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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. There is also fuzzy logic, which attempts to combine some of the design simplicity of logic with the utility of linear control. Some devices or systems are inherently not controllable. 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. 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. 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 . 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.
Logic controls Pure logic controls were historically implemented by electricians with networks of relays, and designed with a notation called ladder logic. Nowadays, most such systems are constructed with programmable logic controllers. 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. Linear controls Linear controls use negative feedback to keep some desired process within an acceptable range. For example, a thermostat is a simple negative feedback control: when the temperature goes below a threshold, a heater is switched on. Another example is a refrigerator, where the mechanism is switched on when the internal temperature rises. However, a simple logic control like a home thermostat doesn't respond smoothly. In industrial furnaces, it's often better to turn the fuel valve open proportionally to the coldness of the furnace. This avoids sudden shocks to the furnace and applies heat more smoothly. A simple proportional feedback system can either be slow to respond, or can tend to oscillate. In the furnace example, the valve may open and shut indefinitely in a cycle as the furnace heats, and then overruns the target temperature. This is bad because it stresses the system. In a furnace, the constantly turning valve will quickly wear out. More expensively, the fluctuating temperature causes expansion and contraction all through the furnace, causing unnecessary, very expensive mechanical wear. Most systems have similar problems. If the sensitivity of such a simple proportional control system is reduced enough to prevent oscillation, the system responds much slower than it could under skillful manual control. To resolve this problem, the most common feedback control scheme has mathematical extensions to cope with the future and the past. This type of control is called a proportional-integral-derivative control, or PID control (pronounced pee-eye-dee). The derivative part is concerned with the rate-of-change of the error: If the measured variable is approaching the setpoint rapidly, then the actuator is backed off to allow it to coast to the required level; conversely if the measured value begins to fall away rapidly, extra effort is applied in proportion to that rapidity to try to maintain it. The integral term magnifies the effect of long-term steady-state errors, applying ever-increasing effort until these reduce to zero. When correctly tuned to the time-constants of the controlled system, a PID control loop can be surprisingly effective at maintaining effortless control. In many real cases, control system designers have to be concerned about practicalities like wearing out control machinery such as valves, by adjusting them too frequently. Therefore, control systems may have a "deadband," a region around the current value in which no control action occurs. In commercial controls, the deadband is programmable. Another common technique is to filter the feedback loop. A filter may reduce the response of the system to undesirable frequencies, to help eliminate instability or oscillations. Most 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. Some feedback controls operate through complex indirect effects. For example, in an airplane's autopilot, the flight plan in the autopilot determines the desired numbers (where to move) that drive everything. The direction of the airplane is controlled by ailerons, elevators, rudders, etc. Each mechanical control has a differential equation that takes the desired movement in six different axes (roll, pitch, yaw, forward, right and down), and calculates the control's position. Usually each input and output number is filtered for particular oscillations of the aircraft or the control part. Military aircraft can be designed so that the system can adjust to the loss of control surfaces when they are shot away. The most complex linear control systems developed to date are in oil refineries (model predictive control). The chemical reaction paths and control systems are normally designed together using specialized computer-aided-design software. When the automated control-system design techniques pioneered by oil refinery controls were applied to aircraft control systems, they caused a revolution, speeding design times by a hundred-fold or more. Now, the core codes of many modern aircraft autopilots are actually themselves coded by computer programs. Feedback loops can be combined and modified in many ways. Usually if a system has several measurements to be controlled, a feedback loop 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. 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. 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. How are these really made? 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. The feedback controls are simulated by having the computer make periodic measurements and then calculating from this stream of measurements (see digital signal processing). 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. 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). See also | ||||||||
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