PHASE 2

GRASPING Mechanism

OUR GOALS

The goal of phase 2 was to design a grasping mechanism for our assembly. It should be compatible with the lifting mechanism we will design in the next phase. Currently, a sample lifting mechanism will be provided for us.

The main priority of our design is to grasp the payload securely without it slipping or falling. We would like the weight of this subassembly to be kept to a minimum. This will allow the links to move more smoothly and will benefit us during the lifting phase of the project.

Constraints

The mechanism must have at least one fourbar linkage
The mechanism must be actuated by one MG-996R servo.
- - The servo cannot be directly attached to a link that contacts/grips the payload.
The mechanism must mount to the test rig shown in Figure 1 using the two #8 screw holes spaced 1 inch apart, vertically. Both holes should be used to prevent the grasping mechanism from swinging while being raised and lowered.
The payload may be picked up anywhere along the midline of the test rig. The 8 inch dimension shown in Figure 1 is for reference only.
Most pieces should be made of ¼" plywood, cut on the laser cutter. The parts may be glued together using wood glue, as needed.
The servo will interface with the provided Arduino shield. The Arduino must be programmed so that rotating one of the potentiometers closes and opens your claw.
The claw will be tested by attaching it to the apparatus shown in Figure 1.

Testing Procedure:

1. Lower the arm with hand open. The hand should not disturb the object.

2. Grip the payload by rotating a potentiometer.

3. Raise the arm 1.5 inches.

4. Lower the arm.

5. Release the payload by rotating the potentiometer.

6. Raise the arm again. The payload should remain undisturbed.

SKETCHES

Matthew DelRosso

Pros:

Simplistic design that allows for one moving arm
One DOF

Cons:

With only one moving arm it would have clearance issues when making room for the payload; extra steps are needed in order for the claw to clear it

Matthew Heras

Pros:

Two arms grip the payload from two sides simultaneously
Gripped vertically rather than horizontally

Cons:

Due to the lack of gears and sliders, the mechanism has a DOF greater than one which makes it unwieldy for this application

Carl Pantano

Pros:

Allows for two moving arms which will allow for better gripping of the payload
Allows arms to clear the object once it has been released
One DOF

Cons:

Has a different approach timing for each arm which could cause the object to be in contact with one arm before the other.

Luke Harvey

Pros:

Two arms grip the payload from two sides simultaneously
Gripped vertically rather than horizontally

Cons:

Concept as it stands requires gears and sliders in order to function properly (these components are not permitted to be used within this phase)

Narrowing down ideas

While brainstorming, we found that the designs from Carl Pantano and Matthew DelRosso were the most preferable due to their practicality and overall function. Both had one DOF and were deemed capable of grabbing our payload securely. Carl's design was further refined and became our design of choice. It incorporated the grasping concepts from Matthew Heras and Luke Harvey, functioned without gears and sliders, and carried over the simplicity from Mathew DelRosso's design. The GLS, as pictured to the right, is derived from Carl's design.

considerations for final design

We decided that a claw that utilized a horizontal grasping mechanism would be more advantageous for this application. Our mechanism should be compatible with other groups' payloads, and this design is well suited for that.

Due to the large number of links involved with our final design, we found it important to have a thinner set of links to keep the weight of the mechanism down.

GLS based off of Carl's design

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

Links = 6

Pins = 7

Total DOF = 3(6-1) - 2(7)

Total DOF = 1

3d rendering

Open position

assembly

Open position

3d rendering

Closed position

assembly

Closed position

completed Design in action!

When assembled for the first time, the prototype had drawbacks such as friction and play in the pin joints that we did not predict with the 3D model. We were able to adjust this with washers and nuts as needed in order to get desired motion.

Reflection on phase completion

The final materialized assembly accomplished most of the goals set out by the team. It was able to effectively grab the payload with enough force to lift it securely. However, significant clearance issues were found during testing. The mechanism made unwanted collisions with the payload during the descent back to the home position. This caused the payload to shift out of the dropzone, and the arm was unable to return to its original position.2

We found that redesigning a single link could solve the aforementioned clearance issues. In addition, a change in hole tolerance could allow the mechanism to grasp the payload with more force. This could also eliminate any jerky movements, allowing for a more fluid and predictable motion.

In our future designs, we believe a more thorough modeling approach would help to avoid any unforeseen clearance issues and create a more robust mechanism.

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