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Classroom Activity: Repeatability

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Activity Info at a Glance
Name: Repeatability
Grade level: middle/high school
Purpose: learn the concept of machine repeatability
Materials: Mimsy, Vorpal Commander on Android device, tape

Description

Students will use the Record/Play feature of Vorpal Commander to explore the concept of repeatability. When a CNC machine or robot carries out a series of pre-recorded actions, how accurate is the playback? The amount that the motion differs from one run to the next is called the repeatability.

Students will use tape to mark a spot on the floor where Mimsy starts. Marking where the backs of the two rear wheels touch the floor is a good method. Then, students will try recording some motions by Mimsy. At first, a simple motion such as simply moving straight forward a couple of feet, then later more complex motions. After recording a given run, students reset Mimsy to the original tape marks then play the recording. A new tape mark is placed on the floor at the end of the playback (the new tape is placed relative to an agreed-upon part of Mimsy, like where the front left wheel contacts the floor. This is repeated several times, with a new piece of tape placed at the end of the run each time. Students then record the distance between the two farthest pieces of tape. This distance is called the repeatability. For example, if the two widest spread pieces of end tape are 3 inches (7.5 cm) apart, then the recording has a repeatability of 3 inches (or 7.5 cm).

Setup

  • Hand out materials to lab teams (Mimsy, Vorpal Commander running on an Android Device, any kind of tape).
  • Teacher explains the concept of repeatability to students.

Conducting the Activity

  • Student teams run a simple test first, with just a short straight line motion, and record data.
  • Student teams may try some more complex recordings, such as a recording involving one turn, then one involving two turns, etc. Results are recorded as before.

Follow Up Assignments

Students will write a lab report summarizing their results. Questions to answer might include:

  1. How do you think different types of movements (straight runs, slight turns, sharp turns) affect repeatability?
  2. What types of tasks might be appropriate for use with pre-recorded actions like these? What limitations are there when using pre-recorded actions?
  3. Do you think the surface on which the robot is running would affect the repeatability? How? What types of surface might have better (smaller error) repeatability, and what kind of surfaces might be worse?
  4. How might sensors be used to improve results over purely pre-recorded motions?

Standards

  NOTE: This section is missing. Please help complete it if you have knowledge of this subject.