Servo motor control with DAQ

[ATMA]

Hi all, i've only been learning Labview for about 3 weeks so please bear with me if my questions sound amatuerish.

I've been assigned with a project where i'm suppose to control a motor using Labview (ON,OFF,FORWARD,REVERSE). Now, based on my research and understanding of motors i've came to this preliminary design proposal:

- Use PCI 6221 DAQ to generate the PWM signals to a driver/amplifier (a Labview VI will generate and control the PWM pulses)
- The driver/amplifier provides the voltage and the amplified current to drive the motor
- Output of the motor encoder is fed back to the DAQ then to PC for further processing

Please verify if this proposal is feasible. The motor in question is a DC servo motor, but the application may be introduced with other motor types.

Secondly, please advice on the driver/amplifier need to drive a motor.

Based on my understanding, the driver is just to make sure the motor gets sufficient current to move it. If my motor requires say 2.5A the amplifier should take the analog output from my DAQ card and amplify to 2.5A and drive the motor. If that's the case, what's the difference if i were to use say at typical op-amp to just increase the gain to 2.5A?

Thirdly, what should i be looking for in a servo motor driver/amplifier? Can someone recommend any vendors or modules?

Many thanks.

[Jochen]

If your control task is really as simple as you describe it (ON, OFF, FWD, REV), your approach should work.
For the case that you need a better level of control, e. g. control acceleration, velocity and position, compensate for varying loads and so on, this approach will cause you a lot of work and headaches as these control tasks require a deterministic closed loop system and some advanced control algorithms.

Even if you run the application on deterministic operating system I strongly recommend using a motion control device that runs all time-critical tasks (control loop, following error and limit switch checking,...) onboard. With a motion control device like the PCI-7340, motion control is as simple as loading parameters for acceleration, velocity and target position (if required) and sending a start command.


Coordinated 2D-move with NI-Motion

Again: This is just a recommendation for a system that requires a higher level of control but if you can anticipate that you may have to meet some of these requirements in the future, you definitely should think of this approach.

Best regards,

Jochen K.
National Instruments

[ATMA]

Thanks Jochen for your prompt reply.

Yes, at the moment (and budget) i believe that's the functionality we can afford. However, i do have 1 concern. The DC motor (48VDC max 2.6A max) do contain an encoder which provides a feedback on the actual position. Naturally, it's not posible to return the encoder output through the DAQ to the PC for the processing. As these stuff are very new to me can anyone recommend any controller/servo amplifier for this proposal?

Can someone comment on my Q2 and Q3 of my first post?

Appreciate it.

[Jochen]

1. You can use an M-Series device to measure the position from an quadrature encoder. Please refer to the examples that ship with LabVIEW.
   
   
2. Depending on the motor type there are a lot of different drives available. Some very simple drives require a PWM input signal and work as you have suggested. Others expect a +/-10 V DC signal and generate the PWM power signal by their own. There are also drives available that require a +/-10 V signal that is directly translated to velocity (e. g. +5V results in 800 RPM). These drives are typically used with brushless DC-motors.
   

Please note that the current in a DC motor is proportional to the motor's torque. That means that in an open loop system the motor's velocity depends on the system's load and damping. Without closed loop control the motor velocity is typically quite inaccurate and not reproducible.

Jochen

[ATMA]

Thanks for replying Jochen. I'll play around with the configurations and see what i can do.

[Sagarmatha]

ATMA - I have a similar application. Would you please share your solution?  - Thanks

[ATMA]

Sorry Dpak27, the source code is proprietary. I don't have the rights to share it.

But it's a very simple solution, you can post your queries and i'll try to help.