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Design Realization lecture 21

Design Realization lecture 21. John Canny 11/4/03. Last time. Mechanics Physics revisited Electric motor characteristics. Next time. Assignment review/crit. Please bring a graphic of your board design and schematic. . This time. Some more physics: Bending and stretching

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Design Realization lecture 21

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  1. Design Realization lecture 21 John Canny 11/4/03

  2. Last time • Mechanics • Physics revisited • Electric motor characteristics

  3. Next time • Assignment review/crit. Please bring a graphic of your board design and schematic.

  4. This time • Some more physics: • Bending and stretching • Construction methods: • Molding • Welding • Structural components • Modular systems

  5. y x Bending: moment of inertia again • The 2D moment of inertia about the x-axis is • For a rectangle: b h

  6. M r Bending of a beam • We can make a beam bend by applying a moment (torque) to its end. The radius of curvature satisfies: which isfor a rectangular beam.E is the elastic (Young’s) modulus. y z

  7. L t M r Beam “spring” • The vertical displacement of the tip t is approximately t  L2/2r • The bending momentdue to force F at tip is M=FL • The spring constant atthe tip is k = F/t which is

  8. Beam stretching • On the other hand the spring constant for stretching a rectangular beam is: • Compared with the bendingspring constant which is: • The ratio is h2/6L2. Assuming L >> h, the beam is much stiffer in stretching than in bending.

  9. I-beams and honeycomb • Bending stiffness can be increased by moving material outward (h3 term). • This is the principle of I-beams and honeycomb.

  10. Mass-spring systems • A mass at the end of a spring can vibrate with a natural frequency which is

  11. Example • Vibration in linear actuators: • Natural frequency of the cantilever, ¼” Aluminum, is about 100 Hz (!)

  12. Example • Diagonal bracing: • Bending becomes a streching mode. Natural frequency increases to over 1 kHz

  13. Vibration in plastics • Metals have relatively low loss when deformed. Vibration is often an issue. • Plastics have much higher loss, so vibrational energy dissipates much faster.

  14. Making shapes: sculpting • Molding kits are available from Tap Plastics and others. • Two part epoxy “magic-sculp” provides a clay-like consistency for about two hours, then sets to a hard plastic model. • The model can still be worked when hard by cutting or sanding.

  15. Making shapes: molding • From a plastic, wood, or metal model, a negative mold can be cast from a flexible plastic such as silicone or urethane.

  16. Making shapes: molding • From the negative mold, copies can be cast. The casting materials are typically urethanes or polyester. Can be clear or colored.

  17. Attaching shapes: Gluing • For plastics, glues are available that closely approximate the strength of the original plastic. • This generally gives best results. Excessive strength in a joint can create strain in the material near the joint.

  18. Attaching shapes: Gluing • Many glues are available for wood. At the high end, epoxies are stronger than the wood itself. • Woodworking “yellow glues” provide very good performance as well and have similar strength to the wood being joined. • Dowels or dovetailing can be used to increase the contact surface – but may not necessarily increase strength.

  19. Welding • Welding is joining pieces of similar metal using molten material of the same type. • The most common joining weld is a “fillet weld”

  20. Welding • The size of the weld adds strength. • The shape of the weld relieves stress that can occur at sharp concavities in a part.

  21. Arc Welding • Creates a thick weld using flux/wire which is part of the electrode:

  22. Oxy-Acetylene Welding • Oxygen-Acetylene torch creates heat, flux/wire is fed separately: • Aside: Oxy-acetylene often used for cutting

  23. MIG (Metal Inert Gas) Welding • Aluminum oxidizes instantly in air and requires an inert gas during welding. MIG welding uses an arc surrounding by gas flow (e.g. Argon).

  24. Spot welding • Normally used to join sheet material. • Sheets are placed together and very high current passed through the join.

  25. Truss systems Trussworks Inc.

  26. Truss systems • Come in either aluminum or steel, 2, 3, 4-leg. • Fast-connectscrew connectionsbetween elements.

  27. Modular construction systems • Can be used to build artifacts from linear pieces.

  28. Modular construction systems • “T-slot” systems comprise extruded aluminum beams that are joined using standard fasteners. Examples of “Minitec” framing:

  29. Modular construction systems • Joining Minitec elements:

  30. Modular construction systems • Minitec includes active elements (motorized motion stages):

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