Brushless servo drive question

[jdholbrook]

Hello,
I'm gathering parts for my CNC router and am at the stage of the drives.
One consideration is going with servos, specifically brushless servos.
I see them on ebay and the prices look nice compared to the DC brushed servos.
In looking over the manual for say a Copley Brushless drive I see that it takes a +10 to -10 volt input to the drive unit for the motor control.
My question is.. How do I get from a step / direction type of control to a variable voltage control?
Is there an interface that I'm not seeing?
Is this the reason brushless servos are not going for as much as brushed DC servos?

Thanks
James

[Al_The_Man]

Originally Posted by jdholbrook Is this the reason brushless servos are not going for as much as brushed DC servos? James

I suspect that the reason DC Brushed are more popular is that matching the drive is not as critical.
With DC Brushless, and particularly AC Sinusoidal, matching the motor to drive gets trickier due to the motor requiring the correct commutation, with a Brushed motor the commutation is in the motor, with the others the commutation is between motor & drive.
Al.

[jdholbrook]

So I should be looking for motors and drives from the same company?
Is there a spec on the motor, such as Inductance, that will help match up the pair?

[jdholbrook]

I dug a little deeper in this forum and came up with this.

quote/
My board which is a controller only (no power mosfets) that takes step and direction inputs like the other 2 thread boards (and geckos, etc) and uses a 12 bit DAC and buffer amp to output an analog voltage which is intended to be connected to a separate motor amplifier, such as those made by Copley, AMC, etc. that accept an analog control voltage. My board takes the encoder feedback from the motor, implements a PID algorithm, and commands the amplifier via this control voltage to move the motor to a new position. Obviously you could use either a brush or brushless motor as long as the amplifier supports the type of motor you desire to use.
/quote

So you would need an additional device between the analog motor controller and the step/direction signals.

[Al_The_Man]

This looks like it takes a step/dir signal and converts to the amplifier types you previously mentioned, ±10v analogue.
No additional device should be needed.
Al.

[stevenmotor]

mct.motor@msa.hinet.net

[unterhaus]

It certainly takes a lot more work to get a brushless servo system going. Two companies make products you might consider. One is the Pixie which converts step/direction to +/- 10v (you need an encoder on the motor) http://www.skyko.com/products/
There is a review and some threads about this device here on CNCZone.

The other is Rutex, who make a number of brushless amplifiers that take step/direction, and also a converter like the pixie.

There are a number of big name companies that have step/direction brushless drives. Off the top of my head, some drives from Electrocraft, PacSci, API Controls, Compumotor, and plenty more will take step/direction. Many of these same drives are re-branded and sold by other established companies. However, you need to read the manuals and often check particular model numbers to make sure with the products from these large companies.