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What is meant by 3-state controller?

3-state controllers have two outputs which may be either switching or continuous (relay contact or e.g. 4 - 20 mA). 3-state controllers are used if the control variable has to be or can be influenced through two actuators with opposing action.
This may be a climatic cabinet with a thyristor power unit for the electric heating and a solenoid valve for cooling. In this example, a 3-state controller with a continuous (analog) output for the heating function (controller output 1) and a switching output for the cooling function (controller output 2) would be the best choice.
On 3-state controllers, the parameters proportional band, reset time, derivative time and hysteresis, familiar from 2-state controllers, can often be set separately for both operating senses. The 3-state controller additionally features the parameter