Servo motor control...

[WilliamD]

I'm trying to wrap my head around how a servo motor works. I've used both servos and steppers with varying controllers. I see how a step motor works, but I don't understand how a servo motor can move so incrementally.

I've been researching and found that RC servo motors are controlled by sending a signal and varying the pulse time. The pulse time corresponds to a angle of rotation. Is this how something like the Gecko's work with DC servo's?

With only having two poles, I would think the motor could only move on full rotation at a time. I know there is the encoder for feedback to tell it how far it has gone, but how do the magnets synchronize with all that. It seems very complicated. Any help and understanding would be greatly appreciated!

[Al_The_Man]

You might want to read up/google PID loop, this will explain to a great degree the servo loop principle.
http://en.wikipedia.org/wiki/PID_controller
Al.

[NC Cams]

Find an application note for the NE544 servo IC. This will show you how they control the RC servos. In RC servo's, the feedback is a simple voltage signal from a potentiometer. The feedback in an industrial servo is digital counts.

Al's link gives insight into the tuning algorythym (sic) which will be of use when you get serious about doing servo tuning.

There is a U-102 Unitrode/TI application note for the UC3637 IC that also gives a lot of insight into how a DC servo works/is driven - at least from the ampifier side.

The motor is essentially a multi-pole low cogging, low inertia motor, hooked to the system to/thru a step down gearing system for more torque, that can accelerate and decelerate quickly in response to the prescribed motion.

[Al_The_Man]

How this for an animation!!
http://users.tinyworld.co.uk/flecc/4...otor031102.swf
Al.

[WilliamD]

Awesome information! The PID loop makes much more sense now. I will definetly be checking into the IC listed. I'm picking apart the animation now. Thanks a lot for your time!

[vger]

Great animation, although it plays a bit fast. That is a 3 phase brushless motor in the animation. Hard drive spindle motors (older ones) are just like that, they may have more poles, but always a multiple of 3. They use hall effect sensors to detect the position of the rotor magnet to help the driver know what to do. A standard DC servomotor will have just 2 leads and will just run at a constant speed with DC applied, and reverse direction with the leads connected reverse polarity. For servo loop operation some sort of feedback is needed. On some systems just a small tachometer generator is attached to the shaft to give only a speed signal to allow the driver to control the speed and/or direction precisely. Others have an angle encoder of one type or another. Most common is an optical encoder that outputs 2 signals that are 90 deg out of phase with one another. Circuits can decode this "quadrature" signal into step and direction information. Some optical encoders have as many as 5000 steps each revolution. The electronics just counts up or down the pulses from the encoder and compares that count to where it should be and outputs to the motor driver to make the motor drive to make the count even. With such a system you can actually manually turn the servo out of position and it will return to the correct place if the system is on. That is not the case with a stepper system unless it has positional feedback like the servo system does. There are microstepping drives for stepper motors that take a standard 200 steps/rev motor and make it do as many as 25,000 steps/rev by turning the individual coils on only partially with many different discrete levels of drive.
Hope I've not confused you..
Steve

[Al_The_Man]

Originally Posted by vger Great animation, although it plays a bit fast.

You can step it in commutation increments.