Features of the Microprocessor Technology and its Introduction to Autopilot Systems - Term Paper Example

INTRODUCTION TO MICROPROCESSOR OF AUTOPILOT SYSTEM 1. Introduction Autopilot systems are generally some form of electrical, mechanical, or hydraulic system that could control a vehicle without human intervention. Although autopilots are control systems widely used in various in industries, autopilots are more identified with ships and aircraft’s automatic navigation. In the early days of aviation, autopilots functions only as wing leveler and yaw dampers. They are merely “proportional” controls that rely on magnetic repeating compass and signal amplifiers. However, in the advent of the microprocessor technology, autopilot soon became a sophisticated piece of equipment capable of holding altitude, maintains heading, change of speed, attitude cruise, climb, descent, turn, and the usual pitch adjustment. Microprocessor controlled autopilots systems are now being use in various critical applications and are commercially available for unmanned vehicles. This paper will discuss the various features of the microprocessor technology and its introduction to the traditional autopilot systems. This includes discussions on the integration of microprocessor controlled autopilot systems and navigation systems in aircraft’s flight management systems particularly on commercial jets and a summary of the microprocessor influence on automatic flights. 2. The Microprocessor The term “microprocessor” came into existence in 1971, when the Intel Corporation of America developed the first microprocessor (Intel -4004) which is 4-bit microprocessor (Singh 2005, p.433). In February 1981, Intel released iAPX 432 (Intel Advanced Processor Architecture) which an advanced design with innovative architecture such as data store using multiple pointer levels, fault tolerance, memory error tolerance, multiprocessing and object-oriented software. However, the product described as a ‘micro mainframe’ that time was discontinued due to performance deficiencies. Intel also announced the 80186 and 80188 high integration 16-bit internal data path microprocessors that were both designed for embedded applications in computer peripherals and other electronic products. This is also the same year that Motorola released the 10 and 12 MHz versions of the MC68000. In 1982, after formalizing the ten-year technological exchange agreement with Advanced Micro Devices (AMD), Intel released the 8026 microprocessor that is four times the power of an 8088. The chip has 134, 000 transistors, a 16-bit internal data path and at a clock speed of 8 MHz that can process 1.2 millions of instructions per second (Allan 2001, p.82). Microprocessor is fundamentally a digital circuit, consisting of thousands of component on a silicon chip with capabilities similar to the Central Processing Unit (CPU) of a computer. A microprocessor is specified by its ‘word’ size such as 4-bit, 8-bit, 16-bit etc., which is the number of bits of data that is processed by the microprocessors as a unit. It is the brain of digital computers capable of arithmetic and logical operations. It is a programmed-controlled device, which fetches data from memory, decodes, and executes instructions. Most microprocessors are simple single chip devices and the basic units or blocks of a microprocessor are ALU or Arithmetic Logic Unit sub-system, Memory sub-system, Input/Output Sub-system, and the Control Unit (Singh 2005, p.433). As we mentioned earlier, the microprocessor is identified with the size of it data-bit which is the size of data that an ALU can work at a time. For instance, an Intel 8085 processor has an 8-bit ALU thus it is called an 8-bit microprocessor similar to an Intel-8286 has 16-bit ALU and it is called a 16 bit microprocessor. The ALU is responsible top perform 16 arithmetic and 16 logic operations on the data store in a general purpose register with the CPU or in a memory device on two 4-bit variables (Singh 2005, p.433). The memory sub-system includes two registers, one for storing data, and the other for storing address. The Memory Address Register or MAR is an 8-bit register, and has three state input, which connects it to the A/D bus. The second register is the Memory Data Register or MDR that also has an 8-bit register, which stores data temporarily before it is finally written in the Random Access Memory (RAM). On the other hand, the input/output sub-system takes binary input from an input device like keyboard, teletype writer, magnetic tape, etc. and is receive by the input register (Singh 2005, p.435). Programming a microprocessor does not mean that the microprocessor alone can be programmed. In fact, it is never used alone since the microprocessor system includes a CPU, and EPROM, RAM, an Input/Output port, a keyboard as input device and 7-segment displays as the output device. An EPROM is always pre-programmed with a suitable monitor program for loading data or instructions into the system and displays it on the 7-segment displays or some other device. In addition, some other useful programs are also entered in the EPROM. The most common and normally used programming language for the system is machine or assembly language (Singh 2005, p.518). In 1985, Intel released the fist microprocessor fully capable of 32-bit processing. The 80386DX microprocessor is fifteen times for powerful that the 8088. The chip has 275,000 transistors, a 32-bit internal data path and at a clock speed of 16 MHz a rating of 6 MIPS. It has a memory addressability of 4 gigabytes available at a clock frequencies ranging from 16 to 33 MHz. In 1986, since IBM is reluctant to incorporate the new Intel microprocessor, Compaq selected 80386DX for their Deskpro computer, which is the first product application of the microprocessor. Two years later in 1988, another ‘86’ based microprocessor was introduced to the public. The 80386SX chip also has 275 transistors but with 32-bit internal and a 61-bit external bus with same clock speed of 16 MHz but with a rating of 2.5 MIPS. The memory addressability of a 80386SX is 16 megabytes and the microprocessor is available at clock frequencies ranging from 16 to 33MHz. Soon after its release, the more powerful 80486DX was introduced with 1.2 million transistors, 1-micron minimum feature size, and a 32-bit bus. This is the first Intel processor to incorporate a Level 1 cache of 8 KB for faster data access. At a clock speed of 25 MHz, the 80486DX has a rating of 20 MIPS. In addition, this microprocessor comes with an integrated floating-point unit and with a memory addressability of 4 gigabytes. It is also available in the clock frequencies ranging from 25 to 100 MHz faster speed than its predecessors do (Allan 2001, p.83). 3. The Microprocessor Control of Autopilot System In engineering and science, the meanings of control systems is those whose major function is to dynamically or actively command, direct, or regulate an activity or an autopilot. On the other hand, a digital computer or a microprocessor is a discrete device because the internal and external signals of a microprocessor are normally discrete-time or digitally coded. A digital control system is therefore a device or a control system that includes digital computers or microprocessors. In fact a “microprocessor is a common component of digital control systems” (DiStefano et. al. 1995, p.12). According to Jones and Watson (1990, p.78), digital control systems are digital autopilot and Landau and Zito (2006, Preface) states that the development of digital computers or microprocessors and their extensive use in control systems in all fields of applications has brought about important changes in the design of control systems. Furthermore, they added, that because of their performance and their low cost makes them suitable for use in control systems of various kinds that demand far better capabilities and performance than those provided by analog controllers. Control Systems or Autopilot Systems primary function is to ensure stability in one or more axes. For instance, when an aircraft encounters turbulence, the altitude indicator sense a signal to autopilot computer indicating the nose has pitched up. The computer processes this signal and sends a signal to the pitch servo telling it to apply down elevator and bring the aircraft back to level pitch. This scenario repeats itself multiple times per second to the keep the aircraft balanced around a fixed altitude. Stability is achieved when the pilot set a point while in a steady state. This the process of homeostasis where stability of an aircraft depends upon a predefined point or preset and it works in almost all primary autopilot functions such as heading and altitude hold (Novacek 2007, p.58; FVA 2007, p.5). The pilots and land the aircraft but normally they engage the autopilot for the flight. The pilot programs the computer controlled autopilot system before takeoff to fly a pre-selected course. On its way, the pilot turns it on and the computer automatically makes the appropriate adjustments. One of the direct benefits of computer-controlled autopilot is reducing a pilot’s workload and fatigue by performing some of the pilot’s important tasks. It can hold altitude and maintain a heading, change of the speed, attitude cruise, climb, descent, turn, trim, yaw, and pitch of the aircraft. More importantly, in case of foul weather, the autopilot can switch to the Instrument Landing System (ILS) and perform an instrument approach to the airport. However, it does not actually land the aircraft, only a human pilot lands an aircraft (McCoy 20067, p.43). The autopilot process is comparable to the processes taking place in our body. For instance, a simple task like taking a drink of water requires our eyes to perceive where the tumbler is and our mind processes the position of the tumbler and ascertains how much force is necessary to pick up the tumbler. Our eyes will then observe while we lift the tumbler to take a drink and this image is sent to our brain where it determines if our arms is indeed going in the right direction or not. If it sees that our arm is heading off-course, it will send a corrective signal to our muscle to move in the right course. This process is known as “feedback” allowing the system to persistently sense and adjust the actions (Novacek 2007, p.58-60). The autopilot system consists of electric actuators or servos connected to the flight controls, the number and location of these servos depends on the type of system installed. For instance, a two-axis autopilot in a helicopter controls the pitch and roll; one servo controls fore and aft cyclic, and another controls left and right cyclic. In the same way, a three-axis autopilot has an additional servo connected to the anti-torque pedals and controls the helicopter in yaw. In addition, a four-axis system uses a fourth servo that controls the group. These servos move the respective flight controls when they receive control commands from a central computer. This computer receives data input from the flight instruments for altitude references and from navigation equipment for navigation and tracking reference. An autopilot has a control panel in the cockpit that allows you to select the desired functions, as well as engage the autopilot (FVA 2007, p.5-10). Traditionally, the control system of an aircraft is divided by functionality into a distinct subsystems such as autopilot and auto throttle. However, there are some issues in implementing each sub-system and this was resolved by federated approach to provide fault containment simply by dedicating a separate fault-tolerant computer in each sub-system (Vincentelli and Sifakis 2002, p.183-184). The basic autopilot system (see Fig. 1.0 below) according to Tetley and Calcutt (2001, p.324) relies on the output of a gyro or magnetic repeating compass that is couple to a differential amplifier along with a signal derived from a manual course-setting control. If nor difference exists between the two signal, no output will be produced by the amplifier and no movement of the rudder occurs. However, when the differential amplifier detects a difference between both sources of data, an output error signal, proportional in magnitude to the size of the difference is applied to the heading of error amplifier. Output of this amplifier is coupled to the rudder actuator circuit, which causes the rudder to move in the direction determined by the sign of the output voltage. The error signal between the compass and the selected course inputs produces an output voltage from the differential amplifier that is proportional to the off-course error. This type of control, therefore, is termed “proportional” control. Autopilot is one of the most significant systems used in ships and aircraft and it has undergone changes from purely mechanical autopilots to electro-microprocessor systems. Today, autopilots are not just use for high steering precision or minimum overshoot but for economic reasons since an optimal well-designed autopilot can shorten journey by 3 to 5% (Cheng et. al. 2004, p.681). The impact of microprocessor since its commercial introduction in 1971 has been very significant predominantly in the autopilot systems of airlines (Hansman 2005, p.1). By the 1980s, autopilot built on microprocessor technology brought many advance capabilities such as programming routes into a flight computer for a fully automatic flight (Crane et. al. 2000, p.8). For instance, the digital databus has change aircraft information architecture tremendously by allowing information to be use by various aspects of the information architecture and facilitate a level of efficient interface and coordination between components that was not feasible in analogue or pneumatic information transmission systems. Another is the sensor, which has been modernized by electronic, and microprocessor enabled sensor systems. Their performance has improved and now holds additional functional features like the linear output, databus compatible output, and automatic compensation. Generally, new classes of sensors are evolving like micromechanical sensors and multi-sensor systems similar to modern air data system that automatically controls thermal effects and compensates for static system and installation errors electronically. More importantly, navigation sensors have also improved and creating a significant impact on vehicle capability. For instance, IRS (Inertia Reference Systems) and lately satellite-based systems like GPS and many other complementary systems have supplemented radio beacon systems such as VOR/DME. Various External Threat Sensors have been developed which considerable had a significant impact on flight safety. Airborne weather radar has reduced the convective weather encounters. GPWS (Ground Proximity Warning) terrain database enhancement and TAWS (Terrain Awareness Warning Systems) undoubtedly has reduced the incidence of CFIT (Controlled Flight Into Terrain). When Air Traffic Control facilities fail, TCAS (Traffic Collision Avoidance Systems) provides a redundant safety net (Hansman 2005, p.4). Another improvement is actuation, which from mechanically driven hydraulic actuators has evolved into a Fly By Wire and Fly By Light systems. FBW, which was initially developed for military aircraft, allows enhanced maneuvering performance. Improvements in auto flight systems control from basic autopilot functions like "yaw dampers" and "wing levelers" to advanced three axis autopilots and approach capability. The conventional mechanically servo controlled throttles has been replaced by FADEC (Full Authority Digital Engine Controllers) that allows automatic throttle functions, fuel efficiency, and enhance engine performance. Auto flight and Navigation Systems are now integrated in the FMS (Flight Management Systems) to enhance trajectory level control and other flight management functions such fuel monitoring, weight estimation, calculation of performance, and automatic tuning of navigation radios (Hansman 2005, p.5). However, the auto flight systems evolution since the introduction of microprocessors into the aircraft control systems as a result has undeniably and significantly increase complexity, as the microprocessor needs sophisticated software to run critical flight instructions (Hansman 2005, p.6). Microprocessor in autopilots systems are now being use in various critical applications. For instance, there are several on-the-market autopilots available for unmanned vehicle and implemented through DSP microprocessors. The MP2028 designed by Micropilot includes user flexibility into the system and they additional input/output ports that allows reprogramming and hardware-in-the-loop capabilities and it is built to utilize a 20MHz 32-bit Motorola processor. Autonomous Unmanned Vehicles build a bigger and more sophisticated commercial autopilot containing eight microprocessors that share computational load. However, this unit only works with handheld GPS units; it has no additional ports for reprogramming and communication with external processors or complementary sensors. The Phoenix autopilot manufactured by O-Navi is fully reprogrammable that employs 32MHz, 32-bit Motorola processor. However, unlike the other unit, it is not capable of hardware-in-the-loop capabilities (Murthy et. al. 2007, p.2). Emerged from a simple device, microprocessor controlled autopilots has also evolved to full control of takeoff and landing of commercial airliners. Beginning with the Boeing 777, full Fly-By-Wire controls became available on commercial aircraft and Boeing test flights of new aircraft are done on autopilot to avoid risking lives of crewmembers (Bushko 2005, p.279). The aircrafts of the new generation are taking a leap forward through flight management systems appeared on the B-767 in 1982. “The relevance of the flight management systems to air traffic control is threefold” (Wickens 1998, p.112. The industry has trying together both vertical and horizontal navigation, thrust control, data storage, optimization, and even auto land. The “autoland” system in the modern jet airliner is a combination of a number of systems mainly autopilot, autothrottles, flight management systems, and instrument landing systems which are all microprocessor controlled. (Wickens 1998, p.113). 4. Conclusion Since its release in 1971, the microprocessor evolved into a more sophisticated and powerful piece of technology that is widely used not only with personal computers but also with aircraft’s autopilot systems. It has brought many advanced capabilities to aircraft such as programmable and fully automatic flights. It has also enhanced the typical Flight Management Systems with the introduction of the digital bus that has change aircraft’s information architecture significantly. It has facilitated high efficiency of interface and coordination between components that is not possible with the traditional analogue or pneumatic information transmission systems. More importantly, it has modernized the aircraft’s sensors like the modern air data systems that are now capable of automatically controlling thermal effects electronically. Moreover, navigation sensors and various external threat sensors improved considerably thus enhancing flight safety and reduced the incidence of CFIT. Another important contribution of the microprocessor to the auto flight systems is the improvement in actuation that has enhanced maneuvering performance of air vehicles. In general, the introduction of microprocessor into the autopilot system brought not only fully automatic navigation and enhanced flight safety but also opportunity for further aviation technology advancement. The microprocessor technology had developed to the point at which individual devices could be linked to form a flight management system rather than a collection of independent, albeit sophisticated, boxes. 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