Phase 3 - Lifting Mechanism

Overview

This phase of the involved creating a lifting mechanism ("arm") to lift the grasping mechanism and payload that were designed in the previous two phases. This phase is incrementally more difficult than Phase 2 since it requires you to perform calculations to ensure your lifting mechanism is designed with enough mechanical advantage that that the provided servo can actuate it. Degree of freedom analysis, graphical linkage synthesis, instant centers, and mechanical advantage are all concepts that are integrated into this phase.

Constraints

• The design must have at least one four bar linkage. 

• The mechanism must be driven by one 180 degree MG-996R servo and activated by a sonar sensor. 

• The lifting mechanism must not have a counterweight in order to move. 

• The lifting mechanism must have mechanical advantage that is not provided through gears. 

• The lifting mechanism must be supported by the 1" x 2" post on the testing rig. 

• 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 your mechanisms. 

• The testing sequence must follow the steps:

  1. 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.

  2. Manually placing the payload in the 3”x3” zone initiates steps a) – g) 

     • Arm moves to down position 

     • Hand closes gripping the payload 

     • Arm lifts the payload 1-3” off the ground 

     • Arm keeps payload suspended for 3 seconds 

     • Arm returns payload to the ground 

     • Hand opens releasing the payload 

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

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

• Lifting mechanism must be operational by Friday, April 7th, 2023. 

Sketches

• James's design was quite simple for what the lifting mechanism is required to accomplish. It had the servo bracket located at the base of the test rig post, which would allow the lifting mechanism to be easily transferred to the phase 4 (the team would not need to rebuild the post once it gets taken away). The only major flaw with James's design is that  his design incorporated a 3D printed joint that connected the lifting mechanism to the grasping mechanism, similar to the one present on the test rig in phase 2. The team deemed this unneeded and therefore unnecessary weight, which needs to be taken into heavy consideration in this phase. 

• Paul's design was very similar to James's design. It also featured the servo bracket at the bottom of the post, which provided the same advantages as James's design. It was also very simple, which would be an advantage during modeling and assembly. The main difference between James's and Paul's design was how they designed the end of the rocker, where the grasping mechanism would be mounted. Paul omitted the 3D printed joint, which would help reduce the weight of the lifting mechanism. However, it should be noted that, because his mounting point was angled downward, the grasping mechanism would be closer to the ground than James's design, meaning that the lifting mechanism would technically have to lift the claw higher to reach the required lift height.  

• Andrew's design was definitely the most unique amongst the four designs, and it was very sleek looking in terms of appearances. However, it did not appear to be the most efficient design when compared to James's and Paul's designs. For one, his servo bracket was screwed into the middle of the test rig post, meaning that, during phase 4, this post would need to be redesigned, cut, and assembled. His design also featured an additional link that ran parallel to the rocker that, while intended to increase lift stability, might backfire and cause unnecessary stress for the servo by creating unnecessary weight. Lastly, Andrew incorporated the same 3D printed joint that James's design featured, which was previously established to be an inefficient means of connecting the grasping mechanism to the lifting mechanism. 

• Jorge's design had aspects that appeared to blend some of the features of previous member's designs. For example, his servo bracket was mounted at a similar location to Andrew's. However, the way he designed his four bar linkage is more similar to that of James and Paul. By this point, the entire group was leaning towards James and Paul's four bar linkage design, so Jorge's incorporation of this feature into his design was further encouragement for the group to continue with it. However by this point we were also all on the same page in regards to leaving the servo bracket at the bottom of the post, so because of this we considered this to be a disadvantage of Jorge's design. 

Decision Matrix

Upon doing the decision matrix we decided that James and Paul were quite close to having the best design, which is reasonable considering how similar their designs were to one another. However, the biggest design difference between the two of them was how they approached the mounting point to the grasping mechanism. By taking the best aspects of both of them, the team would be able to create the best design possible. We decided to go with a mounting point that both was bent upwards to decrease required lift length and eliminated the 3D printed mount to decrease weight. Besides this design difference, the design the team decided to continue with was all that James and Paul's designs had in common. 

Degrees of Freedom Analysis

Graphical Linkage Synthesis (GLS)

Mechanical Advantage Calculations

Tinput= Given Torque of Servo = 0.11 Nm

Required Torque= FClaw × r

FClaw= weight of claw 

r = measured distance from the claw’s center of mass and the point of rotation  

Toutput=Tinput*(H/K)

Toutput  must be greater than FClaw × r 

Mechanical Advantage is generated as the ratio of (H/K) becomes larger

Mechanical advantage is the ratio of torques

MAT-LAB Code/Calculations

GLS w/ Instant Centers in the Lowered & Raised Positions:

MAT-LAB Outputs:

Comparison Photos

Video

Reflection

Overall, the lifting mechanism worked very well. It was able to lift all seven payloads with minimal issue. The sonar did have some problems detecting some of the payloads, particularly the two "hourglass" designs which had little mass at the elevation of the sonar's line of sight. However, some strenuous fine-tuning and repositioning allowed the sensor to detect all of the payloads. Determining the ideal location to permanently mount the sonar on the test bed in phase 4 will likely reduce the issue.  

The speed of the lifting mechanism was another aspect of operation that required some fine-tuning. It was initially quite fast, which caused the lifting mechanism to "slam" the payload into the test rig when it was setting it down. The team tried to slow it down by changing a part of the code, but we initially made it a bit too slow, which made arm traversal a bit wobbly and inconsistent. After some fine tuning, we reached a happy medium that satisfied stability and gentleness.