Servo driver selection

I like the look of the Argon drive, but quite expensive, I can get a drive/servo from china cheaper than one Argon. But would they be as good as a Fanuc motor / Argon drive combo?

I see PICO do a lot of stuff suitable for Fanuc motors, but website does not say what mode their brushless servo amplifier works in, and if it is for AC motors?

Anyone using PICO stuff, any good?

I seem to recall reading somewhere that an AC brushless motor has a sinusoidal wave and a DC brushless motor a trapezoidal wave, can't remember where, and not really sure what difference it would make!! Would it make any difference if you were using step/dir control?

I think so, my motors are 47 volt, and 3 amp stall current

This was the only bit I found confusing about the PICO's. Do you have to do this to all AC servo drives, and does it have any effect on performance?

"The PWM frequency is determined by the motion controller, but should be between 25 and 100 Khz. The duty cycle can be reduced to zero, but must not reach 100%. Off time of at least 500 nS must be provided every cycle to recharge the "bootstrap" capacitors in the high-side transistor's gate drivers."

So you supply the drive with DC, and it then chops the DC into various proportions which becomes AC, so if you were to run the drive at 100% duty cycle, would you just end up moving the DC voltage line to a higher value, i.e. at 0% and 100% you have DC, and anything in between you have AC?

So all the position control is done through LinuxCNC? As one closed loop circuit?

Is this approaching more the setup the likes of Fanuc and Heidenhain use?

Gatesy wrote:

I like the look of the Argon drive, but quite expensive, I can get a drive/servo from china cheaper than one Argon. But would they be as good as a Fanuc motor / Argon drive combo?

I see PICO do a lot of stuff suitable for Fanuc motors, but website does not say what mode their brushless servo amplifier works in, and if it is for AC motors?

Anyone using PICO stuff, any good?

The Pico Systems brushless PWM servo amp is a six-step drive, not sinusoidal. So, it will not be as smooth as a sinusoidal drive. It depends on how the windings of the motor are wound. Some motors run as smooth as can be, some have a bit of "hum" or vibration to them, but it generally is pretty small. You can definitely hear the Fanuc motors hum at certain speeds, that is the disturbance when the drive advances to the next coil. But, the servo loop compensates for most of the effect.

Don't be fooled by that AC vs. DC label, there really is no such thing. Some motors have coils or magnet geometry optimized for trapezoidal drive, some are optimized for sinusoidal. But, do NOT assume when the motor is labeled AC or DC that it clearly tells you which it is designed for. Certainly, all brushless motors are truly AC motors.

Jon

Gatesy wrote:

This was the only bit I found confusing about the PICO's. Do you have to do this to all AC servo drives, and does it have any effect on performance?

"The PWM frequency is determined by the motion controller, but should be between 25 and 100 Khz. The duty cycle can be reduced to zero, but must not reach 100%. Off time of at least 500 nS must be provided every cycle to recharge the "bootstrap" capacitors in the high-side transistor's gate drivers."

OK, first, (replying to an earlier message) there are no steps in the Pico Systems PWM controller/PWM servo amp system. Encoder position is read by the PC, LinuxCNC's PID computes a new velocity and sends it out to the PWM generator, and the servo amp provides current on the motor coils as selected by the Hall sensors.

Jon

Gatesy wrote:

So you supply the drive with DC, and it then chops the DC into various proportions which becomes AC, so if you were to run the drive at 100% duty cycle, would you just end up moving the DC voltage line to a higher value, i.e. at 0% and 100% you have DC, and anything in between you have AC?

if the motor is turning, it is all AC, frequency determind by motor speed. The PWM pulses from the drive are integrated by the motor's inductance into a pretty continuous current flow.

So all the position control is done through LinuxCNC? As one closed loop circuit?

Yes, the position control feed through the PWM controller, and is managed by LinuxCNC.

Is this approaching more the setup the likes of Fanuc and Heidenhain use?

Fanuc sends 6 PWM signals to their servo amp, each drives one transistor. On the Pico PWM amp, there is one PWM and one direction signal, the current is sent to coils as directed by the Hall signals. I can't say how Heidenhain does it.

Jon