Detecting Leveling of Arm and Shoulder

[Johnny101]

Hi guys,

I want to detect the point, during a hand swing motion, when the arm aligns or comes to the level of shoulder like the 1st, 3rd and 5th ones in the figure.
Could using an accelerometer work? If yes how would I position the sensor as the arm could be rotated in any way about its own axis and not necessarily be like in the figure.
A design constraint is that I can place sensors on the arm only nothing on the shoulder.

Thanks in advance.

 

[BradtheRad]

A mercury switch detects positions. These are in 'quiet' light switches, and old-fashioned thermostats.

 

[Johnny101]

@BradtheRad IMHO, mercury switch might not be able to detect position in quick motion. Can you suggest any other method?

 

[BradtheRad]

To detect quick motion, a light sensor is an easy method (unless design constraints do not allow). Mount an led on the ceiling, shining downward. Attach a photosensor on the arm. Fashion a tube surrounding it, so it receives led transmission only when the arm is raised.

The accelerometer probably will work although I have no experience with them.

 

[andre_luis]

Perhaps not applicable to your need, but if there is no restriction as to the complexity and cost, a solution based on recognition of image patterns could be considered. Nowadays, cameras with embedded processing capability are also called "vision sensors." Wearing the arm and forearm with straight lines painted with a color that can be optically filtered by lens, the result is a black background, making visible only those markers, therefore being easier to perform the pattern matching.

 

[Warpspeed]

How about a laser pointer strapped to the arm or wrist ?
The moving spot projected onto a nearby wall would give pretty good feedback as to the raised angle of the arm.

 

[Johnny101]

To detect quick motion, a light sensor is an easy method (unless design constraints do not allow). Mount an led on the ceiling, shining downward. Attach a photosensor on the arm. Fashion a tube surrounding it, so it receives led transmission only when the arm is raised.

The accelerometer probably will work although I have no experience with them.

I think it's a good idea but Design constraint is that sensor can only be placed on arm and it can be used outdoor as well. Can this method be used outdoor somehow?


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Perhaps not applicable to your need, but if there is no restriction as to the complexity and cost, a solution based on recognition of image patterns could be considered. Nowadays, cameras with embedded processing capability are also called "vision sensors." Wearing the arm and forearm with straight lines painted with a color that can be optically filtered by lens, the result is a black background, making visible only those markers, therefore being easier to perform the pattern matching.

According to the design constraint all the sensors should be placed on the arm

 

[andre_luis]

According to the design constraint all the sensors should be placed on the arm

In fact, what I suggested should be considered as a reflector, not exactly a sensor.

 

[chuckey]

If you get half a disc of steel and cut a slot in it from the outer edge heading towards the centre. Pivot the disc on its actual centre, now the slot will be hanging down. If a bulb is put behind it light will shine through the slot only when the top face is horizontal, so if the complete assembley is now strapped to an arm, the presence of the light beam should indicate when the arm is horizontal the light could be detected if the tests are done in the dark, or an opto diode could be used to detect this state and make a bleeper work or transmit an RF signal.
One problem could be the disc swinging wildly, put some paper vanes on it so the air damps it, or make it out of copper and put some magnets next to it.
Frank