Brake vs. Coast in the Drive

[Jared Russell]

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?

[quinxorin]

That's a very ingenious idea! And to answer your question:

Nope. Every speed controller is its own module, and what one does has nothing to do with the others. There is no additional risk of damage.

[Ether]

Originally Posted by quinxorin

what one does has nothing to do with the others.

That's not entirely true. They are all connected to the same vehicle, so putting 2 on Coast puts more electrical load on the 2 on Brake.

But my guess is that they are designed to handle it.

Are the 4 motors being used as 1 motor per wheel (such as mecanum), or 2 motors to control both sides of the robot via a gearbox (like most 6WDs)?

I'm assuming the former, but you can never be sure.

To add on to this discussion: What are the advantages and disadvantages for each, with different drive trains?

[EricVanWyk]

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.

[Jared Russell]

Originally Posted by EricVanWyk

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.

I did not realize that the speed controllers brake during every PWM cycle. I had assumed that they were only shorted when the signal was at/near the "neutral" pulse width.

[Ether]

Originally Posted by Jared341

I did not realize that the speed controllers brake during every PWM cycle. I had assumed that they were only shorted when the signal was at/near the "neutral" pulse width.

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.

link:
http://www.chiefdelphi.com/forums/at...2&d=1295820836

[EricVanWyk]

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.

[jspatz1]

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.

[Hawiian Cadder]

We did this in 2011 with black jags on CAN bus with no ill effects, our motors were 2 cims per side in tough-boxes(2 motors per gearbox) chained at 14.4 fps.

Originally Posted by jspatz1

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.

Would you do it again?

What were the advantages/disadvantages of it?

Did it do exactly what you want, or were there some things you wished it could do, but didn't? If so, what?

Did it put any strain on the motors? By that, I mean did your motors get broken/overheated really quickly?

Thanks!

[jspatz1]

Originally Posted by SuperNerd256

Would you do it again?

What were the advantages/disadvantages of it?

Did it do exactly what you want, or were there some things you wished it could do, but didn't? If so, what?

Did it put any strain on the motors? By that, I mean did your motors get broken/overheated really quickly?

Thanks!

No ill effects. Moderated the braking exactly as we wished. Motors ran cool, no problems. Would do again.

Originally Posted by jspatz1

No ill effects. Moderated the braking exactly as we wished. Motors ran cool, no problems. Would do again.

Thanks!