Servo - February 2009

by Carlos Montesinos and Ricardo Toro

Our philosophy at the Control Systems Laboratory at the University of Illinois is to teach students through interacting and experimenting with their own designs. In our lab, we continuously develop experiments to implement the theoretical concepts learned in class. One of the projects we are currently developing is the Pendulum Aeronautic Robot (PenAeroBot). This system is an inverted pendulum and the idea is to control its angle of inclination, propelled by two motors. The robot uses a digital Proportional-Integral-Derivative (PID) controller to levitate until it reaches the given set point. Figure 1 shows the first prototype and a block diagram of the robot's architecture. Throughout this article, we describe how we built the investigational PenAeroBot and how to implement a digital PID controller. We also show experimental results of the controller's performance and the design of a more flexible PenAeroBot platform for teaching controls systems theory.

The Robot's Structure

The PenAeroBot was developed using standard hobby parts. Table 1 shows a list of all the parts needed to build it. As shown in Figure 1, the robot consists of a digital signal processor (DSP), two DC motors, and two encoders. The DSP is in charge of sampling the encoders, running the PID controller, and manipulating the actuators. The motors are oriented 90 and -90 degrees from the body of the pendulum to maximize the torque exerted on the system. The encoder mounted on the pendulum's base provides inclination feedback while the reference encoder is used to input the desired inclination set point.

Assembling The Robot

Figure 2 shows how to mount the robot. Components (1) and ( 3) are custom-made parts from acrylic. The "L" connectors (2) and ( 4) are standard servo motor brackets. The material and parts may be purchased from US Plastics ( www.usplastic.com) and Lynxmotion ( www.lynxmotion.com), respectively. The brackets have a hole in the center allowing the DC motors to screw in, providing enough support as shown in Figure 3. We used two AstroFlight motors since they are small, light, and powerful. We also used these motors because they are controlled using a custom velocity servo circuit.