Variable resistor control

[bfleish]

Hello

I am using labview 7.1 (can upgrade to 8.5 if need be) and SCB-68 connector block to control a stepper motor through a haydon chopper drive. I can vary the step rate by connecting a variable resistor between two terminals on the chopper drive unit. Can labview do this on its own using the AI/AO or do I need to buy an external variable resistor/potentiometer to control resistance?? I tired to search through this site and help and didn't come up with much. Is it possible just to take a voltage in, multiply it by a number (resistance) and then send the new voltage out and achieve the same result? Any recommendations on what chip I should use if I have to go the external route? Sorry if the question is a bit basic but I am a biomaterials engineer so all this is a bit out of my realm, Thanks.

[johnsold]

First, the SCB-68 is just a connector block.  What kind of DAQ device do you have connected to it?

Second, most DAQ devices produce a voltage or a current rather than a variable resistance.  Some digital to analog converter ICs can be used as variable resistors and you can buy digital potentiometer ICs which can be controlled by digital signals.

Third, some controllers which nominally call for a variable resistance for external control can be driven by a voltage source.  You need to check the data sheet for the controller or talk to the manufacturer.

If you want specifics on creating a drive circuit, you will need to provide the specifications for the controller: What range of resistance? How much voltage is applied to the resistor? Is it grounded or floating?  What are the safety requirements?  Does the speed increase or decrease with increasing resistance or voltage?

Lynn 

[svenn]

Often in my research I want a computer controlled resistor.  Does anyone sell such a device?  Sure I could buy a half dozen digitally controlled resistors, a multitap switch, a USB digital output device, and write some code.  I've done it before for a light dimming demo.  But a cheap little device with a LabVIEW driver would be great to have around the lab.

[tbob]

You can build your own variable resistor by using a switch matrix and discrete resistors.  NI switches have Labview drivers.  Have a common contact on one switch as an input, and another common contact as an output.  Put resistors of varying sizes between the other contacts.  Then you could close the right combination of switches to get the resistance you want.  Like closing the switch to put a 10 ohm resistor in line, and closing another switch to put a 1 ohm resistor in line to produce 11 ohms.  Sort of like those resistance boxes that have a rotary switch to select 1s values, another rotary switch to select 10s values, and so on.  I haven't seen those around in a very long time.

Its not an off the shelf item, but if you would use it a lot, it would be worth building.

[svenn]

Thanks tbob,

I've done that with non-inductive power resistors to make programmable loads. SCXI or PXI cards, wiring, shielding, and such is not as appealing as a little USB box connected right next to my PC board on the bench.

[AnalogKid2DigitalMan]

aka 'motorized pot'. It transports itself across the border....

See http://www.google.com/search?q=%22motorized+potentiometer%22&rls=com.microsoft:*&ie=UTF-8&oe=UTF-8&s...

-AK2DM

[tbob]

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@svenn wrote:

Thanks tbob,

I've done that with non-inductive power resistors to make programmable loads. SCXI or PXI cards, wiring, shielding, and such is not as appealing as a little USB box connected right next to my PC board on the bench.

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True, not nearly as appealing.  But do you have a choice?  Well you do.  Either build it yourself or do without.  See here

[smercurio_fc]

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@svenn wrote:

Often in my research I want a computer controlled resistor.  Does anyone sell such a device?  Sure I could buy a half dozen digitally controlled resistors, a multitap switch, a USB digital output device, and write some code.  I've done it before for a light dimming demo.  But a cheap little device with a LabVIEW driver would be great to have around the lab.

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What about this: http://www.controlanything.com/Relay/Device/POT8PROXR

[tbob]

Smercurio:  that link did not work for me.

[Hooovahh]

The link worked for me, it was a RS-232 8 channel programmable potentiometer.

I've also used a PCI Pickering card that came with LabVIEW drivers.  It worked well enough but I had to write my own wrapper to it.  It is basically just a matrix card with resistors. Not sure if this is the exact card or not.

http://www.epn-online.com/page/new114054/pickering-interfaces-introduces-pci-precision-resistor-card...

The card I used was from 1 ohm to like 18Mohm with 1 ohm incrments.