Pioneer 3™ Operations Manual

Advanced Robot Control & Operations Software
acceleration and deceleration parameters for both translation and rotation are limited
by FLASH constants. ARCOS initializes these values upon controller startup or reset from
related FLASH parameters. The speed limits, either from FLASH or when changed by SETV
or SETRV commands, take effect on subsequent commands, not on current translation or
rotation. The maximums revert to their FLASH defaults when disconnected from a client.
The orientation commands HEAD (#12), DHEAD (#13), DCHEAD (#22) turn the robot with
respect to its internal dead-reckoned angle to an absolute heading (0-359 degrees),
relative to its immediate heading, or relative to its current heading setpoint (achieved or
last commanded heading), respectively. In general, positive relative heading command
arguments turn the robot in a counterclockwise direction. However, the robot always
turns in the direction that will achieve its heading most efficiently. Accordingly, relativeheading
arguments greater than 179 degrees automatically get reduced to 179 or less
degrees with a concomitant change in direction of rotation.
The E_STOP command #55 or the STOP button that is found on some MOBILEROBOTS
platforms override deceleration and abruptly stop the robot in the shortest distance and
time possible. Accordingly, the robot brakes to zero translation and rotation velocities
with very high deceleration and remains stopped until it receives a subsequent
translation or rotation velocity command from the client or until the STOP button is reset.
(See E_STOP and E_STALL later in this chapter.)
PID Controls
The ARCOS drive servers use a common Proportional-Integral-Derivative (PID) control
system to adjust the PWM pulse width at the motor drivers to smooth the power to the
motors. The motor-duty cycle is 50 microseconds (20 KHz); pulse-width is proportional 0-
500 for 0-100% of the duty cycle.
The ARCOS drive servers recalculate and adjust your robot’s trajectory and speed every
five milliseconds based on feedback from the wheel encoders.
The default PID values for translation and rotation and maximum PWM are stored as
FLASH parameters in your robot’s controller and may be changed. You also may
temporarily update the PID values with the ARCOS client commands #84 through #87.
On-the-fly changes persist until the client disconnects. Translation PID values apply to
independent wheel-velocity mode.
The P-term value Kp increases the overall gain of the system by amplifying the position
error. Large gains will have a tendency to overshoot the velocity goal; small gains will
limit the overshoot but cause the system to become sluggish. We’ve found that a fully
loaded robot works best with a Kp setting of around 15 to 30, whereas a lightly loaded
robot may work best with Kp in the range of 20 to 50.
The D-term Kv provides a PID gain factor that is proportional to the output velocity. It has
the greatest effect on system damping and minimizing oscillations within the drive
system. The term usually is the first to be adjusted if you encounter unsatisfactory drive
response. Typically, we find Kv to work best in the range of 10 to 30 for lightly to heavily
loaded robots, respectively. If your robot starts to vibrate or shutter, reduce Kv.
The I-Term Ki moderates any steady state errors thereby limiting velocity fluctuations
during the course of a move. At rest, your robot will seek to “zero out” any command
position error. Too large of a Ki factor will cause an excessive windup of the motor when
the load changes, such as when climbing over a bump or accelerating to a new speed.
Consequently, we typically use a minimum value for Ki in the range of 0 to 10 for lightly to
heavily loaded robots respectively.
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