Accelerometers Gyros?

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drt80
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Posted by drt80: Mon. Mar 30, 2009 - 12:37 PM
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Hi,

I am in a design phase of a project, where I need to do the following:

Sense the rotation of a shaft. The shaft moves very slowly. It moves once every 6-10 seconds and the rotation is about 0.1 degs. I want to sense the rotation in degrees and would want to know the position in degrees (resolution of 0.1 deg) at any given time.

I have been searching here and google, but I have not been able to understand whether an accelerometer or a gyro would be useful here.

I have read about inclinometers from analog devices:

http://www.analog.com/en/mems-an...

These inclinometers internally use 2-axis accelerometers, so I think this can be done. But, just want to be sure.

Can anyone comment on this?

Thanks,
-Deven

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Learning and Information, General Electronics
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dezellis
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Posted by dezellis: Mon. Mar 30, 2009 - 02:01 PM
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Hi,
I would have thought that this application is normally the realm of high resolution rotary encoders but I suspect they are not inexpensive http://www.motioncontrol.com/pro...
I can't really comment on the ADIS16203 solution I have never seen this application before, however, I suspect it will not give the consistency that you probably require.
I am curious regarding the actual application and the shaft size etc.

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ikdor
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Posted by ikdor: Mon. Mar 30, 2009 - 02:37 PM
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All gyros on the cheap side of million dollar ring laser gyros would not work, and even those have drift. Generally it's a bad idea.

You could use a two axis accelerometer glued to a shaft and do a cordic algorithm, but it is unlikely that you will get a usefull resolution close to 0.1 degrees. Check the accelerometer datasheets and calculate it yourself.
Also the electronics will be rotating in that setup, making a mess of the wiring.
A better solution would be a shaft encoder, either an optical or a magnetic one. Be warned though that 0.1 degrees resolution and accuracy are different things.

Good luck,
Igor
Shameless plug: http://www.skf.com/portal/skf/home/product_pages?contentId=001461&lang=en

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drt80
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Posted by drt80: Mon. Mar 30, 2009 - 03:09 PM
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dezellis wrote:
Hi,
I would have thought that this application is normally the realm of high resolution rotary encoders but I suspect they are not inexpensive http://www.motioncontrol.com/pro...
I can't really comment on the ADIS16203 solution I have never seen this application before, however, I suspect it will not give the consistency that you probably require.
I am curious regarding the actual application and the shaft size etc.

Ya.... i have thought of the encoder solution. Actually, I want the solution to be compact. There is hardly any space to mount the encoder on the shaft. The shaft is of 20 mm diameter.

Thanks for your reply,
-Deven

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drt80
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Posted by drt80: Mon. Mar 30, 2009 - 03:14 PM
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ikdor wrote:
All gyros on the cheap side of million dollar ring laser gyros would not work, and even those have drift. Generally it's a bad idea.

You could use a two axis accelerometer glued to a shaft and do a cordic algorithm, but it is unlikely that you will get a usefull resolution close to 0.1 degrees. Check the accelerometer datasheets and calculate it yourself.
Also the electronics will be rotating in that setup, making a mess of the wiring.
A better solution would be a shaft encoder, either an optical or a magnetic one. Be warned though that 0.1 degrees resolution and accuracy are different things.

Good luck,
Igor
Shameless plug: http://www.skf.com/portal/skf/home/product_pages?contentId=001461&lang=en

The resolution is one of the concerns. Other being the movement of the shaft. The movement of the shaft is very less, 0.1 degrees. Will the accelerometer be able to detect the change?

I am trying to find out the algorithms to calculate the actual rotation from the accelerometer parameters. If you know of some websites or name of certain algos, that would be great.

Thanks for your reply,
-Deven

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drt80
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Posted by drt80: Mon. Mar 30, 2009 - 03:15 PM
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@ikdor: I see you mention a cordic algorithm. I will try to find out more about it.

-Deven

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bobgardner
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Posted by bobgardner: Mon. Mar 30, 2009 - 03:19 PM
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You can buy the parallax 3 axis accelerometer at radioshack for about $40. It has a Microchip 12 bit a/d converter on it and the accelerometer. Uses SPI. I have routines to read the accelerometers I can post. Just clamp it in a vise and see how close you can measure tilt.

Imagecraft compiler user

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drt80
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Posted by drt80: Mon. Mar 30, 2009 - 04:55 PM
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Well, I guess I can try it out. But should be a while before I do it. I want to finish my ground work before I order anything.
I am based in India, so need to think before buying (importing).

BTW, I will be interfacing it to one of the AVR32 devices. For trials the AT32UC3A0512 and a different AVR32 for the final design.

-Deven

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barnacle
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Posted by barnacle: Mon. Mar 30, 2009 - 07:09 PM
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Off the wall... how about a vernier scale wrapped around the shaft? A fixed number of narrow stripes running longitudinally along the shaft (i.e. round the circumference) and a scribed transparent grid at a slight angle; observe the position of the moire patterns.

Neil Barnes
www.nailed-barnacle.co.uk

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digitool
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Posted by digitool: Mon. Mar 30, 2009 - 11:12 PM
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Here is a link for some rotary encoders that might fit your needs.

http://usdigital.com/products/en...

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DocJC
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Posted by DocJC: Tue. Mar 31, 2009 - 01:17 AM
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Does it only rotate in one direction?

Is it always 0.1 deg per activation?

How about an optical sensor to detect a Zero Point and just counting the drive pulses to the system driving it?

JC

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glitch
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Posted by glitch: Tue. Mar 31, 2009 - 01:22 AM
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another option is a resolver

Writing code is like having sex.... make one little mistake, and you're supporting it for life.

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drt80
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Posted by drt80: Tue. Mar 31, 2009 - 03:59 AM
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Thanks for the comments!

Now that I have some hints, I will do some search/research for a day or two and get back.

-Deven

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ubarch
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Posted by ubarch: Thu. Apr 2, 2009 - 12:47 PM
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Austria Microdevices makes some very small, relatively cheap encoders that I believe are based on the Hall effect. A magnet is mounted on the moving part, and an IC is mounted in a fixed position close to the magnet. They have shaft encoders and axial encoders. The former involves a magnetic strip that you wrap around the shaft, and the latter involves a small magnet that you attach to the end of a shaft.

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drt80
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Posted by drt80: Fri. Apr 3, 2009 - 08:36 AM
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Update!

I think I will go with the encoder solution. Still do not know which one, but will try to find one locally.

The accelerometer solution is on "hold". At least for now. But, it seems a nice mini project 8).

-Deven

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