Question for the CD graybeards (both figuratively and literally)…
At various times (depending on drive system, robot CoG, and game challenge), our drivers have had a preference for having the drive system speed controllers placed in Brake mode vs. Coast mode. In Brake mode, letting go of the sticks causes the robot to stop fairly quickly, while in Coast mode, the robot rolls to a stop over a sometimes-considerable distance. It’s a preference.
For a robot with 4 drive motors, is there any harm in having one speed controller per side in Coast mode and one in Brake? We tried it briefly this season, and the result was that the robot wouldn’t stop so violently as in 100% Brake mode, but it didn’t roll as far as in 100% Coast mode. My question is, is this a good idea? Is there any risk of damage to the Victors or Jaguars in such an arrangement?
I don’t think it is necessarily a very bad thing, but it will cause some extra battery drain. One motor will be converting the other motors mechanical energy back into electrical energy.
Given the same duty cycle, brake mode and coast mode have different response curves. It is almost like you have them mis-geared together, with one having a higher ratio than the other. Or from another point of view, it is as if you are sending different throttle commands to motors that should be driven in unison.
I have to admit that boiling the math into a forum post is a bit beyond me; There are just too many circular dependencies! What it comes down to is that a coasting motor is floating for the part of the cycle that the braking motor is shorted. It is a similar effect as using a victor and a jaguar in parallel. They provides different effective drive voltages, and this results in a fight.
Shorted but not necessarily braking. In fact, for a black jag at high PWM and low rpm the shorting actually allows the current to keep flowing and providing motive (not braking) torque during the brief PWM “off” portion of the cycle. There’s a thread somewhere that goes into details and explains this.
I’d really love to see a graph of the effect of this hybrid use-case, with feet per second on the X axis and power consumption on the Y axis. Repeat the test for all coast, all brake, and half and half.
We ran exactly the hybrid combination you described in our 2-motors-per-transmission drive train this year, and it worked well all year with no problems. We like the moderate braking it provides without being too severe.