[arvidj]

In another thread Al mentioned ...

... the drive can be used in the torque mode, I generally prefer this method if it is available ...
Al.

I am wondering if he would explain what the advantages of each might be. I have a \KAnalog and the DMM drives can do either. I had planned on Speed mode based on ignorance of Torque mode abilities or virtues.

Obviously there is a vacuum between the ears that awaits filling.

Thanks.

[Al_The_Man]

All the analogue (±10vdc) drives I currently use recommend the torque (current) mode of operation for CNC application.
A few decades ago velocity drives were common, and required a tach back to the drive, the encoder went back to the controller.
With these drives the velocity or inner loop (drive) had to be tuned first and then the outer (PID) loop in the controller was tuned.
It is claimed the velocity drive has looser control around the zero speed point.
I believe the main reason for the switch to trans-conductance or torque amps was due partly to the higher encoder frequency possibility and the sophistication of current DSP's etc?
With these Drives, the motor current is proportional to the ±10vdc signal in.
The switch is reflected in the fact it is very hard to obtain a DC motor with a Tach now, unless ordered as an option.
Going this route has worked out very well for me so far.
If you have the option of both with tach's in place you could do a compare.
If eventually going the Torque mode route you would disconnect the tachs, I usually also remove the tach brushes to avoid possible mechanical complications later.
Al.

[arvidj]

Al,

What you describe related to "A few decades ago ..." appears to be the scenario for DMM systems.

The motor and the encoder are connected to the DMM drive. You can attach a PC to the DMM drive and with the DMM software can control and 'tune' the DMM drvie and motor.

The DMM drive provides synthesized quadrature encoder information to the KFlop\KAnalog and the KFlop\KAnalog would then provide a -10\+10 analog signal back to the DMM drive. The DMM drive can be configured to interpret the analog signal as either 'speed' or 'torque' information.

Everything that I have read talks about velocity ... acceleration and deceleration being changes in velocity, spindle movement as a velocity, etc. therefore I assumed that the information given to the DMM drive would be 'velocity' information. To be honest I could not envision a scenario where 'torque' mode would be used unless one made the possibly invalid assumption that more torque means more velocity.

Where I am having the current 'torque' mode disconnect is "would the KFlop\KAnalog need to know it is sending 'torque' information rather than 'speed' information?"

At first blush my totally uninformed answer would be 'yes, the KFlop\KAnalog would need to know', but after thinking about it a very poorly formed 'maybe not' comes up. My vary naive logic goes like this:

The goal of the KFlop\KAnalog is to drive the -10\+10 analog signal in a way that causes the actual motor position to match the requested position. The KFlop\KAnalog determines the position needs to be changed and the PID process calculates a -10\+10 analog signal value that it believes will make that happen. The difference between 'speed' and 'torque' mode would be the values and filters applied during the PID process calculation. Sort of like "you would teach the KFlop\KAnalog about 'speed' or 'torque' mode as you tuned the PID process.

Now that you know what naivete can create, you can hopefully fill me in on reality.

Thanks,
Arvid

[Al_The_Man]

I have been using motion cards for almost 3 decades, Galil and Acroloop, all using torque mode drives with success.
What I suggest is looking at some of the excellent Galil training videos by Dr Jacob Tal, including motor tuning, there is one on the theory and usage of Torque and Velocity drives that explains it better than I could.
Tutorials - Galil: We Move The World
They are geared to Galil products, but most of the theory and detail applies across the board.
The other thing to point out and Dr Tal touches on it, when the loop is closed in torque mode from motor to controller, simpler PWM amplifier drives can be used that do not posses a DSP controlled loop.
Mach uses these types as the loop is closed in the drive, not the controller.
Al.

[keebler303]

Torque is what makes the motor turn. Having direct command of the torque allows you have the best control over the motor. The controller can make the motor accelerate by commanding the drive to deliver the torque it needs to accelerate, and then hold it at a velocity by commanding a smaller torque. If the controller sees any error, it will automatically command more torque from the motor which will correct the error.

In all digital drives I am aware of, the current loop has the highest bandwidth, the velocity loop lower bandwidth, and the position loop the lowest bandwidth. More bandwidth means that the current loop can react to a command more quickly, thereby better controlling the motor. That is why torque mode is preferred.

Matt

[arvidj]

Al and Matt,

Thank you both for the education.

I watched the Galil video which turned on the lights and then spent more time searching for additional information with Google. I think I now have a reasonable understanding of what is going on and will be changing my configuration between the KFlop\KAnalog from Speed to Torque.

Arvid