Page 201 - Anatomy of a Robot

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186 CHAPTER SEVEN
What Types of Brakes Exist?
Remember the general definition. Brakes are a method of slowing down (or remaining
in place). This is a function that can be implemented in the following ways:
No brakes Okay, we’ve all had bicycles like this. The truth is, aside from scrap-
ing shoes on the ground, it’s possible to slow down just by coasting to a stop. This
does not work real well going downhill, but it works just fine on level ground and
going uphill. Even if the robot has great disk brakes, the control software should
be smart enough to recognize when they don’t need to be used. This sort of brak-
ing action consumes very little energy, but it requires rather sophisticated software.
Here’s an example of the type of software action that could save energy. Suppose
the robot must move 4 feet. Suppose from experience the robot knows it will coast
2 feet once the robot is at top speed and the motor is turned off. It’s likely that the
least energy-expending method of moving is to get to top speed, move for 2 feet,
turn off the motor, and coast for 2 feet until the robot comes to a stop. Other power
expenditure plans may work better, but certainly little power will be wasted in the
last half of the journey. The motor and the brakes will both be off. One thing is for
sure though. The robot will not complete the move in the minimum amount of
time.
Motor braking Just as a motor can be used to accelerate a robot, so too can it
be used to decelerate. Motors can be used as brakes in a couple of different ways.
Because moving coils of wire through magnetic fields cause a current to flow,
some motors become generators when the rotor is spun around. If the motor coils
are shorted out, then a larger current will flow and the motor will resist the spin-
ning motion on the rotor. By definition, this causes braking. More sophisticated
motor control circuits are available that can brake more effectively by driving the
motor coils in an optimum fashion. In fact, the motor can be partially driven in the
opposite direction. The motor then actively counters the robot’s existing motion.
Pad brakes Regular friction brakes of all sorts are available too. We’ve already
discussed ABS brakes and the various forms of braking actions (manual and auto-
matic). It just makes sense to mention them again here. However, one thing hasn’t
been mentioned before. Brakes require cooling. In the worst case, they dissipate
the entire kinetic energy of the robot. Providing for the cooling of the brake pads
(if they exist) must be part of the design.

TORQUE CONTROL
Much like ABS brakes can prevent spinning wheels from locking up, it makes sense to
prevent wheels from spinning during acceleration when they should be gripping the