Rotational speed sensors: limitations, pre-processing and automotive applications

Rotational speed sensors are critical for all vehicular driver assistance systems and other applications such as engine control and robotics. For many embedded control systems, the accuracy of the computed speed needs to be high, and this puts special demands on the pre-processing of the sensor signal. In a wired centralized solution, the computational unit can get access to the raw square wave signal from the sensor. Without a dedicated wire, the sensor must include a timer, and the cog times tk can be communicated to the processor. These cog times are the input to the algorithms described here, whose purpose is to calibrate manufacturing errors in the cog wheel and to interpolate the speed signal to the (usually time) domain of the application. A further purpose was to derive rules of thumb for how the sampling and quantization processes influence the bias and variance of the computed speed signal. Examples of applications to imbalance monitoring, frequency analysis applications, and fault detection were provided.