7.7 - Conclusions & Future Work - Bottle Opener

Conclusion

Ultimately, our team did accomplish the objective. Our bottle opener was able to open 3 bottles of Jarritos during our testing, but not without a few setbacks. Our first setback was due to a misunderstanding of the electrical theory of DC motors. When our motor wasn't connected to the linkage, it would spin at the expected RPM. However, when we connected the motor to our system, the linkage would not move. The weight of the linkages caused a small torque that was well below the 18 kg•cm stall-torque rating of the FIT0186 motor we had sourced, so the system should have worked. This turned out to be because as the load increases on the motor, back EMF decreases, causing the current draw to increase, but our motor controller topped out at 2A which was less than the motor required to begin spinning. Our second setback was due to a calculation error. Our estimate for the torque required to open a crown-cap soda bottle was significantly lower than the true torque required, so the motor would consistently stall even after being supplied with the correct current, causing the system to get dangerously hot after continuous use. Finally, the last setback was that since we produced just barely enough torque to open the bottles, there was significant snap-through after the bottle cap was pried off. This caused the links to move violently, and occasionally come undone.

Future Work

Improved Power Delivery

Our motor control was a L298N. We selected it because it was able to supply 12V, and because we were able to control the voltage across the terminals with digital logic. This would have allowed us to use PID control or some other control loop to precisely move the motors. For our future designs, we will need to design or purchase a controller that can supply at least 7A of current.

Snap-through Reduction

In our future design, there are two ways we plan to limit snap-through in the system. The first method is by using a cycloidal drive train to significantly increase the torque at the input. Our reason for choosing a cycloidal drive is its excellent ability to produce torque in a small package, as well as its ability to handle significant torque. In addition, small hydraulic linear-velocity limiters will be attached to the output link to ensure that after the bottle is opened the maximum speed the link can move is still safely limited passively. The velocity-limiter will greatly improve the safety of the device and the lifetime of the motor, and will reduce vibrations in the system.

We have made the decision not to select a “stronger” motor. This is because motors that require more than 12V and 7A begin to need high power circuits that are dangerous to work with and outside of our realm of experience.