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[on:]

Hi guys,
a very quick question!
I need to measure the position in space of a sphere.
I mean i need the inclination on the three axes X Y Z.
What type of sensor should I use?
Note that I do not need accelleration (therefore accellerometers are not good for me!).

Thanks a lot,
Valerio

[Reply #1 on:]

What dimension does the sphere have? How is it hold in position? What accuracy do you need? What the space the sphere is moved in (dimension)?

[Reply #2 on:]

The sphere is a joint for a robotic arm. More precisely, it is the humeral head:



The sphere will be hold in position through a female joint, so that the only possible motion is rotation in all directions on the spot. About accuracy, a degree it's ok for me.
The purpose of the sensor is to detect the position of the humerus in the 3D space.
Thanks!

[Reply #3 on:]

Do you remember the old mechanical mice? I'd use the same mechanism for this one, just two wheel orthogonally place with contact to the sphere surface and rotation encoders on them. If the sphere surface has a minimal structure on it you could also go for an optical mouse and get the movement with that sensor (they are made to work on almost any surface).

[Reply #4 on:]

So, you aim to measure orientation with respect to earth's z-axis? Meaning, you do not care about the z-direction (what a compass would read out to you)?

When accelerometers are steady, they make perfect orientation sensors for this purpose. If you need the z-direction, you could add a compass module.

But this method is not good when the object is moving back/forth (ex. walking). But you can add a gyro sensor and create a nice filtered system for orientation sensing (getting rid of the compass in this case).

[Reply #5 on:]

http://www.sparkfun.com/products/11072

This is a very small board incorporating a 3-axis gyro and a 3-axis accelerometer.

[Reply #6 on:]

If you are measuring humerus relative to scapula (humerus-scapula) you will get away with only three rotation sensors (X, Y, Z).   However if you are measuring relative to space (humerus-space) then you will also need to sense the scapula orientation relative to space (scapula-space) since any movement/rotation of it will need to be deducted from the humerus-space readings.

[Reply #7 on:]

I like pylon's idea of using an optical mouse.  You could mount the optical sensor in the female joint and read the motion of the head.  However, no matter where you locate it, there will be one axis where the rotation cannot be measured.  Two sensors spaced apart would do it for sure.

[Reply #8 on:]

Wouldn't the suggestion of using mousewheels need to be fixed at axies that emanate from their common (0,0,0)? I don't think that all axis can be used like that if we speak of real life persons: turning the arm around itself (around the up-axis) may not propagate through the skin. and other situations are not favorable.
I cannot advise on the accelerator sensors .. valerio_sperati, it was probably you that got a strong and unjustified headwind here in the forum in another post on a parallel issues. The idea of measuring gravity on two or three axis (from say the elbow) appears very doable as a principle.

[Reply #9 on:]

Thanks a lot for all your suggestions!
I'll take a look to the optical mouse idea, and to the gyroscope sensor too.

Just to make clearer what I need, here there is another image showing which movements I need to track:



More precisely I need to track abduction (rotation on X axis), flexion (rotation on Y axis), and external rotation (rotation on z axis).

About Jackrae suggestion

If you are measuring humerus relative to scapula (humerus-scapula) you will get away with only three rotation sensors (X, Y, Z).

can you advise some model of three rotation sensor?
Thanks again, Valerio

[Reply #10 on:]

I'll talk as if it is to be attached to a live person:
I'll assume that the 'person'/clavicle is situated on the negative side of x, leaving a mouse-roller x-setup on the positive x side. The y-setup has no constraints (can be on both the + and - side). The z-setup will have to point up (negative z direction). They will all have to be attached to the clavicle (negative x side) giving the 'external structure' of the whole setup a rather massive appearance (with simple, but cumbersome mechanics) .. compared to an insignificant 3-axis gyro or acceleration-sensor.

[edit]
ehmm .. three axis fixed on the humeral head pointing strictly at (0,0,0), but the dials/wheels will have to be both aligned to these (collectively dynamic) axis and fixed to the clavicle. .. I think that you are heading into some professional fine-mechanical engineering. Or do I just lack imagination?