Final Design

The final design of the control system of the Hybrid Kapitza Pendulum included an encoder for sensing the angular position of the pendulum, a braided wire and drag chain for connecting the encoder to the microcontroller, a new pendulum arm to minimize the shaking of the pendulum frame, an integrated wiring system connected all sensors and actuators to the microcontroller, a sorbothane base for additional damping of the pendulum base, and a polycarbonate case for safety. A view of how the encoder was mounted to the pendulum is shown in Figure 1.

The pendulum has two input power signals (power and ground) as well as two output signals A and B from two photosensors. All of these cables needed to be connected to our microcontroller for angular sensing. The engineering challenge in this situation was that the encoder would vibrate vigorously along with the pendulum. Hence, the encoder cables needed to be strong to withstand the fatigue of many vibrational cycles. The encoder cable would also be moving along with the servo frame. Consequently, a drag chain would be required to protect the cable from frictional damage while moving as shown in Figure 2. The final design for this wiring included heat shrinks around the encoder cables and a drag chain to protect the cable in Figure 4. The design also included an attachment point to the frame wheel for strain relief in Figure 5. A side view and a back view of the design of the wiring from the encoder to the microcontroller are shown below in Figure 2 and 3 respectively. The implementation of these designs are also shown below.

A new servo arm was required to be designed as the previous servo arm underwent significant deflection during pendulum vibration, reducing the stability of the frame. The old servo arm was made of a thin aluminum sheet which was deemed insufficiently sturdy. The deflection in this arm also caused the front-heavy frame to lean forward, increasing the friction between the frame and the oscillatory motion. The new pendulum arm uses L-brackets for increased strength. A table comparing the moment of inertia of the two arms as well as a performance index (moment of inertia ratio to mass) is shown in Figure 6. Furthermore, a figure of the old servo arm and the new servo arm is shown in Figure 7 and 8 respectively.

After wiring the encoder to the microcontroller, the motors were integrated into the wiring system. An electronic speed controller (ESC) shield was placed on top of the Arduino Due to control the speed of the brushed DC motor. The Due specified a rotational velocity to the ESC through a PWM signal. The ESC then adjusted the voltage to the brushed motor based on the PWM signal. This means that while the closed-loop control code only operated at full speed, it was possible to control the oscillation speed of the pendulum through software if needed in the future. The brushed motor was also equipped with a magnetic encoder, which allowed the system to measure pendulum oscillation speed. Only one signal from the quadrature encoder was used as the direction of the motor was not needed. All digital input pins on the Due could be used for interrupts. All encoder signals made use of this feature to ensure that no quadrature steps were skipped. The servo motor controlling oscillation angle was also controlled through a PWM signal. In order to showcase real-time pendulum feedback and logic states used in the hybrid control algorithm, an LCD screen displayed these variables. The LCD was controlled by the Due through I2C protocol. Through this addition, all motor control and feedback was possible through one sketch in the Due as an integrated wiring circuit was developed as shown in Figure 9.

To ensure the safety of the electronics, all electronic components were contained in a junction box. A double pole double throw (DPDT) emergency stop button was integrated in case the pendulum operations needed to be killed due to a safety issue. Activating this button opened the circuits powering both motors. Each motor was turned on through a rocker switch that lit up blue when powered. Both motor circuits were also equipped with a 5A replaceable fuse in case of an overdraw of current. All wire connections were either soldered or crimped, leaving no exposed wires. Cable glands were used for any wires leaving the junction box. This acted as a strain relief and ensured that no outside forces would damage the wiring within the junction box. Both the Due and LCD screens are mounted using velcro tape. This method was chosen over fasteners as velcro doesn’t loosen to vibration and is easily taken apart for repairability. The junction box was mounted onto a wooden stand through velcro to hold the box up in the vertical position, providing easy access to buttons and an LCD screen. The switches and power inputs were labeled to prevent damage due to user error. Specifically, the servo switch was labeled as #1 because it was important to power the servo motor first before powering on the DC motor. Because the servo motor holds the frame in place, it was important for safety that the DC motor never ran without the servo motor being powered. 

Figure 10 below shows the final system setup.

Sorbothane is a visco-elastic, thermoset, polyurethane material manufactured by Sorbothane, Inc. It absorbs vibrations in the form of kinetic energy and converts it into heat. These unique properties convinced the team that it would be the right option moving forward, as it meant that the material is effective at absorbing vibration. Once the Sorbothane had been placed underneath the aluminum base the pendulum was welded to, there was a significant reduction in vibrations seen while testing the pendulum’s full capabilities. Figure 11 shows a closer view of the Sorbothane base.

The final component of the project was the polycarbonate safety case seen below. This safety case is made out of five 6.35 millimeter polycarbonate panels, connected together with PLA plastic 3D printed three way corner brackets and fasteners to secure all panels together. The case is 0.2921 meters by 0.2921 meters by 0.2985 meters. The bottom is open in order to provide ease of access for maintenance, and the case is secured to the aluminum base of the pendulum through Velcro strips, which have a 110 N strength per 50.8 millimeters. The Velcro is attached at the four corners of the case, so the Velcro has a grip strength of approximately 1.00 kilonewtons, which is enough to withstand impacts in case of any catastrophic events, while also being accessible and easy to detach.