Battery

Tim......

I have noticed the BoeBot behavior changes quite dramatically when new batteries are installed, or when switching betwen 9 volt and 6 vots (4--1.5s).· I have measured the output of the voltage regulator at about 5 volts in both cases.· Is there any way to stabilize Boebot behavior as long as the input·voltage is above 5.2 volts?·

To be a little more clear the changes have to do with how fast the servos turn, and also about making turns.· For example it I program a 90 degree turn with fresh batteries and then change them to batteries that are above 5.2 but well below 6 volts, it is no longer a 90 degree turn.

Steve Joblin

the servos are not powered through the regulator... they are powered direclty from the battery.

The more voltage, the faster the motor spins, so naturally, as the voltage drains in the battery, the motor will spin at differing speeds.

Mike Green

This is normal behavior. The voltage regulator will maintain +5V (approximately) as long as its input is above about 5.5V. The servo motors are (as they should) run off the unregulated power supply input which should range from about 5.5V to no more than 7.2V. More will shorten the life of the brushes in the servo motors. The servos, like all DC motors, will vary in speed and torque depending on the input supply voltage.

The most straightforward way to improve this would be to use rechargable NiMH AA cells with as large a capacity as possible. 2500 to 2800 mAh cells are readily available and you should use 5 of them (not 4) in series. You can get a single AA cell holder from RadioShack and wire it in series with the 4 cell holder that comes with the BoeBot. The 5th cell holder can be glued onto the chassis over the pivot ball. Parallax also sells a holder for a 5th cell that attaches to the 4 cell holder.

This scheme works because NiMH cells have a fairly flat discharge curve. When fully charged, they have a cell voltage well over 1.2V, but this drops very rapidly to around 1.2V where it stays until the cell is nearly exhausted. Because the voltage is stable for most of the battery's discharge, the servo speed and torque will also be stable.

Keep in mind that servos are not very accurate. They're not meant to be. That you get a reproducible 90 degree turn under some circumstances is excellent, but you can't count on it. The only way to do that is to use position feedback like the wheel encoders that Parallax sells for the BoeBot. Even then, wheels can slip and slide on the floor and, even if the wheel position is reproducible, the actual position of the BoeBot may vary.