Velocity control, DC motor

Hi,
Please refer to my attachment, may I ask how to convert my output variable (after coming out from pid vi and marked by biggest arrow) to PWM Duty Cycle duration? The scaling I have done doesn't seem correct.
[This code is modified from the 9505 example, but I am not using the 9505, and I am not doing the position nor current loop. My hardware set up is just a DC motor with a full-H-bridge circuit, a linear encoder and an fpga.]
Thank you.
Message Edited by xf on 04-25-2009 12:59 PM
Message Edited by xf on 04-25-2009 01:00 PM
Attachments:
velocityControl-code.JPG ‏178 KB

Brian,
Thank you for the suggestion about the speed controllers. Can you give me the names of the manufactures of some DC motor speed controllers?
Buying an off the shelf solution is preferred than designing a custom circuit for my application.
As for your ideal about using stepper motors, I am trying to control existing assemblies that already have the DC motors embedded in them. What I am doing is trying to assist the M.E.s in adding control to their proof of concept models and we do not want to turn a custom circuit board and custom software for a micro-controller for each concept model.
Thanks again for your suggestion.
--Jon

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How do you control the motor in the continuous mode ?

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Hellow water2006,
Thank you for using our discussion forums. In regards to continuous mode, are you referring to a constant velocity motion? There are several examples on our website which will give you a good start on setting this up. In particular the "Constant Velocity Move" example which is found at http://zone.ni.com/devzone/cda/epd/p/id/1206
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I was able to quickly use the 9505 Servo Drive example to do closed loop position control on the FPGA but I'm having trouble with figuring out how to modify the code to support velocity control (which I need for a different axis). I'm trying to make my modifications on the FPGA VI itself rather than on the RT Softmotion side.
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If I use the PI Velocity Loop with my Velocity and an arbitrary control setpoint velocity, I can see the velocity stabilizes (motor output looks uniform) but I can't correlate it with the velocity setpoint other than a bigger number is faster. Velocity (from the Encoder loop) seems to readback in values of just 0-2 on my FPGA VI (I don't fully understand the units but I see my position change on my encoder readback in thousands of counts per second). Why are they so different/how do I scale my encoder velocity correctly?
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I was able to get my velocity readings to match my velocity setpoints when using velocity control mode by increasing my Timer Config.Target Period and Encoder 'calls per sample encoder' from 2000 to 40,000. Now my velocity readback is +/- 1 of my velocity setpoint for my drive range. This makes sense to me now - if you read the encoder to quickly it hasn't accumulated enough points to accurately calculate the velocity. It was in the documentation but I didn't fully understand it.
So now I'm down to one question - (from last post):
I still don't understand how to set the velocity of a move if I'm using position control (out of the box example) from within the FPGA code (on the 9505 example). Is the best way to do this just to limit the 'Current Output Range' depending on the max velocity requested? The NI Softmotion RT VIs and Express VIs are able to set 'Velocity' but I'm really unclear what that the SoftMotion RT VIs are actually doing in terms of interacting with the FPGA code. Can you explain how that works?

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Hello,
My application is for a drill, that drills into rocks of various densities for the purpose of collecting rock core samples.
My setup has 2 motors which get controlled, one spins the drill bit at a constant velocity, the other moves the drill mechanism along a Z axis.
For efficient cutting, it is desired to apply a constant force between the drill bit and the rock. I have a load cell which measures the force being exerted by the drill bit on the rock surface, and this force can be adjusted by changing the velocity of the Z axis. So I would like to employ closed loop control to adjust the Z axis velocity to maintain a constant force on the rock.
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If you do need trajectory generation, you would want to keep most of the example code the way that it is, but then program in a 'Force Mode' that utilizes the force and velocity controly as described above. You could think of it as having two different routines programmed side by side.
Regards,
Burt S

Control DC motor position

Dear member
I dont know how to control the position of motor, if inputing the Motor Driver Voltage into PWMA, I can only control the velocity of the motor, what should I do？

Could you please provide some information on your hardware setup? Make, model number, required inputs etc.
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Attachments:
epos and sinewaveform.vi ‏34 KB

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4LeadBipolarSteppingDriver-linengineering--4218m-54p-04.pdf ‏652 KB
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http://zone.ni.com/devzone/cda/epd/p/id/3611
High Performance Stepper/Servo control:
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http://zone.ni.com/devzone/cda/ph/p/id/248
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Hi Everyone,
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I want to build a powder doser which consists of two powder feeders (driven by motors) and laboratory weight controlled by RS232.
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Marek,
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