Deadband

A deadband (sometimes called a neutral zone) is an area of a signal range or band where no action occurs (the system is dead). Deadband is used in voltage regulators and other controllers. The purpose is common, to prevent oscillation or repeated activation-deactivation cycles (called 'hunting' in proportional control systems).

Voltage regulators

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

Gear teeth with slop (backlash) exhibit deadband. There is no drive from the input to the output shaft in either direction while the teeth are not meshed.

Hysteresis is not deadband

Deadband is different from hysteresis. With hysteresis there is no dead zone, the output is always in one direction or another. Devices with hysteresis have memory, in that previous system states dictate future states; in contrast. Examples of devices with hysteresis are thermostats and smoke alarms.

Thermostats

Thermostats exhibit hysteresis. The furnace in the basement of a house is adjusted automatically by the thermostat to be switched on as soon as the temperature at the thermostat falls to 18 °C, for example, and the furnace is switched off by the thermostat as soon as the temperature at the thermostat reaches 22 °C. There is no temperature at which the house is not being heated or allowed to cool (furnace on or off).

Alarms

A smoke detector is also an example of hysteresis, not deadband. The smoke detector at the ceiling of the kitchen starts the alarm as soon as the level of smoke reaches a certain starting value, x, then the smoke detector stays in the alarm position until the level of smoke has been reduced to level y, after which the smoke detector is reset automatically to "normal". The hysteresis here is x minus y.

References

• Johnson, Curtis D. "Process Control Instrumentation Technology", Prentice Hall (2002, 7th ed.)

(The examples in this entry were incorrectly confusing hysteresis with deadband. However the definition was correct. Need to check this text reference.)

See also

• Schmitt trigger