General Robotics

1
General Robotics

• Lab 4 Simple Mechanical Base
• Assigned today, due next week.
• Max Harper
• Some slides borrowed from previous Tas

2
Introduction

• The purpose of this lab is to design a robot
  base that uses feedback control to translate and
  rotate precisely.
• Precise movement is an essential building block
  for later motion planning labs.

3
Challenge Statement

• Upgrade or redesign your mobile base so that it
  can translate and rotate with accuracy.
• Measure the accrued dead-reckoning error without
  feedback.
• Implement feedback control to minimize
  dead-reckoning error.

4
Design

• Many choices for drive
• Tracks
• 2 Wheels
• Skid
• Castors
• Pros and Cons?
• 4 Wheels, fixed
• Friction
• Other?

5
Tank Treads

• 2 treads, each turned by a separate motor.
• Good in loose or steep terrain (Excellent grip
  and high torque).
• Grip means a lot of friction on turns robot
  center drifts.

6
Wheels

• 2 Wheels a proven design
• 2 drive wheels, each driven by a different motor.
• A third point of contact for balance
• Skid
• Castor
• Low rotational friction pros/cons?

7
Skids and Castors

• Skids are easy and reliable cons?
• Castors allow for smoother turning.
• What is a castor?
• Rotates freely around an axis perpendicular to
  the ground.
• Castor cons?

8
Sensors - Encoders

• Shaft of encoder rotates, encoder pulses a fixed
  number of times per revolution.
• Allows the robot to measure the number of
  revolutions each wheel has made.
• Instructions for mounting and using encoders can
  be found through the lab page.

9
Some common problems

• Going straight harder than you think.
• Why isnt my robot going straight by itself?
• Stopping accurately.
• Gear slippage.
• Hey! It worked last night, I swear! what
  changed since then?

10
PID Control

• Proportional
• How fast are we drifting from the center?
• Integral
• How far are we from the centerline?
• Derivative
• Is the drift rate accelerating/decelerating?

11
Grading

• I will specify inches and degrees at run-time.
  Make sure to have a good interface.
• Your grade will be directly proportional to the
  percentage accuracy of your robot.
• i.e. if you are asked to move 10 inches and your
  robot moves 9 inches, you will receive 90
  percent.
• The same goes for turns.

12
Testing

• Translate Write a function that takes as input a
  number (in inches), waits for the start button to
  be pressed, waits five seconds, and then
  accurately drives the robot forward by the
  inputted value in inches (or reverse if the
  inputted number was negative).
• Rotate Write a function that takes as input a
  number (in degrees), waits for the start button
  to be pressed , waits five seconds, and then
  accurately rotates the robot counter-clockwise by
  the inputted value in degrees (or clockwise if
  the inputted number was negative).

13

• Dont hesitate to email me or stop by
• during office hours with questions.
• Thats what were here for!