Author Topic: How to determine the angular position of Shaft?  (Read 679 times)

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Offline jeeloongTopic starter

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How to determine the angular position of Shaft?
« on: January 29, 2013, 05:35:10 PM »
Dear Robot Experts,

I was wondering if could I attach a potentiometer to the shaft of my gearbox or motor that will enable me to determine the angular position of the shaft since the motor/gearbox does not have integrated encoder or potentiometer build in it.

This is crucial for me to create a exoskeleton suit that is worn by an individual.

Is there any recommendation for the purchase of such devices?
Is there any other device that can do similar thing(determine angular position) like rotary encoder(which is possible for me to purchase separately and connect)

 :)
Regards
JL HEE 

Offline jkerns

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Re: How to determine the angular position of Shaft?
« Reply #1 on: January 29, 2013, 06:14:48 PM »
Dear Robot Experts,

I was wondering if could I attach a potentiometer to the shaft of my gearbox or motor that will enable me to determine the angular position of the shaft since the motor/gearbox does not have integrated encoder or potentiometer build in it.

Yes. It's a reasonably common thing to do.

Quote
Is there any recommendation for the purchase of such devices?

Probably about 10K or so would work.  I assume rotation is well under 360 degrees for your application.

Quote
Is there any other device that can do similar thing(determine angular position) like rotary encoder(which is possible for me to purchase separately and connect)
Yes. You can buy encoders.
I get paid to play with robots - can't beat that with a stick.

http://www.ltu.edu/engineering/mechanical/bachelor-science-robotics-engineering.asp

Offline jeeloongTopic starter

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Re: How to determine the angular position of Shaft?
« Reply #2 on: January 29, 2013, 07:21:47 PM »
Thank You Very Much for the confirmation! This means my project is feasible.

I would be purchasing Banebots 4:1 P60 Gearbox and RS555 Brushed Motor (Kit) from Banebots United States and get it delivered to United Kingdom either from robotshop or banebots(US). It provides about 47 Nm of torque which should be sufficient for my exoskeleton to actuate the hip. (Currently working out on more accurate way of calculating torque required for walking)

Hopefully able to find decent hollow shaft potentiometer at decent price. Currently my problem is with the seletion of potentiometer and encoder since, I wasn't sure what does it look like and the dimension provided I could obtain the CAD model of it.

The position of potentiometer/encoder is crucial since the suit will be machined(Probably aluminium) for the thigh link and shin link. And there is only 500 pounds sterling to spend. And I have little background knowledge about electrical stuff(From mechanical engineering). Hopefully my friend will deal with the electronic part.

 :)
Regards
JL Hee

Offline ErikY

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Re: How to determine the angular position of Shaft?
« Reply #3 on: January 29, 2013, 08:01:06 PM »
Just out of curiosity, what type of potentiometer is usually attached to a gear motor for this purpose?

Would it be a linear potentiometer or something else? Also, is there a standard way of attaching this?


Offline jwatte

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Re: How to determine the angular position of Shaft?
« Reply #4 on: January 30, 2013, 12:13:47 PM »
You use rotary potentiometers with axles that can mechanically attach.
However, in industrial control you always use optical or magnetic encoders because they are much more robust.
I suggest looking for ready made rotary encoders. Good quality ones complete with mechanical and electronics start at 50 dollars or so.

Offline ErikY

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Re: How to determine the angular position of Shaft?
« Reply #5 on: January 30, 2013, 01:24:37 PM »
jwatte,

Thanks for this info, as usual, very helpful.

Can I ask, in your opinion, what would be a reason to use a motor and an encoder vs. just using a servo?


Offline jwatte

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Re: How to determine the angular position of Shaft?
« Reply #6 on: January 30, 2013, 09:42:56 PM »
A servo consists of a motor, an encoder, and some controlling logic. There's nothing magic about that.
If you can find a ready-made servo that includes all components and works for you, then that's probably simpler than using separate components.
In bigger industrial control situations, you typically buy a motor to suit the load, an encoder to suit the precision requirements, and a servocontroller that can read the encoder and drive the motor and receive commands from wherever commands come from.

If you're talking about exoskeletons that need to carry both people and load, you're talking about tens of thousands of kilo-centimeters of torque, and it's unlikely you'll find ready-made servos that fit your overall usage profile. In fact, it's somewhat unlikely you'll find many motors and power systems that will fit the general weight requirements at all... There are some Darpa projects around for building working exoskeletons to support things like loading missiles onto airplanes and whatnot. You can find videos on YouTube.

Note that anything that goes onto a human being pretty much *has* to be at the level of sophistication that those research prototypes use. It's not likely something you can build out of cheap, ready-made parts. Yet :-)


 


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