SCH_HALLSENSOR

Summary

This page describes a custom circuit built for the Engineering and Robotics Department at Springside Chestnut Hill Academy located in Philadelphia, PA.

Description

The goal was to accurately measure the RPM rotational speed of a motor and convert it a linearly proportional analog voltage for subsequent measurement by the main robotic software.

The circuit solution uses a Hall effect sensor, type A1302, to pick up the periodic passing of a rare Earth magnet located in a collar mounted on the shaft of the motor. The analog signal passes through a comparator with threshold set to generate a positive voltage in presense of sufficient magnetic field, and a zero voltage in absense of magnetic field. A high speed microcontroller then measures the period of each rotation and calculates the RPM. The RPM is then scaled to an analog voltage for output. There are many subtleties in both the circuit and software that ensure accuracy and rejection of noise; these are not discussed here.

Functional characteristics

Wiring: Power, common, Vout

Power: nominal +12VDC supply (6-20 VDC is fine); draws about 14mA

Vout: 0-5VDC

Output Impedance: 940 ohms (intended to drive high impedance loads)

RPM equation:

RPM = 1230.3*Vout

Minimum rotation speed: 290 RPM (Under 290RPM, software will output zero voltage)

Maximum rotation speed: 6000 RPM

Update rate:

at 300 RPM, about every 2.0 seconds

at 6000 RPM, about every 0.1 seconds

Indicators

Green light blinks each time rotation measurement is successfully made.

Red light blinking means no rotating magnetic signal, magnet polarity reversed, or rotation too slow

Contact

For more information, acquiring additional circuits, or other custom sensing needs, contact tek@estarbase.com