potentiometer for angle measurement

Thread Starter

Gadersd

Joined Dec 8, 2012
98
I am trying to design a circuit that will get the angle of someone's arm. My first thought is to place a potentiometer on a hinge and attach the hinge to my arm. There will be a voltage to binary converter that will read the potentiometer's voltage output and output a binary value representing the angle of the arm. I am worried that the potentiometer will have low accuracy. Is there a better way that will give me better accuracy, or is my idea fine? I don't need it to be very accurate.
 

#12

Joined Nov 30, 2010
18,224
Potentiometer accuracy has always been bad. You can "calibrate in software" or use an op-amp with offset and gain adjustments to convert anything that is pretty linear to a useful result.
 

wayneh

Joined Sep 9, 2010
17,496
It'll work. Just be sure to use a linear pot (not "audio" or "log"), and then calibrate your device for angle versus voltage. You should get a fairly straight line. Don't forget to protect the pot from excess current. You can count on someone turning the dial all the way to either side.
 

wayneh

Joined Sep 9, 2010
17,496
I don't need it to be very accurate.
For what it's worth, there are dedicated angle sensors. The pot approach will not likely be better than ±5%, or maybe ±5°. Maybe better if you have a good calibration curve. I haven't used the sensors but I would imagine ±0.5° is not too difficult.
 

MrChips

Joined Oct 2, 2009
30,708
My opionion is that the potentiometer will work well for your application.
If you wire the potentiometer as a potentiometer (duh!) you wouldn't have a problem with excess current.
 

MaxHeadRoom

Joined Jul 18, 2013
28,617
Accuracy but what resolution?
What are you using to gather and display? Pic etc?
Look up wind vane (360°) sensors, also as well as the encoder solution there is the resolver, you can use the sin/cos signals for any resolution.
Max.
 

djsfantasi

Joined Apr 11, 2010
9,156
@atferrari - I like this one.

@Gadersd - It looks like the turn screw is solderable, allowing one part of the arm to be attached to a small circuit board holding the pot; soldering a brass arm in the slot can be used for the other arm segment to attach.

With a 12 turn 10kΩ trimmer, and an arm motion of 180°, the range of values is 0 - 416Ωs. Each degree corresponds to a ~2.3Ω change. I don't k now if this is sufficient resolution for your application; I expect it would be.

I can draw up a sketch if it would be useful.
 

djsfantasi

Joined Apr 11, 2010
9,156
Or this single turn trim pot gives ~13.8Ω per degree of arm movement around the elbow (this is a 5kΩ pot; there are larger single turn pots with an extended handle available). The handle axis becomes the hinge between the arms.

Other trim pots, single turn, higher values can be found on Jameco here.

I thought further, and velcro straps can be used to hold the measurement arms to the person's arm.
 

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#12

Joined Nov 30, 2010
18,224
OK, how do you go about attaching the pot to one's arm?
You don't. You attach the pot to 2 long pieces, probably metal, and strap the metal to the human. I like DJ's idea of using a circuit board to hold the pot. I don't like the idea of using trim pots. Too fragile and a common 1 turn pot will cover the required range of motion.
 

Brownout

Joined Jan 10, 2012
2,390
OK, how do you go about attaching the pot to one's arm?
The arm has an 'elbow' where one member pivots with another member. Affix the pot body at that exact pivot point, the body attaches to one member and the shaft attaches to the other member. Now, you only need to figure out how to make the physical attachments. Building robots is all about solving issues like this.

Note: only do it this way if it's a 1-turn pot.
 
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