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Transcript
Stages of Control Loop Operations Sayande Adekoye College of North West London Topic Control Systems Using IT – An Introduction Aims To teach IT Introduction to Loop Stability PID,, analogue signals, digital filtering, pulse, operator displays, converting Give students a Task to complete during the PowerPoint presentation, can either work in pairs/individual or groups Level Level 3 Method All Slide Show, Various tasks throughout the presentation, can be used as a group discussion, or as individual/pairs task. Can be handed to students at the end of the lesson for revision purposes. Equipment Laptop Projector Printer Notepad/Pens/Pencils Duration 1 Hour Loop Stability in Control Systems • A feedback control system must be stable as a prerequisite for satisfactory control. • The stability of a control system is often extremely important and is generally a safety issue in the engineering of a system. An example to illustrate the importance of stability is the control of a nuclear reactor. An instability of this system could result in an unimaginable catastrophe. • The stability of a system relates to its response to inputs or disturbances. A system which remains in a constant state unless affected by an external action and which returns to a constant state when the external action is removed can be considered to be stable. Proportional Integral Derivative Control (PID) • Basic idea behind a PID controller is to read a sensor, then compute the desired actuator output by calculating proportional, integral, and derivative responses and summing those three components to compute the output. • A PID loop is a mathematical formula used to drive a process variable toward a particular value (the set point) and keep it very close to that value by controlling an output. In temperature control, for example, the PID's formula controls the output to maintain the desired temperature. The loop compares feedback from an input to the desired set point, compensates for changes in load, such as an influx of cold air, and adjusts the output accordingly. • • • The input is a measurement of process temperature from a temperature sensor (analogue input). The set point is the desired temperature, perhaps from a thermostat (analogue input) or an operator HMI. The output is a heater control (analogue output). • Proportional Effect : The proportional component depends only on the Error, which is the difference between the Set Point and the Process Variable. • Integral Effect : The Integral components integrates the Error over time to overcome the steady-state error. Therefore, the integral response will continually increase over time unless the Error is zero. • Derivative Effect : This anticipates future behaviour of the Error because the response of the derivative component is proportional to the rate of change of the Error. Therefore, in general the derivative action prevents overshoot and eliminates oscillations. Analogue Signals Sampling • Sampling is the method employed in converting analogue information to digital data. A signal is 'sampled' or measured many times a second and the amplitude of the signal is recorded. This amplitude is translated into digital code as a sequence of 0's and 1's. • The process of sampling, by necessity, causes a loss of information. If we are only sampling at particular times, for instance, all the information between those two times is lost. Also, because digital signals have less accuracy than analogue signals, the sampled values may not even be expressed correctly. The effects of sampling on a signal have a number of names including "Sampling noise", "sampling error", or "converted signal degradation". While this may sound like a terrible situation, there are methods to decreasing this error. • The reconstructed signal is then changed into binary code according to amplitude as shown in the expanded diagram of the reconstructed signal shown below. Digital Filtering In signal processing, the function of a filter is to remove unwanted parts of the signal, such as random noise, or to extract useful parts of the signal, such as the components lying within a certain frequency range. • There are two main kinds of filter, analogue and digital. They are quite different in their physical makeup and in how they work. • An analogue filter uses analogue electronic circuits made up from components such as resistors, capacitors and op amps to produce the required filtering effect. Such filter circuits are widely used in such applications as noise reduction, video signal enhancement, graphic equalisers in hi-fi systems, and many other areas. • There are well-established standard techniques for designing an analogue filter circuit for a given requirement. At all stages, the signal being filtered is an electrical voltage or current which is the direct analogue of the physical quantity (for example, a sound or video signal or transducer output) involved. • A digital filter uses a digital processor to perform numerical calculations on sampled values of the signal. The processor may be a general-purpose computer such as a PC, or a specialized DSP (Digital Signal Processor) chip. The analogue input signal must first be sampled and digitized using an ADC (analogue-to-digital converter). The resulting binary numbers, representing successive sampled values of the input signal, are transferred to the processor, which carries out numerical calculations on them. These calculations typically involve multiplying the input values by constants and adding the products together. If necessary, the results of these calculations, which now represent sampled values of the filtered signal, are output through a DAC (digital-to-analogue converter) to convert the signal back to analogue form. Note that in a digital filter, the signal is represented by a sequence of numbers, rather than a voltage or current. Pulse Width Modulation (PWM) • Pulse Width Modulation, or PWM, is a technique for getting a digital signal to generate an analogue output signal. This is usually used to control the average power to a load in a motor speed control circuit. • Instead of reducing the voltage operating the motor (which would reduce its power), the motor's power supply is rapidly switched on and off. The percentage of time that the power is on determines the percentage of full operating power that is accomplished. This type of motor speed control is easier to implement with digital circuitry. Digital control is used to create a square wave, a signal switched between on and off. This on-off pattern can simulate voltages in between full on (5 Volts) and off (0 Volts) by changing the portion of the time the signal spends on versus the time that the signal spends off. The duration of "on time" is called the pulse width. In the diagram the digital signal (solid line) is at a constant frequency while the pulse width is changed (modulated). The dotted line represents the average signal (if the digital signal is converted to an average). The duty cycle represents the amount of time that the signal is high compared to the amount of time that the signal is low. It is important to remember that the signals above can be generated by simply sending binary signals to the output. As mentioned previously ‘1’ represents an ‘On’ and ‘0’ for ‘Off’. 25% 10001000, 50% 11110000 75% 11101110 To get varying analogue values, you change, or modulate, that pulse width. If you repeat this on-off pattern fast enough with an LED for example, the result is as if the signal is a steady voltage between 0 and 5v controlling the brightness of the LED. Operator Displays • With the various displays available, some of the most convenient and appropriate types of displays for your control system is the 16x2 LCD display, along with other similar sized displays, just for their sheer size, where they can be integrated into many larger products such as: washing machines, fridges, printers etc. • These displays can be connected via the GPIO ports of the controller. The picture to the right shows such a display connected to a Raspberry Pi, and below is a demonstration of how multiple ports are used in combination with logic gates to activate certain parts of the display in order to show relevant information. Simulating Control System Operations • A computer model is a computer program that attempts to simulate a real-life system. In other words, it is a ‘virtual’ version of something in the real-world. • The computer model is designed to behave just like the real-life system. The more accurate the model, the closer it matches real-life. • Computer models are cheaper to setup than alternative methods that could be used to: • • • • • To test a system without having to create the system for real (Building real-life systems can be expensive, and take a long time) To predict what might happen to a system in the future (An accurate model allows us to go forward in virtual time to see what the system will be doing in the future) To train people to use a system without putting them at risk (Learning to fly an airplane is very difficult and mistake will be made. In a real plane mistakes could be fatal!) To investigate a system in great detail Electronic circuits can be modelled in real life using a prototyping board - also known as a breadboard. However, this can be time consuming and uses real components, which can be damaged. Computerised simulation software can be used to test circuits without the need to physically build them. In addition, the computer simulation can be saved and edited. Simulation software can also be used to simulate control programmes for programmable interface controllers as well as control systems in general. Converting the Control Model After you have produced a model and ran a simulation of your control system, you can then make a physical prototype of I, using electronic components connected together temporarily to a controller like the Raspberry Pi. The diagram shows such a set using a breadboard connected to the Raspberry Pi. A breadboard lets you insert electronic components and wires. It can be used to string together a temporary version of your circuit. You don't have to solder wires or anything else. If you change your mind you can replace or rearrange components as you like. You typically create an electronics project on a breadboard to make sure that everything works. This option is generally cheap to implement and allows you to develop your control system how you want. Once you are satisfied, you can build more permanent solution of your control system, where you would solder components together onto a PCB board. Task • Write about all of the following topics: –Control operations •Loop stability •Proportional-integral-derivative control (PID) –Sensor signal conditioning •Analogue signals sampling •Digital filtering –Output •Pulse wide modulators (PWM) •Operator displays –Simulation •Simulating control system operation •Converting the control model • You need to: – – – Write a minimum of 1 ¾ pages (in total) to explain each of the topics above Note down references (e.g. web addresses, books etc.) Make sure the paragraphs are in your own words Assessment & Criteria • Explain the stages of control loop operations - P5 • While covering the following topics: –Control operations •open loop control •closed loop control •feedback •loop stability •proportional-integral-derivative control (PID) •proportional control –Sensor signal conditioning •analogue signals sampling •digital filtering –Output •e.g. pulse wide modulators (PWM) •operator displays –Simulation •simulating control system operation •converting the control model Proportional Control • One of the most used controllers is the Proportional Controller, it produces an output action that is proportional to the error/ difference between the desired output and the actual output. Meaning the more the error (or difference between the desired value and actual value) then the more the output action, (this output action may also be known as correcting signal). • In other words, the correcting signal becomes bigger the bigger the error. So, as the error is reduced the amount of correction is reduced and the correcting process slows down. • The principle aim of proportional control is to control the process as the conditions change. Desired Output The controller will check the error (or difference) between the desired and actual (or measured) outputs then tell the control system to change its output accordingly. This will output change will be large if the error is large Error = Desired Output – Actual Output Proportional Control • Example: Cruise Control – In a proportional control system, the cruise control adjusts the throttle proportional to the error, the error being the difference between the desired speed and the actual speed. – So, if the cruise control is set at 60 mph and the car is going 50 mph, the throttle position will be open quite far. When the car is going 55 mph, the throttle position opening will be only half of what it was before. – The result is that the closer the car gets to the desired speed, the slower it accelerates. Also, if you were on a steep enough hill, the car might not accelerate at all. 50 mph 55 mph Throttle Response Throttle Response Proportional Control • Example: Toilet Cistern – The toilet ball floats on top of the water. It’s attached to a lever that controls the amount of water that gets into the toilet tank. – As the ball drops (perhaps after a flush) the valve opens up wide and lets in a lot of water. The tank fills and the ball floats up. As it gets close to the top it starts to close the valve and the water trickles in at a lower rate. Task • • Write about all of the following topics: – – What is Proportional Control Principles of Proportional Control – Uses of Proportional Control You need to: – Make sure the paragraphs are in your own words – Note down references (e.g. web addresses, books etc.) Assessment & Criteria • Explain the principles and uses of proportional control – M3 For further information please contact The STEM Alliance [email protected] or visit www.STEMalliance.uk