[brian257]

Just got my first Automation Technologies 1150 in oz servo motor today. Looks to be a good quality motor. Don't have the big power supply for it yet so just temporarily connected it to a small 60V .5A bench supply expecting it to barely run the motor, but it had a surprising amount of torque with that small amperage.

What surprised me is that this is just a direct drive motor. I am used to servos being a geared motor. Since it is not geared it has hardly any resistance to back driving. Great when I want to use the machine in manual mode, but I am wondering how that will work out on a stopped axis on the machine. I suspect that it will always be trying to back drive, then powering the motor to put it back where it belongs therefore constantly hunting. Don't have that problem much on a geared motor since the gearing keeps it still. Can someone with experience with these tell me how well they just hold in place?

[mactec54]

brian257

I not quite sure what you are meaning, by hunting back driving

If you are using the machine in manual with hand wheels, then the power should not be on the motors

If you are using manual under power, then there should not be any movement from any axes, if they do you will have to tune the drives, No axes should move only if you give it a signal to move

When you turn the power on for CNC use the motors will/should lock up were ever they are, you then home the machine & you are ready to run

[brian257]

I was referring to when the machine is in CNC. A stepper has voltage on it when it is stopped so it holds in place. The servo motor has no voltage when it is stopped so it is free to back drive until it gets out of position and voltage is applied to turn it back the other direction hence a cycle of moving slightly then going back to where it was supposed to be. A highly geared servo like a model airplane servo does not back drive much because of the gearing.

So what I am wondering is if the ungeared servo will be in a constant humm trying to keep position when it is stopped.

[Al_The_Man]

If a servo is tuned properly it will recognize an out of position starting with one encoder pulse, with a torque mode drive, the motor current will be in proportion to the encoder error.
A servo motor is not 'free to back drive' as there is only zero motor current with zero following error.
Al.

[brian257]

So pretty much with the small error values read on the encoder and proportional motor currents it gives a good tight PID loop and will stick where you want it without vibration or hum. I will buy that. Thanks.

[mactec54]

brian257

As Al The Man has said the servo motor is not free to back drive, This could only happen if the drive was badly tuned, The power supplied to the drive was not correct, or the motor was to small for the job it is doing

vibration or hum. No this only happens when the drive is not tuned correct