On off feedback control system is the simplest control system in control system applications . But this feedback control system used in many applications even today. It can understand easily than other complex control theories.
Actuator is turned On or off by the controller depending on error less or greater than the Zero.
On-Off control is only applicable in applications where the controlled variable varies slowly. We can see as examples temperature control applications such as controls in air conditioning and refrigerator, water level control in liquid tanks, electric iron.
Note that on – off control is not at all applicable in motion control applications. Such as driver less cars, electric power generators (frequency control), auto pilot system etc.
ON – OFF Mode of FB Control.
This is the simplest mode of FB control, in. on. Off controller operates on the control variable only when the it crosses the set point (reference)
The actuator has only two states, namely, fully, On and fully OFF
Take a temperature control problem, for instance. Actual temperature can keep around the set point (reference point/ the desired value) by turning off the heating element.
when a measured temp is anywhere above the set point. likewise, the heater is turn on.
ON-OFF Temperature feedback control action
ON-OFF Mode of Feedback control
- The processes temp is continually cycling peak-to-peak and cycling period will depend on the process characteristics &the load. controller has no control has no control over them.
- The idea on-off controller is not practical as sensor noise and other electrical inter refences may cause the set -point (references value). This can detrimental to the devices such as contactors and valves to prevent this the hysteresis can be added to the controller as given in this. This will prevent output from chattering if the peak – peak noise is less than hysteresis.
ON-OFF Temperature control action with Hysteresis
On off feedback control in a refrigerator
- Compressor – Compress the gas refrigerant making it turn to liquid form and heats up.
- Condenser – Heat exchanging pipes where hot gas refrigerant dissipates heat and condenses to liquid at high pressure.
- Expansion valve – Releases high pressure liquid refrigerant into a low-pressure evaporator region making the liquid to evaporate. This make inside the refrigerator cold. The cold gas refrigerant is sucking compressor to start a new cycle.
- Evaporator – Heat exchanging pipes where liquid refrigerant evaporates.
- Refrigerant – Working Liquid that evaporates inside the refrigerator to create the cold temperature. E.g. Ammonia boils and evaporates at – 270F
Refrigerator feedback Control
- The refrigerator thermostat is usually located inside the refrigerator and has a knob that allows users to adjust it.
- Once a user sets the desired temperature. After the refrigerator thermostat maintains that temperature by sensing internal temperature. After prompting the compressor to turn on or off in response to the difference. When the sensed temperature is less than the set value, the thermostat cut off electricity to the compressor.
Standalone air condition unit
Working principle is same as in the refrigerator, In the refrigerator food us cooled inside the air handling unit (AHU) warm air from the indoor room send through the evaporator coils using air circulation fans. Thermistor senses the temperature of the discharged air controller cuts off the power supply to the compressor when this temperature is below the set temperature is below the set point temperature by user. Just like the compressor/condenser part is keep outside the food compartment in a refrigerator the outdoor unit of the air conditioner keeping outside the indoor room unit of the air conditioner is keeping outside the indoor room. (In a shade where air cooling of the hot refrigerant can happen effectively by the cooling fan.
This is the simplest feedback control method and to see more control methods visit these articles.