Synchronizing 3 DC Motors

[Median9]

Hi,

There are three DC motors with their respective drives. They may be set at different or same speeds individually using a potentiometer on each drive. Their speeds have to be controlled from a single point. So if we set Motor A = 100 RPM, B = 150 RPM, C = 200 RPM, and now if we increase the single point control by 10%, Motor A should become 110 RPM, B = 165 RPM and C = 220 RPM.

I have a question. For synchronizing these motors using a single point, what is the most simple and cost effective hardware/software combination in NI?

[Jochen]

Faisal,

before I can answer your question I need some more information from you:

1. Do your drives provide an analog input for velocity control? Is your potentiometer connected to this input and could it be replaced by an analog control signal (e. g. +/- 10 V).
2. I can think of several levels of synchronization. The easiest solution is to change the velocity setpoint for all drives at the same time without acceleration ramps and differentiations regarding varying inertia, motor power and so on. This would be an open-loop type control.
   The most sophisticated solution would be electronic gearing that would result in very precise synchronization (This approach is comparable to a mechanical gear with flexible gearing ratios). This solution would require the usage of feedback signals (e. g. quadrature encoder feedback).
   Which type of synchronization do you need?
   

Best regards,

Jochen Klier
National Instruments Germany

[Median9]

Dear Jochen,

Following are my answers:

1- Yes.

2- To start with, i will go for open loop. We might think of feedback at a later stage.

Sincerely,

[Jochen]

Faisal,

in this case there are two options:

1. If you want to stick with an open-loop solution you could choose a multifunction DAQ board like the NI PCI-6229. Besides other I/Os this board provides 4 analog outputs that you can use to change the velocity setpoints of your motors.
   
   
2. On the other hand you also could use a real motion control device like the PCI-7344. This board provides closed loop control for four axes and supports electronic gearing for tight multi-axes synchronization. With this board you also could start in open loop mode and control the DACs in your program. If you want to switch to closed-loop control later you simply need to add quadrature encoder feedback and switch to closed-loop control which is done by a DSP/µController onboard.

You also could use the PCI-6229 board for closed-loop control but this would mean that your computer needs to run a real-time OS and you will need to program the control loop by your own. Please refer to this link for more information about this option.

Even though Option 2 is a bit more expensive than Option 1 I would recommend Option 2 as the transition from open-loop to closed-loop will be much easier.

I hope that helps,

Jochen

[Median9]

Dear Jochen,

Actually these motors are already running in an open Loop fashion. Set Points are given at three separate places. But they are not yet synchronized. Based on that, i dont see a relative urgency for a precise feedback mechanism.

I have another question:

1- What Software package do i need with NI PCI-6229? Is NI LabView Professional Development System sufficient? Is NI SoftMotion Development Module necessary? Or for that matter, can you please list what other items should i need?

Thanks for the help.

[Jochen]

If you go for Option 1 you only need one of the IDEs that is supported by the NI-DAQmx driver (LabVIEW Base/FDS/PDS, LabWindows/CVI, Visual Studio.NET, C, C++, C#).
The NI SoftMotion Development Module is meant to be used when you want to implement a closed-loop system with "random" hardware and/or advanced control algorithms.

Jochen

Message Edited by Jochen on 09-25-2006 01:21 PM