Dead band

In addition to energy conservation, the variable speed drives offer better control because of a faster response, ie, reduced dead band. They are also sometimes chosen for safety reasons because of elimination of the control station and accompanying valving. The capital saved by use of a smaller motor and elimination of the control valve partially compensates for the cost of the drive. 

With the exception of the dead-band booster, the application of booster relavs has diminished sornewEat bv the increased use of ciir-rent-to-pressiire transducers, electropneiirnatic positioners, and electronic control svsterns. Transducers and valve positioners serve much the same fiinctionalitv as the booster relav in addition to interfacing with the electronic process controller. 

Polymers used for seat and plug seals and internal static seals include PTFE (polytetrafluoroeth ene) and other fluorocarbons, polyethylene, nylon, polyether-ether-ketone, and acetal. Fluorocarbons are often carbon or glass-filled to improve mechanical properties and heat resistance. Temperature and chemical compatibility with the process fluid are the key selec tion criteria. Polymer-lined bearings and guides are used to decrease fric tion, which lessens dead band and reduces actuator force requirements. See Sec. 28, Materials of Construction, for properties. 

Speed variation is the total variation in speed from the set point and includes both dead band and oscillation. 

If a governor starts to cycle after operating for some time, this is generally the resiilt of wear which causes dead band or sticking. Also, all pilot valves should be inspec ted for the effects of dirt in the oil. 

Dead band A range of the controlled variable in which no corrective action is taken. 

Besides, it is also equally important to adjust properly the amplifier gain so as to eliminate completely the dead-band and the oscillations. A recorder having inadequate shielding from the AC circuits may display... 

The mechanical ball and disc integrator is rapid, precise, and accurate. Reproducibility between instruments, individuals, and laboratories is really quite good. The integrator itself is quite economical (less than 1000) but it does require a recorder compatible with the integrator. The limitation in the accuracy appears to be in the recorder itself since it is the recorder pen drive itself that actuates the integrator. Thus any dead band or damping in the response of the recorder will be reflected in the performance of the integrator. 

Dead Band, Dead Space, Dead Zone... 

The output of a dead-zone or dead-band element in response to a sinusoidal input is shown in Fig. 7.82 and is written ... 

A three-state controller is used to drive either a pair of independent two-state actuators, such as heating and cooling valves, or a Didirec-tional motorized actuator. The controller is comprised of two on/off controllers, each with dead band, separated by a dead zone. While the controlled variable lies within the dead zone, neither output is energized. This controller can drive a motorized valve to the point where the manipulated variable matches the load, thereby avoiding cycling. 

One operational problem with analog alarms is that noise in the variable can cause multiple alarms whenever its value approaches a limit. This can be avoided by defining a dead band on the alarm. For example, a high alarm would be processed as follows ... 

Return to normal. The high-alarm return to normal is generated when the value is less than or equal to the high-alarm limit less the dead band. 

As the degree of noise varies from one input to the next, the dead band must be individually configurable for each alarm. 

Static performance measurements related to positioner/actuator operation include the conformity, measured accuracy, hysteresis, dead band, repeatability, and locked stem pressure gain. Definitions and standardized test procedures for determining these measurements can be found in ISA-S75.13, "Method of Evaluating the Performance of Positioners with Analog Input Signals and Pneumatic Output. ... 

A particular type of booster relay, called a dead-band booster, is shown in Fig. 8-88. This booster is designed to be used exclusively between the output of a valve positioner and the input to a pneumatic actuator. It is designed to provide extra flow capacity to stroke the actuator faster than with the positioner alone. The dead-band booster is designed intentionally with a large dead band (approximately 5 percent of the input span), elastomer seats for tight shutoff, and an adjustable bypass valve connected between the input and output of the booster. The bypass valve is tuned to provide the best compromise between increased actuator stroking speed and positioner/actuator stability. 

FIG. 8-88 Dead-band booster relay. (Courtesy Fisher Controls International LLC.)... 

From a dynamic response standpoint, the electronic adjustable-speed pump has a dynamic characteristic that is more suitable in process control applications than those characteristics of control valves. The small amplitude response of an adjustable-speed pump does not contain the dead band or the dead time commonly found in the small amplitude response of the control valve. Nonlinearities associated with friction in the valve and discontinuities in the pneumatic portion of the control valve instrumentation are not present with electronic variable-speed drive technology. As a result, process control with the adjustable-speed pump does not exhibit limit cycles, problems related to low controller gain, and generally degraded process loop performance caused by control valve nonlinearities. 

When constant-speed pumps are used, the booster pump can be started and stopped automatically on the basis of pressure. In this case, an adjustable dead band is provided in the low pressure switch (PSL), which prevents the on-off cycling of the booster pump. The width of the dead band is a compromise as the band is narrowed, the probability of cycling increases, whereas the widening of the band results in extending the periods during which the booster is operated unnecessarily. 

Unless sophisticated speed control power electronics is used, position modulation is achieved via bang-zero-bang control. Mechanical inertia causes overshoot, which is (1) minimized by braking and/or (2) hidden by adding dead band to the position control. Without these... 

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