Technical Field

The invention relates to an apparatus for detecting the malfunctioning of an accelerometer in form of a piezoelectric device, where an electric test signal, preferably in form of a square pulse, can be transmitted from a test signal generator to the piezoelectric device by means of a change¬ over switch capable of performing a change in such a manner that the response signal from the piezoelectric device can be transmitted to an analysis circuit, preferably a frequency-analyzing circuit, so as to be subjected to a malfunction detection.

Background Art

In order to obtain a sufficient measuring accuracy of a vibration measur¬ ing by means of an accelerometer it is necessary to meet two require- ments, viz. the accelerometer must be mounted on a plane surface and the securing moment or force must be suitable. When the securing moment or force is reduced, the resonance frequency drops until no connection exists at all between the accelerometer and the mounting surface. Subsequently, the free resonance frequency of the accelerome- ter is measured. The optimum resonance frequency results in the maxi¬ mum amplitude. A measuring of the resonance frequency of the acceler¬ ometer and the associated amplitude results in an indication of the mounting state of the accelerometer. When the resonance frequency of the accelerometer is measured after a mounting and said resonance frequency is compared with a calibration card, the following factors

apply:

A lower resonance frequency than the indication of the calibration card indicates that the accelerometer has not been sufficiently tightened up. When the measured resonance frequency for instance is 1 .5 times the indication of the calibration card it means that the accelerometer has not been mounted. When the measured resonance frequency is very close to the indication of the calibration card it means that the accelerometer has been sufficiently well mounted.

When the tightening up to the surface is correct, the frequency and the amplitude of the measuring are stored. A change of the frequency during later measurings indicates that something has happened to the tighten¬ ing up. A change of the amplitude without a simultaneous change of the resonance frequency indicates that the sensitivity of the accelerometer has changed.

It is known from EP Publication No. 534,366 to perform a continuous observation of the mounting state of an accelerometer. A test signal is fed to the accelerometer through a capacitor. Such a capacitor should, of course, be avoided. In addition, a rather complicated analysis circuit is required which must be able to distinguish between applied and actual vibration signals. The feeding of test signals and the analysis are carried out simultaneously.

Furthermore US-PS No. 4,418,567 discloses a sensor equipment for detecting the motor knocking in a car motor. This equipment comprises a control circuit generating gate control signals for a splitting up into measuring and test phases. The test phase includes detection of a poss-

ible malfunctioning of the sensor. The splitting up into measuring and test phases is carried out synchronously with the rotation of the motor. The test signal is in fact fed simultaneously with the measuring pro¬ cedure. The testing is merely carried out at a time where it is known that no measuring signal exists. Accordingly, this equipment is limited to a particular use.

In addition DE Offenlegungsschrift No. 3,542,397 discloses a device for testing an accelerometer and in form of a piezoelectric unit. This piezoelectric unit is tested by being mechanically influenced by an addi- tional piezoelectric unit. This additional piezoelectric unit should, how¬ ever, be avoided.

Brief Description of the Invention

The object of the invention is to provide an apparatus for detecting the malfunctioning of an accelerometer in form of a piezoelectric device, and which is far more simple and universal in use than previously known.

The apparatus according to the invention is characterised by the change¬ over switch being adapted to alternately connect the piezoelectric device either to the test signal generator or to the frequency-analyzing circuit and to detect the response signal upon termination of the transmission of the test signal.

Brief Description of the Drawings

The invention is described in greater detail below with reference to the accompanying drawings, in which

Fig. 1 illustrates the resonance frequency versus the tightening up of an accelerometer on a plane surface,

Fig. 2 illustrates a circuit for detecting the malfunctioning of an acceler¬ ometer,

Fig. 3 illustrates state diagrams of electric signals in the circuit of Fig. 2,

Fig. 4 illustrates a simplified embodiment of the detector circuit,

Fig. 5 illustrates the signal above the accelerometer,

Fig. 6 illustrates a preferred embodiment of the detector circuit accord¬ ing to the invention, and

Fig. 7 illustrates state diagrams of electric signals in the circuit of Fig. 6.

Best Mode for Carrying Out the Invention

In order to obtain the highest possible measuring accuracy of a vibration measuring by means of an accelerometer it is important that said accel¬ erometer is mounted on a plane and clean surface, and that the feeding, i.e. the securing moment or force, is suitable. In connection with a predetermined mounting surface the resonance frequency depends on the securing moment or force. Fig. 1 illustrates an example of the rela¬ tionship of the securing moment or force and the resonance frequency of a plane surface. When the securing moment or force is reduced the resonance frequency drops until the connection between the accelerom¬ eter and the mounted site has been eliminated. Subsequently, the free

resonance frequency of the accelerometer is measured. The maximum resonance amplitude is measured at the optimum resonance frequency. A measuring of the resonance frequency of the accelerometer and the corresponding amplitude results in an indication of the mounting state of the accelerometer. When the resonance frequency of the accelerometer is measured after a mounting and when said resonance frequency is then compared with the resonance frequency of a calibration card, the following facts apply: When the measured frequency is lower than the indication of the calibration card it means that the accelerometer has not been correctly secured. When the measured resonance frequency is approximately 1 .5 times the indication of the calibration card depending on the type of accelerometer, it means that the accelerometer has not been mounted at all. When the measured resonance frequency is close to the indication of the calibration it means that the accelerometer has been properly mounted.

When the feeding to the surface is correct, the frequency and the ampli¬ tude are stored in a memory. A change of the frequency during a later measuring indicates that the feeding has been changed. A change of the amplitude without a corresponding change of the resonance frequency indicates that the sensitivity of the accelerometer has been changed.

When an accelerometer is applied a low-impedanced square pulse with a fundamental frequency matching the resonance frequency of the accelerometer, said accelerometer responds by emitting a ringing at the resonance frequency or frequencies, cf . Fig. 3. This ringing can typically be measured approximately 1 /sec after termination of the pulse. When an accelerometer is mounted in such a manner that it is only in contact with the surface in a few points, the response signal includes several

close resonances. In the moment the response signal is received, the accelerometer 4 can furthermore include a low DC voltage which must be filtered off. The response signal can be analyzed for instance by means of an analyzer, such as a frequency analyzer 8. As a result it is possible to determine whether several resonance frequencies apply. The presence of several resonance frequencies indicates that the accelerome¬ ter 4 is in contact with the surface in question on several locations. A neasuring of the response signal at a correct tightening up on a good surface results in an indication of the sensitivity of the accelerometer 4. A change-over switch 1 , 2 switches between a standard measuring state and a testing state by connecting the accelerometer 4 either to a test signal generator 3 or directly to the analyzing circuit 8 or a counter coupled thereto in parallel.

The response signal is subjected to a filtration by means of a band-pass filter 6 in order to eliminate undesired DC and internal resonances in the accelerometer 4, the latter being rather highly positioned. The band-pass filter can for instance cover from 10 kHz to 30 kHz.

Fig. 4 illustrates a simplified embodiment of the detecting apparatus according to the invention. This embodiment comprises an accelerome- ter 4 in form of a piezoelectric device, a change-over switch 1 , 2, and a test signal generator 3 controlled by a logical circuit. The change-over switch 1 , 2 comprises two contact arms which can switch between a measuring and a testing state in the uppermost and lowermost position, respectively, of said contact arms. A test procedure of the test signal generator 3 corresponds to passing five positions a, b, c, d, e of a con¬ tact arm, said procedure being terminated at the lowermost position e where the accelerometer is connected to an amplifier 5. The path of the

signal above the accelerometer 4 is illustrated in Fig. 5 together with the positions of the contact arm. The contact arm can be controlled by a logical circuit.

The amplifier 5 can be a voltage amplifier or a charge amplifier. A high- pass filter 6' serves to filter off low-frequency components before the signal is transmitted to a counter 7 counting the number of zero cross¬ ings. The signal can be transmitted alternately or simultaneously to a measuring equipment 18 communicating with the connection between the amplifier 5 and the filter 6'.

The test signal starts at zero or at a level corresponding to the DC-input level of the amplifier 5 and ends up at the same level after having passed an equal number of positive and negative pulses. When the test signal generator 3 is disconnected the signal level is such that the ring¬ ing signal is symmetrical relative to zero. As a result that the signal anal- ysis can start immediately. The illustrated example includes only one positive and one negative pulse, but the principle is, however, not limited to this number. Another logical control can for instance be adapted to generate n positive and negative pulses.

Fig. 6 illustrates an embodiment of the detecting apparatus according to the invention. The amplifier 5 is a charge amplifier presenting a time constant T = R2-C in the measuring position for the change-over switches 1 , 2, 2a and a time constant of r ₂ = (Ri || F?2)'C ' ^{n "} the testing position. is lower than r ₂ as the measuring position only involves measuring at relatively high frequencies. In addition, the test signal generator formed by a change-over switch 3 comprises only four posi¬ tions. A microprocessor 1 2 connected thereto controls the switching

between the measuring state and the testing state, the pulse generator signal from 3, as well as the signal processing at the measuring equip¬ ment 1 8 and the counting at the counter 7, said microprocessor 1 2 communicating with a decoder for the change-over switches 1 , 2, 2a., a decoder for the change-over switch 3, the measuring equipment 1 8, and the counter 7, respectively, through one or more buses.

Fig. 7 illustrates a state diagram of the circuit in Fig. 6 for

the signal across the accelerometer,

Hi i c, e the control signals to the four analogous gates, where a high level indicates that the gate in question is closed, whereas a low level indicates that the gate in question is open,

e is low whereas a, b or c is high indicating that the amplifier is discon¬ nected. When the amplifier is connected after the activation of the accelerometer through a, b, c, the signal oscillations shown at the top of Fig. 7 are amplified before they are transmitted to the counter 7. The counter 7 is not opened until after a period, preferably 5 to 10 times 7 ^" ₂ which is due to the fact that the oscillations at the beginning of the counting can be at a maximum with the result that a predetermined oscillation period is required in order to obtain a reliable counting. r ₂ can be 1 0 to 20 sec, and

f), respectively, the signal for opening the counter 7.