Linear Potentiometers

epitrochoid
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Joined: Tue Feb 13, 2007 12:45 pm

Linear Potentiometers

Postby epitrochoid » Tue Feb 13, 2007 12:54 pm

Hi I'm using four linear pots for their intended use of measuring shock absorber displacement, but feeding from the 5V ref out on the DL1, all of the pots cap their input to the 5v analogue inputs at 2.547 volts.This happens whether all connected or only having one connected.

The sensors are 3k ohms and the capping occurs at about 2k ohms as viewed from the signal wire and the 5v supply.

The cpping also occurs on the 12v analogue input at the same value.

When read using a multimeter the resistance changes correctly for the full deflection of the sensor arm, and when fed from the 5v output on the logger, reading the signal wire with a multimeter also shows the correct voltage movent from more or less 0 to 5v.

It's a strange one. Any help would be GRATEFULLY appreciated :)

RTUSA
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Location: Richmond, VA, USA

Postby RTUSA » Tue Feb 13, 2007 1:48 pm

Not sure I understand:

Are you seeing a linear voltage change over the full stroke of the pot, min and max at extremes of stroke, ranging from 0-2.547V? Or is it maxing out at less than full deflection?

And, using a meter, you are reading ~5V in the same state as the DL1 records 2.547V?

Are you using pots from RT? Or? What color wires do you have connected to what?

Are you reading the DL1 voltage via Live Monitor or Analysis?
Al Seim
Race Technology USA

epitrochoid
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Joined: Tue Feb 13, 2007 12:45 pm

Postby epitrochoid » Tue Feb 13, 2007 2:40 pm

The voltage should go from 0 - 5v through the full deflection of the pot.

Using a voltmeter across the signal and ground with the positive and ground connected to DL1, I can see the correct voltage change through the range of the pot.

If I connect the signal to the 5v analogue input of the DL1, at half deflection the DL1 analysis AND monitor software record 2.547 and no further movement of the pot shows any change in either analysis (via cf card) or live serial connection.

Is there a setting in the DL1 which puts a range on the inputs? I couldnt find one.

RTUSA
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Postby RTUSA » Tue Feb 13, 2007 3:48 pm

Try connecting the 5V out to an analog input. Does that read 2.547 as well?
Al Seim
Race Technology USA

epitrochoid
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Joined: Tue Feb 13, 2007 12:45 pm

Postby epitrochoid » Tue Feb 13, 2007 4:14 pm

With 5v reference output fed straight into the analogue input (any of the four of them) yields 2.547 volts. Even when there is a voltmeter connected in parallel to this bridge which is reading the correct 5V!

The logger also reads 2.54 (exactly the same) if this voltage is fed into the 12vinputs

RTUSA
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Postby RTUSA » Tue Feb 13, 2007 6:34 pm

Epi:

Something is obviously amiss with the DL1. Could you please connect the shock pot and cycle it while recording the data on a CF card, then email me the data file.

aseim@race-technology.com
Al Seim
Race Technology USA

epitrochoid
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Joined: Tue Feb 13, 2007 12:45 pm

Postby epitrochoid » Tue Feb 13, 2007 7:02 pm

Will do, I'm heading back in tonight.

epitrochoid
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Joined: Tue Feb 13, 2007 12:45 pm

Postby epitrochoid » Tue Feb 13, 2007 9:14 pm

I have sent the run file

I hav updated the analysis software and I have reflashed the DL1 with the latest firmware, no change unfortunately :(

RTUSA
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Postby RTUSA » Wed Feb 14, 2007 3:10 pm

What is the range of resistance on the pots, measured from the 5V in to Ground, disconnected from the DL1?
Al Seim
Race Technology USA

epitrochoid
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Postby epitrochoid » Wed Feb 14, 2007 3:50 pm

3k ohms

epitrochoid
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Joined: Tue Feb 13, 2007 12:45 pm

Postby epitrochoid » Fri May 04, 2007 8:14 pm

for anyone that was following this thread, the solution was that the DL1 had previously been reconfigured for maximum resolution on analogue inputs so the voltage scale was halved to obtain double the resolution for some strain guages.

I sent it back to Race Tech in UK and they have reset it for a princely sum of money. Should be picking it up tomorrow.

FormulaRedline
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Joined: Thu Jan 03, 2008 5:05 am

Postby FormulaRedline » Thu Jan 03, 2008 4:29 pm

epitrochoid wrote:for anyone that was following this thread, the solution was that the DL1 had previously been reconfigured for maximum resolution on analogue inputs so the voltage scale was halved to obtain double the resolution for some strain guages.


Does anyone here have experience using strain gauges with the DL-1 inputs? I am interested in purchasing one for such a use. How difficult is it to reconfigure the unit as this posted talked about? What kind of resolution can I expect in the stock form?

Thanks!

RTUSA
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Postby RTUSA » Thu Jan 03, 2008 4:35 pm

The main thing is to use a strain gauge amplifier that will get the voltage to be logged into a range that can easily be read and resolved by the DL1 (ie volts (0-5 or 0-12) not millivolts.
Al Seim
Race Technology USA

epitrochoid
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Joined: Tue Feb 13, 2007 12:45 pm

Postby epitrochoid » Thu Jan 03, 2008 5:49 pm

If you contact my old formula student team via the contact details on www.formulastudent.strath.ac.uk you might be able to get details on strain gauges, they gauged up our entire 2007 car for chassis design validation quite successfully in the autumn of this year.

FormulaRedline
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Joined: Thu Jan 03, 2008 5:05 am

Postby FormulaRedline » Thu Jan 03, 2008 6:30 pm

RTUSA wrote:The main thing is to use a strain gauge amplifier that will get the voltage to be logged into a range that can easily be read and resolved by the DL1 (ie volts (0-5 or 0-12) not millivolts.

Thank you, that is helpful. Would you happen to have any recommendations for amplifiers people have been using successfully to convert millivolts into the requires 0V to 5V or 12V?

I'm looking for something low cost that can support 2-4 sensors. I haven't sourced the gauges yet, but I'm looking to measure at a resolution of at least 0.2-0.3 micro inches in strain on aluminum. Using some rough numbers for gauges I've looked at, I think that means I'll need about 1 mV resolution. (I'm sort of learning this myself, if anyone thinks I'm completely off here, please speak up :) ) I'm looking for a range of at least 500 units at that resolution, hopefully more. I assume the DL-1 would have no trouble with that?

epitrochoid wrote:If you contact my old formula student team via the contact details on www.formulastudent.strath.ac.uk you might be able to get details on strain gauges, they gauged up our entire 2007 car for chassis design validation quite successfully in the autumn of this year.

I might do that. That sounds like it should have scored them quite a few points in designs (I was on MIT's FSAE team for 4 years). Can I tell them who sent me?


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