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[3] [4] With the invention of the low-pressure stationary steam engine there was a need for automatic speed control, and James Watt's self-designed "conical pendulum" governor, a set of revolving steel balls attached to a vertical spindle by link arms, came to be an industry standard. This was based on the millstone-gap control concept. [5]
The Ziegler–Nichols tuning (represented by the 'Classic PID' equations in the table above) creates a "quarter wave decay". This is an acceptable result for some purposes, but not optimal for all applications. This tuning rule is meant to give PID loops best disturbance rejection. [2]
With the present example, if you turn off the hot then (apart from the water actually in flight) the temperature cannot continue to rise, and so PID control is unnecessary. Previously, when talking about the water as it came out of the tap, the example described a 1st order system with a built-in pure time delay.
The Smith predictor (invented by O. J. M. Smith in 1957) is a type of predictive controller designed to control systems with a significant feedback time delay. The idea can be illustrated as follows.
For example, the position of a valve cannot be any more open than fully open and also cannot be closed any more than fully closed. In this case, anti-windup can actually involve the integrator being turned off for periods of time until the response falls back into an acceptable range.
PID controllers do not have this predictive ability. MPC is nearly universally implemented as a digital control, although there is research into achieving faster response times with specially designed analog circuitry. [3] Generalized predictive control (GPC) and dynamic matrix control (DMC) are classical examples of MPC. [4]
A full PID system can be used, but typically the derivative component is removed (or set very low) to prevent noise from the system or measurements from causing unwanted fluctuations. [ 28 ] A STATCOM may also have additional modes besides voltage regulation or VAR control, depending on specific needs of the system.
In a photoionization detector, high-energy photons, typically in the vacuum ultraviolet (VUV) range, break molecules into positively charged ions. [2] As compounds enter the detector they are bombarded by high-energy UV photons and are ionized when they absorb the UV light, resulting in ejection of electrons and the formation of positively charged ions.