Is there any way to drcease the open loop gain with PCI-7344

[jimmyjimmy2000]

Dear Engineers,
Now we are useing PCI7344 on motor-controlling. In our system, a high resolution encoder works as the feedback sensor, and a third-party of servo works as the velocity loop controller. The feedforward gain of velocity loop is determined by the timing range and the feedback gain of position loop is determined by the resolution of encoder that is selected based on the precision of the whole system. And the nature frequency limmits the bandwidth to about 30Hz.
Because the feedback gain of position loop and feedforward gain of velocity loop are both in a large value and can not change, even if we adjust the KP value on 7344 to 1, the position loop still works unstably. Can you give us some advice to such problem.
Thanks.

[JesseD]

Hello,

Thank you for contacting National Instruments.

Unfortunately, the Kp value cannot be reduced less than 1 (except for using 0, which is probably not what you want).

I'm not sure I fully understand the issue you are experiencing. What is the resolution of the encoder you are using? What values are you using for the control loop gains (PID, feedback, feedforward, etc...)? Do you have a drive that is closing the loop (in addition to the PCI-7344)? Why is the velocity feedforward gain determined by the timing range (why is this a large value that cannot change)?

If you can provide as much detail as possible on the exact setup of your system, we will be able to investigate the issue further.

Best Regards,

Jesse D.
Applications Engineering
National Instruments

[jimmyjimmy2000]

Thank you for your answer,
The resolution of the encoder is 3600000(include the 4x mode in 7344). Now we only adjust the KP value to 1 and all other PID gains are 0. What I mentioned before the feedforward gain is not the adjustable parameter of PCI7344 but the parameter of the servo. That is the proportion of the input to the servo(in voltage) and the output from the motor(in rpm). The max velocity of motor is 120 rmp and the max output voltage from 7344 is 10V. As I know ,if we decrease the proportion by tune the servo, it is equivalent to decrease the KP value in 7344 and will make system stable. but the motor will not reach the max velocity that we need. So we try to find out other way to make the system stable.
I don't know if my description is detailed enough. It's my plesure to provide other details if wanted.
Regards,

[ColinAE]

Hello.

One quick idea that I have is to tweak the DAC limits slightly in the Axis Settings, thus preventing the controller from outputting 10V. Go to MAX and look under the Axis Settings, then the Miscellaneous tab. This will reduce the max tourque applied to the motor, and could solve your problem.

Regards,
Colin C.
Applications Engineering

[jimmyjimmy2000]

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@ColinAE wrote:
Hello.

One quick idea that I have is to tweak the DAC limits slightly in the Axis Settings, thus preventing the controller from outputting 10V. Go to MAX and look under the Axis Settings, then the Miscellaneous tab. This will reduce the max tourque applied to the motor, and could solve your problem.

Regards,
Colin C.
Applications Engineering

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So, to the servo, if we want to drive the motor to the max velocity (120rpm), we must input 10V as the command. I am thinking, if the output from 7344 is limmitted to 5V, the motor can not reach its max velocity. That is no we wantted. Is that right?

[JesseD]

Hello,

It sounds like your motor drive is running in velocity mode rather than torque mode. Is this correct? What make/model of drive are you using? Does the drive close any of the feedback loops? If so, you may want to try setting the Kp to 0.

Note that it is highly recommended that you use torque mode with third-party motor drives.

Regards,

Jesse D.
Applications Engineering
National Instruments