监控与控制系统

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

Worked example: the three-mark "describe the differences"

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

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

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.

Worked example: the automatic door

Worked example: a greenhouse

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.

Marks that slip away

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