[speedydumpster]

Greetings:
I have a problem when tuning my Granite Device VSD-E drive to take an analog input when in velocity mode. Here is what is happening:
1. I tried using Torque mode with Analog input, but I was unable to tune my Dynomotion Kflop controller card in Torque mode. I contacted Dynomotion and they recommended me using velocity mode as it is much easier to tune.

Quote from Dynomotion:
Torque mode is a more complex type of servo to tune as compared to velocity mode. With simple proportional (P) gain a velocity amp system will be stable. Proportional gain means the command to the amplifier is proportional to the error. So as the target is approached it will command slower and slower velocity and therfore slow to a stop. With a torque amp and only proportional gain the system will continue to accelerate faster until it reaches the target which will result in severe overshoot. So with a torque servo some type of damping (D Gain) is required to have a stable system.

2. If I force the drive to use SPI, the motor/drive goes quiet when in velocity mode.
3. As soon as I switch to Analog In, the dithering starts and it's loud. Not only that, but the axis slowly creeps at maybe .5"/min.
4. If I unplug the CMD connector from the VSD-E drive the motor still dithers and the axis still creeps.
5. I tried plugging the motor directly into the drive (no cable for interference), and still the problem persists.

System Config:
1. Windos 7 64 bit
2. Dynomotion Kflop + Kanalog Drive
3. 48V 5.9A continuous brushless AC servos
4. 3 VSD-E Granite Device servo drives
4. WT ZX45 mill
5. 4:1 belt drive on X and Z, 2:1 belt drive on Y.
6. Optical home switches and mechanical limit switches.

I'm thinking that maybe the voltage on the analog input is floating or something, but I'm not an electrical or controls engineer, and don't know how to fix the problem. I belive that I'm so close to having a running CNC mill, but I can't tune everything to work in conjunction with eachother. Can anyone help me?
Thank you.
Dave

[Al_The_Man]

If using a closed loop Dsp controller I prefer that it closes the loop entirely, For this I use analog drives in the Torque mode that do not require any motion profile tuning? I let the controller tune the PID entirely.
Most controller and drive manufacturers recommend the Torque mode for this method of control.
I am not sure if you can set the Granite drives up this way?
Al.

[TomKerekes]

Hi Dave,

It sounds like you just haven't tuned the Velocity loop in the Granite Drive. But you should read their procedures how to do that and/or contact them. The slow creep is not really a problem. It is almost impossible to command exactly zero velocity with an analog signal because there will always be some analog offset or noise. But this will not be a problem when the position feedback is enabled. KFlop/Kanalog will automatically command whatever voltage necessary to hold position.

But as Al explains an alternate approach is to use the amplifier in torque mode if you find the Velocity mode in the Granite drive hard to use. It just means that you will need to find a combination of P and D gains (or P and Lead/Lag compensation) to get a stable system that servos and doesn't oscillate, whereas if the Amplifier is already controlling velocity then simple P gain alone will usually form a good stable position loop.

So try tuning the Granite Velocity Loop first. If that doesn't work switch back to torque mode and we will help you through that.

Regards

[Montabelli]

I have a similar problem, Xerxes suggested I use velocity mode as I have been having issues running my motion master, fanuc 110v ac servos 3 vsd xe drive s galil board, cam soft soft ware. cam soft says my problem is tuning, when I try and tune velocity the axis I am tuning drifts off to the end of travel , hard stop. how do you tune if the axis is at the end of travel?

I originally ran this machine with mach3 and a smooth stepper, it did quite well when I ran it with their experimental tempest planner, but would bog down too much in small segment code.

[Xerxes]

I put some tips to our wiki regarding velocity mode usage:
Configuring VSD-E/XE for velocity mode - Granite Devices Knowledge Wiki

(BTW the wiki is under construction and has only some contents)

[Montabelli]

OK, I am really puzzled, I have been tuning my servos in velocity mode, and I discovered the ratio setting made a huge difference to the settings I could make and the performance of the motor even though the ratio is 1 to 1.

two of my drives were set 10/10 and 1 was set to 50/50. why would there be a difference?

[Xerxes]

The higher divider value gives better internal precision of calculations inside drive. Its recommended to use divider at least 50.