Construction and Control of an Inverted Pendulum

Abstract: The aim of this master thesis is to develop a laboratory process that Saab Ericsson Space AB (SE) can use to demonstrate their fault tolerant computer systems at exhibitions. The process is an inverted pendulum shaped as a rocket to symbolize that the company develops fault tolerant control systems for satellites and carrier rockets e.g. for Ariane 5.

The inverted pendulum is a classical control problem and our task is to develop the mechanical equipment and to control it. The pendulum is made in two dimensions instead of the common one-dimension version. In order to make the model of the pendulum simpler a universal joint was used. That means that the pendulum falls in two separate directions at the same time. This solution makes it possible to split the controller into two and to get the measurements of the fall angles through potentiometers on the axes of the joint. The angle velocity is computed from the angle using an analog filter board.

To move the pendulum, a cross table consisting of two linear actuators is used. The cross table is driven by two step motors and can move the pendulum with smooth motions. The simulations of the system were done in a modelling tool called Modelica. A program written in C-code was made to control the complete system. The control loop is built upon state-space theory. To make the computer and the mechanical process communicate, an I/O-board was developed by SE.

To compare the computer and a person it will also be possible in the future to control the pendulum with a joystick. The mind of a human being works with a fluency around 10 Hz. This can then be compared with the computer's control frequency of 200 Hz.