GB2093946A

GB2093946A - Monitoring shock absorbers

Info

Publication number: GB2093946A
Application number: GB8106047A
Authority: GB
Original assignee: ITT Industries Ltd
Current assignee: ITT Industries Ltd
Priority date: 1981-02-26
Filing date: 1981-02-26
Publication date: 1982-09-08
Also published as: GB2093946B

Abstract

A method of and apparatus for monitoring the in-service performance of a fluid shock absorber. The shock absorber incorporates transducers for generating electrical signals representative of the fluid pressure and the sprung mass/unsprung mass velocity respectively. A circuit (Fig. 3). has peak detectors 22, 25 to ascertain peaks in the two signals which are compared in a differential amplifier 26 the output of which is compared in comparator 28 with a reference value. if the amplifier output exceeds the reference value the shock absorber is deemed to be faulty in operation and an indication of this condition is given. <IMAGE>

Description

SPECIFICATION Monitoring shock absorbers This invention relates to the in-service monitoring of the performance of fluid shock absorbers, such as are used in railway rolling stock.

Shock absorbers are devices designed to produce a resistive force between two parts in relative motion, the force being a predetermined function of the velocity. Typically a fluid shock absorber is a device in which a piston moves in a cylinder and forces fluid through a system of narrow orifices and one-way valves. The resistive fluid forces are normally generated for both directions of relative movement, i.e. compression and extension of a shock absorber. The shock absorber functions properly so long as the fluid force generated by the movement falls within predetermined ranges of values for given relative velocities of the two parts.

The in-service monitoring of such a shock absorber can be accomplished by incorporating in the device a piezo-electric hydraulic pressure sensor and an electromagnetic velocity sensor, these two sensors being connected to a suitable signal processing circuit which compares the fluid pressure and the velocity values and indicates when the shock absorber fails to operate satisfactorily. Such a general arrangement is disclosed in British Patent No. 1 508 527 (K. L.

Ellington-7).

A fluid shock absorber can be designed with the intention of producing a chosen dynamic velocity/pressure characteristic, e.g. a linear characteristic which is identical for both directions of motion. However, in practice a shock absorber may exhibit hysteresis when its characteristic is observed during dynamic cycling.

Figure 1 shows these features for a shock absorber intended to have two substantially linear sections of the velocity pressure characteristic; one during the movement of compression and one during the movement of extension.

It will be noted that the pressure/velocity diagram does not give simple linear graphs, in particular it can be ascertained that for a given velocity the pressures during acceleration and deceleration do not correspond but rather a "looped" form of characteristic occurs owing to the presence of hysteresis.

According to the present invention there is provided a method of monitoring the in-service performance of a fluid shock absorber including the steps of sensing the peak fluid pressure generated within the shock absorber and the peak velocity of the shock absorber during an excursion and determining so the performance of the shock absorber from the sensed values.

In a preferred embodiment of the invention the determination of the peak velocity/peak pressure relationship is effected only when at least one of the peak values itself lies within a range of values.

The above and other features of the invention will become more apparent from the following description of an embodiment of the invention with reference to Figures 2 and 3 of the accompanying drawings, in which: Figure 2 illustrates schematically a telescopic fluid shock absorber with pressure and velocity sensors, and Figure 3 illustrates a monitoring circuit for the shock absorber of Figure 2.

The shock absorber shown in Figure 2 consists essentially of a piston 1 which moves in a cylinder 2. The piston 1 is connected by a piston rod 3 to the sprung mass of a vehicle (not shown) and the cylinder 2, is rigidly supported in an outer housing 5 connected to the unsprung mass of the vehicle.

Movement of the piston 1 within the cylinder 2 forces fluid through pressure valves 7 located in a sealed piston rod guide 8 into a reservoir space 9 between the cylinder 2 and the housing 5. During extension fluid is also drawn from the reservoir into the cylinder 2 via the foot-valve assembly 4.

During compression the compressed fluid in the cylinder 2 is forced through valves 10 in the piston 1. The shock absorber incorporates an annular magnet 11 on the top of the piston-rod guide 8 and an annular coil 12 affixed to the inside of a dust-cover 13. The dust-cover 13, piston-rod 3, and a pole-piece 14 provide magnetic flux paths 1 5 (shown by dotted lines).

The flux paths 1 5 have to be carefully chosen to yield a net useful flux intersecting the coil 12 whose rate of cutting by the windings of the coil produces a voltage indicating the relative velocity of the sprung and unsprung parts of the shock absorber.

The shock absorber also incorporates a piezoelectric pressure transducer 1 6 which generates an electrical output in response to the fluid pressure resulting from the relative movement of the sprung and upsprung masses.

Figure 3 shows the basic elements of a monitoring circuit. The velocity signal from the coil 12 is amplified in amplifier 20 and is passed through a gain control circuit 21 before being applied to a peak detector circuit 22. Likewise the pressure signal from the piezo-electric transducer 1 6 is passed through amplifier 23, gain control circuit 24 and is then fed to a peak detector circuit 25. Each peak detector circuit is fed with a separate adjustable threshold value. The outputs of the peak detector circuits 22 and 25 are fed to the two inputs of a differential amplifier 26. The differential amplifier 26 feeds, via a full-wave rectifier 27, a comparator 28 which also receives a preset limit signal. The output of comparator 28 is then fed via an enabling switch 29 and an integrator 31 to an indicator arrangement 30.The switch 29 is enabled when the two peak detectors 22 and 25 together indicate that peak values have been reached. The peak detectors 22 and 25 are arranged to hold their peak values until one of the input signals, e.g. the velocity signal, falls below a predetermined level. The peak detectors are then reset.

In operation, assuming that the peak detectors 22 and 25 are in the reset condition, each will respond to its input to detect the peak velocity and peak pressure respectively, although these peaks may not be exactly coincident in time.

Adjustment of the relevant gain and threshold levels ensures that a "window" is created so that only if at least one of the peaks occurs in the window will a comparison be made. The circuits and designed so that corresponding velocity and pressure peaks occurring within the window are fed to the differential amplifier. If the shock absorber is functioning correctly these peak values should be substantially equal, so the differential amplifier 26 produces no output or a small output. If the shock absorber performance deteriorates then one of these peak values will rise or fall relative to the other and the output of amplifier 26 will increase correspondingly. The rectified amplifier output is compared with the reference values applied to a comparator 28 and so long as the amplifier output is below the reference value the shock absorber is deemed to be working satisfactorily.If however the rectified amplifier output exceeds the limit value then the comparator 28 produces a fault indication output.

However, for this fault indication to be valid it must occur only when both the peak signals have been.detected, hence the need for the enabling switch 29. If the fault indication is valid then switch 29 is closed and the comparator 28 output is fed to the indication device.

A feature of the circuit is that it can be powered by one of the shock absorber signal sources. For example the circuit can be powered by a capacitor (not shown) which is charged from the velocity signal input and thus acts as a battery. In this case the construction imposed on the sensor by the design and operation of the shock absorber may mean that the current output of the sensor is limited. It will be appreciated that in practice the working life of a shock absorber is measured in years and normally any deterioration in performance is gradual, hence it is not necessary to provide continuous monitoring of the performance. The self-powering arrangement is such that it may take up to 30 minutes or more to achieve a sufficient charge on the capacitor to power the circuit for 0.5 minutes of operation.

During the charging period, i.e. when the capacitor has not reached a satisfactory level of charge, the circuit is switched off. When the capacitor reaches a predetermined level of charge the circuit is switched on and operates until the charge drops to a second predetermined level.

This intermittent operation of the circuit is enough to monitor satisfactorily the performance of a shock absorber in practical operation conditions.

Alternatively,the circuit may be powered from a separate battery. However, to prolong the active life of a battery it may again be preferable to power the circuit only intermittently in the manner described above. Preferably the circuit is only powered when one of the peak values reaches a threshold, e.g. the "bottom" of the window. Only then is power supplied to the differential amplifier 26 and the following portions of the circuit.

Another feature of the invention is the provision of the integrating circuit 31 in which valid fault signal from the comparator 28 is held and integrated with respect to time. The integrator 31 has a control signal leakage so that isolated fault signals occurring at infrequent intervals are treated as spurious signals and are allowed to dissipate. Only when the frequency of the fault signals exceeds a preset rate is the fault condition deemed to be persistent and an indication is generated when the persistent fault condition reaches a level where attention is warranted.

It is to be noted that the detail design parameters of the various elements of the circuit are deemed to be well within the capabilities of those skilled in the art and therefore do not need further explanation.

Claims

1. A method of monitoring the in-service performance of a fluid shock absorber including the steps of sensing the peak fluid pressure generated within the shock absorber and the peak velocity of the shock absorber during an excursion and determining so the performance of the shock absorber from the sensed values.

2. A method of monitoring the in-service performance of a fluid shock absorber including the steps of ascertaining for a given excursion of the shock absorber the peak fluid pressure generated within the shock absorber and the peak velocity of the shock absorber during that excursion and determining whether for a given peak velocity the peak pressure so ascertained falls within predetermined limits, or vice versa.

3. A method according to claim 2 wherein both the peak pressure and the peak velocity are determined as being within predetermined limits for the peak velocity and the peak pressure respectively.

4. A method according to claim 2 or 3 wherein the determination of the peak velocity/peak pressure relationship is effected only when at least one of the peak values itself lies within a given range of values.

5. A method according to claim 2, 3, or 4, wherein the determination of the peak pressure/peak velocity relationship is effected only when both values have been received within a preset time.

6. A method according to claim 5, wherein one or both of the the peak values is held for a preset time to allow for a time interval between the occurrence of the two peak values.

7. A method according to any one of claims 2 to 6, wherein the results of successive monitoring operations are integrated with respect to time.

8. A method of monitoring the in-service performance of a fluid shock absorber substantially as hereinbefore described.

9. Apparatus for monitoring the in-service performance of a fluid shock absorber including means for generating a signal representative of the relative velocity of the sprung and unsprung parts of the shock absorber, means for generating a signal representative of the fluid pressure generated within the shock absorber, and means for determining the performance of the shock absorber from the peak values of the signals.

1 0. Apparatus for monitoring the in-service performance of a fluid shock absorber including means for generating an electrical signal representative of the relative velocity of the sprung and unsprung parts of the shock absorber, means for generating an electrical signal representative of the fluid pressure generated within the shock absorber, means for ascertaining the peak value of the velocity signal for an excursion of the two parts of the shock absorber, means for ascertaining the peak value of the pressure signal for the same excursion of the two parts, means for comparing the two peak values so ascertained and means for generating an output signal when the output of the comparison means exceeds a reference value.

11. Apparatus according to claim 10 comprising peak value determining means and presettable means for adjusting the gain of the signal the peak of which is to be detected.

12. Apparatus according to claim 10 or 11, wherein either or both of the peak determining means includes presettable threshold(s) whereby only peak values in excess of the threshold(s) are determined.

1 3. Apparatus according to claim 1 2 including switching means arranged to inhibit the output signal from the comparison means except when the peak value determining means produce for the same excursion outputs which exceed the respective threshold(s).

14. Apparatus according to any one of claims 10 to 13 wherein either or both peak value determining means includes means for holding the peak value detected until a resetting operation is effected.

1 5. Apparatus according to claim 14 including means for resetting the peak value determining means when a given one of the input signals falls below a predetermined level.

1 6. Apparatus according to any one of claims 10 to 1 5 wherein the means for generating the velocity signal comprises a magnet attached to one part of the shock absorber and a co-operating coil attached to the other part of the shock absorber movable relative to the one part and so arranged in the shock absorber that a net surp!us of flux of one polarity over that of the other polarity intersects the coil throughout the range of movement of the shock absorber.

1 7. Apparatus according to any one of claims 10 to 16, wherein the means for generating the pressure signal comprises a piezo-electric pressure transducer placed within a fluid chamber of the shock absorber.

1 8. Apparatus according to any one of claims 10 to 17, including a capacitor, means for charging the capacitor from one of the signal generating means, and means for powering all or part of the monitoring circuit from the capacitor only when the capacitor charge exceeds a predetermined amount.

1 9. Apparatus for monitoring the in-service performance of a fluid shock absorber substantially as described with reference to Figs.

2 and 3 of the accompanying drawings.

20. A fluid shock absorber including apparatus according to any preceding claims.

21. A fluid shock absorber substantially as described with reference to Figs. 2 and 3 of the accompanying drawings.