[Pat Roche]

Originally Posted by Al Skierkiewicz

Some of the issues you are asking about are typical of tank tread systems. One that you have not mentioned is the bane of tank drive using treads or locked wheels, i.e.stalled motors.
When using treads on carpet, the side friction (during turns) with the carpet is so high that robot has to hop to break free from the carpet surface. Since the motor/transmission is usually capable of enough torque to overcome the friction, the robot hops when turning. However, during these moves the motors are at, or very near, stall. In a four motor drive using the CIMs, that is potentially over 400 amps. In the IFI control system, this causes the battery voltage to temporarily fall below the drop out voltage of 8 volts, and the controller goes into protect mode disabling all outputs and switching to the backup battery. (We don't have any data yet on what the new controllers might do under these conditions.) A wheeled robot in the same kind of drive could have a little less side friction but the problem remains and they also will hop to some extent in turns. Drivers can minimize the hopping by turning in conjunction with forward or backward motion. The AndyMark style wheels that have no side friction were developed for this reason. Everyone tries tank treads from time to time, but the tradeoffs are usually significant enough that teams go back to wheeled drive systems. Any game in which turning is required will have a minimum number of robots with treads. This year's game is a good example.

Your center of gravity has a very impact on this issue also. Assume you have a four cog setup (three similar to a six wheel drive setup and one tensioner) and your weight is either towards the front cog or the back cog you will run into this issue. However by finding the happy medium between center cog drop and your center of gravity as close to the center/low as possible many track systems level out on that center wheel as they reach. My understanding is that this is due to the tread actually sinking a little. This gives the robot a slightly larger footprint to balance on. Account for that and the length of the belt touching the carpet,its like turning a very short but wide wheel base. Another thing that drivers tend to do is not let the robot settle for a second before trying to turn this often throws off the center of gravity way over to the front/rear cogs. To settle the robot you don't necessarily have to stop the machine but just let up slightly to bring down the robots acceleration and allow the robot to tilt back on plane with the center cogs. However I do agree with Al in saying if it is not necessary for the game why implement it. Tread's are superb and often faster at vertical obstacles(stairs, bumps, etc.) but in a flat floor game it tends to be prohibitive.

Best of luck,

Pat