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VII. Conclusions and Future Work

TODO:

• Discuss the power angle problem
• Would we use a curved slider again?
• Friction is bad.
• Future work: stronger gears, milled aluminum walls for slots, allow servo to rotate full 360 degrees, false floor and cosmetic touch-ups, doubling the lid

Ultimately, our prototype functioned how we imagined it and worked. The operation was not as smooth as we would have liked, but the box did open and close, with one motor, using a closed loop mechanism. Not only did we create a functional prototype, we created the mechanism to produce this unique motion by ourselves. We cannot stress enough how proud we are that we designed our own mechanism that achieved the motion we set out to achieve from the beginning of this project. While we would have most likely used a predetermined mechanism if we could have found one to fit our motion, this option was not available. We could not find anything that did what we wanted. Yet, we did not change the motion we wanted. Even though it would have taken much of the time and work out of this project, and we undoubtedly would have produced an exceptional prototype free of gear skips and with minimal frictional effects, we wanted to stay true to this course, Robot Mechanism Design.

In designing our own mechanism, we had to apply many of the topics taught throughout the course, from simple closed four bar mechanisms, to understanding gears, to sliders, to Coriolis acceleration. We went through several designs on how to achieve our motion until we reached our final design which has two sliders with Coriolis accelerations, another slider in the back, rotations throughout, and a gear train driving a two separate mechanism. 

While we succeeded in creating a functional prototype, we did not do it without flaws. Several issues arose during the process that we have mentioned throughout this report, including friction, gear skipping, and the sliding path we chose. We quickly realized friction would be a large issue for us as our mechanism was essentially converting a near horizontal force to push something vertically. To help overcome this, we increased the angle of the rear slider from 90° to roughly 120°. This maintained the lid motion we desired while allowing the box to actually open. The large amounts of friction and geometry of our mechanism also led to the gears being required to transmit much larger amounts of force than we had initially planned for. This did lead to our largest failure, the operation of our lid is not perfectly smooth. The gears do skip at the beginning of the motion where the forces are largest. We have discussed the reasoning and remedies previously, but suffice it to say, this would be the first thing we either redesign (longer gear train to put less strain at each gear for example) or reinforce (a much stiffer and stronger main shaft, a more rigid motor mount). If we did decide to increase the gear train and gear ratio, we would want to use a servo that can go 360° without the space limitation of the large horn that was on the high torque servo we used. This would allow us to have a much higher input speed and would result in less force on each gear. Furthermore, if we wanted to make this prototype into an actual product, we would most likely use metal gears or a much better 3D printer that could give us higher strength and precision. Another manufacturing improvement would be to use a thinner functional wall with much smaller coefficients of friction. We wanted to use aluminum but could not machine it in the time we had. This would allow for cleaner cuts as opposed to the rather rough cuts of the laser cutter on the acrylic, even with sanding. 

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Future Design Changes

Torque

It appears that higher torque would have helped us operate more smoothly. More torque was available: we were not operating the servo at full speed, not for a large range, and we did not use a very high gear ratio. A redesign could raise the servo, or change its long metal horn, to allow for 360 rotation. We could operate the servo at full speed to extract more of its power. And then we could convert this higher power and greater travel into the same ~90 degree crank rotation via a higher gear ratio.

Commercial Bushings

Friction remained a problem through the entire process, including in our final product. Some of this friction was from the curved slider, but some was from our two straight-line sliders (in the expanding crank, and the lid's back-edge). We made our own bushings out of PETG, and we finally tuned our printings and sanded the result to get as close a fit as we could. But a quality commercial linear bearing would have been lower friction. The bearings we were able to obtain were of a low enough quality that we didn't believe they were better than what we were printing (and wouldn't have been directly integrated into the 3D printer parts around them). But higher quality bushings – perhaps on a larger diameter shaft – would have helped.

Metal

Ideally we would have machined the functional walls out of metal, instead of acrylic. We even purchased the aluminum plate to do this, but it would have required CNC milling (in order to achieve a smooth curved path), and we did not have the time to develop this experience. Metal functional walls would have been more rigid, have a lower coefficient of friction, and also could have been thinner, further reducing the friction on the slider pin.

Ideally, all of the gears would have been metal, so that they could handle more torque. It's possible some of the gear skipping is due to the PLA deforming, even with our current low torque.

Gear Skipping

Since gear skipping was our biggest problem in the final build, we would make design changes just to address this. We would switch to metal gears, in case the PLA was deforming. We would increase the stiffness of the drive shaft (better steel or larger) to prevent it from bending. We would redesign the motor mount to attach more rigidly to the floor.

Allowing the servo to rotate the full 360 degrees (by giving clearance for the servo horn, or shortening the servo horn) would have made gear skipping less problematic even when it did happen. In the current design, once a number of teeth have been skipped, the servo horn interferes with the floor and the box will no longer open.

Two Lids

We had explored the idea of making the box with two lids, working as mirror images and parting down the center of a larger box. We have prototypes of this design, and had solved the problem of mirroring the power transmission – having the crank gears directly interacting at the mirror plane, which reverses the direction of rotation in the mirror. This design was ultimately not used in the final build only because it took twice as much time to assemble, twice as much 3D printing time, twice as much plywood, twice as much steel, and would have required twice as much torque. We decided the benefit of demonstrating this trivial power transmission wasn't worth the costs. But in a redesign, with more time and refreshed raw materials, this would be an easy improvement.

Cosmetics

The original design included a false floor, hiding the motor, gears, drive shaft and wiring away from the user. There was nothing preventing this in the final build, but time did not allow us to properly space everything. A redesign would increase the height of the box a little, to allow room for the false floor, and some supports. With this newly-clean interior, we would be able to use the velvet adhesive material we purchased to make the box much nicer on the inside.

The exterior of the box was fine, but a more beautiful product could be achieved with a hardwood outer layer, whether solid or veneer. This could hide the finger joints and allow applying a stain and/or varnish.

Remove The Curved Slider

Many of our challenges stemmed from the choice to use a slider for the front edge of the box. At first it was a right angle, then it became sloped, then it became curved. But throughout the process it was a source of friction and binding. Part of this stems from being unable to buy a commercial solution for a customized curved slider. This meant we had to manufacture our own slider. The final choice of a steel pin in an acrylic slot was the best we found, but still offered considerable friction.

If we were to repeat the design process from the beginning, we might revisit the original ideas of having the lid connected to the box with two links on either end. It's unclear whether a suitable motion can be achieved, but more exploration is warranted.

Final Thoughts

Ultimately, our prototype functioned how we imagined and it worked. The operation was not as smooth as we would have liked, but the box did open and close, with one motor, using a closed loop mechanism.

Not only did we create a functional prototype, we created the mechanism to produce this unique motion by ourselves. We cannot stress enough how proud we are that we designed our own mechanism that achieved the motion we set out to achieve from the beginning of this project. We did not deviate from the proposed motion, even though that would have made the project easier. We wanted to stay true to this course: Robot Mechanism Design.

In designing our own mechanism, we had to apply many of the topics taught throughout the course, from simple closed four bar mechanisms, to understanding gears, to sliders, to Coriolis acceleration. We went through several designs on how to achieve our motion until we reached our final design which has two sliders with Coriolis accelerations, another slider in the back, rotations throughout, and a gear train driving two separate mechanisms.

VIII. Appendix

Arduino Code

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