# Digital Goniometer

Discussion in 'The Projects Forum' started by Ali Inam, Aug 7, 2011.

1. ### Ali Inam Thread Starter New Member

Aug 7, 2011
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Hi guys, I am a student of Biomedical Engineering and am in my 2nd year. I am supposed to make a Digital Goniometer, for those who want to know what Goniometer is:

http://en.wikipedia.org/wiki/Goniometer

Now mostly there are mechanical types of goniometer and we make them electrical by attaching batteries to it and then we get the access to measure the voltage changes by the movement of our arm.

I have decided to make such a Goniometer which will include a double digit seven segment display which will be attached to the goniometer through a Potentiometer, as the arm will move, so will the potentiometer and the voltages will vary according to the change in resistance and the voltage to be displayed in the 7 segment display.

Can anyone please help how to build such a circuit involving 2 seven segment displays to be controlled by a single Potentiometer (POT). I have been searching for different ciruits on google for the last couple of days but stiil I'm unable to find a decent circuit which would give what I really need.

Thanks !

2. ### DickCappels Moderator

Aug 21, 2008
2,659
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You need a digital voltmeter. (Simply) Scale the output of the pot so that the voltage displayed on the meter relates to the angle. For example, 45 degrees might be displayed as 4.5 volts or 45 millivolts. You can use a digital panel meter rather than a DVM to make things look cleaner.

By the way, potentiometers come in many varieties. Trimpots are all linear in my experience, but panel mount pots come in both linear and logarithmic flavors. I suggest you find a linear pot.

It should be a simple project in concept, but as always, you will find the Devil is in the details.

3. ### Ali Inam Thread Starter New Member

Aug 7, 2011
3
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So you mean to say that I should use a digital voltmeter instead of building a circuit involving 2 seven segment displays ???

Jul 7, 2009
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You could also construct the goniometer using a digital encoder and use digital electronics. This would be easier to interface with e.g. a computer if desired. You'll have to zero the thing every time it is powered up unless it's an absolute encoder. The counting and display might be easier because you could use a small microcontroller. The encoder will probably also be more expensive than the pot.

5. ### MrChips Moderator

Oct 2, 2009
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3,365
I also prefer the digital encoder. Using a pot is still analog with a digital readout, though this is easier to implement and you can get fairly good resolution, perhaps up to 12 bits. Your biggest source of error will be the integral non-linearity in the pot.

A more classic solution would be to use a digital encoder which you can buy or you can build your own. This is technically more challenging but you could learn a lot more.

6. ### wayneh Expert

Sep 9, 2010
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I wonder if using a ganged pot (multiple pots on one shaft) would increase precision. Seems like each pot on the shaft would give an independent estimate of rotation angle.

I agree that building a digital display is a waste of time, since you could use a cheap multimeter as a readout. If you don't like making whatever calibration calculation is needed, use data acquisition (such as a LabJack) to get the data onto a computer where you can calibrate it however you like in Excel, as well as record measurements.

If you must use the multimeter display, you could use a simple resistive voltage divider with your pot to get the desired voltage = angle relationship. Worst case, you'll need an op-amp in instrumentation amp mode to set the zero and span of your rig.