A new wheel velocity sensor is described. The sensor represents another application of the ferrite core/magnet technology which has been used for several years in non-contacting keyswitches and keyboards.
The wheel velocity sensor is an active, proximity sensor developed to be used with a variety of exciter rings and tone wheels in anti-skid systems, or as a tachometer sensor, a crankshaft position sensor, or in similar applications where the speed of a rotating vane is to be monitored. The sensor provides an output signal with amplitude independent of the rotation rate of the vane, down to “zero speed”.
One version of the sensor consists of two ferrite toroidal core inductors connected in series with a fixed ceramic permanent magnet providing a magnetizing bias for the ferrite cores. It is a three terminal device and is commonly driven from a balanced signal source. The excitation frequency is not critical, and is usually in the 250 KHz to 1 MHz range. The sensor can be remotely located from its drive and signal processing unit. The maximum operating temperature of the sensor, which is determined by the Curie temperature of the ferrite cores, is over 180°C. Exposing the sensor to higher temperatures causes no permanent damage, as the temperature effects are reversible. The sensor operates with standard exciter rings at air gaps exceeding 0.100″. An integrated circuit has been developed to interface the sensor with standard pulse counting circuits.
The development of the sensor is discussed, and the performance characteristics of the sensor and the interface integrated circuit are described.