Day Fifteen

Agenda

1. Rhino overview
2. SolidWorks overview
3. discussion of technical assignment
4. work time: RPi, SW, design, etc.

SolidWorks demo materials:

1. https://github.com/cmuphyscomp/physcomp-solidworks
2. https://github.com/OrigamiRobotics/romibo-mechanism

Proposal for a Technical Assignment

It’s possible to breadboard an illustrative circuit in five minutes, but less possible for mechanical design examples, so the “quick exercise” model we used for circuits doesn’t work as well. My proposal for introducing mechanical design and fabrication is to incorporate it into a technical assignment which will just be evaluated on technical merit. The purpose of the assignment is to learn and demonstrate technical skills. But even this assignment should allow some free choices, and ideally, it would inspire ideas for more human-centric projects.

Learning goals:

1. use of idiomatic mechanical structures, including bushing and ball bearing joints, turntables, hinges, gears, timing belts, motor mounts, sensor mounting, slender members and I-beams
2. basic use of CAD (of any form)
3. laser cutter use on masonite and acrylic
4. single-axis PD control
5. simple behavior: optimization, memory, or history (needs clarification)

The minimum goal is to build upon the one-in-one-out theme by designing a single-axis controlled mechanism. Even a single actuated freedom can have complex behavior by using time effectively. In a conventional controls class, the feedback would normally be taken directly from the axis in order to control the velocity and position of the actuator. However, successful feedback control only requires that the property being sensed be a function of the system state. So for example, a motor could swing around a sonar sensor taking measurements of the local world geometry, and a controller could send currents to the motor which achieve some desired pointing goal such as aiming the sensor at the nearest object. In general, this is much harder than simply sensing a joint, but it paves the way for more interesting applications.

For an example of what can be possible with just a single motor (and possibly passive freedoms), search for “Acrobot” on line and you can find examples like this this acrobot demo or this one.

Procedure

1. Choose a property to measure with a sensor
2. Choose an appropriate mechanical structure with a single actuated axis which can move that sensor
3. Sketch a complete design on paper for the mechanical structure and components.
4. Design review. (Mon, Oct 20)
5. Revise the mechanical design on paper.
6. Sketch the basic electronic design on paper.
7. Write two-sentence description of proposed behavior.
8. Outline proposed control strategy on paper.
9. Design review. (Wed, Oct 22)
10. Draw in CAD any parts which require fabrication.
11. Fabricate mechanism and electronics. (Mon, Oct 27)
12. Implement control strategy in software. (Wed, Oct 29)

The performance of the device will be evaluated subjectively on the design and on how well it achieves the stated behavioral goal. If the performance can be quantified we can incorporate an objective measure.