BOE Bot calibration help

PSETeacher

I am teaching a unit using the BOE Bot USB edition and my students are running into problems with calibration. Specifically they are calibrating thier robots to go in straight lines with nice 90 degree turns one class but then when they come back next class the programs they wrote previously are no where near what the robot now does. The differences between one day and the next is quite large (ex. one group calibrated it to go straight for 1.5m and now with the same program it is going only about 0.6m) The robots also are curving heavily on commands that previously caused them to move straight. We have troubleshot everything we can think of and I can find nothing that will help except re-calibrating everything which is taking them most of our classtime so they aren't able to move forward with programming it to navigate the perscribed course. Can someone please help me figure out what is happening and/or offer a solution for htis problem?

NWCCTV

Have you tried using fresh batteries each time? Not sure if this is the issue but Servos drain bateries fairly rapidly. Are you using the battery add on Parallax sells? http://www.parallax.com/StoreSearchResults/tabid/768/txtSearch/battery/List/0/SortField/4/ProductID/867/Default.aspx

PSETeacher

I appreciate the suggestion and that was my original thought too but we tried replacing the batteries with fresh ones and still got the same strange results.

Phil Pilgrim (PhiPi)

An encoder would solve this problem nicely. Here's an off-the-shelf unit designed for the BOEBot:

http://www.parallax.com/tabid/768/txtSearch/28107/List/0/SortField/4/Default.aspx

Without an encoder, unfortunately, you're left with the vagueries of an open loop, which behavior you've noted here.

-Phil

erco

For improved open-loop consistency, it's all about precision voltage regulation. Unregulated alkaline batteries will never do the same thing twice. Someone who shall remain nameless (ahem, Duane Degn) has reported significant improvements in repeatability by powering the servos through a voltage regulator. For best battery life, use an LDO regulator with several extra volts above your regulated voltage. For instance, add a fifth AA alkaline battery to the BoeBot and use the onboard 5V regulator to power the servos. For best results, use seperate batteries for the electronics and servos.