PHASE 3

LIFTING Mechanism

OUR GOALS

For Phase 3, the lifting mechanism of our mechanized arm needs to be designed and assembled. Our primary goal for this phase was to generate enough torque to lift the weight of the grasping mechanism along with any other groups' payloads. It should also couple securely with the grasping mechanism. Lastly, we wanted the servo to mount to the lifting mechanism without causing interference with the overall motion.

Constraints

The lifting mechanism:
- Must have at least one fourbar linkage in its design
- Shall not require addition of a counterweight to move
- Must be driven by a second 180° servo motor and activated by a sonar sensor
- Must have mechanical advantage and the mechanical advantage must not be created using gears
- Must be supported by the 1” x 2” post on the testing rig shown in Figure 2
The object must be lifted from and returned to a 3”x3” square area centered 8” away from the post as shown in Figure 2
Most pieces should be made of ¼" plywood, cut on the laser cutter. The parts may be glued together using wood glue, as needed.
All wires must be secured so they do not get caught in the mechanisms.
The testing sequence must follow the steps:

Arm starts in the up position with the hand open and waits indefinitely for a part to arrive. The arm must suspend the hand high enough to provide easy access to place the payload in front of sonar sensor.
Manually placing the payload in the 3”x3” zone initiates steps a) – g):

a) Arm moves to down position

b) Hand 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) Hand opens releasing the payload

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

Manually removing the payload from the 3”x3” zone must return the robot back to the state explained in step 1

SKETCHES

Matthew Delrosso

Pros

Easy to change lengths of components for maximum mechanical advantage.

Cons

Requires construction on both sides of the base mount.

Luke harvey

Pros

Some weight is distributed opposite of the grasping mechanism's weight; possibly counteracts weight for easier lifting.

Cons

Could pose too much weight to pull due to awkward lifting angle.
Possibly limited range of motion.

carl pantano

Pros

Would allow for a simple lifting system from one pivot point.
Reduces quantity of links, keeps weight of mechanism low, and is easier to manufacture.

Cons

Slider increases total friction of the mechanism.
Smaller maximum angle due to slider component.

Matthew heras

Pros

Potential to be compact.
Easy to modify linkage lengths based upon future calculations.
Reduced quantity of links keeps weight of mechanism down and allows easier manufacturing.

Cons

Lower position could present clearance issues.

Resulting design considerations

We chose Matthew Heras's design because it had simplistic action, was lightweight, and allowed for quick calculations. Potential modifications to the lengths of the links and angle of the rocker-elbow would be relatively easy. We utilized the model to ensure no internal collisions would be made. In addition, we made sure that there was enough space for a servo and different mounts.

DOF (Degree of Freedom) Analysis of Matthew's Design

Links = 4; Pins = 4
Total DOF = 3*(4 - 1) - 2*(4)
Total DOF = 1

GLS 1

Our first GLS illustrates the motion of the four links, with a focus on possible clearance issues. After developing this GLS, it was apparent that we would need to offset certain aspects of the design in order to avoid collisions.

To solve this, we attached the crank and servo motor on the outside. We designed a base plate (as represented by the orange rectangle on the right in Figure 1) and attached it to the tower. This allowed us to gain more space for our link placements. The rest of the links were attached on the inside between the two base plates.

GLS 2

Our second GLS was made to show the different key lengths that were used in calculations to determine if our mechanism had the lifting torque required to handle the load.

'h' and 'k' are shown on the far right of the GLS. These were modified several times for both the up and down positions of our mechanism in order to ensure an efficient mechanical advantage ratio.

'L' is shown on the bottom, which is the horizontal distance from our ground pins to the center of mass.

'r' and 'β' are shown on dotted pink construction lines which represent the distances and angles to our center of mass in both the up and down positions.

Figure 1: GLS 1

Figure 2: GLS 2

torque_calculator

Linked document of MATLAB code and outputs based on calculations

Mechanical advantage calculations

The following sample equations and variables were used to calculate the mechanical advantage (all dimensions used for calculations should be taken from Figure 2 above)

T_Out= T_Servo * (K/h)

T_Servo= Torque output from servo motor directly

K =Distance from Crank to Pin I24

h = Distance from Rocker to Pin I24

T_Claw = L*F

L = Length from pivot point to center of mass (line of action)

F = g*mass of claw mechanism (in kilograms)

3D RENDERING

ASSEMBLY

Reflection on results and design

The final assembly was found to complete all the goals set out by the team. It was able to lift both the payload and the claw. Steel dowels were placed through the pin joints to create a fluid motion and eliminate asymmetry. We are satisfied with the design and believe incorporating a custom tower post and gears for the next phase will be straightforward.

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