This page (revision-30) was last changed on 2020-07-17 09:18 by Murray Altheim

This page was created on 2019-12-23 06:53 by Murray Altheim

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SR 04
SR04

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At line 13 changed one line
~[The following text and images are from David Anderson's web site, used with permission.]
~[The following text and images are from David Anderson's web site; kindly used with permission.]
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!! Subsystems
! A. PID Controller
A [Proportional Integral Derivative (PID) algorithm|PIDController] is used to control the main drive motors and maneuver the robot. This useful and subtle control method consists of three subsystems:
# __Shaft encoders__: Optical encoders attached directly to the motor shafts are used to accurately measure the speed of the motors (and by inference, the position of the robot).
# __Pulse Width Modulation__: Hardware timer interrupt generators in the HC6811 chip are used to generate two PWM sign \\ als that control the two L293 H-Bridges, which drive the main motors.
# __PID__: The PID algorithm itself runs in the 20 Hz sensor loop. It samples the shaft encoder variables to determine the current motor speeds, compares these to the requested motor speeds, and adjusts the PWM values up or down to get sampled and requested values to match.
The inputs to the PID control sub-system are the global variables, __velocity__, and __rotation__. Velocity is a signed value that represents the speed at the center of the robot, and rotation is a signed value that represents the difference in velocity between the two wheels. The PID controller reads these values, calculates the proper encoder counts per second for each wheel, and attempts to slew the motors toward the requested values, 20 times per second.
This method of closed loop control has a number of advantages. The velocity of the robot becomes independent of battery voltage, as the PID controller will increase the pulse width to make up for the sagging supply. It is also independent of load, so the robot can move very slowly and still climb over objects and maintain constant speed on an incline. The ability to control the wheels smoothly over a wide speed range translates into the ability to precisely maneuver the robot. The PID controller also provides stability for dead-reckoning algorithms.
! B. Odometry
The encoder counts returned from the [optical shaft encoders|MotorEncoder] mounted on the drive [motors] are also used to track the position of the robot relative to its position when last reset. These are maintained as a set of global variables that are updated at the 20 Hz sensor loop rate.
__X_position__ and __Y_position__ are the Cartesian co-ordinates in inches, and __Theta __is the rotation of the 'bot around its center in degrees. These data provide the "sensor input" for the dead-reckoning behaviors.
! C. Telemetry
A pair of "Lynx" radio tx/rcv modules is used to implement a telemetry back channel. The robot transmits a continuous stream of 80 character ASCII packets at 2400 baud. These consist of odometer and sensor data, battery voltage, state flags, and so forth. These are received and piped directly to the computer screen, or tee'd off to a file for later perusal. The transmitter on the robot draws only about 10 ma and is useful out to around 50 feet, depending on the space.
%%blockquote
Lately I have added a mode that allows the 'bot to wander freely about a space for three minutes, and then sound an alarm and try to dead-reckon back to the origin. This becomes very interesting if the path back to the origin has been blocked. I have seen the robot actually leave the room, go around the house, re-enter through a different doorway and finally back to the origin. — David Anderson
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~[Further descriptions of the SR04's sensors and behaviours is available from
David Anderson's [SR04|http://www.geology.smu.edu/~dpa-www/robots/sr04/sr04.html] web page.]
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This page (revision-30) was last changed on 2020-07-17 09:18 by Murray AltheimTop