Lab 8: Forward Kinematics

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16-311 Introduction to Robotics

• Lab 8 Forward Kinematics

Lead TAs Karim Shaban Hanns Tappeiner Present
ation by Karim Shaban
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Basic Objective From Angles to Position

• Build a 2 DOF planar, jointed robot arm with
  angular feedback
• Angular feedback can come from potentiometers
  (absolute) or encoders (incremental).
• Objective Take two inputs, Theta 1 and Theta 2,
  and drive to a given position. Mark the end
  effector with a pen.
• The length of each link is FIXED (VERY IMPORTANT)
• Pen may be lowered and retracted manually or by a
  motor (extra credit).

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Variables

• Theta 1 Zero when the first link is horizontal
  to the right of the origin
• Theta 1 Range is 0 to 180 degrees
• Theta 2 0 when shoulder, elbow, and pen are
  co-linear
• Theta 2 Range is 90 to -90 degrees
• Counterclockwise is positive

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Parameters

• Link 1 3 ¾
• Link 2 2 ½
• Measure these VERY CAREFULLY
• Hint Print out an image of a protractor and make
  some test points on it.

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Grading

• Stage I (48)
• 4 Sets of pre-defined angles.
• 12 points each for mark being within ¼ circle
• 8 points each for mark being within ½ circle
• Stage II (40)
• 4 sets of points chosen by TAs
• Will be multiples of 5 degrees
• 10 points each for inner circle, 5 points for
  outer circle
• Additional (12)
• 5 for having a motorized marker that works
• 5 for a very accurate arm (TA discretion)
• 2 for speed (just dont go too slow)

½
¼
Target
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Measuring Angles

• Servos NOT ALLOWED
• Potentiometers (single turn)
• Variable resistor that can be used to measure
  angles.
• Handy board will give you a value b/w 0 and 255
  corresponding to voltage output
• Theoretically, handy board value versus angle is
  not a linear relationship
• Practically, may be modeled as linear
• Spreadsheet analysis may be useful here
• Encoders (Incremental Feedback)
• Need to know previous angle to determine new
  angle
• Why is this undesirable?
• Both have advantages and drawbacks
• (Hint Most people use potentiometers)
• Perhaps using a combination of both can help
  accuracy

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Tips / Rules

• When lowered, the pen must be perpendicular to
  the plane no bending the pen to get within
  the circles
• Pen mark must be reasonably small no using
  pens/markers with a 1 inch width
• If your potentiometer is not mounted to a Lego
  block, email us.
• Get started early this lab requires a lot of
  testing
• Doing poorly on this lab will make the next one
  VERY difficult.
• Start by modeling the potentiometer value vs
  angle relationship
• At extreme ends, the potentiometers are basically
  useless (they will output ZERO for the first 30
  degrees or so, and some high value for the last
  30 degrees or so).
• Figure out how to deal with this limitation in
  your design
• (Note the useful range is at least 180 degrees
  we arent that mean)
• The website has a LOT of useful information from
  previous years

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Project Hints

• Should not do
• Dont make a program restart from the beginning
  position
• Must not NOT NOT (did I say not enough?)
  restart/reprogram/power cycle your handyboard.
• Starts at the beginning, and continues to each
  new point
• NO zeroing, restarting or reprogramming between
  points
• Should do
• Start Early! This lab will be harder than the
  previous ones
• Make your program as accurate as possible
• We recommend using floats/doubles as variables
• Make your robot able to be attached to a LEGO
  base plate.
• Used for actual test
• Will be helpful for you, too, when testing!