Demo 2: Pratfall

The second demo introduces laser-cut fabrication of basic mechanical structures. The objective is to build a simple actuated machine which performs a pratfall, i.e., deliberately falls down upon software command.

Please keep your goals simple and commensurate with your skills. If you are new to SolidWorks, please focus on learning to draw parametric parts and choose a simple mechanical principle.

As before, the primary deliverable is a live in-class demo at the start of class on the due date, along with a brief blog entry.

Objectives

  1. Design several laser-cut 6 mm plywood parts to fit together.
  2. Properly mount a hobby servo or DC motor.
  3. Create a dynamic physical process triggered by an actuator.

Deliverables

  1. In-class demo at the start of class on the due date.
  2. Brief blog entry including:
    1. A brief video clip of the mechanism in action.
    2. SolidWorks files as an attachment.
    3. A brief paragraph outlining the chosen operating principles. This prompt already provides the essential absurd justification.
    4. Arduino code. (This is likely to be quite simple.)

Prompts

There are a number of operating principles which might be applied to create this behavior, including:

  1. Articulated support structure. Example: a tripod with one leg formed by the servo; when the servo moves, the triangle of support changes, and when the support shifts out from under the center of mass, the device will tilt.
  2. Changing mass distribution. Example: a rigid structure supporting a servo that can move a ballast mass; when the total center of mass moves outside the support polygon, the device will tilt.
  3. Stored gravitational potential. Example: a servo which pushes a tethered ballast mass off the top, which falls off the edge and pulls over the device.
  4. Stored spring energy. Example: a servo-triggered mousetrap.
  5. Dynamic rocking. Example: a rimless wheel, on which a servo periodically shifts a ballast mass, pumping an oscillation until the device keels over.

Criteria

  1. No glue. Parts can be joined using laser-cuttable features, e.g. press-fit tabs or captive-screw slots.
  2. It’s fine to build a one-shot operation, i.e. something which needs to be manually reset each time.
  3. It’s fine to build off your demo 1 result if you prefer.
  4. The preferred actuation is one or two micro-servos.
  5. If you use a DC motor, you must use a driver circuit and external power supply.
  6. A proper actuator mounting will be rigid and removable. For the micro-servos, the best solution uses the two supplied screws to attach the tabs to pilot holes in the support material. The DC motors have flat faces which can be clamped against the support material using long 2-56 screws through the holes.
  7. No sensors are needed, but can be used if desired.
  8. Other rules are the same as before: live demo in class; cite any sources; properly embed video; make links active; properly format inline code.