The longitudinal slip-ratio is one of several parameters that govern the magnitude of the longitudinal force generated by a tire and probably the most important. As such, the longitudinal slip-ratio of a tire is an essential measurement required in the construction of longitudinal tire models. In laboratory experimental tests the slip-ratio can typically be controlled by controlling the rotational speeds of two motors. However, in in-situ tests, where the slip-ratio needs to be measured, the slip-ratio is estimated from three other measurements namely the wheel speed, vehicle speed and the rolling radius of the tire. While the wheel speed is fairly inexpensive and easy to measure accurately, the accurate measurement of vehicle speed requires an expensive GPS or similar system. The measurement of the rolling radius is often performed statically and assumed constant or can be measured with an expensive laser displacement sensor. Errors in the all of these measurements are compounded when determining the slip-ratio. This paper proposes a method of measuring the longitudinal slip-ratio by performing image correlation techniques on consecutive images of the tire-road interface obtained from a single inexpensive camera. Since the method makes use of one measurement system the probability of inducing errors in the slip-ratio is reduced.

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