A control system is a device or set of devices to manage, command, direct or regulate the behavior of other devices or systems.
There are two common classes of control systems, with many variations and combinations: logic or sequential controls, and feedback or linear controls. A logic gate performs a logical operation on one or more logic inputs and produces a single logic output Feedback is a circular causal Process whereby some proportion of a system's output is returned (fed back to the Input. The word linear comes from the Latin word linearis, which means created by lines. There is also fuzzy logic, which attempts to combine some of the design simplicity of logic with the utility of linear control. Fuzzy logic is a form of Multi-valued logic derived from Fuzzy set theory to deal with Reasoning that is approximate rather than precise Logic is the study of the principles of valid demonstration and Inference. Some devices or systems are inherently not controllable. Controllability is an important property of a Control system, and the controllability property plays a crucial role in many control problems such as stabilization of unstable
The term "control system" may be applied to the essentially manual controls that allow an operator to, for example, close and open a hydraulic press, where the logic requires that it cannot be moved unless safety guards are in place.
An automatic sequential control system may trigger a series of mechanical actuators in the correct sequence to perform a task. An actuator is a mechanical device for moving or controlling a mechanism or system For example various electric and pneumatic transducers may fold and glue a cardboard box, fill it with product and then seal it in an automatic packaging machine.
In the case of linear feedback systems, a control loop, including sensors, control algorithms and actuators, is arranged in such a fashion as to try to regulate a variable at a setpoint or reference value. Feedback is a circular causal Process whereby some proportion of a system's output is returned (fed back to the Input. A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument Reference value is a term used in medicine to denote a laboratory value used as a Reference for values obtained by laboratory examinations of patients or samples (blood An example of this may increase the fuel supply to a furnace when a measured temperature drops. PID controllers are common and effective in cases such as this. A proportional–integral–derivative controller (PID controller is a generic Control loop Feedback mechanism widely used in industrial Control systems Control systems that include some sensing of the results they are trying to achieve are making use of feedback and so can, to some extent, adapt to varying circumstances. Open-loop control systems do not directly make use of feedback, but run only in pre-arranged ways. An open-loop controller, also called a non-feedback controller, is a type of controller which computes its input into a system using only the current state
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Logic control
Pure logic control systems were historically implemented by electricians with networks of relays, and designed with a notation called ladder logic. Ladder logic is a philosophy of drawing electrical logic schematics Today, most such systems are constructed with programmable logic devices. A programmable logic device or PLD is an electronic component used to build reconfigurable Digital circuits Unlike a Logic gate, which has a
Logic controllers may respond to switches, light sensors, pressure switches etc and cause the machinery to perform some operation. Logic systems are used to sequence mechanical operations in many applications. Examples include elevators, washing machines and other systems with interrelated stop-go operations.
Logic systems are quite easy to design, and can handle very complex operations. Some aspects of logic system design make use of Boolean logic. Boolean logic is a complete system for Logical operations It was named after George Boole, who first defined an algebraic system of
On-off control
For example, a thermostat is a simple negative-feedback control: when the temperature (the "measured variable" or MV) goes below a set point (SP), the heater is switched on. A thermostat is a device for regulating the Temperature of a System so that the system's temperature is maintained near a desired setpoint Another example could be a pressure-switch on an air compressor: when the pressure (MV) drops below the threshold (SP), the pump is powered. Refrigerators and vacuum pumps contain similar mechanisms operating in reverse, but still providing negative feedback to correct errors.
Simple on-off feedback control systems like these are cheap and effective. In some cases, like the simple compressor example, they may represent a good design choice.
In most applications of on-off feedback control, some consideration needs to be given to other costs, such as wear and tear of control valves and maybe other start-up costs when power is reapplied each time the MV drops. Control valves are Valves used mainly within industrial plants to control operating conditions such as Temperature, Pressure, Flow, and Therefore, practical on-off control systems are designed to include hysteresis, usually in the form of a deadband, a region around the setpoint value in which no control action occurs. A system with hysteresis can be summarised as a system that may be in any number of states independent of the inputs to the system A Deadband (sometimes called a neutral zone) is an area of a signal Range or band where no action occurs (the system is Dead) The width of deadband may be adjustable or programmable.
Linear control
Linear control systems use linear negative feedback to produce a control signal mathematically based on other variables, with a view to maintaining the controlled process within an acceptable operating range. A linear system is a mathematical model of a System based on the use of a Linear operator. Feedback is a circular causal Process whereby some proportion of a system's output is returned (fed back to the Input. In Mathematics, a continuous function is a function for which intuitively small changes in the input result in small changes in the output
The output from a linear control system into the controlled process may be in the form of a directly variable signal, such as a valve that may be 0 or 100% open or anywhere in between. Sometimes this is not feasible and so, after calculating the current required corrective signal, a linear control system may repeatedly switch an actuator, such as a pump, motor or heater, fully on and then fully off again, regulating the duty cycle using pulse-width modulation. In Telecommunications and Electronics, the term duty cycle is used to describe the fraction of time that a system is in an "active" state Pulse-width modulation (PWM of a signal or power source involves the Modulation of its Duty cycle, to either convey information over a
Proportional control
When controlling the temperature of an industrial furnace, it is usually better to control the opening of the fuel valve in proportion to the current needs of the furnace. For other uses of this term see Industry (disambiguation An industry (from Latin industrius, "diligent industrious" A furnace is a device used for Heating The name derives from Latin fornax, Oven. For other uses see Valve (disambiguation. For the electronic component see Thermionic valve. This helps avoid thermal shocks and applies heat more effectively.
Proportional negative-feedback systems are based on the difference between the required set point (SP) and measured value (MV) of the controlled variable. This difference is called the error. Power is applied in direct proportion to the current measured error, in the correct sense so as to tend to reduce the error (and so avoid positive feedback). The amount of corrective action that is applied for a given error is set by the gain or sensitivity of the control system. In Electronics, gain is a measure of the ability of a circuit (often an Amplifier) to increase the power or Amplitude of a
At low gains, only a small corrective action is applied when errors are detected: the system may be safe and stable, but may be sluggish in response to changing conditions; errors will remain uncorrected for relatively long periods of time: it is over-damped. Damping is any effect either deliberately engendered or inherent to a system that tends to reduce the amplitude of Oscillations of an oscillatory system If the proportional gain is increased, such systems become more responsive and errors are dealt with more quickly. There is an optimal value for the gain setting when the overall system is said to be critically damped. Damping is any effect either deliberately engendered or inherent to a system that tends to reduce the amplitude of Oscillations of an oscillatory system Increases in loop gain beyond this point will lead to oscillations in the MV; such a system is under-damped. Damping is any effect either deliberately engendered or inherent to a system that tends to reduce the amplitude of Oscillations of an oscillatory system
Under-damped furnace example
In the furnace example, suppose the temperature is increasing towards a set point at which, say, 50% of the available power will be required for steady-state. At low temperatures, 100% of available power is applied. When the MV is within, say 10° of the SP the heat input begins to be reduced by the proportional controller. (Note that this implies a 20° "proportional band" (PB) from full to no power input, evenly spread around the setpoint value). At the setpoint the controller will be applying 50% power as required, but stray stored heat within the heater sub-system and in the walls of the furnace will keep the measured temperature rising beyond what is required. At 10° above SP, we reach the top of the proportional band (PB) and no power is applied, but the temperature may continue to rise even further before beginning to fall back. Eventually as the MV falls back into the PB, heat is applied again, but now the heater and the furnace walls are too cool and the temperature falls too low before its fall is arrested, so that the oscillations continue.
Over-damped furnace example
The temperature oscillations that an under-damped furnace control system produces are unacceptable for many reasons, including the waste of fuel and time (each oscillation cycle may take many minutes), as well as the likelihood of seriously overheating both the furnace and its contents.
Suppose that the gain of the control system is reduced drastically and it is restarted. As the temperature approaches, say 30° below SP (60° proportional band or PB now), the heat input begins to be reduced, the rate of heating of the furnace has time to slow and, as the heat is still further reduced, it eventually is brought up to set point, just as 50% power input is reached and the furnace is operating as required. There was some wasted time while the furnace crept to its final temperature using only 52% then 51% of available power, but at least no harm was done. By carefully increasing the gain (i. e. reducing the width of the PB) this over-damped and sluggish behavior can be improved until the system is critically damped for this SP temperature. Doing this is known as 'tuning' the control system. A well-tuned proportional furnace temperature control system will usually be more effective than on-off control, but will still respond slower than the furnace could under skillful manual control.
PID control
Apart from sluggish performance to avoid oscillations, another problem with proportional-only control is that power application is always in direct proportion to the error. A proportional–integral–derivative controller (PID controller is a generic Control loop Feedback mechanism widely used in industrial Control systems In the example above we assumed that the set temperature could be maintained with 50% power. What happens if the furnace is required in a different application where a higher set temperature will require 80% power to maintain it? If the gain was finally set to a 50° PB, then 80% power will not be applied unless the furnace is 15° below setpoint, so for this other application the operators will have to remember always to set the setpoint temperature 15° higher than actually needed. This 15° figure is not completely constant either: it will depend on the surrounding ambient temperature, as well as other factors that affect heat loss from or absorption within the furnace.
To resolve these two problems, many feedback control schemes include mathematical extensions to improve performance. The most common extensions lead to proportional-integral-derivative control, or PID control (pronounced pee-eye-dee). A proportional–integral–derivative controller (PID controller is a generic Control loop Feedback mechanism widely used in industrial Control systems
Derivative action
The derivative part is concerned with the rate-of-change of the error with time: If the measured variable approaches the setpoint rapidly, then the actuator is backed off early to allow it to coast to the required level; conversely if the measured value begins to move rapidly away from the setpoint, extra effort is applied — in proportion to that rapidity — to try to maintain it. In Calculus, a branch of mathematics the derivative is a measurement of how a function changes when the values of its inputs change
Derivative action makes a control system behave much more intelligently. On systems like the temperature of a furnace, or perhaps the motion-control of a heavy item like a gun or camera on a moving vehicle, the derivative action of a well-tuned PID controller can allow it to reach and maintain a setpoint better than most skilled human operators could.
If derivative action is over-applied, it can lead to oscillations too. An example would be a temperature that increased rapidly towards SP, then halted early and seemed to "shy away" from the setpoint before rising towards it again.
Integral action
The integral term magnifies the effect of long-term steady-state errors, applying ever-increasing effort until they reduce to zero. In the example of the furnace above working at various temperatures, if the heat being applied does not bring the furnace up to setpoint, for whatever reason, integral action increasingly moves the proportional band relative to the setpoint until the time-integral of the MV error is reduced to zero and the setpoint is achieved. The European Space Agency 's INTErnational Gamma-Ray Astrophysics Laboratory ( INTEGRAL) is detecting some of the most energetic radiation that comes from space
Other techniques
Another common technique is to filter the MV or error signal. Electronic filters are Electronic circuits which perform Signal processing functions specifically intended to remove unwanted signal components and/or enhance wanted Such a filter can reduce the response of the system to undesirable frequencies, to help eliminate instability or oscillations. Some feedback systems will oscillate at just one frequency. By filtering out that frequency, one can use very "stiff" feedback and the system can be very responsive without shaking itself apart.
The most complex linear control systems developed to date are in oil refineries (model predictive control). Model Predictive Control, or MPC is an advanced method of Process control that has been in use in the process industries such as Chemical plants and The chemical reaction paths and control systems are normally designed together using specialized computer-aided-design software.
Feedback systems can be combined in many ways. One example is cascade control in which one control loop applies control algorithms to a measured variable against a setpoint, but then actually outputs a setpoint to another controller, rather than affecting power input directly.
Usually if a system has several measurements to be controlled, feedback systems will be present for each of them.
Fuzzy logic
Fuzzy logic is an attempt to get the easy design of logic controllers and yet control continuously-varying systems. Fuzzy logic is a form of Multi-valued logic derived from Fuzzy set theory to deal with Reasoning that is approximate rather than precise Basically, a measurement in a fuzzy logic system can be partly true, that is if yes is 1 and no is 0, a fuzzy measurement can be between 0 and 1.
The rules of the system are written in natural language and translated into fuzzy logic. For example, the design for a furnace would start with: "If the temperature is too high, reduce the fuel to the furnace. If the temperature is too low, increase the fuel to the furnace. "
Measurements from the real world (such as the temperature of a furnace) are converted to values between 0 and 1 by seeing where they fall on a triangle. Usually the tip of the triangle is the maximum possible value which translates to "1. "
Fuzzy logic then modifies Boolean logic to be arithmetical. Boolean logic is a complete system for Logical operations It was named after George Boole, who first defined an algebraic system of Usually the "not" operation is "output = 1 - input," the "and" operation is "output = input. 1 multiplied by input. 2," and "or" is "output = 1 - ((1 - input. 1) multiplied by (1 - input. 2)). "
The last step is to "defuzzify" an output. Basically, the fuzzy calculations make a value between zero and one. That number is used to select a value on a line whose slope and height converts the fuzzy value to a real-world output number. The number then controls real machinery.
If the triangles are defined correctly and rules are right the result can be a good control system.
When a robust fuzzy design is reduced into a single, quick calculation, it begins to resemble a conventional feedback loop solution. For this reason, many control engineers think one should not bother with it. However, the fuzzy logic paradigm may provide scalability for large control systems where conventional methods become unwieldy or costly to derive.
Fuzzy electronics is an electronic technology that uses fuzzy logic instead of the two-value logic more commonly used in digital electronics. Fuzzy electronics is an electronic technology that uses Fuzzy logic, instead of the two-value logic more commonly used in Digital electronics. Digital electronics are Electronics systems that use Digital signals Digital electronics are representations of Boolean algebra also see
Physical implementations
Since modern small microcontrollers are so cheap (often less than $1 US), it's very common to implement control systems, including feedback loops, with computers, often in an embedded system. An embedded system is a special-purpose Computer system designed to perform one or a few dedicated functions often with Real-time computing constraints The feedback controls are simulated by having the computer make periodic measurements and then calculating from this stream of measurements (see digital signal processing, sampled data systems). Digital signal processing ( DSP) is concerned with the representation of the signals by a sequence of numbers or symbols and the processing of these signals A sampled-data system is a Control system where a continuous-time Plant (control theory is controlled with a digital device
Computers emulate logic devices by making measurements of switch inputs, calculating a logic function from these measurements and then sending the results out to electronically-controlled switches.
Logic systems and feedback controllers are usually implemented with programmable logic controllers which are devices available from electrical supply houses. A programmable logic controller ( PLC) or programmable controller is a Digital computer used for Automation of industrial processes such as They include a little computer and a simplified system for programming. Most often they are programmed with personal computers.
Logic controllers have also been constructed from relays, hydraulic and pneumatic devices, and electronics using both transistors and vacuum tubes (feedback controllers can also be constructed in this manner). A relay is an electrical Switch that opens and closes under the control of another Electrical circuit. For the mechanical technology see Hydraulic machinery and Hydraulic cylinder Hydraulics is a topic of science and Engineering Pneumatics, Pressurized gas to affect mechanical motion Pneumatic power is used in Industry, where it is common to have factory units plumbed for Compressed Electronics refers to the flow of charge (moving Electrons through Nonmetal conductors (mainly Semiconductors, whereas electrical In Electronics, a transistor is a Semiconductor device commonly used to amplify or switch electronic signals This article is about the electronic device not an evacuated pipe used for experiments in Free-fall.
See also
- Control theory
- Perceptual control theory
- Distributed control system
- Programmable logic controller
- Programmable automation controller
- PID controller
- HVAC control system
- Control engineering
- Sampled data systems
- Building automation
- VisSim
- EPICS
- SCADA
- Coefficient diagram method
- Education and training of electrical and electronics engineers
- Process control
- Process optimization
- Networked control system
- Hierarchical control system
- Motion control
External links
- Semiautonomous Flight Direction - Reference unmannedaircraft.org
- Very new wiki site for all things Controls
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