Topic 7: Control

1. Topic 7: Control

2. The term for a device that converts one type of energy to another is transducer. Both sensors and actuators are transducers. Overview Inputs Sensors capture analog data, such as temperature, pressure, light. This is converted to digital by a AD (analog-to-digital) converter, so that it can be processed by a computer. Processing The microcontroller or CPU makes a decision on the basis of the data input. Ifsome threshold is reached, thensome action is taken. Outputs The system affects the outside world in some way, e.g. turning on a heater, moving a robot arm, adjusting the speed of a motor. The generic term for the mechanism by which a control system acts on its environment is an actuator.

3. Examples of control systems The following are cited by the IB as examples of control systems. You should make sure that you can describe each one of them in terms of inputs, processing and outputs. • Automatic doors • Heating systems • Taxi meters • Elevators • Washing machines • Process control • Device drivers • Domestic robots • GPS systems • Traffic lights

4. Automatic Doors Inputs • Optical motion sensors emit infrared or microwave radiation and detect it rebounding off an approaching person. The amount of radiation detected is converted to a digital value by an AD converter. Processing • If the value corresponding to the amount of radiation reflected back into the sensor exceeds some pre-set threshold, a command is issued by the microcontroller to open the doors. Outputs • Motors attached either to a hydraulic arm, belt or cable pull or slide the doors open. After some pre-set delay, the doors close again.

5. Heating Systems Inputs • Temperature sensors detect ambient temperature and convert the analog data into digital so that it can be interpreted by the microcontroller. (There are lots of ways to measure temperature but a common one is a thermistor, whose resistance changes with temperature.) Processing • If the temperature value is above or below some pre-set threshold, then a signal is sent to the appropriate temperature control device. Outputs • Turning a fan on/off • Turning a heater on/off • Turning an air conditioner on/off • Opening/closing windows

6. Taxi Meters Inputs • Sensors attached to the transmission of the car measure the distance travelled. This is converted into a digital value by an AD converter. • A timer measures the journey time elapsed. Processing • If the taxi travels a preset distance, then an amount is added to the fare total. • If the taxi waits for a preset period, then an amount is added to the fare total. • In practice, the microcontroller will use some algorithm to determine, from the combination of distance travelled and time elapsed, when to increase the fare total. Outputs • An amount is added to the fare total and a small digital display shows the running total.

7. Elevators Inputs • Passengers press buttons to call the elevator. • Passengers press buttons to tell the elevator where to go. • A load sensor converts the current weight of the occupants of the elevator, converts it to a digital value using an AD converter, and sends this value to the microcontroller. Processing • The microcontroller calculates where to go and in what order according to three pieces of information: • Where passengers are waiting • Where passengers want to go • Where the elevator is currently • If the load value exceeds a pre-set threshold, the microcontroller produces some sort of appropriate output. Outputs • The elevator is moved using either a hydraulic ram or a steel rope on a motor-driven pulley. • The doors are opened using cables on a motorized pulley that slide the doors on rollers. • A warning or alarm may sound in the event of excessive load, and the elevator may cease to function until the load is reduced.

8. Washing Machines Inputs • The programme number is entered by the user through buttons • The amount of water in the drum is detected by a pressure sensor beneath the drum (AD conversion) Processing • The microcontroller responds to the programme choice by activating the appropriate cycle • When the amount of water reaches a pre-set level, the microcontroller issues a command to shut off the water supply Outputs • The water supply is turned on when a cycle begins and shut off when it reaches the correct level • The drum turns according to the cycle chosen

9. Device Drivers Inputs • Requests issued by the operating system or application software Processing • Translation of the request into low-level instructions understood by the hardware Outputs • The hardware reacts appropriately

10. Domestic Robots Few models exist yet. Examples are: • Vacuum cleaners • Lawn mowers • Pool cleaners • Security patrollers Inputs • Vacuum cleaners, lawn mowers and pool cleaners all sense the perimeter of the area in which they are to work. They do this with a combination of cameras and pressure sensors. • Security robots also detect movement and/or heat. Processing • Domestic robots build a map of their surroundings from their sensory input. They use this information to decide where to move and how to act. Outputs • Motors controls wheels and cleaning/mowing equipment. • Security robots can also take video of suspicious activity or alert a human.

11. Global Positioning Systems Inputs • High-frequency, low-power radio waves are sent to and received from at least three geostationary satellites. Processing • The amount of time taken for the signal to come back from each satellite allows the handheld unit to calculate its own position. Outputs • The basic information provided by a GPS is your location, but some can also calculate speed and journey duration, together with a route travelled.

12. Traffic Lights There are different types of traffic lights, some for traffic only, some for pedestrians to cross. Inputs • A button is pressed by a pedestrian when they want to cross the road. • Some traffic lights can sense when a car is waiting. • Some traffic lights can be overridden by humans who are watching the traffic through cameras. • Some traffic lights just work on timers. Processing • The system receives a signal from its sensory input requesting that it initiate its sequence. The system calculates how long since it last changed. If this time was sufficiently long ago, it initiates its sequence. Outputs • The lights change according to a pre-set sequence. • A sound alerts visually-impaired pedestrians.

13. Process Control (Generic)This is a generic term for all control systems that perform some task in a way that is regulated by the conditions of the system. Use this example to help you describe a system you have not encountered in the examples provided. Inputs • The system gathers data about the current state of the system from sensors that convert analog data to digital data so that it can be read and understood by the CPU or microcontroller (AD conversion). • Examples are heat, light, pressure, gas concentrations, movement, distance, pH, salinity, saturation, etc. Processing • The microcontroller compares the current state information with pre-programmed thresholds and acts accordingly. • For example if it is too cold, then a heater is turned on, if it movement is detected, an alarm goes off, etc. Outputs • The digital signal from the microcontroller causes some effect in the physical world through an actuator (digital-to-analog or DA conversion). The effect of the actuator is to regulate the state of the system.

14. Microprocessor versus Microcontroller • A microcontroller is made up of a microprocessor (the CPU), plus some ROMand RAM, plus I/O ports, all integrated into a single chip. • Essentially a small CPU with a program written into its ROM • Part of an embedded system (a dedicated, single-purpose computer system forming part of a larger mechanical device)

15. Feedback Input (Sensor) Processing (CPU) Output (Actuator) F E E D B A C K

16. Hardware associated with control systems • Input • Sensors • AD Converters • Processing • Microcontroller/Microprocessor • Output • Actuators • Transducers • Fan, sprinkler, alarm, etc

17. Related topics • Given a description of a control system, you may be asked to: • Construct a flowchart • Write pseudocode

18. Example question • An air conditioning system is activated when the room temperature exceeds 24 degrees Celsius. It is deactivated when the room temperature falls below 22 degrees Celsius. • (a) Explain the role of feedback in this system. • (b) Explain why an analog-to-digital converter is required. • (c) Identify three hardware devices that the system might use. • (d) Draw a flowchart to model the system. • (e) Construct pseudocode for the algorithm described above. • (f) Explain why there is a difference between the temperatures at which the system is activated and deactivated.

19. Social and ethical issues of embedded systems • Tagging prisoners •  Fewer parole violations • Invasion of privacy • Surveillance and CCTV •  Reduction in crime • Invasion of privacy • Information can be misused (e.g. blackmail, discrimination, marketing) • Safety systems •  Greater levels of safety • Overreliance •  Threat from hackers

20. Distributed computing • A distributed system is made up of separate autonomous computers that communicate and coordinate their actions using messages such that they appear to the user to be a single integrated computing facility.

21. Comparison Centralised Distributed Multiple autonomous components Components are not shared by all users Resources may not be accessible Software runs in concurrent processes on different processors Multiple Points of control Multiple Points of failure • One component with non-autonomous parts • Component shared by users all the time • All resources accessible • Software runs in a single process • Single Point of control • Single Point of failure