CHAPTER 1: REAL TIME COMPUTER CONTROL

Transcription

1 CHAPTER 1 Page 1 ENGG4420 LECTURE 2 September :49 PM CHAPTER 1: REAL TIME COMPUTER CONTROL REFERENCES: G. F. Franklin et al., ``Feedback Control of Dynamic Systems,`` 5th Edition, Pearson, S. Bennett, ``Real Time Computer Control,`` Pearson Education. R. Muresan, ENGG4420 Lab Manual, 2010.

2 CHAPTER 1 Page 2 SECTIONS SECTION 1.1: Real Time Definitions and Development SECTION 1.2: Dynamic Models SECTION 1.3: Feedback Control SECTION 1.4: PID SECTION 1.5: Tuning SECTION 1.6: Implementing Real Time Control Algorithms

3 CHAPTER 1 Page 3 SECTION 1.1 REAL TIME DEFINITIONS AND DEVELOPMENT The application of digital computers to industrial control began in the late 1950s. In late 1960s the real time operating systems were developed. By 1970, it was becoming possible to consider having two computers on the system, one simply acting as stand by in the event of failure. The development of microprocessor in 1974 made possible the use of distributed computer control systems. COMPUTER CONTROL TASK OPERATION Sequential mode: plant input, control, plant output and communication tasks are all being carried out in turn and, with the sequence then being repeated indefinitely. Parallel mode: various tasks are executed concurrently. Computers with a single processor unit are in fact performing as pseudo concurrent operation. Concurrency (or pseudo concurrency) problems: Task synchronization Sharing of resources between tasks.

4 CHAPTER 1 Page 4 What is Real Time? Real time is a quantitative notion of time and is measured using a physical (real) clock. Within this context: real time response is the ability to reliably and, without fail, respond to an event, or perform an operation, within a guaranteed time period. Ex. chemical plant... Logical time (or virtual time) deals with a qualitative notion of time and is expressed using event ordering relations such as before, after, sometimes, eventually, preceeds, succeeds, etc. While dealing with logical time, time readings from a physical clock are not necessary for ordering the events. Ex. library software... A system is called a real time system, when we need quantitative expression of time (i.e., real time) to describe the behaviour of the system. Both the computer system and the controlled system (environment) use the same time scale. Any system whose behaviour can completely be described without using any quantitative expression of time is not a real time system.

5 CHAPTER 1 Page More Definitions of Real Time Systems General Definition: Any system in which the time at which the output is produced is significant i. ii. This is usually because the input corresponds to some movement in the physical world, and the output has to relate to that same movement. The lag from input time to output time must be sufficiently small for acceptable timeliness. Real time systems are those which must produce correct responses within a definite time limit otherwise performance degradation and/or malfunction results; Or, a real time system reads inputs from the plant and sends control signals to the plant at times determined by plant operational considerations not at times limited by the capabilities of the computer system. A real time program is a program for which the correctness of operation depends on both the logical results of the computation and the time at which the results are produced.

6 CHAPTER 1 Page 6 EXAMPLE APPLICATIONS OF REAL TIME SYSTEMS INDUSTRIAL APPLICATIONS: major usage of realtime systems Example 1: Chemical Plant Control Type of process control application Real time computer periodically monitors plant conditions readings of pressure, temperature, and chemical concentration of the reaction chamber Automation system decides based on the readings the corrective actions necessary at that instant in order to maintain the chemical reaction at the certain rate. The actions decided upon must be carried out within certain predefined time bounds Typical time bounds for chemical plants are from a few microseconds to several milliseconds.

7 CHAPTER 1 Page 7 Example 2. Automated Car Assembly Plant Example of plant automation system The work product (partially assembled car) moves on a conveyor belt Several workstations are place along the conveyor belt each workstation performs specific operations: fitting engine, etc. At each workstation a sensor senses the arrival of the next partially assembled product There is time constraint imposed on the workstation computer work must be completed before moving to the next step Time bounds are typically 100x of ms. Chassis Fit Engine Fit Door Fit Wheel Conveyor Belt Spray Paint Finished Car

8 CHAPTER 1 Page 8 Example 3: Supervisory Control and Data Acquisition (SCADA) SCADA are a category of distributed control systems and in general monitor and control a large number of distributed events In SCADA systems, sensors are scattered at various geographic locations to collect raw data that is processed and stored in real time database Example of SCADA is the Energy Management System (EMS) EMS helps to carry out load balancing in an electrical energy distribution network Another SCADA example is a system that monitors and controls traffic in a computer network makes the router change its traffic routing policy dynamically based on sensed load in different segments of the network. TIME CONTRAINTS in SCADA applications is that the sensors must sense the system state at regular intervals (every few ms) and the data must be processed before the next state is sensed.

9 CHAPTER 1 Page 9 MEDICAL: few examples are: robots, MRI scanners, radiation therapy equipment, bedside monitors, and Computerized Axial Tomography (CAT) Example 4: Robot used in recovery of displaced radioactive material Radioactive material is used in various treatments in medical field If the radioactive material gets dislocated and falls a robot is used to restore the radioactive material to its proper position. The robot walks into the room containing the material, picks it up, and restores it to its proper position. The robot needs to sense the environment frequently and based on this information it needs to plan its path. The real time constraint on the path planning task of the robot is that unless it plans the path fast enough after an obstacle is detected, it may collide with it. Time constraints in this case are a few ms.

10 CHAPTER 1 Page 10 PERIPHERAL EQUIPMENT: there are many peripheral equipment that contain embedded realtime systems such as: laser printers, digital copiers, fax machines, digital cameras, scanners etc. AUTOMOTIVE AND TRANSPORTATION: a few examples in this area are: automotive engine control systems, road traffic signal control, airtraffic control, high speed train control, car navigation system, and MPFI engine control system Example: Multi Point Fuel Injection (MPFI) System MPFI is a real time system that controls the rate of fuel injection and allows the engine to operate at its optimal efficiency, and reduce pollution. A MPFI system injects fuel into the individual cylinders resulting in better power balance among cylinders as well as higher output. The computer primarily controls the ignition timing and the quantity of fuel to be injected The actions of the computer are determined based on sensor's data from the engine: Ambient temperature, engine coolant temperature, exhaust temperature, emission gas contents, engine rpm, vehicle road speed, crankshaft position, camshaft position, etc.

11 CHAPTER 1 Page 11 TELECOMMUNICATION APPLICATIONS: examples in this class are: cellular systems, video conferencing, cable modems, etc. Example: A Cellular System Popular means of mobile communication a cellular system usually maps a city into cells. In each cell, a base station monitors the mobile handsets present in the cell. The base station performs various tasks such as locating a user, sending and receiving control messages to a handset, hand off calls, etc. When a mobile user moves away, its received signal strength (RSS) falls at the base station. The base station monitors RSS and when it fall under a certain threshold value, it hands off details of the ongoing call of the mobile to the base station of the cell to which the mobile has moved. The hand off must be completed within a sufficient small predefined time interval so that the user does not feel any temporary disruption of service during hand off. Typically, call hand off is required to be achieved within a few ms.

12 CHAPTER 1 Page 12 AEROSPACE: there are various real time systems in aerospace: avionics, flight simulation, airline cabin management systems, satellite tracking systems, on board aircraft computer, etc. Example: On Board Aircraft Computer On board computer implements the auto pilot mode the on board computer takes over all controls of the aircraft including navigation, take off, and landing. In the auto pilot mode the computer periodically samples velocity and acceleration of the aircraft, amplitude etc. The sampled data is used to compute X, Y, and Z coordinates of the current position. The computer compares the current position with the pre specified track data. Before the next sample data values are obtained the computer computes the deviation from the specified track values and takes any corrective actions that are necessary. Sampling of parameters and data analysis need to be completed within a few microseconds.

13 CHAPTER 1 Page 13 INTERNET AND MULTIMEDIA APPLICATIONS: realtime applications in this area include: video conferencing and multimedia multicast, Internet routers and switches, etc. CONSUMER ELECTRONICS: in this area there are numerous applications of real time systems: set top boxes, audio equipment, Internet telephony, microwave ovens, intelligent washing machines, home security systems, air conditioning and refrigeration, toys, cell phones, etc. Example: Cell Phones A cell phone at any point in time carries out a number of tasks simultaneously such as: convert input voice to digital signals, convert electrical signals from the microphone to output voice signals, sample incoming base station signals in the control channel, etc. A cell phone response to the communication received from the base station within certain specified time bounds. Example: a base station might command a cell phone to switch the ongoing communication to a specific frequency (within a few ms).

14 CHAPTER 1 Page 14 DEFENCE APPLICATIONS: real time systems include: missile guidance systems, anti missile systems, satellite based surveillance systems, etc. Example: Missile Guidance System capable of sensing the target and home on it Homing becomes easy when the target emits either electrical or thermal radiation. The missile guidance is achieved by a computer mounted on the missile. The mounted computer computes the deviation from the required trajectory and effects track changes of the missile to guide it onto the target. The time constraint on the computer based guidance system is that the sensing and the track correction tasks must be activated frequently enough to keep the missile from straying from the target within a few hundreds of microseconds or less depending on the missile speed. OTHER EXAMPLES: from every day life we have the railway reservation system as a real time system but it is in a different sub category...

15 CHAPTER 1 Page 15 A BASIC MODEL OF A REAL TIME SYSTEM

16 CHAPTER 1 Page 16 A. B. C. ENGG4420: LECTURE 2 CHARACTERISTICS OF REAL TIME SYSTEMS TIME CONSTRAINTS: every real time task is associated with some time constraints: a) Deadline associated with tasks common constraint; b) Delay; c) Duration It is the responsibility of the Real Time Operating System (RTOS) to ensure that all tasks meet their respective time constraints. NEW CORRECTNESS CRITERION: correctness in real time system implies both logical correctness of results, and correctness of time at which the results are produced. a. A logically correct result produced after the deadline would be considered incorrect. SAFETY CRITICALITY: In many real time systems the issues related to safety and reliability are bounded together to generate safety critical systems safety critical system is required to be highly reliable. a. Safe system is one that does not cause any damage even when it fails b. Reliable system is one that can operate for long durations of time without any failure