Embedded Motion Control 2011
Under construction, references and figures needed.
Introduction
An important question scientists are eager to answer is whether there exists water on the planet Mars. If water is present, perhaps also life is possible on the red planet. In 1997, the first NASA probe landed on Mars. In January 2004, two Mars exploration rovers landed on the Martian globe [citation needed]. The golf cart-sized rovers were autonomous, robotic dune buggies that carried their own science, power and communications equipment. Since January 2004, several experiments and measurements have been performed which indicate that water had been around on the planet for a while. In August 2005, a third exploration rover was sent to Mars, this rover will join Mars Global Surveyor and Odyssey in their exploration of the planet. In the upcoming years, more inquisitive spacecrafts will follow them to investigate Mars, perhaps solving the mystery of what happened to the planet’s water and preparing for the arrival of the ultimate scientific explorers: humans [8]. In the Dynamics and Control Laboratory (WH -1.13) a reproduction of the Mars exploration rover is present. This reproduction is made out of LEGO, especially using LEGO Mindstorms, as shown in Fig. 1.1. The Mars-rover is equipped with two programmable interfaces (LEGO RCX 2.0), a camera, three light sensors, two motors, two encoders and a temperature sensor.
The LEGO Mars-rover is located on a Mars landscape containing three lakes as shown in Fig. 1.2. Now it has been established that water existed on Mars, the next mission is to find the locations of this water and to discover its properties. In this course this is done by determining the temperature of the water in the three lakes of the landscape. However, no software is present on the programmable interfaces of the Mars-rover. This software has to be designed and developed during the course in order to complete the mission. The earth computer can upload programs to the programmable interfaces of the Mars-rover. The interfaces can send and receive data to and from the earth computer. The programs are written in C using the BrickOS environment.
The course will consist of lectures and workshops which will introduce you in the interesting world of embedded motion control. The mission will be performed by groups of 4 people which will be assigned a group tutor. Each group will have weekly meetings with the tutor regarding questions about the mission. Four weeks after the beginning of the course each group will present their software design to the other groups. At the end of the course a final contest will be held where all groups have to perform the mission. Every group also has to deliver a website-as-report, which describes the complete embedded motion design, including an evaluation of the obtained results.
The remainder of this course guide is as follows. The goal of the mission will be described in Chapter 2. Since only one complete setup exist, each group is given an experimental kit at the beginning of the course of which the contents are described in Chapter 3. The experimental kit can be used for developing. The installation procedure of the necessary software to program the RCX is given in Chapter 4 together with some examples. Chapter 5 describes the use of the library which obtains the camera information and the startup procedure of the earth computer and of the library needed to communicate between the 2 RCXs and between the RCX and the earth computer. Additional information regarding the course and some tips and tricks can be found in Chapter 6. Appendix A and Appendix B contain the C application programming interface (API) and the BrickOS API. BrickOS is an alternative operating system for the LEGO RCX which provides a C development environment for the RCX.
Goal
The general setting of the Mars-mission has been described in the introduction. The overall goal of the Mars-mission can be described as
- Program the Mars-rover in order to find out if there exists water on Mars. If water is found perform measurements to find out its state.
A description which is more applicable to the course and the setup as shown in Fig. 2.1 is
- Write software for the Mars-rover such that it finds the three lakes, measures the temperature of the water and sends the outcome to the earth computer.
This goal can be divided into several subgoals
- Getting familiar with the LEGO working environment.
- Learn the C programming language.
- Design of the software architecture.
- Programming of the mission software.
- Completion of the mission.
At the end of the course the Mars-mission will be performed by all groups in a final contest.
Hardware
In this chapter the different LEGO parts are discussed in more detail.
RCX
The RCX contains a CPU, display, memory, three actuator output ports and three sensor input ports. The output ports (A, B and C) can be operated in three modes: on, off or floating. When an output is turned on, it can run either in a forward or reverse direction. The output power level can be adjusted using pulse width modulation (PWM). The sensor ports (1, 2 and 3) can be used for both passive and active (powered) sensors. What kind of sensor is used has to be told to the RCX by a program. For each sensor, the sensor type and sensor mode have to be configured. The sensor type defines the interaction. The sensor mode determines the interpretation of the sensor values. The LCD can display four-digit signed numbers, single-digit unsigned numbers and special indicators. The RCX can communicate with a computer or another RCX through infrared. The RCX is powered by six AA batteries.
Temperature sensor
The LEGO temperature sensor measures using a thermistor (temperature sensitive resistor), i.e. the resistance of the thermistor changes with temperature. The resistance is measured by a circuit and converted to a temperature. It is a passive analog sensor. The temperature sensor is calibrated from -20° Celsius to +50° Celsius. The RCX can read and display the temperature in either Fahrenheit or Celsius.
Light sensor
The light sensor is an active sensor used to detect the amount of lightness and darkness. When the sensor is pointed at a surface, it reads the amount of reflected light. A light surface or a bright light results in a higher value than a black surface or shadowed object. It can also respond to blinking objects. The light sensor both emits and detects light. Therefore it can measure the amount of light in a room or the reflected amount of light from a surface. The light sensor can be used for line detection, edge detection or light/dark seeking purposes. In this course the light sensor will be used for edge detection.
Rotation sensor
The rotation sensor measures rotation with a resolution of 16 increments per revolution. The sensor uses two light diodes and an encoder disc with 4 slots which results in a quadrature signal with the mentioned resolution of 16 inc/rev. The resolution can be altered by applying a transmission between the motor and the rotation sensor. The maximum working speed of the rotation sensor equals 500 RPM. Note that at very low or very high speeds it is possible that counts are missed.
Motor
The motor has an internal five-stages gear reduction to increase the torque and a weight of 28 g. If no load is attached to the motor, the rotational speed equals 340 RPM and the no-load current equals 3.5 mA [citation needed]. The rotation speed of the motor is proportional to the voltage applied to it. The stall torque (locked motor axle shaft) of the motor equals 5.5 Ncm, the stall current equals 340 mA. Despite of the presence of an internal thermistor, avoid long stall periods in order to prevent overheating of the motor [citation needed].
USB IR tower
The RCX communicates to a computer through IR light. To receive the signals from the RCX, the USB IR tower can be used. For computers which already have an IR port, still the tower has to be used. Most standard IR ports use IrDA for communication, while the RCX uses a simpler proprietary protocol. The IR tower programs can be used to download control programs to the RCX and to upload data to the computer. The IR tower and the RCX must face each other to establish communication.
Connecting wire
The connecting wire can be used to connect the motor to the RCX or to extend the connection wires of the various LEGO parts. No specific orientation of the connectors is required.
Installation
Embedded_Motion_Control/Lego_Mindstorms_Installation