Vorpal Robotics Wiki
Log in

Vorpal Hexapod Activity: Engineering Concept: Repeatability

From Vorpal Robotics Wiki

Vorpal Hexapod Quick Links:

Required Items

  • One or more Vorpal Hexapods with Gamepad equipped with SD card
  • Some small objects to use for obstacles
  • Masking tape or other tape to mark robot starting and ending positions. Preferably a kind of tape you can easily write on.
  • A marker that can write on the tape. For masking tape, most pens and even pencil will work, for other types of tape you may need a Sharpie type marker.

Activity Procedure

  • Set up a very simple obstacle course which includes a start line, just one obstacle (could be anything like an eraser or book) and a finish line.
    • The idea is that Vorpal will have to start at the start line, make just one turn, then end at the finish line.
  • Put Vorpal at the start line in a "Walk 1" mode. Mark where the front two legs touch the floor with tiny pieces of tape.
  • Using RECORD mode, move Vorpal around the obstacle and to the finish line. End the recording when any part of it crosses the finish line.
  • Mark the front two leg positions on the floor with tape. Mark the tape "L1" and "R1" for the left and right sides, run number 1.
  • Now, reset Vorpal back to the starting point with the legs resting on the original two pieces of starting tape. Play back your recording.
  • Mark the new positions of Left and Right leg on the floor. This time mark the tape "L2" and "R2" for left and right run number 2.
  • Repeat a few more times, at least 4 total.
  • What do you notice about the "spread" between the L and R marks at the finish line?
    • Measure the maximum distance between all the L marks, and the same measure for the R marks. How large are these numbers?
  • In Engineering, the concept of "repeatability" is used to measure how accurate a machine can repeat a set of motions or operations (such as cutting operations, 3D printing, etc.)
    • The numbers you measured give you an idea of how much error there is in the set of motions you did to reach the finish line.
    • Smaller numbers are of course better!
  • Discussion: What could you have done differently to make the motion more repeatable?
    • What factors do you think affect repeatability?
    • Brainstorm some ideas, then try it out and see if you beat the numbers in the first try!


  • Trying to move very fast, for example using W4 "Scamper" mode, is a bad idea if you want to move accurately!
  • Trying to minimize the number of turns is probably also a good idea.
  • If the floor is very slippery, that could cause a lot of error in motions. If carpet is available, it might be interesting to note the differences between a tile floor (or large table top) and carpeted floor.


  • Adding more complexity to the motion, for example requiring two turns, is more challenging. How does the more complex track affect accuracy?
  • How does the distanced traveled affect repeatability? Try just a straight run with no turns, and repeat for 4 feet and 8 feet distances (or any other distances you choose).

Optional: Using ScratchX

  • If you program the movements using Scratch, is that better than recorded manual control? Try it and see!