Final Project Report

Design Process:

Throughout our process, we had many ideas that were scrapped for various reasons. Some of the most significant reasons were difficulty and time restraints. Initially, we wanted to develop a fully functional hand, but our plan was to use only one servo motor to power our mechanism. At one point early in development, we had an idea to connect the four fingers to one axel and have them bend in unison while including a bevel gear that would be attached to the thumb to transfer power to it. This plan proved to be very tedious given the relatively short amount of time we were given, so we continued our brainstorming. We eventually settled on a two-pronged claw rather than a hand, since using only two “fingers” would be easier to power with only one motor. This combined with a slider crank mechanism would allow the claw to open and close to grab various objects.

Since we had the claw’s main function as picking up and dropping objects, we wanted the claw to be able to close and open to specific positions rather than just pressing a single button and having the claw closing and opening completely. To achieve this, we used an Arduino board to program a joystick to control the servo motor. The joystick has two inputs: input values of 0 to 1023 in the X direction and 0 to 1023 in the Y direction. For our claw, we really only needed one of these inputs, so we decided on the Y direction inputs for no particular reason, it just made more sense having the joystick move forwards and backwards to close and open the claw, rather than moving side to side. 

Kinematic Analysis:

For Kinematic Analysis, we will split it into two parts. The first is the palm of the claw, which contains the slider crank mechanism that transfers motion from the motor to the finger. The second analysis will be from the prototype and explain the motion of the finger. 

Analysis of the slider crank mechanism:

After we have analyzed the slider crank mechanism, we can look at the analysis of the double chebyshev slider system. This was already calculated in the prototype, but the work will be put here for ease of reference.

Kinematic Analysis of Finger Mechanism:

In conclusion: these two systems can be related to each other by understanding that lever D in the slider crank is link A in the finger mechanism. Thus angle 4 in the slider crank is angle 2 in the finger mechanism and omega 4 in the slider crank is omega 2 in the finger mechanism. The Mechanical Advantage is graphed below:

Implementation:

We started with link manufacturing. Using laser cutting, we cut basic link shapes using baltic birch wood. Afterwards, assembly was quite tedious, though it wasn’t very difficult after we had already assembled the links in Solidworks. We used basic 3mm screws along with nuts and low friction washers to keep the links in place. The low friction washers were used to minimize friction between the links so that the single servo motor wouldn’t have to produce extra torque to move the assembly. After assembling the two fingers, we attached the ground links to a base plate that would have the slider crank built into it and also hold the servo motor. The motor would then move within a specified range to open and close the claw using a single link that connected both fingers to the motor.

For electronics and circuitry, we used a simple arduino board. As stated earlier, the arduino was programmed to allow a joystick to control the motor within a specified range of 180 degrees. We used the Y input values of 0 to 1023 of the joystick to control the claws position. Using the Arduino coding application, we design the program to take the input values, depending on the joystick position, and give us an output value between 0 and 180, corresponding to an angular position that the servo would move to.

Arduino Code that we used^

Final Demonstration:

As can be seen, a major issue was the backwards bending of the finger digits. This was caused by the mechanism going beyond it prescribed angle and locking out. Additionally, due to the low mechanical advantage, the friction of the mechanism was a significant factor. This affected the opening motion more than the closing.

Conclusion and Future Works:

In the end we accomplished our primary goal of making a grasping mechanism capable of holding onto objects. However, the extra goal of giving the mechanism the form of a human hand proved to be too ambitious in the limited time available for the RMD final project. 

We learned several important concepts whilst working that will be useful in our careers as engineers. The most impactful on this project specifically was the method of turning simple circular motion into a complex velocity profile through the manipulation of linkages. Without the double chebyshev system that made up the claws fingers, it would be impossible to make a swinging motion with just one motor input. Another important lesson learned was the need to conceptualize not just the shape of a mechanism, but also the forces and motion involved. The linkage system created greatly amplified velocity at the cost of torque, which needed to be accounted for in motor selection. This ended up being the fatal flaw of the mechanism. The mechanical advantage was so low that the friction of screws stalled out the mechanism. 

The members of team fifteen would like to thank our TA Connor Hennig for his recommendation of chebyshev linkages for the finger mechanism. These proved vital in creating the velocity profile reminiscent of a human finger.