Servo driver selection

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

Gatesy wrote: I seem to recall reading somewhere that an AC brushless motor has a sinusoidal wave and a DC brushless motor a trapezoidal wave,

That's a difference in the power supply, not the motor, though. You can drive an AC servo trapezoidally or drive a bldc sinusoidally.

Would it make any difference if you were using step/dir control?

None at all, that is a difference in the how the servo amp is controlled, not how the amp controls the motor.

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."

Gatesy wrote: Do you have to do this to all AC servo drives,

Nearly all servo drives require this, but it isn't always your job to ensure it.

The Pico (and Mesa) drives are "dumb drives". They just do exactly what you tell them to. There is no internal position loop, they don't handle end-stops, they just drive a motor at the current/voltage that you tell them to.

The Pico drive uses Hall sensors to commutate the motor. The Mesa drives don't even do that, the 7i39 and 8i20 are commutated in HAL too.

In some ways this is an advantage, it gives you much finer control, and you can see what is happening with HAL tools. It also save money.

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