need help with motion control signal levels

[jd710]

I've done mostly digital work and have inherited a motion control design after a fellow employee moved on and I'm having trouble comprehending the basic concepts of a motion control design and was hoping someone might be able to help.

What I have is a PI current control loop (summing the current feedback from the motor, and the set current) which creates an output voltage which in turn is summed with a triangle wave to create the PWM output to the motor transistors.  Moving back one stage in the circuit , that set current is created from a velocity loop whose output is the sum of a voltage set point and the voltage feedback from an encoder.

So...  my question... in general what should the inputs and outputs of these two stages be?    Should the set voltage and encoder voltage be equal and opposite creating essentially a 0 output?  If so, is it this zero output when summed with the 38khz(pwm frequency) current feedback that sets the output level of the current loop?   Or does there need to be some output from the velocity loop such that it is equal and opposite to the DC portion of the current feedback?  Any help would be appreciated.  Also, if anyone knows of a good basic text or website on the subject.

thanks

[GiGe]

Hi,

What hardware are you using to do your motion control?  Are you doing it with one of our Motion Controllers or a Data Acquisition card? 

It also sounds like you are trying to do analog feedback, if you are doing this using a motion controller then this is how to configure that.  

I hope these documents help. 

Have a great day.

GG

[jd710]

GG,

thanks for the reply and info, I've looked through some of it previously but I'll have to take some more time.

Unfortunately, the previous designer decided it was much easier to create a circuit, tons of vhdl code in some fpga's and have mountains of software written to make a motor turn forward and backward, rather than just purchase a drop in controller from someone who knows what they are doing.  In the end I may just chuck it all and start over but I'm told it was kind of working at one point.  What I was hoping is that someone with much more knowledge than I, might be able to give a quick explanation as to what signals levels I should expect to see in a "working" system.

Yes it is all analog feedback (the encoder counts are converted to a voltage through a D/A).

For example I have a 5V set voltage into the velocity loop amplifier which has a gain of 10.  This gain of 10 I'm assuming is to amplify the error between the 5V input and a -5V feedback from the encoder??   So in a steady state with no speed changes the output of this stage would be 0?   This output from this velocity loop is fed to the current loop (in a PI configuration) with a proportional gain of 20.  The other input to the current loop is the current feedback from the motor.  should these two inputs be essentially equal and opposite?  At some point a voltage has to be output from this stage to create the PWM (a summation of that output voltage and a triangle wave).  But any DC voltage is just pegged to the rail by the essentially infinite gain of the integrator section of the current loop.

Anyway, I think this should be fairly straight forward but it's confusing me.

If you have any insights it would be appreciated.

[GiGe]

Hi,

Sorry, I am also a little confused; I am more familiar with the NI motion controllers.  You may want to consider using the PID toolkit in LabVIEW to control your system. 

GG