when the cable is undamaged.

Claims

1. Sensor device comprising a sensor cable having electrodes separated by a dielectric, and selfdiagnosis means, the self-diagnosis means comprising an oscillator circuit comprising an oscillator connected to an electrode and adapted to oscillate in conjunction with the cable, thereby producing a test signal at a frequency determined by the integrity of- the sensor cable for detection of damage thereto, the cable providing inductance or capacitance forming part of the oscillator circuit.

2. The sensor device of claim 1 wherein the sensor generates an operational signal during normal operation for the purpose of determining the operational response of the sensor, and the oscillator generates a test signal that has a frequency out of the bandwidth of the operational signal when the cable is undamaged.

3. The sensor device of claim 2 wherein an operational amplifier circuit is provided to process the sensor operational signals, the operational circuit comprising a filter for filtering the oscillator signals.

4. The sensor device of any one of the preceding claims wherein the device comprises a direct current offset voltage circuit for detecting leakage currents between the electrodes.

5. The sensor device of any one of the preceding claims wherein the sensor cable ~~ comprises a termination capacitor between electrodes at an end of the cable, the capacitor forming part of the oscillator circuit

6. The sensor device of any one of the preceding claims wherein a termination inductance is provided between electrodes at an end of the cable, the inductance forming part of the oscillator circuit.

7. The sensor device of any one of the preceding claims wherein an operational amplifier circuit is connected to one end of the cable for processing the sensor operational signals, the circuit comprising an operational amplifier

8. The sensor device of claim 7 wherein the operational amplifier circuit comprises a reference voltage at an input side of the operational amplifier to create an offset direct current voltage at the output of the amplifier.

9. The sensor device of claim 7 or 8 wherein an input pole of the amplifier is set at a reference voltage to create an offset direct current voltage at the output of the amplifier.

10. The sensor device of claim 7,8 or 9 wherein the amplifier comprises a filter for eliminating the test signal from the operational output.

11. A sensor device constructed and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings