Two-Position Control

INTRODUCTION :

Two-position control compares the value of an analog or variable input with instructions and generates a digital (two-position) output. The instructions involve the definition of an upper and lower limit. The output changes its value as the input crosses these limit values. There are no standards for defining these limits. The most common terminology used is setpoint and differential. The setpoint indicates the point where the output pulls-in, energizes or is true. The output changes back or drops-out after the input value crosses through the value equal to the difference between the setpoint and the differential.
Two-position control can be used for simple control loops (temperature control) or limit control (freezestats, outside air temperature limits). The analog value can be any measured variable including temperature, relative humidity, pressure, current and liquid levels.
Time can also be the input to a two-position control response. This control response functions like a time clock with pins. The output pulls-in when the time is in the defined on time and drops out during the defined off time.
 
                                                  
Figure shows an example of two-position control in a home heating system, where the thermostat is set to energize the heating system when the space temperature falls below 70 F and turn off when the temperature rises to 72 F in the space. This is an example of a setpoint of 70 F with a two-degree differential.