A Didactical Mobile Robot, Technical Solutions and Experiments Mircea Niţulescu Automatic Control Dept., University of Craiova, Faculty of Automation, Computers and Electronics, 5 Lapus, RO - 1100, Craiova, (ROMANIA) E-mail: nitulescu@robotics.ucv.ro Abstract. Line tracker is a didactical mobile robot that can move on predefined trajectories marked on a surface using contrast color strips. Basically, it is a differential mobile robot designed only to follows different trajectories in continuous mode. Two robots line tracker were butt in our laboratory using acquisition kit sets. These robots are simply and efficient models to illustrate other sophisticated methods and strategies for navigation, control or planning of a real automated material handling system based on a set of mobile robots. This paper present some technical details of this mobile robot, some experiments made in our laboratory for different predefined routes and evaluates some possibilities to add new capabilities. 1. INTRODUCTION In present, free ranging mobile robots are still uncommon even in highly automated factories, but they will have certainly an important role in the factory of the future. The CIM concept will integrate the control and the planning of an automated material handling system based on a set of mobile robots. An automatic guided vehicle system is a material handling system that uses independently operated selfpropelled vehicles that are guided along predefined pathways. Automatic guided vehicles (AGVs) are often used in Flexible Manufacturing Systems for servicing CNC machine tools or other auxiliary device [1], [2], [3], [6]. AGVs can be programmed for automatic routing and positioning, and many are now designed to operate under the control of a supervisory computer system. They are essentially mobile pallets carrying platforms in the production activities. The definition of the pathways in industrial medium is generally accomplished now using wires embedded in the floor or reflective paint on the floor surface. Guidance is achieved by sensors on the vehicles that can follow the guide wires or paint. The vehicles are powered by means of on-board batteries that allow operation for several hours between recharging. There are a number of different types of AGVs, all off which operate according to the preceding description. Historically, the usually types in industrial medium can be classified [3] as follows. Driverless trains were the first type of AGVs to be introduced. This type consist of a towing vehicle that pulls one or more trailers to form a train and it is use in applications where heavy payloads must be moved large distances in warehouses or factories with inter-mediate pickup and drop-off points along the route. AGVs pallet trucks are used to move palletized loads along predetermined routes. In the typical application the vehicle is backed into the loaded pallet by a human worker, who steers the truck and uses its forks to elevate the load slightly. Then the worker drives the pallet truck to the guidepath, programs its destination, and the vehicle proceeds automatically to the destination for unloading. AGVs unit load carriers are used to move unit loads from one station to another station. They are often equipped for automatic loading and unloading by means of powered rollers, moving belts mechanized lift platforms or other devices. The assembly line AGVs is designed to carry a partially completed subassembly through a sequence of assembly workstations to build the final product.
However, for full flexibility, free ranging AGVs or mobile robots perhaps provide the most attractive option for the future. These vehicles require no hidden underfloor cables or tracks but instead are able to use their own on-board guidance systems to make their way through the factory. Figure 3 presents some detail concerning the body structure of the mechanical design. Left Wheel There are a variety of methods whereby AGVs can do this, and a combination is frequently used to design a real mobile robot: guidance, position reference beacons, imagining and vision, internal sensors monitoring. The last method is the simplest to implement but is not absolute in the sense that errors can occur due to wheels slipping or skidding. Method based on vision is the most attractive for true autonomy but no commercial system of this type exists at present for factory use. Robot front Gears and shafts Left DC Motor Using pre-defined trajectories marked on a surface using contrast color strips or reflective paint on the floor surface is more economically in comparison with the method based on wires embedded in the floor. Line tracker didactical mobile robot is based on this system. Gears and shafts Castor point Left DC Motor 2. MECHANICAL PARAMETERS OF THE MOBILE ROBOT Line tracker is a differential mobile robot. To assure the isostatic equilibrium, it has two driving wheels and a supplementary sliding point (equivalent to a supplementary castor wheel). It is a product offers for customers by OWI Inc., model MV-963 as a kit-set and two exemplars was built in our laboratory. Figure 1 present the general aspect of this robot. Right Wheel Fig.2. Principle of mechanical design. Right side panel Body panel Fig.1. The mobile robot Left side panel Because it is a didactical model, the mechanical design is as simply as possible. Figure 2 presents the mechanical block diagram of the line tracker robot and Fig.3. Body structure of the mechanical design.
Line tracker transmits power from to each wheel through combination of gears and shaft. The shaft is used to change position while the amount of power stays the same. When power is transmitted by rotation of gears, change of rotating speed and change of torque also occur. If different sizes of gears are engaged, rotation speed increases or decreases between two adjacent gears. The rotation speed of the, which is about 11.300 rpm, is reduced by a combination of gears in order to control speed. The total reduction ratio for each driving wheel is 1 / 150, while the total torque assured for one driving wheel is 3.000 g x cm. The height, length and with of the robot are respectively 110 mm, 155 mm and 135 mm. 3. ELECTRICAL PARAMETERS OF THE MOBILE ROBOT The mobile robot is powered for electronic parts by a DC 9 V battery and by a DC 3 V battery for the two differential driving wheels. The electric current consumption is approximate 20 ma for the electronic circuit and approximate 320 ma for the two differential driving wheels. Periodically, the batteries must be manually removed and recharged. 4. ELECTRONIC CIRCUITS The general block chart of the electronic circuits is presented in Figure 4. As we can see in this figure, the electronic circuit is designed to control the mobile robot on the pre-defined trajectories. It is composed by six elements, each of them with a special functionality. 4.1 Sensor section Sensor section (see Figure 5), the eyes of the robot, is a photo interrupter which projects and receives infrared rays. The pre-defined trajectory is usually marked with a black line drown on the white floor to assure the necessary contrast. While black absorbs infrared rays, white reflects them. The robot distinguishes black and white by projecting infrared rays from the photo interrupter. There are two reflecting photo interrupters that trace black lines. Each of them is also a filter to cut off visible light as that diffused reflection can be rejected. The information is transmitted to a special electronic circuit, which is its brain, while s and wheels, as part of mechanism, work as its feet. R 1 Sensor (output) Sensor section RPI 1 RPI 2 Reversal section Comparison section Power source 4.2 Comparison section Fig. 5. Schematic diagram for sensor section. Amplification Section (Left) Amplification Section (Right) R 2 VR R 3 Left DC Motor Right DC Motor From Sensor - + (Output) Fig. 4. Electric and electronic block chart. Fig. 6. Schematic diagram for comparison section.
Voltage of power which came from two photo interrupters are compared in this section, and finally the voltage is selected to be sent to next section (see Figure 6). Phototransistor of photo interrupter gains more current from a white surface. It is followed by a greater change of voltage drops at resistance between R 2 or R 3 and the variable resistance VR. On a black surface, the amount of voltage drop is smaller. To check the direction of motion, the operational amplifier compares this voltage drops. R 5 Left DC R 7 TR 1 R 8 R 9 TR 3 R 11 Left DC (output) 4.3 Reversal section Output voltage from the comparison section turns into a signal to rotate left. Low level indicates rotation and high level indicates stop. Motor right must move in reverse to left. Current amplification circuits consist of the same structure. Therefore, an operational amplifier reverses signals from the comparison section to rotate right. Figure 7 presents schematic diagram for this section. R 6 Right DC TR 2 R 10 TR 4 R 12 Right DC (output) Fig. 8. Schematic diagram for amplification section. From Comparison Section R 13 - + (Output) (internal) R 4 C (battery) R 14 Fig. 9. Schematic diagram for power source. 5. EXPERIMENTS Fig. 7. Schematic diagram for reversal section. 4.4 Amplification section A large amount of current is necessary to rotate s. Signals from the comparison section or reversal section are not strong enough to rotate s. For each section two transistors are employed to amply current of signals so that s right and left can rotate. 4.5 Power source The power source consists of 3 V from two batteries and 9 V from the other battery. The 3 V is for rotating s and 9 V is spent only for the electronic circuit. Electronic noise from the is filtered here so as not to give adverse influence to all other sections. A series of experiments was make with this robot in our laboratory. After these experiments some conclusions are: In concordance with the physical position of the two photo interrupters, the optimal width of the pre-defined trajectory is between 5mm and 8 mm. If this interval is exceeding, the functionality of the mobile robot is a little unsteady for right lines. The minimum rotating radius is approximately 60 mm. If this value is lower. The mobile robot looses the pre-defined trajectories. The optimum distance between the sensor and the floor is 3-4 mm. The quality of contrast between the pre-defined trajectory and the floor is very important and critical. For a lower contrast it is possible for robot to loose the trajectory.
Fig. 10. Mobile robot on a right trajectory. Fig. 13. Mobile robot on a complex trajectory. If the robot doesn't follow the trajectory, it is possible to adjust the semi-variable resistor VR (see Fig. 6) for calibration. Figure 10 present a test make with the mobile robot using a pre-defined plane and right trajectory. Many experiments were make using different width of the trajectory. Figure 11 present a test make with the mobile robot using a pre-defined circular trajectory. Figure 12 present a test makes with the mobile robot using two crossing pre-defined plane trajectories. Figure 13 present a test make on a complex plane trajectory, like an eight number, including circular arcs, straight lines, and crossing routes. Fig. 11. Mobile robot on a circular trajectory. 6. CONCLUSIONS This mobile robot is a good choice for didactical activities. It is possible to illustrate some fundamentals in mobile robotics for the students: strategy for control on pre-defined trajectories, basically for electronically control design, sensor functionality, basically for differential mobile robot movements, etc. If it is possible to use a sensor system with more that two photo interrupters (for example a linear multi photo interrupters or a linear camera), other strategies can be developing using a personal computer and the existing electronic circuit of this mobile robot. Fig. 12. Mobile robot on crossing trajectories. Some tests were makes so for non horizontally trajectories. The torque developed by the two driving wheels and the necessity to eliminate the slippage with the floor surface introduce in this case supplementary limits.
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