Magnetic field change detection circuitry having threshold establishing circuitry

Apparatus for detecting a tooth of a body, such tooth having an edge terminating in a notch, such tooth and notch being disposed along an edge of the body, such apparatus, comprising:

• a pair of magnetic field sensors disposed laterally along a direction of movement between the body and the pair of magnetic field sensors;
• a magnet disposed at the reference position to provide a magnetic field through the body and the pair of magnetic field sensors, the magnitude of the field passing through the body and each one of the pair of sensors being related to the relative position between the tooth and such one of the pair of sensors, each one of the pair of sensors producing an output signal related to the magnitude of the magnetic field passing such sensor;
• a differencing circuit fed by the output signal produced by each one of the sensors for producing a difference signal having a peak when the tooth is positioned between the pair of sensors;
• a peak detector for detecting the peak in the difference signal and for producing a peak detection signal indicating detection of such peak;
• a threshold circuit fed by the differencing signal for producing a gating signal when the magnitude of the difference signal exceeds a threshold level;
• a gate, the gating signal enables the peak detector produced peak detection signal to pass through the gate to an output of the apparatus when the gating signal indicates the magnitude of the difference signal exceeds the threshold such peak detection signal being inhibited from passing through the gate to the output of the apparatus in the absence of such gating signal;
• and
• a threshold level adjusting circuit for establishing the threshold level in accordance with the magnitude of the difference signal.

Apparatus for detecting a tooth of a body, such tooth having an edge terminating in a notch, such tooth and notch being disposed along an edge of the body, such apparatus, comprising:

• a pair of Hall effect cells disposed adjacent to, and laterally disposed along, the edge of the body;
• a magnet positioned to provide a magnetic field through the edge of the body and the Hall effect cells, the magnitude of the field passing through the Hall effect cells being related to the relative position between the tooth and the Hall effect cells, such Hall effect cells producing an output voltage related to the magnitude of the magnetic field passing through the cell;
• a differencing circuit fed by the pair of Hall effect cell produced voltages for producing a difference signal having a peak when the edge of the tooth is positioned between the pair of Hall effect cells;
• a peak detector for detecting the peak produced by the difference signal and for producing a peak detection signal indicating detection of such peak;
• a threshold circuit fed by the differencing signal for producing a gating signal when the magnitude of the difference signal exceeds a threshold level;
• a gate, the gating signal enables the peak detector produced peak detection signal to pass through the gate to an output of the apparatus when the gating signal indicates the magnitude of the difference signal exceeds the threshold such peak detection signal being inhibited from passing through the gate to the output of the apparatus in the absence of such gating signal;
• a threshold level adjusting circuit responsive to the difference signal for tracking maxima in the difference signal and for establishing the threshold level of the threshold circuit in accordance with the tracked difference signal maxima.