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Topic: Using an Arduino for a Project (Read 2827 times) previous topic - next topic


Here's my problem. We have to create a new goniometer (device physical therapists use to measure joint angles) and I came up with the idea to use the law of cosines to do this. My professor suggested that I use an arduino set up with a rotary encoder in order to measure distance. I was wondering if this was even possible and if so can someone please direct me to somewhere that can show me how to do this? I am completely new using Arduinos so any info is appreciated! Thanks


I came up with the idea to use the law of cosines to do this

How does the law of cosines help?

use an arduino set up with a rotary encoder in order to measure distance




Mar 29, 2013, 02:53 am Last Edit: Mar 29, 2013, 06:47 pm by Coding Badly Reason: 1
I obviously have no idea what is going on haha. My professor said that it may be possible.

Here is a link to the image (link to scamming website removed)

The I need to use an arduino to measure the distance that goes from the upper arm to the lower arm.

Moderator edit: image attached to post.


Hi vedet44,

I believe you are trying to determine the distance the subject can move his arm by measuring the angle that the arm moves (at the elbow).

The law of cosines will allow you to calculate this value if you know the length of the subject's forearm, upper arm (bicept) and the angle they form.  (See http://en.wikipedia.org/wiki/Law_of_cosines)

You can measure the arm parts with a ruler, and the angle with the Arduino. I'd suggest a potentiometer rather than a rotary encoder though.

There are 10 types of people in the world, those that understand binary, and those that don't.


We are actually looking to use the law of cosines to calculate the angle of the elbow joint. I need something to allow for the measurement of the distance between the patients upper arm directly to the forearm. Would a potentiometer be better for measuring distances?



Like a digital ruler or tape measure?

Closest I could find was this

Some form of linear encoder would have better durability than a pot.


To determine the angle at the elbow using the law of cosines you have to measure three distances: from shoulder to elbow, elbow to wrist/hand and from wrist/hand to shoulder. If you are looking for a device that is simpler (or at least no more complex) than a goniometer I don't think you're going to do it with an Arduino and the law of cosines.



Mar 29, 2013, 05:47 pm Last Edit: Mar 29, 2013, 05:50 pm by Lakes Reason: 1
This gives an angle readout


Coding Badly

@vedet44, never again post a link to that site.  Files can be attached to posts (as I've done to your earlier post with your image).


@vedet44, never again post a link to that site. 

What was wrong?
Gone for good

Coding Badly

What was wrong?

The site in question allows scammers to advertise: "Your computer has a virus. Click here to remove it."  "Your butt is fat. Click here to shrink it."  Etcetera.

The site in question uses a never ending stream of pop-ups to advertise.


The purpose of this project is to design a new goniometer (in my opinion that is incredibly dumb because the current goniometer is so simple). I have no experience using an arduino but my professor recommended it and said that if a string was wrapped around a rotary encoder you could code it so the rotary encoder would be able to measure the amount of string that was unwound. It's looking like that may not be possible haha.


So, if you want to do this type of project you ask "What do I want to measure" and then you go and find a sensor which does that.

If I understand you correctly, you want to measure an angle, right? So why not use a potentiometer? It is a sensor which measures angles/rotation.

I did something similar, take a look at these images: http://fkeel.blogspot.co.at/2010_09_01_archive.html


Using a rotary encoder to measure it will also work. If you want to do that, start by searching for a sensor which measures rotations. check out sparkfun.com they have something like that.

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