Re : Closed Loop or Open Loop

[Oompa]

Dear All,

I'm currently using the DAQ card PIC-6221 to control a DC servomotor with an encoder displacement feedback. I'm using the labview program with a motor amplifier to start the motor and stop it at a specific rotational displacement, where it works by stopping the motor when the encoder feedback value reaches the input desired displacement value. The final displacement of the motor before it stops always exceeds the desired value marginally. May I know for my case,am I using an open or closed loop control? Is a PID controller a MUST for closed loop control? Is my DAQ card capable of desigining a PID controller? If so may I know how?

Also, my project requires me to toggle between the current and voltage mode control. May I know if that is possible to carry out digitally using the Labview Program as my current motor amplifier requires me to push a switch manually to toggle between Voltage and Current mode. Finally, I hope my last question is not out of this world but is it possible to use 2 amplifiers(drivers) for a single motor such that there's a chance to use both the current and voltage modes of control?

Really looking forward to the replys of you kind souls out there! Thanks!

Regards,

Oompa

[JB]

The PCI-6221 has no ability to work as a PID controller. Obviously you could develop a software PID but this will be more difficult to realize than below solutions and the performance may be insufficient.

1. Replace the PCI-6221 by a motion controller board (eg PCI-7342) which is designed for these tasks and use a simple amplifier.
   
2. Keep the PCI-6221 but use a servoamplifier that can control the motor in both modes. Doing so, the regulation is done by the servoamplifier itself. The PCI-6221 is used to generate the speed/current setpoint (analog output) and to set the mode (digital output).

[Jochen]

Oompa,

In general I widely agree with JB's suggestions. I'll just add some more comments to your questions.

• If you need closed loop control depends on what you try to accomplish. If you just want to generate a certain torque (current mode) that is proportional to the output voltage of your control device, you may not need to run a closed loop system. A similar statement is true for the voltage mode.
• Closed loop control is especially important if you need to control the position, or if you need to run a special trajectory profile (position/velocity/torque over time)
• Maybe it's possible to control the current/voltage mode of your drive with a digital signal. You will have to find out in the manual or check with the vendor.
• Using two drives with a single motor doesn't seem to be recommendable. If you want to do something like that, you will need to switch the power signals with relays between the two drives, but as you are working with inductive loads this might become quite tricky and there is a risk to burn both, the motor and the drives...

Regards,

Jochen Klier
National Instruments

[R_J]

Hello

I want to know the Working of motor & drives as a system. I am new to motors & drives. Please share your knowledgement with me.

Regards,

J

[Jochen]

Please use this tutorial as a starting point.

Regards,

Jochen