Monitoring and control systems
Introduced| English | Meaning |
|---|---|
| preset value/ˈpriːset ˈvæljuː/ | A value chosen or stored in advance for use as an initial setting or comparison target. |
| actuator/ˈæktʃuːeɪtə/ | A device that converts a control signal into a physical action, such as movement. |
| monitoring/ˈmɒnɪtərɪŋ/ | Read sensors, record or display, at most warn. |
| sensors/ˈsensəz/ | Devices that detect physical quantities and produce signals representing their measurements. |
| control system/kənˈtrəʊl ˈsɪstəm/ | control system |
| feedback/ˈfiːdbæk/ | Information about an outcome returned to a person or system to guide subsequent action. |
| thermistor/ˈθɜːmɪstə/ | A resistor whose resistance changes strongly with temperature, commonly decreasing as temperature rises. |
| ADC/ˌeɪ diː ˈsiː/ | A device that converts an analogue input signal into a digital representation with finite resolution. |
| feedback loop/ˈfiːdbæk luːp/ | A control process in which measured output is returned to influence subsequent input or action. |
The teacher who was tired of being interrupted
- In 1883 Warren Johnson, a college teacher in Wisconsin, was fed up with the janitor walking into his classroom to check the heating.
- He built a device with a strip of two metals that bent as the room warmed and cooled. At a set temperature it closed a circuit and rang a bell in the boiler room.
- A sensor, a preset value 预设值, a signal to someone who acts, and a room that is measured again afterwards: the first automatic control loop in a building.
- Today the loop is a processor and an actuator 执行器 instead of a janitor, but the parts are the same.
Monitoring and control
- Both kinds of system read sensors 传感器. The difference is what happens next.
- A monitoring 监控 system collects, records or displays the readings and takes no action, though it may raise a warning: a weather station, a patient monitor that beeps.
- A control system 控制系统 compares each reading with a preset value and, if it is outside the range, sends signals to actuators that change the physical process: a thermostat switching a boiler.
Monitoring reports; control acts and then measures again
The key difference between a monitoring system and a control system is that a control system:
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Both read sensors, but a control system uses the data to act (via actuators), while monitoring only collects and reports.
A pure monitoring system takes action through actuators to change its environment.
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Monitoring only collects and reports data — it does not act. Acting through actuators is what makes it a control system.
Worked example: the three-mark "describe the differences"
- A monitoring system only measures, records or displays the readings, and at most raises a warning.
- A control system compares each reading with a preset value and, if it is outside the range, sends signals to actuators that change the process.
- The change is then measured again, so a control system contains feedback 反馈 and a monitoring system does not.
- Three points, three marks. The test for any system is the second point: does it act on the reading?
A control system compares each sensor reading with a ____ value and acts only if the reading is outside the range.
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The comparison with a stored, preset value is the mark that separates a control system from a device that merely reacts.
Sensors
- A sensor turns a physical quantity into an electrical signal: temperature (a thermistor 热敏电阻 or thermocouple), pressure (a strain gauge), infra-red, sound, light, moisture.
- Most sensor signals are analogue, so an ADC 模数转换器 (analogue-to-digital converter) turns them into binary before the processor can compare them with anything.
- Name the sensor for the quantity: a "temperature sensor" is a fair answer, "thermistor" is a better one.
A thermistor: its resistance changes with temperature
A sensor:
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A sensor measures the world and outputs a signal. An actuator does the reverse — it turns a signal into an action.
Actuators
- An actuator does the reverse of a sensor: it turns a signal from the processor into a physical action.
- Motors open doors and valves, heaters and coolers change a temperature, pumps move water, a buzzer or lamp warns a person.
- The processor's output is digital; where the actuator needs a varying signal, a DAC converts it first.
A motor: an actuator that turns a signal into movement
Match each device to whether it is a sensor (input) or an actuator (output).
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A sensor turns a physical quantity into a signal; an actuator turns a signal into a physical action.
Which is an actuator?
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An actuator carries out an action (motor, valve, heater, buzzer). The others are sensors/inputs.
The control loop
- The sensor measures. The ADC converts the reading to digital. The processor compares it with the preset value.
- If the reading is outside the range, the processor sends a signal to an actuator, which changes the process.
- The sensor measures again, and the loop repeats. That repeated measuring after each action is the feedback loop 反馈回路.
The control feedback loop
Tap round the loop a thermostat or autopilot repeats. A control system doesn't just read the world — it acts, then re-measures, correcting itself again and again.
Put the stages of one pass round a control loop in order.
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Measure, convert, compare, act, measure again. The last step is what makes it feedback.
Worked example: the automatic door
- Describe how a system opens a door when a person is within 2 metres and closes it when nobody is.
- An infra-red or ultrasonic sensor measures the distance to anything in front of the door. The ADC converts the analogue reading and sends it to the processor.
- The processor compares the distance with the preset 2 metres. If it is less, it sends a signal to the actuator, a motor, to open the door.
- The sensor keeps measuring. When no reading below 2 metres arrives, the processor signals the motor to close the door. The repeated measuring is the feedback.
Which of these belong in the automatic door system? Select all that apply.
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Sensor, converter, comparison value and actuator are the working parts. A printer would be monitoring bolted on; it plays no part in opening the door.
Worked example: a greenhouse
- Keep a greenhouse between 20 °C and 26 °C. A thermistor reads the temperature; the ADC digitises it; the processor compares it with the two preset limits.
- Below 20 °C: a signal to the heater actuator switches it on. Above 26 °C: a signal to the motor opens the vent and switches the heater off.
- Inside the range: no action, but the reading is still taken. Then the loop repeats.
- Every control-system answer has these five parts: sensor, ADC, comparison with a preset, actuator, and measuring again.
Why feedback matters
- Without feedback the system cannot tell whether its action worked, so it cannot correct itself or know when to stop. A heater with no temperature feedback heats forever.
- Feedback also lets the system respond to the world changing on its own: a door opened by hand, a cloud over the greenhouse.
- A monitoring system has no feedback because it changes nothing. It can only report or warn.
Why is feedback essential in a control system?
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Feedback closes the loop: the actuator changes the environment, the sensors re-measure it, and the system adjusts — e.g. a thermostat stops heating once the target is reached.
Match each system to its type.
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Raising an alarm is still reporting. Only a system that changes the physical process through an actuator is control.
Marks that slip away
- A monitoring system does not use actuators. If your answer has the weather station switching something on, it has become a control system.
- Sensor signals are analogue. Say the ADC converts them before the processor compares.
- The comparison is with a preset value. "The processor decides" is not a mark; "compares the reading with the stored 2 metres" is.
- Say that the loop repeats. The feedback point is the one most often left out.
You've got it
- monitoring = read sensors, record or display, at most warn; control = compare with a preset value, act through actuators, measure again
- a sensor turns a physical quantity into a signal, through an ADC; an actuator turns a signal into an action
- the loop: sensor → ADC → compare → actuator → measure again; the repeat is feedback
- describe any control system with those five parts, in that order