There are two common types of controllers, with many variations and combinations: logic controls, and feedback or linear controls. There is also fuzzy logic, which attempts to combine the easy design of logic with the real-world utility of feedback controls.
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2 Linear controls 3 Fuzzy logic 4 How are these really made? |
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 usually respond to switches or photoelectric cells, and cause the machinery to perform some operation. Logic systems are great for sequencing mechanical operations in places like elevators and factories, but notably poor at managing continuous process controls in such places as oil refineries and steel mills.
Logic systems are quite easy to design, and can handle very complex operations.
Logic systems may be designed with a system similar to boolean logic.
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, control starts. In household thermostats, a simple switch turns the furnace on.
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 that can shatter brickwork or surprise and possibly hurt people near the furnace.
A simple proportional feedback system tends 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.
Often, if the response of the system is slowed down enough to prevent oscillation, the system doesn't respond fast enough to work in normal situations.
To resolve the problems, the most common feedback loop scheme has mathematical extensions to cope with the future and the past. This type of loop is called a proprtional-integral-derivative loop, or PID loop (pronounced pee-eye-dee). If the error curve is graphed over time, the past is considered by adding a number proportional to the area under the curve over a certain amount of time in the past (this is the "integration" part). The future is considered by the adding a number proportional to the slope of a line tangent to the error's curve at the present time (this is the "differential" part). A PID loop always adds its result to the current output, so that it effortlessly floats to a new steady output level.
Most real feedback loops are concerned about wearing out control machinery like valves, by adjusting them many times per second. Therefore, they often have a "deadband," a region around the current value in whcih no control action occurs. In commercial controls, the deadband is programmable.
Another common method is to filter the feedback loop. A filter eliminates undesirable frequencies (cycles) from the system under control, which perfectly eliminates oscillations. Many systems 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, back and up), 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. 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.
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 multipled by input.2", and "or" is "output = 1 - ((1 - input.1) multipled 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 whole point is to make complex control system design possible for less-skilled people. It does this.
Since modern small computers 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 meaurements, and then calculating from this stream of measurements (See digital signal processing).
Computers emulate logic devices by making measurements of switch inputs, then calculating a logic function, 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 tubes.
Feedback controllers have also been constructed hydraulic and pneumatic devices and electronics using both transistors and tubes.
Logic controls
Linear controls
Feedback loops can be combined and modified in many ways. Usually if a system has several mesurements to be controlled, a feedback loop will be present for each of them.Fuzzy logic
How are these really made?
See also distributed control system, programmable logic controller, PID loop, HVAC control system, embedded system, digital signal processing