GPS System Design and Control Modeling Chua Shyan Jin, Ronald Assoc. Prof Gerard Leng Aeronautical Engineering Group, NUS Abstract A GPS system for the autonomous navigation and surveillance of an airship is developed. It comprises of electronic components, mechanical components and software. The basis for selecting the various components will be discussed. A system view of the entire closed loop control implemented by the various components and the software code will also be looked at. Introduction Figure 1: A 5.5 foot long airship An airship is one of the flight platforms available for conducting aerial missions. Some of the advantages it possesses over other systems such as fixed wing aircraft and rotary wing aircraft are low power consumption, low noise signature, low cost, vertical take-off
and landing and the ability to hover over a point. Combining this with the knowledge of some of the drawbacks of airships, which would include large profile, limited payload and susceptibility to wind, there are certain military situations where the airship could be used, such as for surveillance purposes. An autonomous airship based on GPS allows for the airship to operate without a human operator. This minimizes the danger to humans, reduces human error and brings down labor costs. A possible deployment situation would be for the routine surveillance of secured strategic points and areas. The bird eye view provided by the airship would mean a larger area is surveyed and no human troops need to be deployed. Furthermore, the low costs would mean several airships can be deployed and only a single operator is needed to monitor the video images sent back as they are autonomous vehicles. Overview of the project The current project calls for the development of a GPS flight navigation and surveillance system that is then to be fitted onto an airship, which has also to be designed and fabricated. Initially, project goals were stated, that is to construct an airship which is able to navigate and survey autonomously. The components needed to achieve the goals can be divided into 3 distinct categories which work in tandem: Electronic components, Software codes and Mechanical components. These were developed simultaneously as necessary to achieve the various functions spelt out in the section Software Code.
Electronic Components As this project largely revolves around the ability to navigate and survey autonomously, the GPS receiver and antenna are of high importance as these determine the accuracy and reliability of the navigation system. However, this must to be balanced with the weight constraint faced when designing a system for use with a relatively small airship. The 6 key factors for consideration are: Weight, Voltage required, Accuracy, Customer support, Size, Cost. After considering 6 different brands of receivers with their respective antennas, Trimble Lassen SQ was deemed to have best met the criteria and selected. Trimble Lassen SQ GPS Receiver Compact Unpackaged GPS Antenna Trimble Lassen SQ has a light weight of 5.7g, low power requirements of 143mW, good accuracy of: <6m (50%) and <9m (90%), good customer support by Dinkum Technologies, small form factor of 26mmx26mmx6mm and low cost of S$107. Motorola was considered seriously as it met most of the criteria as well. However, there was a Minimum Order Quantity and this did not allow for development. Differential GPS, while giving higher accuracy of around 1m, is heavy and not suitable for small airships. However, it might find application in other situations. When selecting a microcontroller, the factors for consideration were: sufficient RAM space, ability to support serial input, ability to support I2C input, development support and cost. The PIC 18F452 was chosen as this had a sufficient RAM of 1536 bytes, which was needed for a reasonable number of waypoints and to be able to receive the long string of characters output by the GPS receiver. Also, it is able to support serial and I2C
input. It is widely used and there are forums that provide much troubleshooting information. It is reasonably priced at S$15 for a PDIP (from Farnell), which is easy to use for development as it fits easily into a breadboard. Also, there had to be a way to program the appropriate code into the microchip. The MPLAB ICD2 was chosen to be the programming and debugging device. Instructions came with the programmer as to how to build the appropriate connecters and set up the circuit. An integrated circuit was used to convert signals from the microcontroller that were in TTL levels, 0 to 5V, to RS 232 levels, -12 and 12V, for interfacing with the laptop. The MAX 232 chip was chosen for this purpose. There was also the issue of there being only 1 serial receive and transmit pin on the microcontroller. But there are 2 different sources of serial inputs, the laptop and the GPS receiver. A mechanical switch was used as a device to enable selection between which of the 2 devices was connected to the microcontroller. There is also the need to use a H-Bridge motor driver. The maximum output current of the microcontroller is 25mA and is insufficient to drive the motors directly. Therefore, the L293D, was used to output a much higher current to the motors.
An electronic compass, CMPS03 is used to allow the airship to be able to determine its current bearing in relation to the earth s magnetic field. This knowledge can then be used for navigation and also for surveying. This electronic compass is small and light. Rechargeable lithium ion batteries are used as these provide high capacity to weight ratio. The batteries are also the heaviest single component and care must be taken to reduce this weight as much as possible. Mechanical Components The mechanical motion of the airship is achieved by using motors. 2 DC motors are used to drive the propeller blades that provide the vectored thrust. As the DC motors are the components of the airship that drain the most power from the batteries, light and low power consumption motors have to be used. Small Mabuchi N20 motors are used as these are light and require little power input. Another similar DC motor was used at the rear of the airship as a rudder. This allowed for the steering of the airship and when used together with the electronic compass, provided autonomous yaw control for the airship. A servo motor was used to control the pitch angle of the DC motors to the horizontal. This allows the airship to climb, decent or maintain a horizontal path as needed. The servo motor used was Futuba sub micro servo RCM 306, which weighs a light 6g and provides sufficient torque of 0.8 kg.cm. The angle which the servo motor turns to is determined by the pulse width of the input signal provided by the microcontroller. This pulse is generated using a software code.
Software Code The software code was written in C programming language using a PICC C-Compiler. C was chosen in preference of assembly language as it is a higher level language, making it much easier to write and implement the mathematical functions needed for navigation. Also, it is sufficiently low level as to allow for sufficient control on the code. Authentication Input Waypoints Lift Off Navigation Surveillance Landing Figure 2: Overview of algorithm This is the main sequence of actions that is written for the autonomous flight navigation and surveillance. These actions are implemented as separate functions in the written code. As can be seen from the above flowchart, there are closed loops functions that provide for the system to navigate to the next waypoint and survey the desired spot for the desired length of time. We shall explore the navigation and surveillance algorithms, as these are the 2 more important functions. The navigation algorithm is a closed loop control function as shown below in Figure 3. The system compares the desired MGR, with the 3 parameters being longitude, latitude and altitude, with its current sensed position. The current position is derived from the signal output of the GPS receiver, which is in NMEA (National Marine Electronics Association) 0183 protocol, GGA format. This specifies what the corresponding fields
stand for. The comparison yields an error, which if is above the tolerance level, will actuate the rudder motor and the servo motor for yaw and pitch control respectively. The yaw control is implemented by the electronic compass working with the rudder motor. This allows for the target longitude and latitude to be achieved, within a specified tolerance level, The surveillance is also implemented by a closed loop control function. The function required is to hold a certain yaw angle for a determined amount of time. This is very similar to the yaw control implemented in the navigation section. It must be noted that the current navigation algorithm was designed for a no wind scenario. As there is a serious weight constraint faced because minimizing the profile is of high importance, wind counter measures could not be implemented. Preliminary tests on the 5.5 foot long airship in Figure 1 showed a wind force of 3.5N in moderate wind environment. It must be noted that the 2 thrust motors provide a combined thrust force of 0.14N. However, more on wind counter measures will be discussed in the Future areas for development section.
Figure 3: Autonomous GPS Navigation Control Loop Microcontroller + H Bridge Driver Desired Pitch Calculation Desired Pitch Voltage Microcontroller Thrust Motors Pulse Voltage Rotor RPM Servo Motor Propellers Pitch Angle Thrust Force Calculation Wind Force Propeller Force + + Airship Force Dynamics Waypoint Lat, Lon, Alt + Error Desired Yaw Calculation Desired Yaw + Error Microcontroller & H Bridge Driver Voltage Rudder Motor Rotor RPM Propellers Thrust Airship Yaw Dynamics Sensed Yaw Electronic Compass Yaw Angle Sensed Position Lat, Lon, Alt GPS Receiver Actual Position Lat, Lon, Alt
Future Areas for Development As autonomous airships have not been heavily researched into, there are still many areas for future research and development. Some of the possible areas are: 1. An auto-focus camera mechanism. Studying how the auto-focus in cameras work might provide some clues. 2. A smart vision recognition algorithm that allows the airship to maintain surveillance of a particular target. 3. Wind countering measures (Hardware). A wind counter motor could be implemented to provide a thrust force equal and opposite to the sensed wind force. 4. Wind countering measures (Software). A certain navigational algorithm could be used to allow the airship to maintain a straight course to its next waypoint as much as possible. The Kalman Filter could be studied and considered if appropriate. 5. Incorporating gyroscopes to maintain roll stability. 6. Incorporating sensors for obstacle detection and avoidance