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by jacquesm 3331 days ago
No, they are not, servos simply do not step, and whatever pulse there is is PWM to control the torque without overheating the driver in an analogue domain.

There is no reference based on a 'step' or a 'pulse', there is only the desired position and the actual position and the difference between the two can be quite large, much larger than with a stepper which can by definition be at most one step ahead or behind or it is game over.

This gets very interesting once you start to combine servos on multiple axis and you want to limit your maximum error compared to some desired trajectory.
1 comments
pasta 3331 days ago
I know servos don't step, but doesn't the decoder 'count' steps?

But I totaly agree with you, they are completely different. Maybe in this case it was not very clever to oversimplify the problem.

By the way: you point out what I think is great about this business. So much work has been done to control motors very accurate. It's amazing what CNC machines can do today. And also how software optimized motion paths (gcode). And now this hardware. Great stuff!

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jacquesm 3331 days ago
Encoders (not decoders) typically contain a glass disc which contains two 90 degree out-of-phase sinusoidal patterns of light/dark, these are digitized to some arbitrary precision to give you a number of 'steps' around one revolution of the encoder axis. The 90 degree out-of-phase signal is used to be able to determine the direction of motion.
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pasta 3331 days ago
Ah! Thank you for that info. I always thought they used binary disks (black and white or holes).
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TaylorAlexander 3331 days ago
So the term "losing a step" is used on stepper motors, which can be mechanically overpowered to lose positioning. Encoders however will typically always measure the position accurately even if you overcome motor torque and disturb the motor. So in that sense "lose a step" simply does not apply here. Steppers can lose positioning by being pushed beyond their holding torque, while brushless servos will not lose their positioning in the same situation.
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jacquesm 3331 days ago
The fix for that was to bolt an encoder onto the stepper just as with the servo, but in practice these systems do not work very well.
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