PSU and DMM tunning loop

[AndyTT]

Hi All,

I am attempting to setup a vi to control the output of a PSU using a DMM. Within my application I want to be able to precisely set a Voltage to within 10mV. This will come in handy when I am not able to directly measure the output of the PSU.

I am sure this has been done a thousand times before but I cannot find any examples.

I have attached my example circuit so far and would like to know if I am on the right track or maybe there is a better way to do this task? The code is only a rough outline and does not contain the VI to setup the PSU or DMM as yet.

When I debug the code it seems to run as intended but I have not considered any fault conditions or put safety limits in place.

Thanks for any comments

[GerdW]

Hi Andy,

when you really want to use the DMM to precisly control the output of your PSU you should:

- change the order of DMM and PSU: read DMM first, then set the PSU

- put a PID control between DMM and PSU: use the DMM value as pv, have the sp as control and wire the PID output to your PSU…

[AndyTT]

Hi GerdW,

Thanks for the reply.

Interesting you say use a PID, for some reason I thought this was a bit overkill at first. Do you mean creat a software PID using labview? or some built in add on that does PID control?

If I take a measurement from the DMM without setting up the PSU first there will be nothing at the input to the DMM to measure?

Thanks

[GerdW]

Hi Andy,

Interesting you say use a PID, for some reason I thought this was a bit overkill at first.

Why? It's just a subVI with three inputs (sp, pv, gains) and one output (PSU setting)…

You could stay with your scaling (gain/offset) - this basically is just a P control loop!

Do you mean creat a software PID using labview? or some built in add on that does PID control?

LabVIEW comes with a PID toolkit, AFAIK for free in recent versions…

Which LabVIEW version do you use? Which kind of license?

[Hooovahh]

So this is a neat exercise and probably useful in some cases.  But for me wouldn't getting the power supply calibrated be a better option?  The calibration sheets I get from a new, or newly calibrated meter are in the sub millivolt range.  The length that the meter stays within calibration can vary of course but 1 or 2 years is a pretty normal amount of time that I'd expect a PSU to stay within the calibrated range.  And if you purchase some high precision equipment you can do the calibration yourself.  I have done calibration routines like this in the past but it was in places where a front panel dial could adjust the scaling in unexpected ways.

[AndyTT]

Hi Hoovah,

Thanks for the reply. I am not trying to calibrate the PSU using a DMM. This was more for an application where I want to measure some point in a circuit (e.g. a voltage divider) and want to tune the PSU output until I reach a certain voltage at the divider output.

What are your thoughts on using PID control?

Thanks

[GerdW]

Hi Andy,

I want to measure some point in a circuit (e.g. a voltage divider) and want to tune the PSU output until I reach a certain voltage at the divider output.

When you combine the PID with some senseful output limits (input parameter of the PID VI) and maybe an OutputRateLimiter (a different function in the PID toolkit) you can even make your VI/hardware setup more fail safe…

(All these options in addition to all safety features of your PSU!)

[Hooovahh]

Oh so sorry.  I miss read this as some kind of way of tuning the PSU, when really this sounds more like you wanting to get a desired output out of some circuit, by adjusting the input.  PID sounds useful in this case.