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Key Concepts

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Stiction. Stiction is the static friction that needs to be overcome to enable relative motion of stationary objects in contact.[1] The term is a portmanteau of the term "static friction",[2] perhaps also influenced by the verb "stick". Any solid objects pressing against each other (but not sliding) will require some threshold of force parallel to the surface of contact in order to overcome static cohesion. Stiction is a threshold, not a continuous force. In situations where two surfaces with areas below the micrometer range come into close proximity (as in an accelerometer), they may adhere together.

At this scale, electrostatic and/or Van der Waals and hydrogen bonding forces become significant. The phenomenon of two such surfaces being adhered together in this manner is also called stiction. Automobiles[edit] This is why driver training courses teach that if a car begins to slide sideways, the driver should try to steer in the same direction as the slide with no brakes. Examples[edit] Blish lock. Deadband. Input and output of the deadband operator. Voltage regulators[edit] In some substations there are regulators that keep the voltage within certain predetermined limits, but there is a range of voltage in-between during which no changes are made, such as, maybe, between 112 to 118 volts (deadband is 6 volts here), or 215 to 225 volts (deadband is 10 volts here).

Backlash[edit] Gear teeth with slop (backlash) exhibit deadband. Hysteresis Vs. Deadband is different from hysteresis. Thermostats[edit] Simple (single mode) thermostats exhibit hysteresis. A thermostat which sets a single temperature and automatically controls both heating and cooling systems without a mode change exhibits a deadband range around the target temperature. Alarms[edit] A smoke detector is also an example of hysteresis, not deadband. References[edit] Johnson, Curtis D. See also[edit] Schmitt trigger. PID controller. Some applications may require using only one or two actions to provide the appropriate system control. This is achieved by setting the other parameters to zero.

A PID controller will be called a PI, PD, P or I controller in the absence of the respective control actions. PI controllers are fairly common, since derivative action is sensitive to measurement noise, whereas the absence of an integral term may prevent the system from reaching its target value due to the control action. History and applications[edit] PID theory developed by observing the action of helmsmen. The Navy ultimately did not adopt the system, due to resistance by personnel. Electronic analog controllers can be made from a solid-state or tube amplifier, a capacitor and a resistor. Most modern PID controllers in industry are implemented in programmable logic controllers (PLCs) or as a panel-mounted digital controller. Control loop basics[edit] The sensed water temperature is the process variable (PV).

Where : Error and . . Control theory. The concept of the feedback loop to control the dynamic behavior of the system: this is negative feedback, because the sensed value is subtracted from the desired value to create the error signal, which is amplified by the controller. Extensive use is usually made of a diagrammatic style known as the block diagram. The transfer function, also known as the system function or network function, is a mathematical representation of the relation between the input and output based on the differential equations describing the system. Although a major application of control theory is in control systems engineering, which deals with the design of process control systems for industry, other applications range far beyond this. As the general theory of feedback systems, control theory is useful wherever feedback occurs; a few examples are in physiology, electronics, climate modeling, machine design, ecosystems, navigation, neural networks, predator-prey interaction, and gene expression.

Overview[edit] Gain scheduling. One or more observable variables, called the scheduling variables, are used to determine what operating region the system is currently in and to enable the appropriate linear controller. For example in an aircraft flight control system, the altitude and Mach number might be the scheduling variables, with different linear controller parameters available (and automatically plugged into the controller) for various combinations of these two variables. A relatively large scope state of the art about gain scheduling has been published in (Survey of Gain-Scheduling Analysis & Design, D.J.Leith, WE.Leithead).[1]