What Is Brake/Coast Mode?

I was surprised to find out that our Spark controllers were capable of being in a Brake/Coast mode and even more surprised when I didn’t know what that meant. I did some research and found out that it’s a preset mode through a small button on the actual controller, but I couldn’t quite figure out what its actual use is. It sounds like it’d mostly be used for motors on the drivetrain, but what exactly does it do and what other applications are there?

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Brake mode takes your brushed dc electric motor and turns it into an electric generator, which takes the kinetic energy from the physical system attached to the motor and converts it to electric energy into the circuit.

Depending on the year, This year I’m not sure I’d use brake mode in my drive train, it makes stopping from full more difficult to control.
EDIT - definitely wouldn’t use it the drive train of on a tall robot with higher center of gravity and grippy wheels. that’s a recipe for disaster.

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Coast mode allows the motor to “coast” or “idle” when power is no longer applied. This means running the motor then cutting power output to it will not stop “on a dime”, but will instead continue to spin by inertia.

Brake mode does the opposite. What it won’t do, however, is completely offset gravitational forces. For example, using it on an elevator will make it drop more slowly, but won’t prevent it from dropping at all (depending on gear ratios of course).


Brake mode senses the motor moving and applies power to keep the motor in place, It wont hold you up in place of a ratchet but it might hold a light intake with the proper gearing in place. Coast mode is the opposite, it allows the motor to move with out applying power to hold it in place.

To clarify, brake mode isn’t an “active” feature. It uses the physical properties of the motor and back-emf to naturally resist motion.

Physically you can experiment with this by attaching a wheel to a CIM motor, and spinning the wheel. Brake mode is similar to shorting the wires (touching them together) of the CIM motor; the wheel stops faster. Coast mode is similar to leaving the wires open (unconnected to anything).

We switched the drive motor controllers in our robot to Brake last night, and our driver seems to be fine with it. He has a year of driving experience, though.

Try it both ways, for a while, and see which works best for you, overall.

Brake mode quits working when the robot is disabled, doesn’t it?

That’s what I meant, thank you for correcting me

Last time I experimented with it was with the old Victor 884s back in 2008… These were set with a jumper. Pushing the robot in brake mode was a pain, and the back-emf powered the Victors to turn on.
I don’t have a spark to experiment with, from the manual (REV-11-1200-UM-00):

When not driving the motor, the SPARK will short the motor terminals to dissipate electrical energy, effectively braking
the motor. Alternatively, the SPARK can be put in a Coast Mode which allows the motor to spin down at its own rate.
Press and release the MODE button to toggle between brake and coast mode. When in Brake Mode (default), the Status
LED will display a solid or blinking blue color. When in Coast Mode, the Status LED will display a solid or blinking yellow
color. See section 2.6 STATUS LED for more information.
This mode is saved in memory and persists through a power cycle.

I think it depends on which motor controller.


The only motor controller that I have seen this behavior with are the sparks, which is why I won’t use them. All of the other motor controllers I’ve used (various Talons and Victors) brake mode persists when the robot is disabled/e-stopped or the pwm cable comes loose.

Note you can set one motor controller per side to brake mode and the other to coast mode for something in between having no braking force and having maximum braking force. We did that with our robot from the game that shall not be mentioned (RR) to eliminate the tipping problem but still have some braking action to improve the autonomous repeat-ability.

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Thanks for the info, that I’m too lazy to look up.


Some background on what’s going on w.r.t. brake/coast mode inside the motor controller.

It involves how the transistors of the motor controller are configured when commanded to idle (zero) and/or disabled. They’re either configured for slow or fast decay of the motor current. Slow decay is brake mode and fast decay is coast mode.

In brake mode the bottom transistors of the H-Bridge turn on, virtually shorting the motor leads together (the current circulated through the motor as if the leads were directly connected together — by following a path from one lead, through the transistor to ground, back through the other transistor, and through the other lead). In coast mode, the current isn’t recirculated and instead the opposite pair of transistors are turned on, and current is dumped back in the battery.

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