This patent relates to a method for evaluating the efficiency of vehicle shock absorbers to facilitate their adjustment or replacement as required, and the device for implementing the method.
The known devices used to evaluate the efficiency of a vehicle""s shock absorbers are based on measuring and sensing instruments of electromechanical type. They generally comprise a movable platform on which the wheels of the vehicle front or rear axle are rested. Having positioned the wheels on the platform, suitable mechanical means transmit a stress to the platform, which transmits it to the vehicle via the suspensions. The oscillations which the platform undergoes as a result of the transmitted stress are sensed by suitable means, these oscillations being proportional to the oscillations of the vehicle. By analyzing the maximum amplitude of the oscillation and its damping time, the effectiveness of the vehicle suspension (shock absorbers) can be evaluated.
The known devices suffer however from a series of problems which limit their accuracy. In said devices the evaluation of the vehicle suspension efficiency done in the described manner is influenced by the deformations which the tyre undergoes during the measurement.
To take account of the deformations undergone by the tyre, the manufacturers of these devices take account of the fact that the frequency of the tyre oscillations is different from that of the vehicle oscillations. However, it has been found that the results obtained with devices such as that described vary greatly, in accordance with the tyre inflation pressure.
The object of this invention is to overcome the drawbacks of the known art within the context of a rational and reliable solution which is able to shorten the time used to verify the suspension efficiency, and does not require the provision of a measuring site.
The invention attains said object by a method for verifying the shock absorber efficiency, and by an apparatus for implementing the method, which is able to measure and also take account of the deformations undergone by the tyre during the measurement.
To this end the apparatus of the invention uses image acquisition means connected to a processor to process the acquired images.
The method of the invention comprises the following operational steps:
identifying, by virtue of image acquisition means, at least one point of the vehicle rigid with the shock absorber lower connection, and at least one point of the vehicle rigid with the shock absorber upper connection,
associating said two points with a Cartesian reference system in order to identify their height from the ground and hence their distance apart,
subjecting the vehicle or at least one wheel thereof to a non-horizontal forcing stress,
memorizing a succession of photograms in order to acquire the equations of motion of said at least two points,
calculating the equation resulting from the difference between the two equations of motion,
determining from said equation the parameters necessary to evaluate the shock absorber efficiency.
In greater detail, the method of the invention comprises acquiring and processing a certain number of images at least of a portion of the vehicle close to that shock absorber the efficiency of which is to be verified, in order to identify at least one reference point of the vehicle rigid with the shock absorber lower connection, and at least one reference point of the vehicle rigid with the shock absorber upper connection.
The reference point of the vehicle rigid with the shock absorber lower connection can be a chosen point on the wheel rim, such as a point on the wheel rim edge. Instead, the reference point of the vehicle rigid with the shock absorber upper connection can be any chosen point on the vehicle chassis close to the shock absorber under examination.
Said reference points are determined according to the invention by seeking and identifying within the images those regions in which the transition between the grey levels is a maximum, as explained hereinafter.
In detail, to speed up the processing of the acquired images and to be certain that the point rigid with the shock absorber lower connection is a point on the wheel rim edge, the invention, after identifying a certain number of points apparently pertaining to the wheel rim edge, calculates the equation of the conic section, such as an ellipse or a circumference, passing through these points and, as the reference point rigid with the lower connection, chooses on the wheel rim at least one point pertaining to that conic section, or related to it, such as the centre of the conic section itself.
Instead, said at least one reference point rigid with the vehicle chassis is chosen, according to the invention, about a point having a height greater by 20% than the height of the highest point of the major diameter of that conic section representative of the edge of the vehicle wheel rim.
Having identified said at least two reference points and associated them with a Cartesian reference system, the method of the invention subjects the vehicle to a non-horizontal stress and at the same time acquires a certain number of images of the event, i.e. of the oscillations undergone by the vehicle following the transmitted stress, to be able to construct the equation of motion of the two individual points.
It should be noted that in processing the acquired images to determine the equation representative of the motion of said at least two points, any vehicle movements in a horizontal direction due to the transmitted stress will be compensated.
According to the invention, the number of images sufficient to reconstruct the equation of motion of the two points must be between 20 and 150 images per second.
Knowing the two equations of motion the method of the invention calculates the equation resulting from the difference between them, to determine the relative oscillation between the two reference points alone. This oscillation represents the oscillation of the vehicle suspensions, following the application of the stress, net of the deformations undergone by the tyre.
Knowing this equation, the shock absorber efficiency can be verified by analyzing suitable specific parameters, such as the maximum amplitude of the oscillation and the time required to damp the oscillation amplitude to below a determined value.
The method of the invention is implemented by an apparatus comprising at least one image acquisition device, such as a photographic camera or video camera, installed on a movable support element and intended to frame at least a portion of the vehicle close to the shock absorber the efficiency of which is to be verified. In particular the lens of said image acquisition device is able to frame the vehicle wheel at that shock absorber of which the efficiency is to be verified, and at least a portion of the vehicle chassis close to that wheel. Said image acquisition device is also connected to a processing station preferably provided with a display for displaying the results.
It is apparent that the invention can achieve the same result by using either an image acquisition device for each wheel or a single image acquisition device plus the use of suitable optical paths achieved with the aid of fixed and/or movable mirrors and possible shutters, which enable all the wheel to be seen in succession from the filming point.
The stress which is transmitted to the vehicle, or to a wheel of the vehicle, can be caused either manually by the operator, or by usual means of mechanical type, for example a hydraulic or pneumatic jack positioned below the vehicle chassis and arranged to raise the vehicle chassis from the ground.
The special characteristics of the device and of the method are specified in the claims.