Using variable reluctance sensors for differential odometer applications

Applying a variable reluctance sensor to a differential odometer application requires special considerations. Due to widespread use in anti-lock brake systems and the need for cost effective designs, variable reluctance sensors have advantages over more expensive active sensors. Unfortunately, both mechanical and electrical parameters can adversely affect the output of a variable reluctance sensor. The output signal varies with the rate of change of the flux, mechanical spacing and magnetic source variation. At low vehicle speeds the output signal, which is a function of the rate of change of the flux, tends to approach zero and signal-to-noise ratios become significant. Since changes in vehicle direction typically occur at lower speeds, differential odometers require good low speed performance. In addition to speed, differential odometers rely on two sensors, therefore sensor variation must also be compensated for. Consequently, using variable reluctance sensors to perform differential odometer calculations requires signal processing, special circuitry and pulse train processing to extract an acceptable level of information. This paper introduces a signal conditioning circuit that operates over a wide frequency and amplitude range. The output of the circuit can be connected to the input of a microprocessor. Passive components can be adjusted to work with different applications.