Potentiometer experts out there?

[GaryR]

I use an AIM MXL Pista to gather data in my race car. For the steering position I use a string potentiometer which is attached to the steering shaft a couple inches up from the lower U-joint, accessed through a hole created by a 1980's AC hack job. The unit is broken and I want to replace it but would rather not spend $250 to do so. In searching online I find all kinds of string pots, some with more string length (which would be better for me), but not sure of the electronic side of things. I can cut/splice in my special connector but don't want to fry anything. This is the layout of the AIM sensor plug -

Function
Pin 1 - Analog signal 0-5 V
Pin 3 - Not connected
Pin 2 - GND
Pin 4 - V reference (4.5 V)

So am I crazy in thinking I could use "brand X" and save money? Anyone point me to ones that will work?

Thanks!

[356RS]

I think the most important specification you will need to know would be the impedance "OHM" of your broken pot. Most pots will handle your voltage requirement, then you can just find an X brand to replace it with.

[vintage guy]

You need to match the total resistance (across the full pot resistance) and the taper (resistance vs. position) of your original pot. There are linear tapers and various logorithmic tapers available. You probably have a linear taper. If you can then resolve the mechanical issues, you should be good to go.

[GaryR]

This is all the other info from the AIM site, the pot itself has a sticker on it: Model LX-PA-10, that's it..

Electrical characteristics Value
Application Car
Nominal Resistance 10 kΩ
Tolerance on resistance value ±5 %
Precision (%) 0.030

Mechanical characteristics Value
Mechanical travel 1080 ° / 5 revs
Temperature range From -55 °C to 125 °C
Dissipated power at 40 °C 2.4 W
Dissipated power at 70 °C 1.5 W

[Rusty Heap]

The AIM MXL Pista data collection unit most likely has a "tare" function, where regardless of voltage input from the steering feedback pot, you can press "Zero Unit" for when the wheels are pointing straight. That then sets the reference voltage, and anything under/over that is that much steering input left or right.

[GaryR]

Originally Posted by Rusty Heap

The AIM MXL Pista data collection unit most likely has a "tare" function, where regardless of voltage input from the steering feedback pot, you can press "Zero Unit" for when the wheels are pointing straight. That then sets the reference voltage, and anything under/over that is that much steering input left or right.

Sort of. You set it up as a mid-point potentiometer and calibrate from there, that's the easy part.

Dug a little, found this -
http://www.sal.wisc.edu/pfis/docs/rss-vis/archive/public/Product%20Manuals/unimeasure/pdf-lx-pa.pdf

[GaryR]

And this -

http://www.unimeasure.com/obj--IG_pdfs/LX-PA-series.PDF

[Grady Clay]

I agree with Chuck above.
It is probably a linear taper.

Are there other identical ‘string potentiometers on the car? If so, just measure the total resistance of one of them – between pins #2 and #4.


The issue matching the resistance of the potentiometer is two-fold. If way too low, the extra current can put a strain on the limit of current supplied by the “V Reference (4.5 V). This could limit the total number of sensing devises used by the system.

If way too high, the ‘input impedance’ of the voltage measuring device starts to become an issue.

Close resistance is good enough. It doesn’t have to be exactly the same.

Try measuring the resistance across pins #2 and #4 with a multimeter. That is the total resistance of the device. This may be an open circuit because of the failure.

Measure the resistance between pins #1 and #2 while moving the ‘string’ through its full range. If you get a varying reading, position the ‘string’ at about half-range. That resistance measurement should be about half the total resistance of the device.

Repeat for pins #1 and #4.

If any of these three techniques work, you now have the total resistance of the device and can buy something equivalent.

You can calculate the Watt rating of the potentiometer using the formula:
W = V^2/R; where V = 4.5 Volts (V^2 = VxV = 20.25 V^2) and R = (total device resistance) Ohms. Most potentiometers are ¼ Watt or so. Your calculation will probably show it dissipates micro-Watts or mili-Watts. Power (heat) dissipation should not be an issue.

I hope this helps.

Best,
Grady

[GaryR]

Can't do any measuring with the string or moving the pot as it's broken but i'll try going pin-to-pin and see whats what. Ideally I will find out who out there sells a draw wire LX-PA-10! Thank you all for the input so far!

[Grady Clay]

Electrical characteristics Value
Application Car
Nominal Resistance 10 kΩ
Tolerance on resistance value ±5 %
Precision (%) 0.030

Mechanical characteristics Value
Mechanical travel 1080 ° / 5 revs
Temperature range From -55 °C to 125 °C
Dissipated power at 40 °C 2.4 W
Dissipated power at 70 °C 1.5 W


OK,
This tells me that the total resistance of the device is 10 kΩ (10,000 Ohms). This resistance can vary by 5% (9.5kΩ to 10.5kΩ), normal, no big deal part.

The “Precision (%) 0.030” is about the deviation from perfectly linear. This is fairly high quality. The linearity deviation will cause an error in reading when the steering is off-center. You can buy a potentiometer with more ‘precision’ (less than 0.030% of value) but have a needless added expense.

I don’t understand “Mechanical travel 1080 ° / 5 revs”.
“1080°” = 3 revolutions. “5 revs” = 1800°. What is it?

The “Dissipated power at 40 °C 2.4 W” and
“Dissipated power at 70 °C 1.5 W” doesn’t make sense.
With 4.5 V and 10kΩ, the power dissipation is 2.025 mW (0.002 Watts). Perhaps they meant ‘2.4 mW’?

The temperature coefficient makes sense as the resistance increases with temperature so the power dissipation goes down. Normal for a resistor.

The “Temperature range From -55 °C to 125 °C” is high quality and suitable for “Application Car”.

Best,
Grady

If the ‘string’ is broken, is it from the string being stressed at the end of its travel?

When the steering is at lock (both directions), the string potentiometer should be able to travel more (both directions). This prevents damage to the device.

G.

[GaryR]

Grady - it's the pot itself, seems jammed up. New the string moves very smoothly in/out but now its hard to move and the string most likely broke because of that. I took it apart and put a new end on the string but it's NG.. Just noticed the brand, UniMeasure made in the USA!

[Grady Clay]

Unimeasure:
Linear Potentiometer, Position Transducer, Displacement Sensors, String Pot

Best,
Grady

[GaryR]

Yes, found their site and just wrote them on how to purchase. I'll just use the pigtail from the one I have on it (if I can buy one and IF its cheaper i'll buy two)! Thanks Grady, all I needed was a push!

[GaryR]

To wrap this up I got a prompt reply from unimeasure. That sensor sells for $160 + shipping direct from them and I will have to attach my pigtail, which for the approx. $92 difference I will be glad to do!