Phase III

Objective

Create a lifting apparatus that will raise and lower the grasping mechanism and payload to and from the constraints of the drop zone

Explicit Constraints

Must incorporate at least one fourbar linkage in its design
Have a mechanical advantage in which the output force is greater than the input force
Must lift grasping mechanism where the payload is at least 1" above the platform
The lifting mechanism must be supported by the 1" x 2" post on the testing rig

Implicit Constraints

The arm should move the grasping mechanism in a way as to not strike the payload
Keep servo motor travel between 0º to 170º (it is unlikely that the servo will be able to rotate the horn to the full 180º due to motor design limitation).
Keep grasping mechanism close to post, as the servo motor would do less work this way

Pros: Crank and coupler are almost collinear in down position (good mech. adv.)

Cons: Mech. Adv. may be too weak in up position, design might be improved by putting the crank up top

Pros: Compact design that saves space, easy to construct, relatively light

Cons: Almost a parallelogram offering very little mechanical advantage, May cause grasping mechanism to swing, will not be able to lift claw without gears

Pros: Simple/straight forward design, keeps grasping mechanism straight, does not move payload for initial pickup

Cons: Grasping mechanism may swing when lifted, coupler is lengthy (stress on coupler)

Pros: Allows ample room to position over the payload without hitting it, simple design allows the construction to be easy and light weight.

Cons: The ground link is large and could be heavy, the weight shifts a lot as the linkage moves and could make the mechanism more unstable.

Design Considerations

When we chose our final design for lifting mechanism, our group combined ideas from all members, however, the final decision closely resembles Aaron's design configuration; have the servo motor and crank above of rocker arm, although the grasping mechanism was placed on the opposite side of the payload. The major discussion points/ideas that went into the design of the final lifting mechanism are as follows:

Incorporate Aaron's servo motor placement to avoid awkward linkage placement and give crank ample space to freely rotate (regardless of its length)
Have the lifting mechanism raise/lower the grasping mechanism from the left side (in front of) the payload as to not come into contact with the payload when the grasping mechanism is being lowered

Degrees of Freedom Analysis:

3(linkages - 1) - 2(pins - full sliders) - half sliders

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

Final Decision

Up & Down Positions of the GLS

The image of our Graphical Linkage Synthesis above shows the up and down position of our lifting mechanism. The design is largely reminiscent of Aaron's design, mainly the position of its linkages and the placement of the servo motor. However, after seeing the lifting mechanism' path of motion, we noticed that the grasping mechanism may come into contact with the payload, which may result in a failure of properly being able to actually lift the payload. Thus, we flipped the grasping mechanism and have it hold the payload from the front (left) side to avoid this issue. The servo motor is not expected to rotate its full 180° since the payload reaches it's 1" minimum height well before the servo reaches its maximum angle.