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5.3.3 Control-feedback


Revision Points:  (Full course)
  • Candidates are expected to:
    • describe the main components of the control-feedback loop of a closed system: input, process, output, feedback.
    • identify a typical application using physical variables such as controlling a greenhouse environment
    • identify a typical application using documents as feedback such as utility billing systems;

A simple feedback loopDEFINITION - Feedback in a computer controlled system takes place when the output from the system has a direct effect on the input.

In many computer controlled systems sensors provide the inputs. If such a system uses feedback then there are also sensors monitoring the outputs.

For example:

  • A sensor monitoring the temperature of a greenhouse that is heated by a computer-controlled heater.
  • A sensor monitoring the level of water in a washing machine that is filled by a computer-controlled pump.

Because the inputs that monitor the outputs are fed back into the system (hence the term feedback) they can be used to control the outputs.  For example:

  • If the input sensor detects that the greenhouse is too hot then the output, the heater, is turned off.
  • If the input sensor detects that the washing machine is full then the output, the pump, is turned off.

Feedback loops in a greenhouse control system:

Inputs:

  • Buttons - used to set the timing and the settings at which the various outputs are triggered.

Inputs with feedback:

  • An automated greenhouse watering system in actionTemperature sensor - measures the air temperature (a physical variable).  Use with a feedback loop to turn on the air heater if the temperature is too low and open the window vents is the temperature is too high.
  • Air humidity sensor - measures the air humidity (a physical variable).  Used with a feedback loop to spray a water mist into the air if the humidity is too low and open the window vents is the humidity is too high.
  • Soil moisture sensor - measures the soil moisture level (a physical variable).  Used with a feedback loop to add water to the soil if it is too dry.
  • Light sensor - measures the light entering the greenhouse (a physical variable).  Used with a feedback loop to close the window shades if the light level is too high and provide artificial lighting if the day length is too short.

Processing:

  • An automated greenhouse vent in the open positionComparing the temperature of the greenhouse with the upper and lower limits that have been set.
  • Comparing the humidity of the greenhouse with the upper and lower limits that have been set.
  • Comparing the soil moisture with the upper and lower limits that have been set.
  • Comparing the light levels in the greenhouse with the upper and lower limits that have been set.
  • Comparing the time with the on/off times set by the operator.

Outputs:

  • LCD screen - used to display the time, the current temperature, humidity and light readings and the limits that have been set.
  • LED's to indicate readings are within the set limits.
  • Heaters - to heat the air in the greenhouse.
  • Lights - to illuminate the plants.
  • Motors - used to open and close the vents in the greenhouse roof.
  • Water pumps - used to spray a water mist into the air or water the plants.

Feedback loops in a central heating control system:

A typical home heating controllerInputs:

  • Buttons - used to set the time and heating options as well as override the programmed timer.

Inputs with feedback:

  • Temperature sensor (hot water tank) - measures the water temperature (a physical variable). Used with a feedback loop to turn off the water heating once the correct water temperature is reached.
  • Temperature sensor (room thermostat) - measures the room temperature (a physical variable). Used with a feedback loop to turn off the central heating once the correct room temperature is reached.

Processing:

  • Comparing the temperature of the room with the set temperature.
  • Comparing the temperature of the hot water with the set temperature.
  • Comparing the time with the on/off times set by the operator.

Outputs:

  • LCD screen - used to display the time and the program settings.
  • LED's to indicate if the water heating (HW) and central heating (CH) are switched on.
  • Boiler on/off switch - used to turn the boiler on and off.
  • Central heating pump - used to pump hot water around the boiler.

A typical utility billUsing documents as feedback in a utility billing system:

Inputs:

  • The Meter reading - entered into a computer database, along with the customer reference.

Documents as feedback:

  • A bill with an estimated reading - the document has an estimated meter reading and an estimated payment due.  The customer can fill in the actual meter reading and return the document.  This feedback will result in a new output - the correct bill.

Processing:

  • Calculating the payment due.
  • Mail-merging the meter reading, the payment due and the customer's name and address to form a personalised bill ready for printing.

Outputs:

  • The printed bill.

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