Page 89 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Chapter 5

lue braking resistor across the armature. While a braking resistor or other form of
braking will reduce the freewheeling speed of the robot, it will waste power. For this
reason, brakes of any sort must be applied only when needed.

The ideal way to reduce the freewheeling velocity of a drive servo is through the use
of circuitry that directs the back EMF of the motor into the battery. In this way, the
battery recovers some energy while the robot is braking. The common way of doing
this is through the use of a freewheeling diode in the motor control bridge.

I have not found simple freewheeling diodes to provide an adequate amount of
braking in most downhill situations. This is because the back EMF must be greater
than the battery voltage and the effective braking resistance includes both the
battery internal resistance and the motor resistance. Thus, voltage-multiplying
circuits are usually required if this type of braking is to actually be accomplished.

Drag variations
A robot drive or steering servo is usually position commanded. The moment-to-
moment position error of a cruising mobile robot is usually not critical. As long as
the robot’s odometry accurately registers the movements that are actually made, the
position error can be corrected over the long term. For this reason, drag variations
are not usually critical to normal driving.

However, sometimes a robot will be required to stop at a very accurate end position
or heading. One such case might be when docking to a charger. In these cases, the
closing error becomes critical.
Variations in drag caused by different surfaces and payloads can also cause another
significant problem. If a robot’s drive system is tuned for a hard surface, and it finds
itself on a soft surface, then the servos will not have adequate gain to close as accu-
rately.
If the robot stops on such a surface with its drive servo energized, then the position
error may cause the servo to remain stopped but with a significant current being
applied to the motor. This type of situation wastes battery power, and can overheat
the motor. If the robot’s gains are increased for such a surface, it may exhibit insta-
bility when it returns to a hard surface.

One solution to this situation is to ramp up the error proportional gain of the servo as
its speed command reaches zero. When the P-gain has been at its upper limit (typi-
cally an order of magnitude higher than its base value) and long enough for the

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