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Wheel Speed Sensor For ABS systems based on AD22157

Published on Jan 20 2011 // Acceleration and Speed Sensor Circuits

Wheel Speed Sensor For ABS systems based on AD22157 1

The AD22157 is a mixed signal magnetic field transducer
designed for applications where both speed sensing and direction
sensing of a ferrous target wheel are required over a wide
speed range.

APPLICATIONS
Automotive
Wheel speed and direction sensing
Transmission speed sensing
Industrial
Incremental position sensing
Proximity switching

The sensor combines integrated bulk Hall cell technology and instrumentation circuitry to minimize temperature related drifts associated with Hall cell characteristics. The sensor is compensate to work optimally with SmCo magnets. The architecture maximizes the advantages of fine line CMOS and high voltage DMOS allowing the device to operate accurately in demanding environments. Principle features of the AD22157 include an adaptive differential zero crossing detector which accurately determines the position of target wheel edges. This architecture eliminates the effects of package and thermal stress on the Hall sensor array resulting in 2% repeatability of the time interval from rising edge to rising edge of the sensor output.

 Wheel Speed Sensor For ABS systems based on AD22157 2

CIRCUIT OPERATION
The AD22157 is a two wire current modulating transducer which generates current pulses in response to spacial differential change in a magnetic field. A typical application is wheel speed sensing where the field to be sensed is generated by the interaction of a permanent magnet behind the sensor and a notched or hole stamped ferromagnetic target wheel in front of the sensor. Under these conditions the sensor must reject that portion of the ‘bias’ field which is constant, and amplify the remaining differentially
modulated portion of the field and determine accurately the position of edge transitions on the wheel.

Wheel Speed Sensor For ABS systems based on AD22157 3

SIGNAL DETECTION
The bias field rejection is accomplished by a spacial differential measurement of the field using integrated Hall plate structures within the silicon substrate. A linear array of three Hall cells is used. The AD22157 is designed to give optimum quadrature signals at a tooth/ notch pitch of 5mm. Each of the three Hall devices is constructed of four individual plates of 200um diameter connected in parallel and spatially orientated in each of four cross quadrature positions in order to relieveprocess gradient induced offsets in the Hall signal voltage. The Hall plate arrays are biased by three matched current sources. The sensitivity of the plates to magnetic field is 5uV / Gauss at thiscurrent. The three Hall effect sensors are connected to instrumentation amplifiers as two pairs with the center plate shared between the two amplifiers. In this configuration two spacial differential magnetic signals are transformed into electrical signals whose peak to peak amplitude is directly proportional to the differential magnetic field component and the Hall plate bias current. Pitch matching the Hall array to the wheel results in an approximately sinusoidal field variation being sensed by the spatial differential
array.