Lifting Mechanism

This phase of the project is dedicated to designing the lifting mechanism which will provide an upward and downward motion needed to move the claw that was designed in the previous phase allowing the payload to be lifted from the ground.

Explicit Design Constraints

Must have at least one fourbar linkage in its design
Can't have an addition of a counterweight to move
Must be driven by a second 180 degree servo motor and activated by a sonar sensor
Have a mechanical advantage and it cannot be created using gears.
Must be supported by the 1” x 2” post on the testing rig.
The payload must be lifted from and returned to a 3”x3” square area centered 8” away from the post
It must be created from 1/4" plywood
The arm should start in the up position with the claw open and wait for the payload to arrive. The arm must suspend the claw high enough to provide easy access to place the payload in front of a sonar sensor.
After the payload is manually placed in the zone the steps should go as followed:

a) Arm moves to down position

b) Claw closes gripping the payload

c) Arm lifts the payload 1-3” off the ground

d) Arm keeps payload suspended for 3 seconds

e) Arm returns payload to the ground

f) Claw opens releasing the payload

g) Arm moves to up position, without disturbing the payload, allowing easy access for a user to remove the payload

Implicit Design Considerations

Claw Constraints
- Operated by one 180 degree MG-996R Servo that can't be directly attached to a link
- Mechanism must mount to test rig
- Payload must be picked up anywhere along midline
- Shouldn't include gears
- Overall want a simple design

Ginger's Design

Pros: Has good mechanical advantage

Fairly easy to design

Easily Attachable to claw already designed

Cons: Limited range of motion

Mary's Design

Pros: Easy to design

Easy to attach claw to

Cons: Limited area to grab from

Might hit payload on way back up

Noah's Design

Pros: Mechanical Advantage is Present

Would not be hard to implement with claw

Cons: Mechanical Advantage may not be high enough

Limited Height to Raise too

Design Direction

The team decided to go with a design similar to Noah and Ginger's design for the lifting mechanism. This decision was made because the lifting mechanism that Noah and Ginger designed would be easy to implement with the claw the team designed in Phase 2, and the lifting mechanism also had good mechanical advantage in order to pick up the payload.

Degrees of Freedom Analysis

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

DOF = 1

∴ The structure is a mechanism.

Graphical Linkage Synthesis

Key

Down Position

k = 7.52

h = 1.52

k/h = Mechanical Advantage = 4.95

Up Position

k = 8.46

h = 2.46

k/h = Mechanical Advantage = 3.44

GLS Considerations

The Graphical Linkage Analysis above is largely based on the first design from the brainstorming section, with a four bar linkage that is attached to a hole in the claw, with a second hole that uses a redundant rocker to add stability to the claw. While both instant centers are highlighted, the one used to calculate mechanical advantage in this design is I24, which is dimensioned in the GLS. Overall, the design chosen translated well from brainstorming to a GLS.

Mechanical Advantage Variables

Sample Calculations

For the mechanism to work, the absolute value of torque being produced by the servo and the mechanical advantage (T_Out) must be larger than the absolute value of torque produced by gravity and the claw (T_Claw).

T_Claw= r x F

r = r x cos(beta) , r x sin(beta,

F = Mass of Claw x g

T_Out = Torque of the Servo x Mechanical Advantage

T_Servo = Servo Lift Power (in this case, 0.11 Newton Meters)

Mechanical Advantage = k/h

The mechanism lifts if the absolute value of the torque of the claw is less than the torque output absolute value.

MATLAB Code

Outputs Below

Torque_estimator.pdf

Torque_estimator (2).pdf

Torque_estimator (1).pdf

Lifting Mechanism In Action

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