Current Output Adjustment

[brethigh]

This may seem like a trivial question, but please bare with me (I'm still a NI/LabVIEW beginner).  I have a test stand that uses a potentiometer to adjust current going to a set of electronic brake that's attached to a motor shaft.  We're trying to upgrade this stand to have a GUI where anyone can input their desired value of current and that value is supplied to the brakes to apply torque on the motor shaft.  My current hardware consists of a cDAQ-9174 and miscellaneous modules of analog signal recording and digital input/outputs.  Is this something that I can do, relatively easily, with LabVIEW?  Ideally the users could input their desired torque value to be applied on the motor shaft, but I feel like current would be easier to manipulate, at least in terms of programming the GUI and just have the users do the math to convert desired torque to current.

[cbutcher]

Hi Brethigh,

I'll start with the end bit and then move to the first bit:

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@brethigh wrote:

Ideally the users could input their desired torque value to be applied on the motor shaft, but I feel like current would be easier to manipulate, at least in terms of programming the GUI and just have the users do the math to convert desired torque to current.

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If the math is simple for people to do (i.e. you could describe some equation etc) then you'll be definitely able to put it into LabVIEW, probably without significant difficulty.

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@brethigh wrote:

I have a test stand that uses a potentiometer to adjust current going to a set of electronic brake that's attached to a motor shaft.  We're trying to upgrade this stand to have a GUI where anyone can input their desired value of current and that value is supplied to the brakes to apply torque on the motor shaft.  My current hardware consists of a cDAQ-9174 and miscellaneous modules of analog signal recording and digital input/outputs. 

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How are you currently controlling the potentiometer? Is this with your digital I/O, or are you using e.g. a screwdriver and a physical actuator?

If the latter, would you be able to instead replace some aspect of the system (digital potentiometer for example?)

Having some robot arm with a screwdriver will be fairly tricky. Having a digital device that varies resistance is likely much easier to control with LabVIEW 🙂

[RavensFan]

If the potentiometer is just the old school electro-mechanical way of varying a control signal to the motor such as 0-10V, then you could replace that pot with an analog output channel.

[crossrulz]

From a safety perspective, I would replace the cDAQ with a cRIO.  One of the cRIO-903x series would work well since you can use the embedded GUI.  But then you can have watchdogs and other safety functionality put into the FPGA of the cRIO to turn off the motor and other devices if something starts going bad.

[brethigh]

The current pot that we're using is a physical dial that you turn by hand and there's a digital display that shows the motor's controller current increase or decrease based off the torque that's being applied to the motor shaft.  We'd like to move away from manually turning a dial to inputting the desired torque, hit go, and the software does the rest.  We (at least me) are absolutely not opposed to moving to a digital potentiometer to achieve the end result.

[brethigh]

"From a safety perspective, I would replace the cDAQ with a cRIO.  One of the cRIO-903x series would work well since you can use the embedded GUI.  But then you can have watchdogs and other safety functionality put into the FPGA of the cRIO to turn off the motor and other devices if something starts going bad."

We actually have a cRIO, but I don't even have a full license of LabVIEW yet...I'm still trying to convince my supervisors it's a worthwhile investment (and my trial period is almost up!).  Our motor controller, coupled with other safety features that have/will be implemented on the test stand are adequate to ensure safety.  Though I do appreciate the concern - I enjoy going home every night in one piece.  

[RavensFan]

Another thing you should look into is if the motor controller has a serial interface.  They often have a command set that lets you put them into a digital control mode where you can write commands such a start and stop, enable, speed, and direction, and read back parameters such as voltage and current.  If you do go any kind of digitally controlled path whether serial or cDAQ or cRIO, make sure you have a actual physical E-stop button in one or more locations so that you can disable the motor in the event that your PC decides to freeze up or go on some virus scan and won't let you control the motor anymore.

[brethigh]

Yes, that is definitely something that we're accounting for.  The motor and controller are products that my company is producing and currently still in the R&D phase, though soon will move to the design validation testing phase.  Our current test stand is for an older product and it's being re-purposed and upgraded.  Safety is always the most important aspect because, like I said in my last post, I enjoy going home every night in one piece.