Knowledge Base

Hall Effect Sensors are used to establish proper motor commutation on Brushless Servo motors.  To be effective, the Hall Effect Sensors must be properly aligned with the servo motors phases.  Motor manufacturers typically align the Hall sensors at the factory so customers do not need to.  But if the hall sensors are removed from the motor or become out of alignment for any reason, realigning them is mandatory for proper operation of the motor.  Here is a Hall sensor realignment process for a typical brushless servo motor.

Equipment Required:

1. Additional motor to back-drive the motor that needs Hall alignment.  Tape one shaft to the other and back-drive in velocity mode.
2. Method to power the halls (Vcc and Gnd on the feedback connector)
   1. Servo drive that can power the halls, OR
   2. 5V power supply (verify your Hall sensors spec.  Kollmorgen AKM’s use 5V Halls.)
3. 2 Channel isolated oscilloscope to measure motor back-emf (bemf) and a Hall channel
4. Pull-up resistor for the Hall channel.  2.2kOhms should do fine. (OR if the servo drive can monitor and output the Hall channel, that would be easier)

Objective:

Looking at chart 1 below, note that the Blue plot (Motor Phase U bemf, which is phase U with respect to V) crosses zero at the same point that the square-wave brown plot (Hall U) changes state.  That is our objective; to match the motor phase U zero crossing point to Hall U within +/- 10° electrical (See example calculation below to determine the mechanical tolerance in degrees).  If we do not keep within the +/- 10° electrical spec, motor performance can suffer.

Electrical Degrees Example Calculation

Given: The desired commutation angle setting is within +/-10° electrical and 360° Mechanical = 1 motor rotation

If motor pole pairs = 5, then;

Chart 1:

Proper motor and hall phasing for Servo Motors.

Procedure:

1. Mount the Halls or commutating encoder (an encoder with hall sensors built in) on the motor
   1. Hall sensors only: Mount the Halls on the motor stator
   2. Commutating encoder: Mount the encoder on the motor rotor, keeping the encoder rotor moderately loose
2. Using oscilloscope channel 1, scope across the motor’s U and V channels
3. Power the halls, use a pull-up resistor from Hall U to 5V, and use oscilloscope channel 2 across Hall U and gnd.
4. Back-drive the motor at low and constant speed
5. Adjust the Halls to match the motor phase with the Hall output.
   1. Hall sensors only: Adjust the sensors position on the stator until motor phase U is aligned with Hall phase U, then tighten the Halls down.
   2. Commutating Encoder: Adjust the encoder rotor until the motor phase U is aligned with Hall phase U, and tighten the rotor down.