Page 88 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Closed Loop Controls, Rabbits and Hounds

maximum deceleration is usually specified, beyond which the robot will not attempt
to brake. In this way the robot will slow as much as possible before the impact. If a
mechanical bumper is designed into the front of the robot, it can be made to absorb
any minor impact that cannot be avoided by the control system.

Freewheeling
Perhaps the most difficult control problem for a drive servo is that of going down a
ramp. Any back drivable drive servo will exhibit a freewheeling velocity on a given
ramp. This is the speed at which the robot will roll down the ramp in an unpowered
state. At this speed, the surface drag and internal drag of the servo are equal to the
gravitational force multiplied by the sine of the slope. The freewheeling speed is thus
load dependent.
If a robot attempts to go down a ramp at a speed that is greater than its natural free-
wheeling speed for the given slope, then the servo will remain in the forward driving
quadrant. If the robot attempts to go slower than the freewheeling speed, then the
servo will remain in the braking quadrant. The problem comes when the speed goes
between these two conditions. This condition usually occurs as soon as the robot
moves over the crest of the ramp and needs to brake.

Under such transitions, both the quadrant discontinuity and drive/brake nonlin-
earity will act on the servo. This combination will make it very difficult to achieve
smooth control, and the robot will lurch. Since lurching will throw the robot back
and forth between driving and braking, the instability will often persist. The result
roughly simulates an amphetamine junky after enjoying a double espresso. If the gain
ramping trick described above is not adequate, then it may be necessary to brake.

My dearly departed mother endeared herself to her auto mechanic by driving with
one foot on the gas and the other on the brake. When she wished to go faster or
slower she simply let up on the one while pushing harder on the other. This method
of control, however ill-advised for an automobile, is one way of a robot maintaining
smooth control while driving down ramps.

One sure-fire method of achieving smooth control on down ramps is to intentionally
decrease the freewheeling velocity so that the servo remains in the drive quadrant.
To accomplish this, one can use a mechanical brake or an electrical brake. The
simplest electrical brake for a permanent magnet motor is to simply place a low va-

71