2) Design Process - RA

Size

Initially, the size of the running figure was chosen to be about 20 cm tall like in figure 1 because this automaton was intended to be easily carried around by hand to any place and demonstrate its movements.  A typical smartphone size of about 15 cm is portable with one hand, so the team deemed the figurine size of 20 cm plus several centimeters of space for motors and gear transmission should be portable enough for two hands. Gears and links were dimensioned to make the figure about 20 cm. 

Figure 1. Initial estimate of figurine size

The motor casing was simply designed to be as small as possible just to encase the motor only. A CAD model of the design is shown in Figure 2.

Figure 2. Motor mount box

The figurine stand was designed to be taller than 10 cm to allow the figurine’s leg to move freely in air without any ground contact. A CAD model of the figure mounted on a stand is shown in Figure 3:

Figure 3. Figurine mounted on a stand

Motor

The linkages and gears that needed to be motorized were planned to be made from either plastic or wood, and their sizes were small, ranging from 3 mm to 70 mm. These can easily be lifted with the 12V DC motor provided in class with rated torque at 4.5 kg-cm. Also, it was readily available in class. Due to its adequate power and availability, the team decided to use the motor. 

The motor was placed on the ground below the figurine. The team decided to use only plastic and wood to build the automaton system, so it believed the entire automaton would be more stable when the motor is placed on the ground. The plastic and wood would not be strong enough to raise the relatively heavy motor to any height above the ground.

Lastly, the motor was oriented vertically to reduce the number of gears and thus friction in power transmission.

Figure 4. Bevel gears on the center of figurine.

Figure 4 shows the motor oriented vertically and has a shaft that extends to a bevel gear to transmit power to the figurine limbs. If the motor had been sideways, more gears had to be introduced to transmit power from the motor on the ground to the raised figurine. Another option would have been a belt drive in a sideway orientation, but the team did not have the parts for the belt drive, and was not familiar with the configuration of it for utilization.

Material

The main criterion for material selection was availability. Strength of the material was not considered since the automaton was not expected to experience any significant external stresses. Therefore, wood and plastic that were readily available in school and could be manufactured quickly and easily with laser cutter and 3D printer were chosen as parts materials. Also, these two materials were light in weight, so it was expected that it would not impose significant loads on the motor we used.

Linkage Designs

The key to automaton’s natural arm movement was enabling back and forth movement of linkages with a single direction power input. The motor transmitting power to the arm linkages was intended to provide power in a single direction only. No controller was to be implemented to switch power direction.

One way to achieve the reciprocating movement is using Grashof double rocker linkages, where in four bar linkages, the sum of the lengths of the longest and the shortest link is greater than that of the other two links.

Creating natural leg movement was more complicated. Because the team aimed design the leg hanging in the air where one end of the leg is not attached to any ground, the same four bar link design used for the arm could not be used. We took inspiration from the following videos to implement slider, and complex leg links to our design:

Link 1: https://www.youtube.com/watch?v=Gk5PAmg7oR0

Link 2: https://www.youtube.com/shorts/wtS-GFIEyO8

Figure 5. Arm and Leg Linkage Designs

Click the video below to see the final CAD model of the linkage movement.

Video 1. CAD motion analysis of Running Automaton.