PID Loop (again)

[Tks]

Hi,

I have the following situation.
Doing a retrofit with panasonic MHDA 1Kw drives.
They are in velocity control mode.

The driver i use is de dspmc (from vital systems) using the plugin for Mach 3.

Now i'm in the situatie that i have the system up and running.
But the double PID loop is giveing some pain...

how would one attack this problem?
The panasonic drives have a auto-tune wizzard.
Which sets some couple of gains.

But then i have also the mach 3 plugin in which i need to setup the PID vars.
Also i miss a goal to what i need to tune for. Because maybe is 0.1mm @ 1250mm/sec/2 and 2000mm/min to much asking from the machine??

also i get some high frequency whineing sound... is this bad??
When autotune runs you only can hear the spindle (axle) a bit...

Hope someone can point me in the right direction of how to tune such double loop system.

For example what is an acceptable following error?By which speed and acc?

[CNCMAN172]

TKS,

I have been using Panasonic 750W drivers with MACH3 for years and have been very successful. The internal tuning wizard works great and you can control the stiffness and the wizard will tune the driver to each given situation. I have always used the parallel port to provide step and direction inputs into the driver. I recently purchased a USB pulse engine made my Dynomotion called KFLOP, however have not had a chance to install the unit. I also did a good deal of investigation into Vital Systems unit as well as unit from AJAX.

I spent a good deal of time asking some key questions which might help point you in the right direction. Normally these higher end units want to interface with the Panasonic driver using +- 10V signals and have the driver be in velocity or torque mode. The MACH3 plugin is the linkage from MACH3 to the driver and MACH3 does all the calculations on where to go next and these Vital System type units become the pulse engine that execute the moves.

Normally when you use one of these you do not use the tuning provided by the Panasonic driver, however you can use these values as a starting position. Normally it is not a double loop you have a closed loop where the Panasonic driver is being feed +- 10 v signals to tell it where to move the motor. The Vital System is providing this information and it must be tuned to provide the right kind of response for your given system.

If you are getting a great deal of whining from the motors this indicates the gain is probably too high, the noise is normally the high speed oscillation where the motor is attempting to find the requested position. You can also tune it so high the unit will go into oscillations and jerk the machine around. It is always best to slowly increase the gain until it provides the right performance. Most servos will have a slight hum when tuned correctly but not a loud like what your describing.

The following error should be very small, but not so small that it can never be achieved. If you look at all these parameters on the Panasonic driver after it tunes the motor you will have a very good starting point. Make suttle changes slowing one at a time and log each one on a paper so you can keep track. You can also use a oscope if you have one available to examine the gain visually and this can help get it to the correct level. You can google tuning a motor with an oscope or look in the panasonic manuals and find examples of how this is done.

Another question that might be useful is where are your encoders being feed into? The encoders should be going back to the Panasonic driver some system allow you to drive the encoders into the control system. The AJAX system always has the encoders handled by the motor driver. They do have an option where you can take the encoder outputs from the driver and feed them back into the AJAX unit to actually make use of something close to double ended control.

Hope this helps some.

Russ

[Tks]

Hi Russ,

Thank you for your response.

I indeed have the encoders fed into the drives and then into the motion controller (DSPmc) it works quite good. Its just that the system is now in double loop. THat means that the panasonic drive needs to tune out the throttle versus speed. And the dspmc uses the throttle response versus distance error (infact speed!) so the clue is. Do i just need to auto-tune and do the rest with the dspmc? The DSPmc lets you tune the complete PID settings...

the high tone is visible in the tuning plugin from dspmc its the speed which goes rapidly up and down...

if you might get a new idea/sight hope you remember to post it over here.

[CNCMAN172]

THS,

Well your breaking some new ground maybe someone who is running the DSPMC can comment.

The Mach3 plugin is taking the step and direction information and sending it to the DSPMC controller. THe DSPMC controller inturn commands the Panasonic drive by issues +-10V signals, where zero volts or very close is stopped.

It sounds like your getting the sound when you tune the motor using the DSPMC plugin? Does the plugin have a built in tuning wizard? Many times they have you plug in the controller to a serial link on a PC and run some tuning software to tune the unit.

I think I would use the autotune from Panasonic to collect all the right information on PID curves and write it all down. Then I would attempt to tune everything using the DSPMC. You also might want to go to the AJAX CNC site Ajax CNC - CNC Control Retrofit Kits for Mills, Lathes, Routers & ATCs - Mach3 Ready and look under their support forum as many people are asking similar questions with their unit and my guess is the approach would be very similar to what you need to do on your unit.

The high whinning clearly sounds like the gain is set to high. I think they had a tuning pdf on their site which you might find useful. Hopefully, I will get a chance to start playing with my KFLOP in the next couple weeks.

Good Luck

Russ