University of Moratuwa

Transcription

1 University of Moratuwa B.Sc. Engineering MAP BUILDING WITH ROTATING ULTRASONIC RANGE SENSOR By A.C. De Silva (EE) E.A.S.M. Hemachandra (ENTC) P.G. Jayasekara (ENTC) S. Kodagoda (ENTC) R.L. Peiris (EE) Department of Electronic and Telecommunication Engineering May 2006

2 Table of Contents 1.0 Introduction Overall Functionality Modular Functionality Ultra Sonic Range Sensor Unit Stepper Motor Drive Unit Darlington Mechanism Limit Switch Mechanism Microcontroller unit Mapping Software Software Functionality Probability Model Sonar Sensor Model Sensor Fusion Modes of Operation GUI Appendix ULN SRF05 16

3 1.0 INTRODUCTION As the title of this project implies, main function of this device is to sense obstacles around the platform and build a map as to where they are situated with some interpolations. The basic construction of the platform is an ultrasonic range sensor mounted on a shaft of a stepper motor so that the sensor could be rotated. Two limit switches are used to identify the two extreme positions of the motor so that intermediate positions could be calculated. A microcontroller is used to issue commands to the stepper motor, to exchange commands with the sensor and to communicate with the pc. The pc is extensively used to build a map according to the data received from the microcontroller. The basic operation is illustrated in Figure 1. Limit switches Sensor Micro Controller Motor Motor Driver IC Figure 1 The map building system with the ultra sonic range sensor includes electronic circuitry, mechanical manipulation, firmware, software and probability theories (sensor fusion). The platform contains circuitry and mechanical devices. A PIC 16F877A was used to control hardware and link with the pc software. Also two ICs ULN2003 and MAX232 were used to drive the motor and to do serial communication with the pc respectively. All these ICs with other circuit elements were mounted on a PCB. The designing of the PCB was done using Protel software. A mechanical manipulation was used in the form of a spur gear between the motor shaft and the sensor shaft. The gear reduction was 1:5. This increased the resolution of the sweep angle and obviously reduced the speed of rotation. Also placing of limit switches to obtain extreme positions of the rotating sensor was done accurately by maintaining proper angles. Firmware was used to implement the algorithm for the sensor, stepper motor and for serial communication. PIC C was used as the programming language. Precise timing signals were provided to the ultra sonic range sensor with firmware to obtain obstacle distance with great accuracy. Also continuous pulses were given to the stepper motor via firmware. Another major

4 portion of this project was implemented in a pc. That is to build the actual map to identify where obstacles are situated. Its algorithm was implemented using Visual Basic. A user interface was designed to view the map as well as to issue some commands to the device from the pc itself. This includes start, stop and sweeping mode (i.e. full sweep or sweep within a desired angle). As an enhancement, a probability model was implemented within this software to get an accurate map. Conditional probability was used to get separate sensor readings and Bayes rule was used for fusion of several sensor readings. With this more accurate map was obtained including the shape of the obstacle. 2.0 OVERALL FUNCTIONALITY When the start command is issued from the pc the sensor shaft rotates to its initial position until it triggers the limit switch. At this position the angle is zero. Then the PIC starts to issue pulses for the stepper motor through ULN2003. At every step the ultrasonic range sensor is triggered to identify any obstacles within that angle. As the timing diagram shown in Appendix Figure 2, a triggering pulse of 10µs is given to the sensor from PIC. Then the sensor itself issues an 8 cycle sonic burst from its transducer. The microcontroller enables the interrupt to get the echo signal. It calculates high time duration of the interrupt signal and with some manipulation with speed of sound and temperature of air the obstacle distance is calculated. Likewise average of 4 distances in 4 step angles is calculated and sends to the pc along with the step angle via serial communication. Within the pc according to the probability model, an approximation of the obstacle is plotted along with the raw distance plot. When the other obstacle distance for the other step angle is received by the pc, it uses the sensor fusion model and calculate the most probable distance for the obstacle to be, and will be plotted on the interface. This process continues through out an angle of When the sensor shaft reaches the other limit switch, the direction of rotation is changed. There is another mode to sweep the sensor with in an angle which the user desires. The sweep angle can set by the user from the software so that the sensor shaft sweeps only within that specific region.

5 2.1. MODULAR FUNCTIONALITY Ultra Sonic Range Sensor Unit (SRF 05) The most important module in this map building device is, non other than the ultra sonic range sensor unit. It is being used in its operation Mode-1, which is the simplest mode. In Mode-1, separate trigger and echo pins are utilized for the purpose. The mode pin is left unconnected. One of the microcontroller output pins is connected directly to the trigger input. Whenever, the distance to the obstacle is needed to be measured, the microcontroller is programmed to set the trigger input pin to go high for 10µs. Then the SRF05 starts the ranging. The SRF05 will send out an 8 cycle burst of ultrasound at 40 khz and raise its echo line high and then listens for an echo. The echo output is directly connected to one of the input pins of the microcontroller. Whenever, an echo comes from an obstacle, the echo line goes low and this is handled by the microcontroller. The operation of the microcontroller will be illustrated later. Figure 2- SRF05 Timing Diagram, Mode 1

6 SRF 05 +5V +5V Echo o/p Trigger i/p Mode 0V Figure 3 - Ultra Sonic Range Sensor Unit Stepper Motor Drive Unit The sensor unit is rotated by mounting it on a stepper motor unit. The microcontroller sends control signals to the ULN2003 IC, which is a Darlington array, which in turn drives the stepper motor. The stepper motor is powered by a 12V power supply. To increase the resolution, gear wheels have been utilized. With this gear reduction the stepper motor can operate at a resolution of approximately Teeth = 79 Teeth = 16 Figure 4 - Gear wheel Setup

7 SRF05 Gear Drive Stepper Motor Figure 5 Mechanical Setup of the Stepper Motor Drive Unit Darlington Mechanism ULN 2003 To sink additional currents, generally, Darlington mechanisms are made use of. This prevents microcontroller providing large currents for the stepper drive. Stepper Motor Darlington Mechanism Microcontroller Figure 6 Block Diagram of the Stepper Motor Drive Unit

8 Limit Switching Mechanism The sensor unit is only allowed to rotate a total of This is because the sensor can cover the missing 60 0 with the help of its beam width. Thus, a 60 0 is excluded. Figure 7-Limit Switch Operation With the help of two limit switches the stepper motor is allowed to sweep the desired angle. When the limit is reached the relevant limit switch sends an interrupt to the microcontroller to so that the direction of rotation can be changed. Limit Switches Interrupt Microcontroller Change of Direction Motor Drive Figure 8- Block Diagram Operation

9 Microcontroller Unit The microcontroller used for this system is a PIC16F877A by Microchip Corporation. The microcontroller unit accounts for these events. a. Stepper Motor Driving - Speed - Direction of rotation b. Ultra sonic Range Sensor Interfacing - Sensor Triggering - Obtaining the round trip time for the echo c. PC interfacing via RS232 - Output the observations to the PC - Responding to the PC instructions PORT D of the PIC was used to send control signals to the stepper motor driver system. By reversing the order of control bits the rotating direction of the motor can be reversed. Also by driving all bits to zero the motor can be stopped. A reading from the range sensor for the round trip time is obtained at each step angle that is each step. Four such readings are averaged inside the PIC to obtain a more accurate reading. The round trip time is in the class of µs. The PIC performs its operations according to the commands sent by the PC program. For instance the PC program can either direct the system to function in the global or local mode by sending a control character using the RS232 serial link between the PC and the hardware sub system. When the system is switched to local mode the PC program sends the location and sweep information in addition to the control signal. Apart from this the PC can direct the shaft to move to the initial position or stop. Above all these the RS232 link is used to feed the PC with the data obtained from the sonar sensor.

10 2.2. MAPPING SOFTWARE The software was written using Visual C#.NET 2005 and has several functionalities. These are Communication with the PIC Micro-controller using serial communication Sending the user commands to the Micro-controller in the form of ASCII characters Obtaining the delays of the echoes for each angle and keeping the information in an updated array after converting the delay (ms) into distance (m). Updating the display (map) based on the distance information kept on the array. Calculate a probability of occupation based on the sensor model (for the Ultrasonic Range sensor) for each of the positions and use the Bayesian Model to combine the results of the different readings (Sensor fusion) Software Functionality

11 2.3. PROBABILITY MODEL Sonar Sensor Model In this model the Sensors range area is divided into three regions based on the delay (i.e. the alleged distance of the obstacle). The Whole area is divided into a grid (whose no of elements will decide the resolution of the Mapping model) and each of the squares is assigned a probability based on its position with relative to the sensor reading. The probability of occupation will be calculated differently based on which region it occupies. Region I R r β α + R α P( Occupied ) = Max 2 P( Empty) = 1 P( Occupied ) Occupied Region II R r β α + R α P( Empty) = 2 P( Empty) = 1 P( Occupied )

12 Region III The model does not assign a probability to this region as no information can be derived from the sensor after the first echo. R = Range of the Sonar sensor r = Distance to the Grid the Grid Element β = Sonar Beamwidth α = Angle to Grid Element Max Occupied = Highest Possible Probability Sensor Fusion The grid which covers the entire possible sweep area is filled as the reading for each angle comes from the Micro-controller. The overlapping regions can be combined using any of number methods such as Bayesian, Dempster-Shafer and HIMM (Histogramic In Motion Mapping). Of these the Bayesian method has been employed to combine the different readings Modes of Operation There are two modes of Operation available in the software. These are, Full scan or Global Mode This allows the system to scan the entire area (300 0 ) that is limited by the limit switches. In this mode the area can be entirely viewed (360) using the probability Model. Local Scan Mode This allows the system to scan a specific area (a limited degrees) if a user needs to scan a particular segment.

13 2.5. GUI GUI Operation Global Mode Operation To operate in the Global Scan Mode select and click on the button Then the sensor shaft will return to the starting position indicated by the arrow and start scanning for the full range. Local Mode Operation To operate in the Local Scan Mode select. Once this option is selected click on the button and select the range required to on the map (on the left) and click on the button. Probability Model In order to activate the probability model click the This can be shown in two methods. button. Color graded: This can be activated by selecting the. This will assign different colors to the grids based on the probability. Monochrome: This is the default mode which will show grid elements with 0.5 or greater probability as black and others as white.

14 3.0 APPENDIX 3.1. ULN2003 Seven Darlingtons per package output current 500mA per driver (600mA peak) Output voltage 50V. Integrated suppression diodes for inductive loads outputs can be paralleled for high current. TTL/CMOS/PMOS/DTL compatible input pinned opposite outputs to simplify layout. Description The ULN2003 is a high voltage, high current Darlington array containing seven open collector Darlington pairs with common emitters. Each channel rated at 500mA and can withstand peak currents of 600mA. Suppression diodes are included for inductive load driving and the inputs are pinned opposite the outputs to simplify board layout. This versatile device is useful for driving a wide range of loads including solenoids, relays DC motors, LED displays filament lamps, thermal print heads and high power buffers. The ULN2003A is supplied in 16 pin plastic DIP packages with a copper lead frame to reduce thermal resistance. This is available also in small outline package (SO-16) as ULN2003D. Pin Connection

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16 3.2. SRF05 - Ultra-Sonic Range sensor The SRF05 is designed to increase flexibility, increase range, and to reduce costs still further. The Range is increased from 3 meters to 4 meters. A new operating mode (tying the mode pin to ground) allows the SRF05 to use a single pin for both trigger and echo, thereby saving valuable pins on your controller. When the mode pin is left unconnected, the SRF05 operates with separate trigger and echo pins, like the SRF04. The SRF05 includes a small delay before the echo pulse to give slower controllers such as the Basic Stamp and Picaxe time to execute their pulse in commands. Mode - SRF04 compatible - Separate Trigger and Echo This mode uses separate trigger and echo pins, and is the simplest mode to use. All code examples for the SRF04 will work for the SRF05 in this mode. To use this mode, just leave the mode pin unconnected - the SRF05 has an internal pull up resistor on this pin.

17 Figure 2