CS · Chapter 7

Cambridge AS & A Level · Part 1 · Chapter 7

Monitoring and Control Systems

Sensors that watch, actuators that act, and a program in the middle that never stops looping. This page has every figure, question and exam question from the chapter, with labs for feedback loops and bit flags.

Guru Jazzy

Chapter 7 opener: an automated, sensor-controlled environment

🎯 Learning objectives

By the end of this chapter you should be able to:

show that you understand monitoring and control systems
show how bit manipulation can be used to monitor or control a device

📡 7.01 Monitoring systems

A monitoring system watches some physical property of a system. It can be used in two ways:

to build up a record of the system's condition over a period of time
more often, to detect the moment a property goes outside its safe or desired range: for example, a CPU that is running too hot

Discussion point: Give yourself one minute. Think about what is being measured, and count how many different kinds of monitoring system you can name.

Some ideas to compare with yours

CPU temperature in a laptop · a patient's heart rate and blood oxygen in hospital · river levels for flood warnings · air quality (PM2.5) in a city · a smart-meter recording electricity use · weather stations · a burglar alarm · smoke and carbon monoxide alarms · tyre-pressure warnings in a car · a fitness tracker counting steps · earthquake seismometers.

Sensors

With a person in charge, you might read a temperature from a mercury thermometer. In this chapter, a computer or microprocessor is doing the job, so it needs a measuring device whose value can be sent to it. That device is a sensor. For temperature, a thermocouple is one example: its output voltage changes with temperature.

Sensor: a hardware device that measures a property and sends a value to a controlling computer.

Tip: A sensor has no built-in intelligence. It cannot take any action on its own when something goes wrong. If the temperature becomes dangerous, it is the computer that sounds the alarm.

Kinds of sensor

Named after what it measures	Named after how it works	Named generically
temperature, pressure, humidity, carbon monoxide, pH, sound, light	Infrared: a passive infrared (PIR) sensor just measures the infrared it receives; an active one sends out infrared and measures how much is reflected back	Motion sensor: different motion sensors actually measure different physical properties

🎮 Pick the sensor

Which sensor would a computer need for this job?

Question 7.01 How many different types of motion sensor do you know?

Show some answers

Passive infrared (PIR): detects the heat of a moving body (security lights, alarms).
Active infrared beam: something breaking the beam is detected (lift doors, shop doors).
Microwave / radar: measures reflected microwaves (Doppler effect), as in automatic doors.
Ultrasonic: times reflected sound pulses (car parking sensors).
Accelerometer and gyroscope: measure the device's own movement (phones, fitness trackers).
Pressure mat: detects a person stepping on it.
Camera / video motion detection: compares successive images.

🎛️ 7.02 Control systems

A control system does everything a monitoring system does, and it can change what is happening. To act on the system it needs an actuator: an electric motor connected to a controlling device. The actuator might switch something on or off, or adjust a setting.

Sensor: a hardware device that measures a property and sends a value to a controlling computer.

Actuator: a hardware device that receives a signal from a computer and adjusts the setting of a controlling device.

Discussion point: Look back at the monitoring systems you listed. How many were really control systems? Could the true monitoring systems be changed into control systems?

A way to decide

Ask: does the system change the thing it is measuring? A laptop that speeds up its fan when the CPU gets hot is a control system. A river-level gauge that only sends warnings is monitoring. It would become control if it also opened flood gates (an actuator), and its next reading would show whether that worked.

🎮 Monitoring or control?

🔁 A computer-controlled environment Figure 7.01

**Figure 7.01**A computer-controlled environment: sensor → ADC → computer → DAC → actuator

Follow the loop round the diagram:

The sensor in the controlled environment produces an analogue measured value.
An ADC (analogue-to-digital converter) turns it into a digital value for the computer.
The computer checks the value against the desired range.
If action is needed, it sends a digital control signal to a DAC (digital-to-analogue converter).
The DAC's analogue control signal drives the actuator and its controlling device (heater, fan, valve…).

The diagram shows the ADC and DAC as separate boxes. In real systems they are usually built into the sensor or actuator itself.

The process never stops. At regular, timed intervals the computer asks the sensor for a reading. If the value is out of range, the computer starts a control action. The next timed reading then shows the computer what effect that action had. This is feedback, and a control system cannot work without it.

♻️ Closed-loop feedback control Figure 7.02

**Figure 7.02**Closed-loop feedback control: the controller compares the actual output, read by a sensor, with the desired value

A closed-loop feedback control system is a special case where the feedback directly drives what happens next:

A microprocessor acts as the controller.
It compares the actual output, read by the sensor, with the desired output.
It sends the actuator a value that depends on the difference between the two.

🔬 Lab: the greenhouse controller

The controller reads the temperature sensor every sample, compares it with the target and switches the heater (the actuator) on or off. The night outside gets colder as time passes. Turn feedback off to see what happens to a heater on a fixed timer.

Target 22 °C Sample every 2 min

Press Run to start the control loop.

Question 7.02 Where would you be likely to find a closed-loop feedback control system?

Show some answers

Air conditioning or central heating with a thermostat.
A car's cruise control, which holds a set speed up and down hills.
An oven or a 3D printer's heated nozzle and bed.
An aircraft autopilot, or a drone holding its height.
A greenhouse controlling temperature, humidity and watering.
Industrial processes: chemical reactors, pH dosing in water treatment.

🔢 7.03 Bit manipulation to control devices

The controlling computer runs a real-time program that loops forever. As the sensors report, the program sets Boolean flags. With two properties to monitor and control, it could use four flags:

IF SensorDifference1 > 0 THEN Sensor1HighFlag ← TRUE
IF SensorDifference1 < 0 THEN Sensor1LowFlag  ← TRUE
IF SensorDifference2 > 0 THEN Sensor2HighFlag ← TRUE
IF SensorDifference2 < 0 THEN Sensor2LowFlag  ← TRUE

Another part of the program keeps checking whether any flag is set. In machine code, each flag can be a single bit of a byte. The fragments below use the three least significant bits (positions 0, 1 and 2) of the byte at address 0034 as flags.

Goal	Code	How it works
Clear every bit (e.g. at switch-on)	LDD 0034 AND #B00000000 STO 0034	AND with 0 always gives 0, so every bit becomes 0.
Toggle one bit (a problem has appeared, or been solved)	LDD 0034 XOR #B00000001 STO 0034	XOR with 1 flips a bit; XOR with 0 leaves it alone. Only bit 0 changes.
Set one bit to 1, whatever its value (report a condition repeatedly)	LDD 0034 OR #B00000100 STO 0034	OR with 1 forces a 1; OR with 0 changes nothing. Only bit 2 is set.
Test one bit	LDD 0034 AND #B00000010 STO 0034	AND with 0 clears the other bits and keeps bit 1. Comparing the result with 2 (binary 00000010) shows whether bit 1 was set.

In each case, LDD loads the byte into the accumulator, the logic instruction changes it, and STO stores it back at the same address.

Mask: the operand of a bitwise operation. It picks out which bits are affected.

Rule of thumb: OR 1 = set · AND 0 = clear · XOR 1 = toggle · AND 1 = keep (test)

Reflection point: Do you see that a bitwise logic operation works on every bit separately? In effect, all the bits in the accumulator are processed at the same time.

🔬 Lab: the flag register

Tap bits of the register (address 0034) or the mask to change them, then apply an operation. Every bit position is processed at once.

Mask (operand)

🎮 Choose the operation and mask

Pick the bitwise operation, set the 8-bit operand, and check. Your mask must work for any starting value of the register.

🌡️ Packing a reading into two bytes

Exam Question 3 below stores one greenhouse reading in two bytes. This drill practises the format:

Byte 1: bits 7–1 show the location (location 4 sets bit 4). Bit 0 is a flag: 0 = not yet processed, 1 = processed.
Byte 2: the temperature as an 8-bit two's complement integer, so readings below zero are allowed.

🎮 Encode the reading

🚨 Tracing the intruder alarm program

This is the program from exam Question 4. Four sensors are stored in bits 0–3 of SENSORS. The program counts how many sensors are triggered by testing one bit at a time: VALUE takes the values 1, 2, 4, 8 as its mask. Step through it, or pick other sensor values.

Sensors triggered (tap to change)

Instruction	SENSORS	COUNT	VALUE	ACC

📌 Summary

A monitoring system needs sensors.
A sensor measures a physical quantity: temperature, humidity, pH, infrared, pressure, sound, carbon monoxide and many more.
A monitoring and control system needs sensors and actuators.
Its program runs in real time, in an infinite loop that reads the sensors at timed intervals.
If the sensor values show that action is needed, the program sends signals to the actuators.
Bit manipulation in an assembly language program can be used to monitor or control devices.

🃏 Key-term flashcards

Tap a card to flip it.

✅ Quick quiz

📝 Exam-style questions

Write your answer first, then open the model answer. These are my mark-scheme-style points, not official mark schemes.

(i) A monitoring system.

(ii) Sensors.

(iii) Any two, about 3 marks each: name, what it measures, how the value is used.

Temperature sensor: measures the air temperature in the barn. Its value goes through an ADC to the computer, which displays it and flashes the warning if it is outside the range the hens need.
Light sensor: measures light level. Hours of light affect egg laying, so the computer warns if the barn is too dark or bright.
Humidity sensor: measures moisture in the air. Too humid or too dry triggers the warning on screen.
Gas sensor (e.g. ammonia or CO₂): measures air quality, so poor ventilation can be spotted.

(i) A (monitoring and) control system.

(ii) Actuators.

(iii) The computer reads the sensors at regular intervals and compares each value with the stored acceptable range. If a value is outside the range, it sends a signal (through a DAC) to the right actuator, e.g. to switch a heater, fan or lights on or off. The next reading gives feedback on whether the action worked.

Part	Operation	Operand	Why
(a)	AND	00000000	Anything AND 0 is 0.
(b)	OR	00000100	OR 1 sets bit 2; OR 0 leaves the other bits unchanged.
(c)	XOR	00110000	XOR 1 flips bits 4 and 5 (1→0, 0→1); XOR 0 leaves the rest.

(a) A monitoring system (data logging).

(b) Any three, each with a justification:

Temperature sensor: to measure the temperature in the greenhouse.
ADC: the sensor's output is analogue and the computer needs digital values.
Computer / microprocessor or data logger: to request a reading every hour and process it.
Storage (e.g. flash memory or a hard disk): to keep all the readings for the whole season.

(i) Bit 5 is set, so the reading came from location 5. Bit 0 is 1, so the reading has been processed. (Byte 2 = 00011000 = 24 degrees.)

(ii) Byte 1 = 01000000 (bit 6 set, flag 0). Byte 2 = 11111011: +5 = 00000101, flip the bits to 11111010, then add 1.

(i)

LDD 106            // byte 1 into ACC
AND #B00010000     // keep only bit 4
CMP #B00010000     // was bit 4 set? (= denary 16)
JPE LOCATION4      // jump if it came from location 4

(ii) OR #B00000001 sets bit 0 to 1 and leaves every other bit unchanged.

Cambridge International AS & A Level Computer Science 9608, Paper 31 Q5, June 2015

(a)(i) A monitoring system.

(ii) It only reports a condition by sounding an alarm. It does not change the environment, and there is no feedback controlling a process.

(b) Any two, each with a reason:

Passive infrared (PIR) sensor: detects the body heat of a person moving in a room.
Pressure sensor under a mat or floor: detects someone stepping on it.
Magnetic contact switch on a door or window: detects it being opened.
Sound sensor: detects breaking glass or noise in an empty house.

(i) Key values as the program runs:

Pass	VALUE (mask)	SENSORS AND VALUE	COUNT after
1	1	0	0
2	2	2	1
3	4	0	1
4	8	8	2

When VALUE = 8, CMP #8 is true and the program jumps to EXIT, which loads COUNT: ACC = 2.

(ii) #1. JGT ALARM jumps when COUNT > 1, i.e. two or more sensors.

(iii) The instruction CMP #8 (after ZERO: LDD VALUE) becomes CMP #128, so the mask goes on up to bit 7.

Cambridge International AS & A Level Computer Science 9608, Paper 31 Q6, June 2016

Guru Jazzy · Cambridge AS & A Level Computer Science · Chapter 7 Monitoring and Control Systems · Figures from the Cambridge coursebook (Langfield & Duddell)