Arm / Lifting Mechanism

Objective:

Design and produce the Lifting ("Arm) Mechanism to move the Claw Mechanism

Design Constraints:

Mechanism must be actuated by one 180-degree MG-996R servo
Mechanism must mount to the test rig with two #8 screw holes spaced 1 inch apart, vertically
Materials include: 1/4" plywood, wood glue
Must be able to lift the claw with Mechanical Advantage
3 weeks to finish Phase 3

Design Considerations:

Design #1

Pros: easy design

Cons: the servo interfered with the base

Design #2

Pros: robust design

Cons: two four bar linkages and more math involved / pushes the claw outward when raising

Design #3

Pros: robust design / lowers claw into the base for tight maneuvering

Cons: two four bar linkages and more math.

Final Design Chosen was #1 with modifications. The coupler was extended to create an L-shape in order to attach to the claw with 2 screws for rigidity. The design produced the most mechanical advantage while being simple.

Degrees of Freedom Analysis for Design #1:

4 Linkages; 4 Pin Joints; 0 Full Slider Joints; 0 Half Slider Joints

DOF = 3( L - 1 ) - 2( Jp + Jfs ) - Jhs

DOF = 3 (4 - 1) - 2 ( 4 + 0 ) - 0 = 1 --> Mechanism

Graphical Linkage Synthesis (GLS):

Arm in Down Position

Arm in Up Position

Torque Calculations:

Tclaw = r x F

r = [r ∗ cos(β) r ∗ sin(β) 0] & F = [0 -mlift*g 0]

Tout = T servo * (k/h)

Tdifference = Tout + Tclaw

Mechanical Advantage Calculations:

<--- Down Position

Mechanical Advantage

= [ r(crank) / r (rocker pin to COM) ] * [ k / h]

<--- Up Position

<-- Torque MATLAB SCRIPT:

MATLAB Output (Down):

Tclaw Tout Tdiff

0.2347 0.4793 0.714

Motor Can Lift Claw

MATLAB Output (Up):

Tclaw Tout Tdiff

0.1676 0.3421 0.5096

Motor Can Lift Claw

Design Renderings:

Arm in Down Position

Arm in Up Position

Final Arm Images:

Arm in Down Position

Arm in Up Position

Comparison Photos:

Reflection:

After reconfiguring our design once and making a few small changes, the arm moves as intended in its respected positions. One small change we made was offsetting the servo from the base. Our GLS originally had it on top of the base which was not going to work in the real world. If we were to make any more changes during this phase it would be to make the arm more rigid by adding additional support arms connected from the base to the claw.

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