Using Digi-Key Shaft Encoders

[Kevin Watson]

Originally Posted by Dave...

The 6639S-1-103 Bourns pot is 10K and not available in a 100K. Will adding 90K resistors in series be acceptable to the RC? It seems like we would lose quite a bit of the RC's resolution.

No, you don't need to use the additional 90K. 10Kohms across 5v means that only 0.5mA of static current will flow through the potentiometer for a power dissipation of 2.5mW. The potentiometer is rated far above this value and therefore safe to use.

Originally Posted by Dave...

1. Some type of encoder/pot to measure steering angle (slow rpm, continuous turn with good resolution ~3°)

To get good resolution you'll probably need an encoder because the linearity of non-wirewound potentiometers is questionable.

Originally Posted by Dave...

I like the idea of a continuous pot because it doesn't take up CPU power and takes almost nothing to program, and does not rely on starting postition (absolute).

Yes, these are good attributes. If you really need the accuracy or just want to learn how robotics engineers do it in the real world, I'd suggest at least looking at an optical encoder. Here are a few ways to get around the starting position problem: 1) Make sure the wheels are straight when you start the robot 2) Use a potentiometer to find the zero position at startup, then use the encoder for steering angle. 3) Have the computer run through a calibration sequence at startup, stepping the steering angle in one direction until a switch is tripped at a pre-determined angle. 4) If your steering mechanism is robust, you can run through a calibration sequence that steps the steering in one direction until you run into a hard stop which causes a sudden increase in motor current that can be detected with the current sensor.

Originally Posted by Dave...

http://www.rec.ri.cmu.edu/education/...tent/index.htm site has some information listed for encoders, but I didn't find where to obtain their encoders and they were not in the Edubot or KOP. Where do we obtain these parts?

Digi-Key has many. An example is the grayhill 61K128-050.

Originally Posted by Dave...

Sample code from FIRST or Innovation FIRST would help teams to concentrate on building robots.

See http://kevin.org/frc for an encoder interface example.

-Kevin