\ 16-311 Lab 4: Low Level Mobile Base Control
 
Lab 4: Low Level Mobile Base Control

Lead TAs: Michael Edelson, Karim Shaban,
Igor Avramovic

Lab 4 Demo Times

Introduction:

The purpose of this lab is to design a feedback controlled robot base that will be used in subsequent motion planning labs. PowerPoint presentation

Challenge Statement:

Step 1:
Upgrade or redesign your mobile base so that it can translate and rotate with accuracy.

Step 2:
Measure the accrued dead-reckoning error (do not use feedback)

  • Record the location of the point of contact for each wheel. Drive the robot forward 18 inches, drive in reverse 18 inches. Repeat forward and reverse nine more times. At the end of the 10th lap, stop the robot and record the point of contact for each wheel. Report what happens.
  • Record the location of the point of contact for each wheel. Drive the robot forward 12 inches, turn ninety degrees to the left, repeat three more times to make a square. Drive around the square five times. Record the location of the point of contact for each wheel. Report what happens.

Step 3:
Implement some form of feedback to minimize dead-reckoning error.

  • 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).

Evaluation:

Your grade will be directly determined by 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 a 90.

Design Tips:

  • Make sure your robot really goes in a straight line because you will need this for future labs
  • Avoid a four-wheeled base to minimize errors from slippage when turning. Two drive wheels and a single castor for balance works well.
  • Keep your encoder as close as possible to the actual wheel in the gear train. This will minimize errors between the wheel and the encoder.
  • When testing, check the grading sheet to find out what values could be asked. Test either all possible values, or a decent sampling of those values, for both translation and rotation.
  • Have your encoder spinning faster than the wheel by as much as possible. This will reduce the amount the robot moves for each encoder signal.
  • Encoder info:
    • Click for info (note that you should hot glue over the wires to the encoder to help prevent leads from breaking off.)
    • Click here for a diagram
    • Click here catalogue info (Part number 3315C-1-006-ND)
  • Tips from Chau
  • Using a PID-based Technique For Competitive Odometry and Dead-Reckoning. This is a great page for those interested in learning more about the PID control scheme mentioned in class.
  • Make sure your robot can go backwards as well as forwards with the same accuracy. This is usually preferable to turning 180 degrees if you need to turn back (less error).
  • The encoders we gave out are **analog** encoders. At least one group tried to use the digital encoder libraries for the Handyboard with not-so-successful results. For those who haven't noticed, analog encoder libraries are available for the handyboard and are linked off the Handybaord web site (and of course our links page).
  • Lab 4 Grading Sheet