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Topic: Angular position sensor? (Read 6 times) previous topic - next topic

louarnold

Jul 16, 2012, 05:20 pm Last Edit: Jul 16, 2012, 05:54 pm by louarnold Reason: 1
This is for several uses on a robot. I am looking for a sensor that provides a code for the angular position of a shaft - such as a steering shaft or a disk that rotates a camera mounted on it. The idea is to determine the angle when the shaft is NOT turning. This is not for detecting high-speed changes. What is available?

Years ago, this was done with a shaft encoder - a metal disk with printed patterns that passed under a light sensor as it rotated. The position coding was obviously not very accurate, and got worse as the disk got smaller in diameter. I would like something that is a bit more modern in technology. What's available?

johnwasser

If you have access to the end of the shaft you can mount a magnet and use a special Hell-effect angle encoder.

What you want is called an Absolute Shaft Encoder.
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AWOL

Quote
special Hell-effect angle encoder

"Hall Effect"
"Pete, it's a fool looks for logic in the chambers of the human heart." Ulysses Everett McGill.
Do not send technical questions via personal messaging - they will be ignored.

johnwasser


Quote
special Hell-effect angle encoder

"Hall Effect"

Oops. :(
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louarnold


If you have access to the end of the shaft you can mount a magnet and use a special Hell-effect angle encoder.

What you want is called an Absolute Shaft Encoder.

Well, the hall effect sensor requires rotation - it senses speed.
I do want an absolute shaft encoder, but not in the old style that uses a coded pattern on a disk. I wanted something using a more modern technology - something that sense changes in fractions of a degree, perhaps. Does such a device exist?

AWOL

Quote
it senses speed.

No, it senses magnetic fields.

You need a shaft encoder.
I don't know of any solid-state ones, so a Gray code encoder is probably the only sensible absolute position sensor.
"Pete, it's a fool looks for logic in the chambers of the human heart." Ulysses Everett McGill.
Do not send technical questions via personal messaging - they will be ignored.

louarnold


Quote
it senses speed.

No, it senses magnetic fields.

You need a shaft encoder.
I don't know of any solid-state ones, so a Gray code encoder is probably the only sensible absolute position sensor.

OK. I'll keep my eyes open.
Thanks very much.

johnwasser

You have not said what kind of resolution you need. 

Austria Microsystems makes the Hall Effect absolute encoders in 8, 10, 12, an d 14 bit resolution (256, 1024, 4096, 16384 steps):

          http://www.ams.com/eng/Products/Magnetic-Encoders/Rotary-Encoders

DigiKey has the AS5263 (12-bit) for $10.22 each: http://www.digikey.com/product-detail/en/AS5263-HQFT/AS5263-HQFTCT-ND/3028680
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louarnold

#8
Jul 17, 2012, 08:48 pm Last Edit: Jul 17, 2012, 08:58 pm by louarnold Reason: 1

You have not said what kind of resolution you need.  

Austria Microsystems makes the Hall Effect absolute encoders in 8, 10, 12, an d 14 bit resolution (256, 1024, 4096, 16384 steps):

         http://www.ams.com/eng/Products/Magnetic-Encoders/Rotary-Encoders

DigiKey has the AS5263 (12-bit) for $10.22 each: http://www.digikey.com/product-detail/en/AS5263-HQFT/AS5263-HQFTCT-ND/3028680

Ahhh, that's a good question.  I'm not really sure about angular resolution. Placing a gear around a steering shaft and using a stepping-motor driven worm gear, what would be a realistic resolution? Would 1 degree be enough? I wonder.

The sources are good. I'll have to look at them more closely, but now I know that they really exist, so thanks very much. I note they seem to be ICs. How do they connect to a shaft?

Can a hall-effect device be made to sense angular position without having rotation - so that when the device powers up, it knows what direction the single tire steering wheel is pointing?

johnwasser

> Placing a gear around a steering shaft and using a stepping-motor driven worm gear, what would be a realistic resolution?

If your stepper does 200 steps per revolution and your worm gear has 100 teeth that is 20,000 positions.  Perhaps you would be happy with 4096 positions.

> How do they connect to a shaft?

Magnet on END of shaft.  IC centered over magnet.

> Can a hall-effect device be made to sense angular position without having rotation - so that when the device powers up, it knows what direction the single tire steering wheel is pointing?

Yes.  That's why they are called ABSOLUTE angular encoders.  They tell you absolutely which way the shaft is pointing.
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louarnold


> Placing a gear around a steering shaft and using a stepping-motor driven worm gear, what would be a realistic resolution?

If your stepper does 200 steps per revolution and your worm gear has 100 teeth that is 20,000 positions.  Perhaps you would be happy with 4096 positions.

> How do they connect to a shaft?

Magnet on END of shaft.  IC centered over magnet.

> Can a hall-effect device be made to sense angular position without having rotation - so that when the device powers up, it knows what direction the single tire steering wheel is pointing?

Yes.  That's why they are called ABSOLUTE angular encoders.  They tell you absolutely which way the shaft is pointing.

I think you meant the shaft gear when you said 100 teeth. Geeee, Is that enough? (Just joking. I should have done the math before asking.)

For the magnet on the end of the shaft...any pictures, what does it look like when it's attached? I assume the sensor is attached so the magnet rotates under/over it?

johnwasser

> For the magnet on the end of the shaft...any pictures, what does it look like when it's attached?

Instructions for mounting a Magnetic Rotary Encoder 1.0 of a GM3 gear motor: http://reprap.org/wiki/Magnetic_Rotary_Encoder_1.0

> I assume the sensor is attached so the magnet rotates under/over it?

Yes. Either under or over.  They show a cylindrical magnet but it is magnetized across a diameter, not from flat to flat as most cylindrical magnets.
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louarnold


Yes. Either under or over.  They show a cylindrical magnet but it is magnetized across a diameter, not from flat to flat as most cylindrical magnets.

Its not quite clear in the picture. The magnet and motor seem to be in a plastic case, but that isn't mounted close to the sensor. I assume that's not easy to show.
However, the URL is great to have. I'll book mark it.

johnwasser

Here is an example diagram:

http://team358.org/files/programming/ControlSystem2009-/MagneticRotaryEncoderSetup2.jpg

Another example of use:

http://www.electric-web.org/rotary_encoder.htm

This page has a picture of an evaluation board:

http://www.digikey.com/product-detail/en/AS5048-DB-1.0/AS5048-DB-1.0-ND/3188614
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louarnold


Here is an example diagram:
http://team358.org/files/programming/ControlSystem2009-/MagneticRotaryEncoderSetup2.jpg
Another example of use:
http://www.electric-web.org/rotary_encoder.htm
This page has a picture of an evaluation board:
http://www.digikey.com/product-detail/en/AS5048-DB-1.0/AS5048-DB-1.0-ND/3188614

That mounting system will certainly work for what I have.
I bookmarked all the URLs as well as the FRC home page.
Thanks very much!
Regards,
Lou.

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