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

[Joel J]

Originally Posted by Kevin Watson

If you really, really want to use a potentiometer, why not use the Bourns 6639S-1-103 which is available at Digi-Key (search for 6639S-1-103-ND). I'd rather use an encoder.

-Kevin

I am in heavy deliberation at the moment. I am trying to decide whether or not I wish to use a potentiometer or encoder for something relating to the subject of this thread. I am pretty much at a loss for good reasons (I have never used potentiometers or encoders before), so I am wondering why you have a bias that steers more towards an encoder than to a potentiometer.

[D.Viddy]

Originally Posted by Joel J.

I am in heavy deliberation at the moment. I am trying to decide whether or not I wish to use a potentiometer or encoder for something relating to the subject of this thread. I am pretty much at a loss for good reasons (I have never used potentiometers or encoders before), so I am wondering why you have a bias that steers more towards an encoder than to a potentiometer.

Because almost all potentiometers that will find will not revolve all the way around. Also pots wear out very easily when attached to a constantly rotating shaft. Last years controller was too slow for using an encoder. This years processor has no trouble keeping up with a 256 tick encoder. We have ours controlling our motors beautifully right now. Dead recoding is incredably precise with them. It's not a bias, it's just one is better than the other.

[Joel J]

Originally Posted by D.Viddy

Because almost all potentiometers that will find will not revolve all the way around. Also pots wear out very easily when attached to a constantly rotating shaft. Last years controller was too slow for using an encoder. This years processor has no trouble keeping up with a 256 tick encoder. We have ours controlling our motors beautifully right now. Dead recoding is incredably precise with them. It's not a bias, it's just one is better than the other.

Ok, thanks.

[Kevin Watson]

Originally Posted by Joel J.

I am in heavy deliberation at the moment. I am trying to decide whether or not I wish to use a potentiometer or encoder for something relating to the subject of this thread. I am pretty much at a loss for good reasons (I have never used potentiometers or encoders before), so I am wondering why you have a bias that steers more towards an encoder than to a potentiometer.

As Dylan and others have pointed out, potentiometers have problems when used for positioning/control. Encoders, on the other hand, are designed for this type of application. You can find some example encoder code here.

-Kevin

[Greg Powers]

mechanical or optical encoders? i know the mechanical encoders are alot cheaper and i was wondering how much that degraded quality

does anyone know the pro's con's between the two?

we plan on using them to compensate for steering problems and atonomous and aren't sure which we want.

[steven114]

Mechanical encoders don't last nearly as long as optical ones do - check the specs on them for that measurement.

[Daniel]

Have you read the white paper titled "Quadrature Encoders"?

This should help you a lot. email me if you have any specific questions or comments.

Daniel

[sudeepr71]

After reading this forum, I still have a few questions.
What exactly is a encoder?
And how can I use it to measure the speed of the robot?

[Alan Anderson]

Originally Posted by sudeepr71

What exactly is a encoder?
And how can I use it to measure the speed of the robot?

There are a few different kinds of encoders. What most people here are talking about are "rotary shaft encoders", which can tell you the angular position of a rotating shaft. Usually that's done by counting pulses as it turns. Some encoders have thousands of steps per rotation, giving a very precise measurement. At the other end of the precision scale, last year's TechnoKat robot used six black stripes on the wheel hubs with Banner sensors to measure wheel rotation.

To measure the speed, you just use the difference in position between two measurements at different times.

[sudeepr71]

Originally Posted by Alan Anderson

At the other end of the precision scale, last year's TechnoKat robot used six black stripes on the wheel hubs with Banner sensors to measure wheel rotation.

To measure the speed, you just use the difference in position between two measurements at different times.

On your last year's robot you used it to see how far to go and then turn or something for the autonomous mode? We tried that and found that it wasn't as accurate as the line following.

So to find the speed, one would have two encoders at different times and subtract them?

[WizardOfAz]

Originally Posted by sudeepr71

On your last year's robot you used it to see how far to go and then turn or something for the autonomous mode? We tried that and found that it wasn't as accurate as the line following.

So to find the speed, one would have two encoders at different times and subtract them?

Just one encoder to get distance and speed. You can use the timer support to get an accurate time base and distance/time to get the speed.

Bill

[IrisLab]

I'm having trouble with the links to digikey at the first of this post. Does anyone know the part numbers that the links refer to?

Or alternatively, any recommends for digi-key optical encoders?

Thanks.