Two-Position Control
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.
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 3, 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.
Floating Control
Floating control is a control response that produces two possible digital outputs based on a change in a variable input. One output increases the signal to the controlled device, while the other output decreases the signal to the controlled device. This control response also involves an upper and lower limit with the output changing as the variable input crosses these limits. Again, there are no standards for defining these limits, but the terms setpoint and deadband are common. The setpoint sets a midpoint and the deadband sets the difference between the upper and lower limits.
When the measured variable is within the deadband or neutral zone, neither output is energized and the controlled device does not change - it stays in its last position. For this control response to be stable, the sensor must sense the effect of the controlled device movement very rapidly. Floating control does not function well where there is significant thermodynamic lag in the control loop. Fast airside control loops respond well to floating control. An example of floating controls is shown in Figure 4.
Floating Control
Floating control is a control response that produces two possible digital outputs based on a change in a variable input. One output increases the signal to the controlled device, while the other output decreases the signal to the controlled device. This control response also involves an upper and lower limit with the output changing as the variable input crosses these limits. Again, there are no standards for defining these limits, but the terms setpoint and deadband are common. The setpoint sets a midpoint and the deadband sets the difference between the upper and lower limits.
When the measured variable is within the deadband or neutral zone, neither output is energized and the controlled device does not change - it stays in its last position. For this control response to be stable, the sensor must sense the effect of the controlled device movement very rapidly. Floating control does not function well where there is significant thermodynamic lag in the control loop. Fast airside control loops respond well to floating control. An example of floating controls is shown in Figure 4.
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