Feedback

In the past I would have said I was more familiar with the kind of feedback that came from an electric guitar and amplifier.  Lately I have become more familiar with the kind that shows up on servomotors and linear feedback on actuators.  Feedback technology has become more sophisticated and some vendors like Timken Motion have entered the market with lower cost solutions that make it easier to do closed loop position control.  This is really good news for the industry.

As with most things in the mechatronic world, feedback requires a great deal of finesse.  The majority of servo motor feedback devices are based on Hall Effect transistors.  These devices have become very popular because they can be applied to a wide range of sensing applications where magnetic fields are involved.  Among which there are less costly current sensors available which is a great help in the motor control world.

There are design issues with Halls that require consideration.  The most important is hysteresis error.  Since the role of the Hall Effect switch in a servomotor application is to sense the position of the rotor, any angular error in the sensor results in errors in turning on power transistors to the stator windings.

You wouldn’t think it a significant issue, but I remember a debate with a colleague who had just completed testing of a sensorless motor drive circuit.  The idea behind the circuit was to eliminate the need for the Hall Effect sensors in a brushless dc motor drive.  The drive worked quite well, but the puzzle was why the sensorless drive was more efficient than the sensor drive.   After much debate, we concluded that the sensorless circuit was more accurate with regard to calculating the rotor position than the Hall Effect version.

I wouldn’t normally consider this conclusive, but there are a number of aspects of the motor drive that are impacted by higher resolution in the feedback.  I recently worked on an actuator where we used a 1K magnetic encoder based on the fact that the output position was based on a gear reducer and lead screw and the feedback resolution was meaningless.  But it turned out that because we were operating at relatively low motor speeds, the controller did not have enough data to properly regulate the motor current loop.

The normal Hall Effect feedback for timing of motor currents is 120 degrees per rotation. This means there are only six edges per revolution of the motor from which timing can be extracted for energizing the windings.  At low speeds this seems entirely inadequate, and I am sure that there are other algorithms used to manage the motion.

Notwithstanding some weaknesses in the fundamentals of Hall Effect devices, a good understanding of the device and clever circuit design has created some leading edge technology that is enabling the motion control industry to move forward.  Let’s continue the trend.

 

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