Quote:
|
Originally Posted by JBotAlan
EDIT: Just out of curiosity, how did you learn how to do these calculations--the life of the battery? I really don't know any of this. I don't imagine it's that hard, but I just never learned it and I recognize this is some of the stuff I should know being that I'm dealing with electrical this coming season.
Thanks
|
A lot of this is not calculable. It comes from experience. There is so many variables from match to match and robot to robot it is almost impossible to take everything into account. A couple of rules of thumb help to analyze what is taking place.
1. Tank style steering will put the drive motors into stall when ever they turn unless something is done to reduce the drag (friction) of the side motion of the wheels. More turns in a match will draw stall currents more often, reducing terminal voltage, available battery power and overall battery life.
2. Drive trains that are optimized for pushing only will draw high currents when running flat out at their highest speeds. Making some trade offs in speed vs. pushing will help in current drain. Drivetrains that are designed for high speed (greater than 12 fps) will likely draw extreme currents when starting out from a dead stop and when pushing, bumping into field objects or driving up ramps. Most mechanical experts design for 8-12 fps.
3. Software modification of drive signals will help in current draw. Ramping up to full speed for instance will smooth out the current demands. Dual speed transmissions will also help by giving the designer two separate optimizations for the drive motors.
4. Six motor drives generally will be less effective electrically due to the additional friction of the extra gears, balancing of the loads, etc. Although they will be the greatest pushers if the wheels can couple to the floor, it has been rare that a robot would only push and do nothing else. If you are in stall every time you start a motor, then you are adding two more loads to the battery. Stall currents are listed in the motor spec sheet. 6 x 129 amps for Chalupa drives exceed the max current spec on the battery. Repetitive current demand near the max reduces battery life. I have seen six motor drives that were not able to move the robot if minor damage to the robot caused some additional friction in the drivetrain.
5. One only needs to observe robot operation during practice to recognize problems. An aggressive practice where the robot stalls, falters or halts for a few seconds, is drawing too much current. A tank style robot that "hops" when turning has too much side friction and can be expected to be at max current every time the robot hops up or down. Any odd problems in robot operation such as software failures, "no data radio", intermittent IO tally LED operation or other apparent failures can be attributed to electrical system failures due to low battery.
6. During practice, a robot driving without opponents ought to last at least 6-8 minutes and preferably 10-15 before the battery runs down. Any less, and your design is inefficient and you are reducing the life of the battery. Our batteries have about a 400 charge/discharge cycle life if used in the normal operating range. A robot that eats a battery in 2 minutes has likely cut that life to 100 cycles or less and no amount of charging is going to fix that. It is also going to fail without warning. Check your design by using a current probe or amp clamp to check overall currents while the robot is practicing and check each individual motor current. Keep a list of these currents. If during a competition something seems wrong, a quick check of motor current can identify bent mechanical systems or wheels that have been pushed out of alignment. Currents should be nearly identical from side to side. If they are not, you may have an electrical problem, bent parts or a bad motor.
Hope this helps.