Condition indicating device for wheeled vehicle shock absorbers

According to the invention, an accelerometer (G) is attached on the spring suspension (U) of the wheel (H) in question and capable upon passing of the vehicle over road irregularities to emit a decaying A.C. signal (S) with information on the oscillations of the axle (A) and the damping of the tire. After integration in an integrator (I) the signal is a measure of the difference between the damping of the shock absorber at expansion and at compression. This signal is compared in a comparator (K) with a reference signal (V.sub.ref) for the permissible damping difference and results in alarm (L) upon achieving said difference.

This invention relates to a device for indicating the condition of shock 
absorbers of wheeled vehicles by using an acceleration-sensitive means, 
which is rigidly attached near the shock absorber and emits a signal 
resulting in indication when a predetermined acceleration value has been 
exceeded. 
When designing shock absorbers, they are to be dimensioned so as to yield 
maximum damping without causing the tires at upward motion to loose road 
hold, and so as to damp sufficiently softly to prevent the vehicle from 
being bumpy at downward motion. These two criteria imply, that the shock 
absorber must have properties, which are different for compression and 
expansion. A worn absorber more and more loses its capacity of damping 
oscillations. As the absorber works predominantly at compression, the 
damping for compression consequently decreases relatively rapidly. At a 
new absorber the difference between the damping at expansion and at 
compression is great, but decreases with increasing wear. It was, 
therefore, desired to obtain a reliable indication of worn shock 
absorbers, preferably from the driver's seat. 
One indication arrangement of the kind referred to above in the 
introductory portion is known from GB-PS No. 1 483 231. The 
acceleration-sensitive means there is located on the vehicle body in order 
to be actuated by the mechanic oscillations of the body via the shock 
absorber. When the maximum amplitude of the body oscillation--i.e. the 
first oscillation--at the passing over a distinctive road irregularity 
exceeds a certain value, the means becomes operative for releasing a 
signal to an indication circuit. Thus, the influence of the shock absorber 
on the first body oscillation at road bumps is measured, which renders an 
unsafe and gross indication. 
By the present invention, however, an indication is obtained which is both 
safe and sensitive, and which is based on measuring the effect of the 
shock absorber on oscillations, which are generated in the tire surface 
proper and transferred to the wheel axle. In accordance therewith, the 
present invention is characterized in that the acceleration-sensitive 
means includes an accelerometer, preferably a piezoelectric element, which 
is rigidly attached adjacent the wheel hub and continuously measures the 
oscillations of the wheel axle and emits a correspondingly varying 
electric signal to an integrator, the output signal of which is compared 
in a comparator with a reference signal corresponding to the predetermined 
acceleration value, in order upon achievement thereof to release 
indication, for example lighting-up of an alarm lamp.

In the drawing, a car wheel H with axle A and stationary spring suspension 
means U and absorber spring indicated at D is shown. The wheel load, and 
therewith the damping, being approximately symmetric, it is considered 
sufficient to carry out the measuring of the absorber behaviour at a 
single one of the four wheels. According to circumstances, however, it may 
be motivated to measure at several wheels by using identically alike 
measuring arrangements. 
For the indication of the absorber wear, a measure of the aforesaid 
difference between the damping at expansion and at compression can be 
obtained by measuring the oscillation of the axle, in order to obtain the 
difference between rising and dropping speed. The axle oscillation is a 
measure of oscillations generated in the tire surface and depending on the 
tire properties (damping and natural oscillation frequency) when the 
vehicle passes over road irregularities. For measuring the axle 
oscillations influenced also by the shock absorber, an acceleration 
transducer G, a so-called accelerometer, is attached to the wheel 
suspension U. Said accelerometer may be a piezoelectric quartz element, 
which in the embodiment shown is mounted facing upward on the wheel 
suspension U, and at its free end supports a metal plate. When the base 
point of the piezoelectric element is accelerated at the passing of the 
vehicle over road irregularities, the point motion is transferred 
vertically via the piezoelectric element to said plate. The mass inertia 
of the plate opposes the acceleration motion and thereby momentarily 
compresses the element. The piezoelectric effect thereof results in a 
measurable current impulse S, which is proportional to its compression 
caused by the road bumps via the tire, axle and absorber spring, and the 
amplitude of which decays due to the damping of the axle oscillation. 
The output signal S of the transducer (piezoelectric element) G is passed 
to a low-pass filter LP in order to eliminate disturbing shock transients 
in the signal. The output of the filter LP is connected to the input of an 
amplifier F, the output of which is connected to the input of an 
integrator I. The output signal V.sub.a therefrom then is a measure of the 
difference of the absorber and is fed to one input of a comparator K. To 
the second output thereof a reference voltage V.sub.ref is fed which 
corresponds to an acceptable minimum value of the absorber difference. The 
reference voltage V.sub.ref is derived from a region of the 
damping/frequency characteristic of the tire where the damping is changed 
little with the variations in the natural oscillation frequency of the 
tire, while the output signal V.sub.a refers to a region where the 
frequency dependency of the damping is high. When the signal V.sub.a in 
question has diminished to the reference voltage V.sub.ref, i.e. the 
difference has decreased to the permissible minimum value, the comparator 
emits an output signal to an indicator, for example a lamp L, which 
preferably is mounted on the instrument panel of the car, in order to warn 
the driver on the poor condition of the shock absorber. 
The invention is not restricted to the embodiment described above, but a 
great number of different modifications can be imagined within the scope 
of the invention. It is especially possible to use a separate voltage 
source for generating the reference voltage V.sub.ref, instead of deriving 
it from the tire characteristic. Furthermore, instead of utilizing the 
reduction of the signal to the comparator down to the reference voltage 
for indication, the increase of the signal up to the reference voltage can 
be utilized. Of course, accelerometers of a kind other than piezoelectric 
ones can be used.