Abstract
Recording vehicle speed accurately has always provided a challenge for the data collection engineer. Many ways have been tried to do this apparently simple task but the fact remains that no one solution satisfies every need.
Sometimes, engineers want to know wheel speed from the very start of rotation. This is useful for characterisation of the behaviour of traction control devices since wheel slipping can occur at very low speeds. Traction control devices often use speed difference as a control parameter. Similarly, the performance of anti- lock braking systems is assessed, by analysing what happens during the braking event through to the point where the wheels are stationary.
In other cases, engineers require that there should be good resolution of wheel speed at high vehicle speeds. The detection of cornering can be done by measuring small differences in wheel speed.
In order to measure both low and high speeds accurately the measurement system will have to handle speeds from one rpm up to 1500 rpm. This is a dynamic range of 30dB.
This paper will review the methods used in vehicle testing to measure rotational speed and show the advantages of each method. Particular emphasis will be placed on the post processing of speed signals in analysis software to improve accuracy.