Autonomous Mobile Platform

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Autonomous Mobile Platform II

Krzysztof Jaskot, Artur Babiarz
Institute of Automatic Control Silesian University of Technology Gliwice, Poland e-mail: [email protected], [email protected]

Abstract—The paper presents design of autonomous mobile platform based on the all terrain 1/8th scale four wheel drive radio control model. In this paper was considered problem of automatic control of mobile platform using information from GPS system, electronic compass and encoder. The mobile platform is equipped in two-stroke glow engine, heavy-duty drive train and wide-track suspension and controller based on ARM7 microcontroller and using MaxStream XBee Pro 2.4GHz radio modem communication module. The base station equipment is also described. Communication protocol between mobile platform and base station is presented. The paper presents also an application of electronic compass to measure azimuth of mobile platform. Problem of speed and distance control is described. Autonomous mobile platform is a machine that can operate in a human-made environment. By control in this case we will understand to be able to avoid collisions with obstacles (other mobile platforms and walls) during drive. Results of real application are also shown. Results of work on autonomous mobile platform that can operate in human environment are presented. The obtained properties of the system have been effected that it can be used for future research and autopilot design project. Keywords: mobile microcontroller platform, sensors, communication,

provides only information from the electronic compass. This information is needed to determine the azimuth of mobile platform. II. MOBILE PLATFORM

As described in the article [2] mobile platform has been built using a remote-controlled car on a scale 1/8th (length 55cm, width 43cm) and it was delivered by the HPI Racing. Selected terrain model with an independent suspension and four-wheel drive (4WD), because we wanted to create an autonomous platform that can operate in open terrain. The appearance of old version of mobile platform with installed controller and the GPS system is shown in figure 1.

I.

INTRODUCTION

The aim of the project was to create an autonomous mobile platform, which could operate in open terrain. Autonomous mobile platform is a machine that can operate in a human-made environment [4,5,6,7]. The key to autonomy is a control system built on the basis of information concerning the position and goal. Use GPS NMEA (National Marine Electronic Association) protocol allows you to obtain information in text form about the current location of the object. After adding information about the intermediate target points (Waypoint), we can receive information about the current direction [8]. After the experiments conducted and described in the article [2] been amended accordingly to increase the accuracy of determining the direction of movement and speed control. The work was considered problem use information derived from the GPS and IMU (Inertial Measurement System) [3] as a source of control signal. In addition, we uses also information from the speed sensor. IMU currently

Figure 1. Old version of mobile platform.

As the propulsion system used in this model, two-stroke internal combustion engine with a capacity of 3.5cm3 and power 2HP. This allows the dispersal model to speed about 60km/h. In addition to the chassis and drive train in the composition of the platform includes two servos, which are responsible for controlling the throttle/brake and course. These two servos give us the ability to control the platform traction. After the experiments described in the article [2] we change the appearance of mobile platform. We add aluminum frame with installed GPS and 2.4GHz antenna. Aluminum frame prevents any interference generated by working servos, engine (gearbox). We also add new

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controller and IMU system. The appearance of new version of mobile platform is shown in figure 2.

readings from sensors, providing two-way wireless communication with a PC (telemetry, simulation), I2C management. The process of autonomous control based on sensor readings of the following: GPS - provides information on the location of the car on the globe, can also be a source of information about vehicle speed and orientation relative to the direction north. Electronic compass (IMU) - allows you to specify the orientation of the car towards the north. Rotation sensor - provides information about the angular velocity of the drive shaft speed of the car and indirectly, used as a source of feedback for the speed. Other sensors, such as: temperature sensors [1], ultrasonic rangefinders, may be connected to an external derived I2C bus driver.

Figure 2. Mobile platform.

Where: 1- aluminum frame, 2- GPS, 3- antenna 2.4Ghz, 4main controller, 5- IMU, 6- batteries. III. CONTROL SYSTEM

Figure 3 shows a block diagram of the main controller. The heart of the autonomous vehicle is mounted in the cardriver will be built using a microcontroller AT91SAM7S, managing servo, all sensors, power supply and wireless communication. To build a control system for mobile platform we uses microcontroller AT91SAM7S256 (ARM7 Core) it was delivered by the ATMEL. He is responsible for collecting information from sensors, processing them, the exercise of control algorithms and generate servo control signals. In addition, it enables wireless communication, reading values of the individual, shared variables and modify them. Basic features of the microcontroller is: RISC architecture, maximum speed clock 60 MHz 256 KB FLASH program memory, 64KB SRAM memory, interrupt controller, three 16-bits timers, four 16-bits PWM modules, three USART interface, USB controller, I2C bus, two SSP, SPI bus, 11channels 10-bits A/D converter [9]. The microcontroller of our choice has also some other interesting features, such as on board USB controller that together with SAM-BA Boot Assistant provides an easy and fast way of programming the ARM. We have also the JTAG interface, which provides hardware debugging capabilities. The main functions of the controller are: collecting information from sensors, generating servo control signals through the implementation of control algorithms based on

Figure 3. Block diagram of the controller.

Main controller using the AT91 microcontroller is shown in figure 4. The controller is equipped with a plate operator panel with LCD, LEDs and push buttons for menu operation serving. With them you can read some of the performance of the program, namely: as GPS data, compass reading. During normal operation, the buttons are not available because the case is sealed. Then the display shows the next cycle menus that contain the most relevant data. Sample screens are shown in figure 5. The aim of the work is to create an autonomous vehicle, but at the prototype stage and testing is necessary to reduce its autonomy and ensure operator efficiency even take control of the car, which for various reasons, may start to behave unpredictably. Application of five-channel FM RC controller allowed the design of switch mode AMS (Autonomous/Manual Switch).

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Figure 4. Main controller with user interface.

Two channels, as in the case of a simple remote control, are used to transmit information about the location and direction of the throttle servo. A third channel, acts as a switch to allow the change of source signals servomechanisms between the apparatus of the FM (manual) and microcontroller (autonomous). In addition, upon failure of the FM signal from the apparatus or GPS, followed by AMS emergency stop of the vehicle. AMS switch is built using 8bit microcontroller from Atmel - ATMega8. It is also connected to the I2C bus of main controller.

In addition to the equipment fitted on mobile platform available to the operator is base station, which includes: remote control transmitter – manual control and auto/manual switch, radio communication module XBee PRO 2.4GHz – sending/receiving data to/from the vehicle, computer witch control application providing remote viewing operating parameters of the control system mounted on a platform. Figure 7 shows a system using wireless communication modules, XBee PRO. These modules operate in transparent mode. This means that any data sent to the first module will be unchanged at the output of the second module. Communication with the modules is done using a UART protocol, which is easy to implement in the microcontroller AT91SAM7S. From the PC side it is necessary to convert the signals. After the application of FT232RL FTDI and appropriate drivers on your computer, you can create a virtual serial port.

Figure 7. Full duplex communication between PC and controller.

Figure 5. User interface.

Communication between the controller and the computer is realized in the form of data frames. An example of a data frame containing the request and response are shown in figure 8.
Request: Response:

The controller consists of two mounted in a housing designed for the project PCB and LCD. In total, this creates a three-layer structure. Connection with external devices is via connectors placed on the body. Furthermore, inside the controller can be mounted battery with dimensions 65x40x10mm. Visualization of the main controller is shown in figure 6.

Figure 8. Example of a data frames.

Figure 6. Visualisation of main controller.

This frame allows you to read off the value of n variables. In the body frame includes a request ID variable in hexadecimal, separated by commas. The answer of controller is frame containing a comma-separated variables. ARM7TDMI processor architecture allows you to install an RTOS (Real Time Operating System). The operating system allows the division of tasks performed by the microcontroller processes. All tasks are performed simultaneously which is referred to as multitasking. The operating system also provides mechanisms for communication between processes and synchronization tasks. The core of the system takes over responsibility for the allocation of CPU time for the process, taking into account the priorities of processes and supporting a notification of interruption. The application of controller operates under the FreeRTOS system [10]. It is optimized for embedded systems with low hardware resources consists of only three source files written in C. FreeRTOS also makes dynamic

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memory management, create lists and introduces features to process execution pauses for a specified period of time. Figure 9 shows the diagram of mechanisms for exchange of data between the key elements of the microcontroller software to which they are: processes - constantly running in a loop programs, drivers - used, among others by processes to communicate with microcontroller peripherals. Autonomous vehicle program consists of 6 processes which have been divided between tasks. The process of "GPS" is responsible for operating the GPS receiver. When you receive the correct line of NMEA 0183 (every 5ms) is processed, and read data update the corresponding variables. The process of "IMU" collects data from the IMU device driver via UART0, every 20 ms is sent to query the value of the magnetic compass reading. The process of "interface" is used to refresh the driver of the operating panel is currently displayed on the LCD. The process of "LED" acts as a signal generator, reporting via LEDs on the correct behavior. The process of "control" uses data to assess the state in which currently there is a car (including position, speed, azimuth), and then using the implemented algorithms update a servo settings. The process of "communication" is the basis of telemetric system of the car.

minimize the difference between the azimuth obtained in the calculations in the perception stage, and a given azimuth.

Figure 10. Results of test GPS system.

The latter is the direction from which the car should move to reach the waypoint in a straight line. This system is a PID regulator, extended with additional capabilities.

Figure 11. Block diagram of the azimuth control system.

The second controller shown in Figure 12 controls the speed servo uses a PI regulator. Preset speed depends on the quality of data obtained in the stage of perception. It is estimated based on the number of satellites used by the GPS receiver to measure the position.

Figure 9. Division of microcontroller programs and threads used for peripherals and their drivers.

IV.

RESULTS
Figure 12. Block diagram of speed control.

Test drives using the built controller were implemented in several stages. The first was the verification of the system of GPS and radio communications – transmission measurements to the base station. At this stage, was carried out manual control of direction and throttle. The results of this test are shown in figure 10. Test results for the manual control shows a high accuracy to obtain information from GPS. The second test phase was to incorporate automatic direction and throttle control. Information on the target points WP1÷WP7 was recorded in the memory of the microcontroller. In an exemplary control algorithm using two independent controllers. First, the block diagram shown in Figure 11, controls the servo direction. It seeks to

If their number is less than 4, the quality is considered satisfactory and speed reference is 0. Otherwise, it is different from zero and can take two values. The value of fast (1m/s) applies only when the car is already headed toward the via point and not located closer than 2m from it. If the distance is less than 2m predetermined value of speed is 0.25m /s. This helps the controller to turn the direction of the correct execution. Example of autonomous waypoints navigation (Fig. 13) shows positions and orientation taken from GPS and

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electronic compass. During drive we also recorded measurements such as velocity (GPS, encoder), azimuth (GPS, electronic compass), servos (Figure 14).

Figure 14. Examplary measurments recorded during autonomous drive (Azimuth –GPS, electronic compass; Speed – GPS, encoder; Servos).

ACKNOWLEDGMENT This work has been supported by Ministry of Science and Higher Education In the years 2010 – 2012 as development project OR000132 12. REFERENCES
[1.] Babiarz A., Jaskot K.: Temperature control system for glow engine, Zeszyty Naukowe Politechniki l skiej, seria Automatyka z.150 , s. 115-121, Gliwice 2008. [2.] Babiarz A., Jaskot K.: Autonomous Mobile Platform, International Carpathian Control Conference ICCC’ 2010, s. 179-183, Eger, Hungary. [3.] Babiarz A., Jaskot K.: The inertial measurement unit for detection of position. Electrical Review, ISSN 00332097, R. 86 NR 11a/2010, pp. 323-334. [4.] Baker C. R., Dolan J. M.: Street Smarts for Boss Behavioral Subsystem Engineering for the Urban Challenge, Robotics & Automation Magazine, IEEE, Carnegie Mellon Univ., Pittsburgh, PA, March 2009 [5.] Behringer R., Maurer M.: Results on Visual Road Recognition for Road Vehicle Guidance, Proceedings of the 1996 IEEE Intelligent Vehicles Symposium, Tokyo, Japan 1996 [6.] Braun T., H. Schäfer, K. Berns: Topological Large-Scale Off-road Navigation and Exploration – RAVON at the European Land Robot Trial 2008, The 2009 IEEE/RSJ International Conference on Intelligent Robots and Systems, St. Louis, USA, October 2009 [7.] Harkins R. et al., Design and testing of an autonomous highly mobile robot in beach environment, Proceedings of the World Congress on Engineering and Computer Science 2008 WCECS 2008, October 22 - 24, 2008, San Francisco, USA [8.] Jaskot K.: Implementation of GPS information to control of UAV model, ZN Politechniki Rzeszowskiej, Mechanika z.71 Awionika, Rzeszów 2007, ISBN 02092689. [9.] AT91 ARM Thumb-based Microcontrollers, ATMEL [10.] FreeRTOS – ARM7, www.freertos.org

Figure 13. Example of waypoints navigation.

V.

SUMMARY

The proposed solution allows to control an autonomous mobile platform using a GPS and electronic compass. Further development of the mobile platform will be implemented will support the addition of sensors (eg laser scanner, ultrasonic range finder), allowing to overcome obstacles and to develop new control algorithms. After applying the sensor system may need to change the glow engine in the electric motor with reverse gear.

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