IB Computer Science Topic A1.3.6: Use of Control System Components Notes

Control System Components: How a Machine Senses, Decides and Acts

1. A control system replaces a human decision with a mechanical one, so it must be able to measure the world, compare the measurement against a target, and change something physical.
2. Those three abilities are what the five components provide, and the model that connects them is input, process, output and feedback.
3. The same five components appear in a thermostat and in a self-driving car, which is why the model is worth learning once.

The Input, Process, Output and Feedback Model

Input

1. Input arrives from two sources, and a control system usually needs both.
   1. A set point comes from the user and states the target, such as 20 degrees on a thermostat dial.
   2. A measurement comes from a sensor and states the current condition, such as the 17 degrees the room is actually at.
2. A system given only one of the two cannot act, because it either has no target or no idea where it currently stands.

Process

1. The controller subtracts the measurement from the set point to produce an error value.
   1. With a set point of 20 degrees and a measurement of 17 degrees, the error is 3 degrees and its sign says the room is too cold.
2. The control algorithm converts that error into a decision about what to do and how strongly to do it.
3. The error value is the single number the whole system is built to drive towards zero.

Output

1. Output is a signal sent to an actuator, which then makes a physical change.
   1. The controller closes a relay, the relay opens a gas valve, and the boiler fires.
2. Output may be a simple on or off, or a proportional value such as a motor speed or a valve position.

Feedback

1. Feedback sends the new measurement back to the input stage, which turns a single pass into a loop.
   1. The sensor now reads 19 degrees, so the error falls to 1 degree and the controller keeps the boiler on.
   2. The sensor then reads 20 degrees, the error reaches zero, and the controller switches the boiler off.
2. Without this step the boiler would run for a preset time and stop, whether the room had reached 20 degrees or 30 degrees.

The Five Components the Syllabus Names

Controller

1. The controller is the processing element that holds the set point, receives the sensor measurement, runs the control algorithm and issues the output signal.
   1. In a washing machine it is a microcontroller, a single chip carrying a CPU, memory and input and output pins.
   2. In an industrial plant it is a programmable logic controller, chosen because it tolerates heat, vibration and electrical noise.
   3. In a self-driving car it is a bank of processors, because the decisions involve image recognition rather than one subtraction.
2. The controller is the only component that makes a decision, and the other four either feed it or carry out its instructions.

Sensors

1. A sensor measures a physical quantity such as temperature, pressure, light, distance or soil moisture.
   1. A thermocouple reports furnace temperature as a small voltage that rises with heat.
   2. An inductive loop under a road reports whether a vehicle is sitting above it.
2. Sensors are the only route by which the outside world reaches the controller, so a failed sensor makes the system blind rather than merely less accurate.

Actuators

1. An actuator turns the controller's signal into physical movement or physical change.
   1. A solenoid valve opens or closes to start and stop water flow in an irrigation system.
   2. A servo motor turns a steering column to a commanded angle.
   3. A heating element converts an electrical signal into warmth.
2. Sensors and actuators face in opposite directions, since a sensor converts a physical quantity into a signal and an actuator converts a signal into a physical change.

Transducers

1. A transducer is the general category, and both sensors and actuators are kinds of transducer.
   1. A piezoelectric element converts pressure into a voltage, which makes it a sensor.
   2. The same piezoelectric element converts a voltage into movement, which makes it an actuator.
2. Naming a component a transducer says what it does to the signal, not where it sits in the loop.

Control Algorithm

1. The control algorithm decides what output the error value should produce.
   1. On-off control switches the actuator fully on or fully off, which is what a basic thermostat does and why room temperature oscillates around the set point.
   2. Proportional control makes the output proportional to the error, so a large error produces a strong response and a small error a gentle one.
   3. PID control combines the current error, the accumulated past error and the rate at which the error is changing, which is why a well-tuned PID loop settles without overshooting.
2. The algorithm is the part a computer scientist writes, and it is where the same five components produce a system that is smooth or one that hunts constantly.

Open-Loop and Closed-Loop: Whether the Output Is Ever Checked

The distinction is whether feedback exists, and everything else follows from it.

1. A microwave heats for three minutes whether the food ends up warm or scalding, because nothing measures the food.
2. A thermostat keeps measuring, so it still holds 20 degrees when someone opens a window.
3. Open-loop is not a worse design, it is a cheaper one that is correct whenever the disturbance is small or the operator is watching.

Tracing a Complete Loop

1. An automatic greenhouse vent shows all five components in one cycle.
   1. Sensor: a thermistor reports 31 degrees inside the greenhouse.
   2. Controller: a microcontroller holds a set point of 26 degrees and computes an error of 5 degrees.
   3. Control algorithm: proportional control converts an error of 5 degrees into a command to open the vent halfway.
   4. Actuator: a linear motor drives the vent to that position.
   5. Feedback: the thermistor now reports 27 degrees, so the error drops to 1 degree and the vent is eased almost shut.
2. Remove the thermistor and the system becomes open-loop, opening the vent on a timer whether the day is hot or cold.
3. A1.3.7 works through eight real applications of this model, from autonomous vehicles to automatic doors.