angle sensors ?

Search for absolute rotary encoder.

thank you but It is not what I need

I need to measure the angle of the knee joint when the motor move the leg the knee is bending like an angle 'X' the head angle of a triangle when the base length change ' by moving forward or backward by the motor' the angle of the knee ' the angle is opposite to the base' is change I need to measure that angle

I'm having difficulty visualising the set-up - can you post a drawing or photo?

As long as you can get something in .jpg .gif or .png format (by saving from a graphics program, screen capture or importing from a camera or scanner), this classic thread from Sparkstation explains how to post it.

The absolute rotary encoder is designed for measuring angles - fix the body to one member, & the shaft to the other (may need some gearing), & hey, presto! They're used extensively in robotics. An alternative is a rotary potentiometer, giving an analog reading that can be converted to an angular measurement.

If I understand your description you have two triangle sides articulated and you change the triangle basis. You want to measure the angle at the articulation between the 2 above mentioned arms/beams/bars. You have 2 possibilities:

- the 2 sides have same length. One articulation will most probably be fixed and the other side will slide. You can at the fixed articulation use a sensor as mentioned above and measure the angle to the perpendicular to the basis. It will be 1/2 of the angle between the 2 bars. It will go then from 0° to 90°.

- you cannot do this you have to use a "floating" sensor at the very place where you want to measure between the 2 arms. You will have the full angle. I wrote "floating" since the sensor will move in the plane as the articulation between the bars will do.

If the speed is not important the cheapest way (but not the less precise) will be a rotary potentiometer. It has the advantage of a high output signal and a simple module for acquisition. It allows also a simple zero calibration.

thank you nick name ,

but I am not quit sure what do you mean ?

But I used an encoder to measure the distance that the motor move 'L' so I have the length of 'L' and used the equation : sin X/2= (L/2)/H

But the problem that I need H to be variable with the patient size and I can't measure it to change its value every time.

And I did not understand what do you mean by "floating sensor"

thanks.

Ok let me explain:

according to your sketch you should have 2 beams supporting the leg to which you fasten it and via a screw drive you straiten it or bend it. Assuming that the 2 beams are equal in length you are in front of a triangle with 2 equal sides. The angles between the basis and each of the beams are equal and since the sum of angles for a triangle is 180° each of them will be 0.5*(180- Ax) where Ax is the knee angle. So that if you have a potentiometer connected to the support of your screw and the rotor of it turns as the beam left you will measure the angle between H and L. This angle is Ahl=90-Ax/2 so that back you get Ax = 180-2*Ahl. This pot is fixed since it is on the frame. If you want to measure directly the Ax angle then you need a pot between the 2 H beams this pot will move with the knee thus I called it floating since it is not fixed in space but moving and requires also a free cable which could suffer from the bending and generate troubles.

I hope it is now clear, if not write again the details you did not understand.

You can use inclination sensor for measurement of angle. This sensor measures angle of x,y and z axis and gives analog output which you can use for display on scaling meter of control any movement.

Please see

www.sensorsystems.it

or

www.am.pepperl-fuchs.com

You have not mentioned your specific application. Both accuracy and resolution are important in choosing an instrument, so I am sending a link to a standard sports medicine type ROM setup

http://books.google.com/books?id=5MTcORR6lvUC&pg=PA208&lpg=PA208&dq=electro+protractor+rom&source=bl&ots=39jwEowY8l&sig=0-_L0eJK_UZdYmOWw7xnnjicrQ8&hl=en&ei=8ekXSuPbMKGGNan82YwP&sa=X&oi=book_result&ct=result&resnum=2

By the way, I trust the 180^o is just a bit of overspecification, and that you're not really gonna bend a knee that much.

Thank you all for answer

My application is a continuous passive motion machine for the knee it should move the leg to specific flexion angle and extension angle,

In the fig:

'H, H' is the patient hip leg from the hip to the foot and the' X' angle is the knee angle which I need to measure, 'L' in the moving part moved by a DC motor

So I need a measuring method that dose not depend on the 'H' length beques the 'H' length is variable with the patient tall, the angle should be depend on the 'L' .

Is the inclination sensor is suitable for my application?

Thank you all again

No, the inclinometer will not work. Now that I see what you're doing, go back to NickName's rotary pot. That will provide exactly the kind of feedback you need to control the motor

With the only known variable being L, one side of a triangle, you just don't have enough information to find any of the angles. My feeling is that your best bet would be to measure the hip-to-knee and knee-to-foot lengths, and use the cosine rule to find your angle.

If you're using a PC to control/monitor the equipment, it should be easy enough (?) to take a quick measurement (with e.g. a dressmakers tape measure) and enter the data before starting.

nik name said:

the 2 sides have same length. One articulation will most probably be fixed and the other side will slide. You can at the fixed articulation use a sensor as mentioned above and measure the angle to the perpendicular to the basis. It will be 1/2 of the angle between the 2 bars. It will go then from 0° to 90°.

I didnot get that part

how can I measur the angle?

and what is the "floating sensor"

plzzzzzz help

My bad. I missed JohnDG's post about rotary pots also. Sorry. I owe you a "GA" in the future.

Anyway, put your listening ears as Judge Judy likes to say. Three good engineers have now said rotary pot (they make them especially for this kind of application - linear, low-noise, center-tap). Unless you have a burning desire to do something else, use a rotary pot.

Here's alink to a typical supplier. Caveat: I've never dealt with these folks; I know nothing about them.

http://www.positek.com/

Thank you all for answer

I am sorry for the delay in answer

I understand that I need a rotary pot that will measure the angle

I looked for it and I find a lot of things like that picture but I am not sure if they are good for my application also I do not understand how can I use them.

I know I look so stubbed by keep aske question but I am not familiar with these types of sensor.

Thanks again you all have been so helpful

Oops, thought I had replied to this, but it must not have gone.

I would use a single turn linear potentiometer, something like those once made by Bourns or New England Instrument. I believe those brands may now belong to someone else, but I haven't checked to see. You would ideally want something like the 41 mm housing with a groove in it for mounting.

You want good linearity, say, 0.5%.

It should have a grounding terminal.

Resolution should approach infinity.

I'd guess perhaps 1000Ω, but talk to your electronics guy.

You'll apply +V on end, -V on the other and take the angle-dependent voltage off the wiper for feedback and measurement.

Two things to really watch. (1) You don't want to hyperextend the knee, so put stops on your mechanism. Consider a sacrificial, low strength link. (2) The knee doesn't swing in a perfect plane and you shouldn't force it. You need some flexibility in your leg clamps. Talk to an orthpoedics guy about this.

Finally, everything I mentioned here can be done digitally. I'm not smart enough to do that but there are people on here who are.

Finally, quit criticizing yourself for not knowing. All good engineers are dumb. When you're dealing with the life or well-being of other humans, you always assume you don't know for sure and double-check and ask others.

I can not thank you enough

I think I finally have a good idea about what I need.

And don't wary I already talked to an orthopedic and you don't have to wary about the knee it is perfectly save in my hand.

finally thank you for making me fill not so subbed after all