15.2 -- Project Prototype

Kinematic Analysis

During our planning process, we initially considered a simple rectangular motion for the robot, as shown in the profiles and animation. Through some decision-making and redesign, we decided to go with different motion profiles to represent the intended output for our design best.

Fig. 1: Position, Velocity, Acceleration, and Force profiles for rectangular motion

Fig. 2: Buttering Animation in rectangular motion

Fig. 3: Mobility, Kinematic Relations, Velocity and Acceleration, and Force and Torque Analyses

Iteration Documentation

To begin our mechanism design process for the ButterBot, we started with Fig. 4 below. To pick up a small slab of butter, we decided on two essential motions: directly out in a horizontal path so the arm extends toward the butter and scoops it, and directly up vertically to pick up the butter. This gave us an initial motion path of a rectangle, which we iterated on and can be seen through Fig. 4, 5, and 6.

Fig. 4: Initial ButterBot mechanism design

Fig. 5: Second iteration of mechanism design

Fig. 6: Third, more complex iteration of mechanism design

After much deliberation and research, however, we decided to stray away from these complex designs and go with something more simple. Research led us to a simple four bar lift, which can be seen in an example below (Fig. 7). The benefit of this mechanism is that it can scoop out in front and up, which is essentially what we wanted for our robot. Additionally, we liked that this mechanism would allow the butter to be picked up and lifted in the same, horizontal orientation, preventing slip during transportation.

Fig. 7: Four-bar lift inspiration from YouTube video

YouTube video link

Now that we had our desired mechanism for our robot, we starting creating models in CAD and ended up coming up with Fig. 8 and 9, which uses a crank to operate the mechanism, however we would add a dc motor and gear ratios for more torque as actuation for our end product. This design includes four bearings at the joints for smooth motion, and includes five different parts that are 3D printed, including the four main linkages as well as the crank. We intend on attaching this mechanism to the top of our robot, which we will build similar to the robot car we created earlier in the semester, which will take care of the forward motion for us that we need to scoop under the butter. Once the bot scoops under the butter, the actuation would occur to lift up the butter, and the bot would return to the user.

Fig. 8: CAD design of the four-bar lift in lowest position

Fig. 9: CAD design of the four-bar lift in highest position

Physical Prototype

Fig. 10: Physical prototype in lowest position

Fig. 11: Physical prototype in lowest position, backside

Fig. 12: Physical prototype in highest position

With our final prototype design, we decided to go with PLA to 3D print our mechanism, as it is quick to fabricate and snap together with bearings. Based on this prototype, we learned that even with such a thin wedge as the one we printed out, scooping under the butter would be very tedious and possible unsuccessful. To fix this, we decided that going forward, we will design an end effector gripper that would replace this wedge on the link, which will be powered by a small servo to open and close around the butter. This introduces new potential issues that we anticipate, but plan on addressing such as: How far should the gripper close? What size should it be? How much clamping force do we need to prevent slip, but not crush the butter?

Draft Bill of Materials

MG90S Metal-Gear Micro Servo ($8) - small servo motor for the gripper to open and close around the butter block
6–12 V DC motor ($15) - motor that provides torque at low speed. Powers the 4-bar linkage to raise and lower the butter.
4 set of 3-6V DC motors ($10) - same motor and wheel set as used for the previous build 2 robot car.
Elegoo UNO R3 ($17) - used for the logic and as the brain of the bot.
2.4 GHz RC Transmitter + Receiver Kit (FlySky FS-i6, $50) - remote controller and receiver pair to control the robot.
2S 7.4 V Li-ion/LiPo Battery ($18) - rechargeable main power source for all motors, Arduino, and RC electronics.
Li-ion/LiPo Balance Charger ($20) - recharges the 7.4 V battery.
5 V 3 A Buck Converter (LM2596, $7) - voltage regulator that steps the 7.4 V battery down to 5 V for the Arduino, receiver, and servo.
Main Power Switch ($5) - toggle or rocker switch to turn the entire system on or off.
Battery Connector Pair (XT30 or Deans, $7) - quick-disconnect plugs to connect the battery to the circuit.
M2/M3 Screws, Nuts, and Standoffs ($10) - hardware for assembling the printed parts and mounting motors or boards.
Shaft Couplers or Set-Screw Hubs ($8) - connects the 4-bar motor shaft to the printed linkage or gears.

Our total bill with all of these materials is $175, but we already have some of these things from past builds and projects like the Arduino board, the dc motors, and hardware, so we strive to cut down to the $150 budget by our next build.