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RepRap motherboard to an AC servo motor and driver

Posted by firstyear 
RepRap motherboard to an AC servo motor and driver
July 03, 2010 08:35AM
Hi there,

I've just been given 3 AC servo motor's and just don't know what to do with myself...eek.
Has anyone managed to hook up a RepRap motherboard to an AC servo motor and driver? Or managed to find a driver that can work between the motherboard and an AC servo motor? Thanks...
Re: RepRap motherboard to an AC servo motor and driver
July 05, 2010 05:34AM
Hi everyone, I'd really love to make a reprap and really really hope I can use the AC servo motor's.
From what I've read so far the reprap divers and or geckos etc only support either steppers or DC Servos at the moment, so if anyone has any ideas I'd be highly grateful...
Re: RepRap motherboard to an AC servo motor and driver
July 05, 2010 05:36AM
whoops, that was supposed to say driver's not 'divers' ..ha.
Re: RepRap motherboard to an AC servo motor and driver
July 05, 2010 08:32AM
You can buy off the shelf servo motor drivers that will take step and direction signals and emulate a stepper, so you would be able to connect those to the mother board in place of the stepper motor drivers. I don't think they are cheap though. Some servo motors have them built in.

Re: RepRap motherboard to an AC servo motor and driver
July 05, 2010 09:09AM
I fear you have to craft a solution for yourself. This may be a starting point: [www.design.ucla.edu]

Generation 7 Electronics Teacup Firmware RepRap DIY
Re: RepRap motherboard to an AC servo motor and driver
July 07, 2010 06:53PM
According to Wikipedia,
Two-phase AC servo motors

A typical two-phase AC servo-motor has a squirrel cage rotor and a field consisting of two windings:

1. a constant-voltage (AC) main winding.
2. a control-voltage (AC) winding in quadrature (ie, 90 degrees phase shifted) with the main winding so as to produce a rotating magnetic field. Reversing phase makes the motor reverse.

An AC servo amplifier, a linear power amplifier, feeds the control winding. The electrical resistance of the rotor is made high intentionally so that the speed/torque curve is fairly linear. Two-phase servo motors are inherently high-speed, low-torque devices, heavily geared down to drive the load.

This implies that an AC servo motor is quite different in operation from a DC stepper motor. Stepper motors have highest torque at low speed, and lose power as they rotate faster (step pulses become short and inductance limits current). This high torque is needed to move the entire axis assembly for several of the axes, and the extruder requires quite a bit of force to push the hot plastic through the tiny orifice. Steppers are also very good at moving to an exact position and staying there, whereas AC servo motors are more intended to match a speed and stay there. If your motors did not come with gear reduction heads, I fear that you will never get the precision positioning needed for at least the X, Y, and Z axes motors. You will also need to generate the base AC power to run the main coils, and then generate a second AC power form of variable amplitude and 90 degrees out of phase with the first. This is theoretically possible with two switching power supplies that generate phase controlled AC. There are also COTS AC servo motor controllers, but htue look VERY expensive and professional.

I fear you are going to spend much forcing the AC servo motors to fit a spot that can be more easily and cheaply filled with DC stepper motors. Especially if you do not already have gear reduction drives for each one.I found 4 NEMA 17 steppers on eBay for about $50, and right now single motors as low as $7. There are many 4 motor kits plus combined stepper driver to convert a machine to CNC, but those add up to more like $130.

Re: RepRap motherboard to an AC servo motor and driver
July 07, 2010 10:15PM
Thank you so much for your replies and advice, it's really appreciated and has given me ALOT to think about..

Im starting to build a 3x3x3ft gantry cnc/reprap and was thinking along the lines of using the AC Sevos for their torque.
I guess I need to figure out if it's too much for what I want to create; and if so maybe just invest in a good set of steppers that plug and play with the reprap motherboard and probably geckos.
I've read that the commercial cnc machines use AC Servos, so thought there must be a good reason (service life/torque/speed/dependability/repeatability) so will firstly look into cheap AC drivers if possible, otherwise I'm thinking 425oz steppers with geckos.
Also need to figure out if if its primary use will be cnc (driven by mach3) or printer (driven using skeinforge + Replicator G.)

Guess I really want to make as few mistakes at the beginning as possible....gotta watch the peso's.
Re: RepRap motherboard to an AC servo motor and driver
July 08, 2010 04:28PM
I was Googling your issue and found a page that discusses the difference between DC stepper motors and AC Servo motors for CNC control.
AC Servo vs DC Stepper

Yes, on high end systems, they use AC servo motors almost exclusively. But entry level, and probably most home brew CNCs use steppers because that are cheaper and simpler.

From what I am understanding, the motor in an AC servo motor system is only one part. To make it a "servo", there must also be a feedback system. One end of the motor shaft must have some kind of encoder, most often an incremental optical encoder. This gives the AC Servo motor a maximum resolution, just like the step size in a stepper motor. This becomes and error signal, feed back into the Servo driver to help drive the motor until it reaches the zero error position and holds there. The device that takes the encoder output and uses it to drive the AC Servo motor until it reaches the desired location or speed is the Servo Driver. This driver generates the main power signal as the main AC plus the 90 degrees out of phase, variable amplitude and direction control AC signal that is used to control the motor. This feedback must be controlled by a PID to prevent overshoot and oscillation. In fact, the AC Servos are much more sensitive to buzzing, chattering, and other oscillations right around the current 0 speed point and so require anti-backlash and pretensioning to minimize this oscillating around the set point. The overall CNC controller then sends a voltage or digital value to the AC Servo driver box to add in the with error signal and control the speed and direction of the AC Servo motor. The parameters of the PID must be carefully adjusted otherwise the servo will be very slow to reach its position, or will overshoot many times before finally settling for a spot in the middle. And all this is made worse if there is an slop or backlash in the mechanical system.

The end result, when you have the AC motor, the optical encoders, the AC Servo Motor Drivers, carefully calibrated PID(s) and tight, no backlash mechanical drive trains, you get superior speed and accuracy, but at considerable expense. AT the other end of the scale, the stepper motors, running open loop so no oscillations or PID tuning and very simple, cheap stepper driver chips can provide pretty good speed and precision, at much lower cost.

I hope this helps.

Re: RepRap motherboard to an AC servo motor and driver
July 08, 2010 09:52PM
Do these AC servo motors have any kind of make and/or model names and numbers on them?
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