analog feedback on a stepper

[Endeavour-1770]

Hi,

I'm running a motion control system with the MID7602, PCI7352 and a stepper motor plus encoder. The problem I have is the folowing:
With the stepper I drive a linear, leadscrew alike, mechanism to bend some probes. Those probes will provide in return a voltage to tell me how far they are being bend. The question is, how could I implement a function with the ADC as primary feedback to tell the motor to stop motion at a given voltage-level? Furthermore is there a chance of suppling a vi with similar functions, so i could understand the mechanism better?

Thanks!





Message Edited by Endeavour-1770 on 03-19-2008 08:26 AM

[DanielHermyt]

Hi,

how are you working at the moment? Do you use LabVIEW?

Using LabVIEW there is a library with motion functions, that contains e.g. "Read ADC".

There are a lot of motion examples that are searchable with the NI Example Finder (LabVIEW->Help->Find examples). Perhaps you can find something there as a starting point.

If you could post your actual implementation, I could have a look at it.

[Endeavour-1770]

Hi!

Yes, I do use LabView 8.5 with the NI-Motion 8.5 Module and I'm aware of the ADC read function, but maybe I didn't make my problem very clear. Looking for some examples within the LabView help ended up in no examples as with the Read ADC being integrated in the primary feedback. In the NI-Motion manual were a few examples, but there the read voltage from the ADC is just feedback as a position in a way that wont work for my setup.

The thing i want to do is to move a pushing rod towards a sample...the sample will be bend a return a voltage "somehow" (not predictble) proportional to the bending...i already read back that voltage with the Read ADC function, so that is no problem...what i want to do then or next, and that is with what i'm struggeling, is...to tell the motor to move the pushing rod towards the sample until i reach, due to bending, a certain voltage value (predefined) like a trigger and then stop the motor...similarly as i'd do by telling the motor via the Move to Postion VI to move into a certain position and reading back the value of the position with an encoder all the ime until the final position is reached. As far as i understood i could feed that ADC value into the Move to Position VI and the value of the ADC will be interpreted into a position as if there was an encoder, but  that's not what i want to do...I want to know how to tell the motor to move until it reaches a defind voltage value?

[DanielHermyt]

Hi Endeavour-1770,

please excuse my delay but I have been out of the office.

Your description of the procedure as you described it is exactly the procedure that I wanted to suggest you to do. Why isn't it possible in your case?
To define a specific "end-voltage" you could use a case-structure and compare your actual voltage with a reference voltage and to use the TRUE or FALSE as input for the case selector.

[Endeavour-1770]

um...that is something i already tried and i guess I'll have to stick with it, as it seems to have no better solutions to solve the problem, but the problem comes with the signals that I'm measuring... they are in the range of some micro down to a few pico volts and noise is having great affects and therefore the motor could stop too early or too late...I think and i guess you'll have to admit...a case structure is not a real feedback loop as it is implemented in the Load Target Position VI, or is it really that simple? Even if i'd use a case structure, is there such a function to tell the motor to move clockwise or counter clockwise, without telling him a target position to move to, only giving him the direction?...i guess there isn't, so I'd have to trick it by telling the motor to move a 1000000 steps e.g. cw, which i then hope will be far above the step range that is necessary to approach the desired trigger level, right?

anyway, thank you very much...thought there is a nice way to solve it