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IK Target Classifier

Run the IK Target Classifier MicroSim Fullscreen

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Description

Move a target around a two-link arm and see the ring of positions the tip can reach, with the elbow-up and elbow-down arms that reach it. Then classify eight targets as having two solutions, one solution, or being too far or too close to reach. Used in Chapter 12.

The links are 0.116 m and 0.135 m long, the lengths of the SO-101's upper arm and forearm. The tip can be no farther from the shoulder than l1 + l2 = 0.251 m (the arm straight) and no nearer than l2 - l1 = 0.019 m (the arm folded back). The distance of a target from the shoulder is r, the square root of x² + z².

Lesson Plan

Grade level: Middle school and up (age 12+). Time: 10 to 15 minutes.

  1. Explore. Drag the target, or use the two sliders. Where are two arms drawn? Where is there only one? Move the target to the outer circle and then close to the shoulder, and watch the arm straighten and fold.
  2. Find the rule. The only thing that matters is r, the distance from the shoulder to the target. Compare it with 0.019 m and 0.251 m.
  3. Eight targets. Choose Eight targets in the menu. For each target, work out r, then press one of the four buttons. Pressing a button commits your answer. You get one attempt at each, and 7 of 8 is mastery.
  4. Think about it. A real arm adds a rule of its own: a pose outside a joint's limits is not a solution. Which of the two poses would you choose if the arm must not swing its elbow far?

The picture is a schematic, drawn with the shared robot-arm drawing library. The small zoom box appears when the target is within 0.05 m of the shoulder, where the inner circle is too small to see clearly.

Specification

The learning objective, evidence of mastery and content for this MicroSim are specified in Chapter 12: Kinematics: Where Is the Hand and How Do I Get There.