Wheel Encoder

AIman

I would like to make a wheel encoder that works for a 10 inch wheel and is accurate to 1/64 of an inch. The wheel encoders I saw on the Parallax website work for 6 inch wheels and have an accuracy to 1/2 inch. I have to control both foward and backward movment as well as swivel wheels that give side to side control

Can I make an encoder that is as accurate as I want?

Is there another way to do this?

tonyp12

Here is a place that sells products to make your own steel discs, maybe they would know if 1/64" is possible
http://www.renaelectronics.com/product_stencil.htm

John Abshier

For a 10 inch wheel and 1/64 inch accuracy you need an encoder with 2011 counts (pi * 10 inches * 64). I doubt that you can make one. There are encoders on the market that would meet or exceed your requirements. U.S. Digital is one source that I have used with success. You would have to couple the encoder to the wheel or motor shaft.

John Abshier

Ray0665

You might want to check out this thread

http://forums.parallax.com/showthread.php?118906-Stingray-Motor-Shaft-Encoders

schill

As mentioned in the referenced thread, if you need that many counts per revolution you will typically monitor the motor revolutions instead of the wheel revolutions. They will be different if you are using a geared motor with a gear ratio other than 1:1.

If it has a gear ratio of 100:1, then the motor will turn 100 times for every turn of the wheel. A 64 cpr (count per revolution) encoder on the wheel would only measure 64 counts per turn of the wheel. But, a 64 cpr encoder on the motor shaft would measure 6400 counts per turn of the wheel.

In order to do this, you will either need a motor with a built-in encoder or one with a shaft extending out the back of the motor (usually for purposes such as this) to attach the encoder.

If you are driving 10 inch wheels, I expect you are probably using a geared motor to generate the torque you require.

AIman

Yes a geared motor is required.

Could I simply mount the encoder wheel to a gear?

AIman

I just got what you meant by putting the endocder on the motor.

I will do some research and see if that will work.

Thanks much all.

GordonMcComb

Two part reply:

Part 1: You can achieve the 1/64" *resolution* with a 512 CPR disc, as long as you use quadrature encoding. (Though it would be best to double the CPR to 1024.)

Part 2: You will simply not be able to get 1/64" *accuracy* with any kind of wheel encoder, depending on distance of travel, the errors can become quite large. This is due to simple mechanics of wheels moving over a surface. Encoder errors are unbounded, meaning that every slip, every turn, the error grows. Over a distance of 10-20 feet the error can easily grow by fractions of an inch. If your 10" wheel is not *exactly* 10 inches, you can easily be 1/64"+ off each rotation, and the error grows every time the wheel turns.

Some applications provide more potential for accuracy. For example, if you're creating some kind of measuring device, you could conceivably try a machined wheel with a high-quality rubber tire of a known durometer. Have some means to measure circumference before taking the reading. Over a consistent type of surface -- say hard flooring or lumber -- you might be able to hold to a 1/64" accuracy over (say) a 10 foot distance.

Part 2B: Putting encoders on the ungeared side of the motor puts a lot of demand on the encoding electronics. Either way, a transmissive encoder affords more accuracy. I echo John's suggestion of US Digital, and they also sell the quadrature interface chips that makes counting at high speeds much easier.

PJAllen

GordonMcC,
Part 2 -- that's some good stuff.
Plain truth like that forces people out of the "crystal spheres" paradigm.