Lots of encoder chat here lately, so I'm starting a·
thread on the specific encoders I used on my Homebuilt platform. I had great results.
friend "Hollywood" made the attached encoder disks·
in Adobe Illustrator, the one I used has 36 b/w stripes and the new one has 72. Ten and five degrees, respectively.
An encoder site with some info: http://groups.csail.mit.edu/mac/users/pmitros/encoder/
this sensor : http://www.junun.org/MarkIII/Info.jsp?item=48·
That tiny chip has internal signal conditioning and Schmidt triggers. It·
a clean digital signal that I fed directly into a Stamp input pin.
A word about the encoder disks: they must be mounted PERFECTLY to achieve consistency. Perfectly flat, true, and concentric. Mount your paper encoder disk centered perfectly on a strong, flat background. Wobby or eccentric encoder disk will give the same results as wobbly or eccentric wheels. Do not skimp or rush this step. Make your encoder disks PERFECT.
Some printers may print these as ovals, which are utterly useless. Use calipers or a compass and verify that your encoder disks are PERFECTLY circular.
Properly implemented, more stripes can yield better resolution and an improvement in accuracy, but in the end it comes down to a demand for absolute mechanical perfection in your drive system.·
use a motor encoder and get hundreds of pulses per revolution (PPR), it doesn't·
guarantee you will get incredible repeatable accuracy. Wiggle your wheels in all directions, be brutal. Any play, wiggling or flexing anywhere is the beginning of the end. All the processing power in the world can't make up for a sloppy transmission or·
wheel. There are many mechanical factors.·
Transmission backlash (gear slop) kills repeatability. Any wobble, slop, or end play in your·
wheel axles kills repeatability.·
Wide tires and tire tread work against repeatability.·
Pneumatic tires work against repeatability, they are mushy and usually far from circular. Higher tire pressures may lead to better results. Tank treads and 4-wheel skid steering are very difficult to get any repeatability from whatsoever.
Make your encoder disks as large as possible and locate the·
encoder sensor as close to the outer diameter of the wheel as possible. This gives you a wider stripe to read, no matter how many stripes you have. The sensor must be oriented properly. The two dots (LED & PTX) on the sensor must be oriented radially, not circumferentially for best repeatability. A line connecting the two dots should intersect the center of your axle. Experiment with the gap between your sensor and encoder disk for best output·
results. I used about 0.080", but·
that may change for different paper and·
Two pics of my encoder installation attached. This allows my robot to drive a very straight line: http://www.youtube.com/watch?v=Trv682XZ_8c
Hope this info is of some help.
·"If you build it, they will come."
Post Edited (erco) : 1/22/2010 6:20:19 AM GMT