# Is it a good idea to use potentiometer to measure angle?

I'd like to measure an angle between two elements on hinges using arduino. Can I use a rotary potentiometer?

If I take reading at two edge positions for calibration (0 and 90 deg.) can I assume that the resistance will change linearly between those positions?

• Make sure that you use a linear potentiometer and not a logarithmic one.. Oct 24, 2011 at 21:05
• @m.Alin, that is how you take away the chance for someone to have to learn to cope with coding to resolve that! Think of a perfect learning exercise. Oct 25, 2011 at 6:53
• @m.Alin - trivial?... Oct 25, 2011 at 7:23
• It's worth noting that common potmeters have a very limited life, so if this is for a robot arm moving all the time I'd suggest an optical rotary encoder. Oct 25, 2011 at 7:42
• @stevenvh That's a much more knowledgable shopkeeper than I ever encountered when I was a teenager learning about electronics. Oct 26, 2011 at 4:14

I've had to do this before where the shaft was connected to a continuously rotatable pot. That's a normal pot with two wipers 1/2 turn apart.

I found that the pot was reasonably linear, but not good enough for what we were trying to do. I added a calibration procedure and ended up doing a piece-wise linear lookup. If I remember right for that pot and the accuracy we wanted, a calibration point every 20 degrees or so seemed to be good enough. Any one pot stayed pretty consistant once calibrated. I'm sure they would wear more over time and require re-calibration, but initially a single calibration adjusted the system well enough to ship. We did specify calibration at regular intervals when other maintanence was already scheduled to be performed.

Long term pots are probably not a good measure of angle unless you recalibrate regularly after a cerain amount of use. The wiper rubs against the slider, which eventually wears off material and changes the resistance.

If you need long term consistancy, get a rotary encoder.

There are two problems with a pot as a sensor.

1. The wipers can generate electrical noise as the wiper scrapes along the track.
2. A pot is a fairly high impedance sensor, and so the signal wires can pick up noise if they are long.
3. This high impedance also causes problems if you are sampling the pot with an ADC, due to the ADC's sample and hold capacitor.

The electrical noise problem can simply be solved with a small capacitor between the wiper and ground. About 10nF should be enough. Larger values are better, but will limit the rate at which the signal can swing.

Adding a unit-gain op-amp means that you can drive the signal along a long wire. It also drives the signal nicely into the ADC's sample and hold capacitor, giving a good noise free reading.

Using a linear taper rotary potentiometer is a workable idea if it provides the linearity that you need. You may need to determine this experimentally, depending on what potentiometer you choose. Higher end potentiometers that are designed to be used as angle sensors have guaranteed linearity specifications, low end pots will not.

Depending on your application, physical orientation, accuracy needs, and budget; you could also consider an optical shaft encoder or inclinometer.

The resistance will also change with temperate, meaning even at the beginning accuracy could be somewhat suspect even after calibration.

For example if zero ohms is at 0 degrees and 1M ohm is 90, that would affect the current and subsequently the power being dissipated over that resister (putting some resister in series is a good idea I would imagine) The difference would probably be trivial if designed right I imagine.

• Most pots are 3 terminal, 2 terminals go across the entire resistor and one is in the middle on the wiper. This means that you connect power the the first two terminals and measure your output voltage from the wiper. Oct 25, 2011 at 7:13
• Increased local heating will still occur between the wiper and power terminals, though, @Kortuk. I never worry about this source of error. Oct 25, 2011 at 14:10
• @tyblu, a well-designed circuit will not draw any significant current through the wiper. Therefore any heating is even along the entire pot's resistance. So the voltage output is still nicely ratiometric even with resistance change from self-heating. Oct 26, 2011 at 4:17

Its a complete no-no...I tried using one for a balancing bot, attached it to a light pendulum and it failed miserably.There is definite lag due to friction, and its not sensitive enough.I strongly recommend using a gyro.One very cheap alternative is using a WiiMotion Plus( $10-$15) for

• Pots don't have lag. When the wiper moves, the resistance changes immediately. There may be lag due to low pass filtering in the circuit. There may also be a small dead band, but most pots are quite "tight" in that a small rotation of the shaft does result in wiper movement. Oct 26, 2011 at 12:01
• Lag, I meant was the physical lag (due to friction), and not the electrical one! Oct 26, 2011 at 14:31
• What physical lag? The wiper on most pots is quite tightly connected to the shaft. When the shaft turns, the wiper moves. For it to move later it would have to deform significantly. It doesn't work that way. Also, friction is a resistive force to the rotation of the shaft, but doesn't cause lag. Friction is not a time-related thing. Oct 26, 2011 at 15:38
• @OLin, just a guess, but judging from the application description, Rupin may be talking about the static friction that had to be overcome before the shaft moved at all. This would result in jerky movements that could be called "lag" if you were expecting smooth movement & measurements.
– wjl
Oct 26, 2011 at 18:12
• @wjl: Perhaps, but that's a system issue, not one inherent to pots. The OP wants to measure the angle between two hinges. With the pot connected to the hinges, it might keep the hinges from moving for small applied force. However, if the hinge moves the pot will move, and so will measure angle without lag. The OP asked for angle measurement, not change in angle as result of some force. Oct 26, 2011 at 19:33

The correct device to measure angle is called a rotary encoder. There are two kinds, absolute, or incremental. Incremental encoders output the increment of angle on the shaft, and used to measure angular motion. Absolute encoder output the actual position on the shaft, and are used to measure absolute angle.

A pot can be used for this, but only if you do not need a lot of precision. Pot resistances have thermal drift, resistance also drifts with wear, they get dirty, etc. Just a pot and a lookup table would need constant recalibration, unless you only want to know if a hinge is opened or closed. If that is the case, a simple pushbutton with an appropriate length shaft will work just fine. If you need to actually measure the angle, use a rotary encoder.

Pots are used for position measurement in some applications, but as others have observed not every pot will do, and even then failures are possible.

A few years ago, I had to fix a mobile boom crane. The position of the boom was measured using a cable to a spring loaded drum, and a pot measured the angle and number of turns of the drum.

The pot track had failed, so the anti-tilt detector prevented the crane from lifting anything and sounded the alarm all the time.

In this application the cycles are small compared to a machine where an encoder would be used, and the effect of wiper friction was not significant.

So it depends on many factors, as the other answers show, but it also is affected by how long you want it to work for, whether this is an installation you are selling to someone or is for yourself or is just a test.

It depends on how accurate you can measure the resistance of say , 1 degree. the larger the potentiometer diameter, more resolution you have.

• Potentiometers are analog. Any carbon resistance track type will vary proportional to position regardless of diameter. A wire-wound type might have a tendency to show discrete steps but a larger diameter would probably be for higher power and would use larger diameter wire and have the same problem as the smaller one. May 15, 2016 at 9:28