ADVANCED THEORY – Monitoring and control systems Monitoring systems A computerised monitoring system is what is says a system designed to 'watch' or monitor some state external to the computer system.A house fitted with a burglar alarm system is a monitoring system.The inputs to the computer system are one or more sensors to detect the presence of an intruder movement will 'trigger' the sensor which will send a signal to the computer system.This signal would be interpreted by software and could trigger the sounding of an alarm or report on screen some warning message. In this and other examples, the process is 'automatic monitoring'- meaning there is no human interaction in the monitoring process.The monitoring will be done with periodic sampling of inputs.The process will decide on the polling frequency, that is the time interval between samples. Other monitoring systems include: • a pollution or weather monitoring system, in which data readings from sensors are fed back to the computer system remotely and processed by software to produce graphs (of temperature and wind speed, for example) • a car dashboard display showing the temperature outside the car Control systems Control systems are either'event-driven' or'time-driven'. a computer which is acting as the control system is called a direct digital control computer is the automated control of a condition or process by digital device/computer In an event-driven system, the controller, in response to some event, alters the state of the system: • A robot loads a part into a work area and the part is sensed as present. • The level of liquid plastic in the hopper of an injection-moulding machine is monitored by a low-level switch. When the level reaches a critical low point, the control system opens a valve to start the flow of more plastic into the hopper. • parts moving along a conveyor past an optical sensor are counted. Some control systems are 'time-driven'.The controller takes action at a specific point in time or after a certain time has lapsed: • The factory buzzer sounds a bell at specific times of the day to indicate the start of shift, start and end of the break period and end of the shift. • Heat-treating or paint~spraying operations must be carried out for a certain length of time. • In a washing machine. the agitation cycle is set to operate for a certain length of time, before the controller stops this phase of the wash cycle. All process control systems have the following features in common: • the continual measurement of input variables, such as temperature, pressure and flow rate. • the execution by software of some optimising strategy and the implementation of safety procedures. • the actuation of devices such as valves, switches and output warning lights. Compiled By : Anjalee Rajapakse 1 ADVANCED THEORY – Monitoring and control systems • the generation of equipment performance data showing information about the state of the process in real time. A typical petro-chemical plant may have as many as 2000 parameters which are sampled to control the process. Measured parameters include temperature, pressure flow rates, liquid levels and chemical concentration of liquids. The time between samples will vary between under one second for example, every two minutes.The Direct Digital Control (DDC) computer may control hundreds of feedback loops. If process parameters exceed normal or safe limits then an alarm will be raised and require human intervention.The DDC computer may be one of a number each control a separate process, with each DDC connected to a central computer. The central computer will have authority to change the parameter settings for the various process control loops. Feedback All the above systems use feedback. Feedback from sensors are processed by software. Outputs from the system are then fed back as an input together with new data from the input sensors. Hardware requirements Transducer: a device which converts one form of energy to another Sensor: a device which receives a signal and responds to it Actuator: a device which 'actuates' or moves something connected to it Transducers A transducer is any device which converts one form of energy to another, examples include: • A light bulb converts electrical energy to light by passing an electric current through a filament • An electric motor converts electricity into some form of mechanical energy to produce motion. • A microphone converts sound wave energy into electrical signals. • A loudspeaker converts electrical energy into sound. Compiled By : Anjalee Rajapakse 2 ADVANCED THEORY – Monitoring and control systems Sensors A sensor is a device which receives a signal and responds to it.The sensor must detect the presence of or change in some energy leveI. The signal is then converted to a form which can be understood and quantified by the connected computer system.The sensor is therefore performing two tasks: • acting as a transducer to convert from one energy form to another • quantifying the signal. The signal is produced by some form of energy such as pressure, heat, light or some chemical reaction.The sensor converts this energy into a quantified analogue or digital representation of the input signal.Think what happens when you touch a hot plate. The sensor is the skin on your fingers which then sends a signal to the brain. It is the brain which then decides 'that was and causes the output – the pain! Sensors can be classified as: • thermal: a thermometer measures temperature, a thermocouple gauge measures temperature by its effect on two dissimilar metals, a calorimeter measures the heat produced from a chemical reaction • mechanical: a pressure sensor measures pressure, an altimeter measures altitude from some fixed point, a liquid flow sensor measures flow rate, a gas flow sensor measures velocity and flow rate of a gas. • electrical: an ohmmeter measures resistance, a voltmeter measures current, a galvanometer measures voltage. • chemical: an oxygen sensor measures the percentage of oxygen in a gas and a carbon dioxide sensor detects the presence of CO2 • optical: a light sensor detects light levels, a photocell is a variable resistor which responds to a change in light level, an infra-red sensor detects infra-red radiation waves. Actuators An actuator is a device which 'actuates' or moves something connected to it. An actuator uses some form of energy to produce the motion, and so an actuator is a specific example of a transducer. Most actuators are 'electro-mechanical transducers’: • A strain gauge converts the deformation of an object into an electrical signal. • A galvanometer produces an electric current in a coil. The current produces a magnetic field which is used to create movement. • A generator converts some form of mechanical energy into an electric current. • A motor converts an electric current into mechanical energy to create movement. Another type of transducer is a 'thermo-electric transducer': • A thermocouple converts heat entry into electrical energy • A thermistor is a variable resistor which is affected by temperature change. It converts heat energy to electrical energy Transducers, actuators and sensors are now produced on both the macroscopic (visible by the human eye) and microscopic scale. Nanotechnology is making possible devices on the nanoscale. Regardless of their scale, these devices operate using the same principles. the device is produced on the microscopic scale then the software which communicates with the devices can similarly be contained on a small chip circuit. Compiled By : Anjalee Rajapakse 3 ADVANCED THEORY – Monitoring and control systems Converters and multiplexers Quantities in the physical world, such as temperature and pressure, are analogue by nature. Such sensed data will need to be passed through an analogue-to-digital converter to be sent to the DDC computer A digital-to-analogue converter converts digital data to analogue quantities. All process control systems have the following features in common: • the continual measurement of input variables, such as temperature, pressure and flow rate. • the execution by software of some optimising strategy and the implementation of safety procedures. • the actuation of devices such as valves, switches and warning lights. An example monitoring program Consider the following fragment of pseudocode: EndReadingSensor <— FALSE ReadingOutOfRange <— FALSE REPEAT CALL SensorRead(SensorValue) IF SensorValue > MaximumAllowed THEN ReadingOutOfRange <— TRUE Reading <— 'H' ELSE IF SensorValue < MinimumAllowed THEN ReadingOutOfRange <— TRUE Reading <— 'L' ENDIF ENDIF IF ReadingOutOfRange THEN CALL WarningDisplay(Reading) ENDIF ReadingOutOfRange <— FALSE FOR TimeFiller <—1 TO 999999 ENDFOR UNTIL UNTIL EndReadingSensor Note the following features of the program: • There is an infinite loop. • The loop finishes with another loop that does nothing other than create a delay before the outer loop repeats. • When the sensor reading indicates a problem, the loop calls a procedure to handle whatever notification method is to be used. • Following this call, the loop continues so the Boolean variable has to be reset to prevent the warning procedure being repetitively called. Compiled By : Anjalee Rajapakse 4 ADVANCED THEORY – Monitoring and control systems An example monitoring and control program Consider a system which is controlling an enclosed environment. The environment has a sensor to monitor a property and an actuator to control that property. The following fragment of code might be used: EndReadingSensor <—FALSE READ DesiredOutputLevel REPEAT CALL SensorRead(SensorValue) SensorDifference <— DesiredOutputLevel -SensorValue IF ABS(SensorDifference) < DesiredOutputLevel/100 THEN SensorDifference <—0 ENDIF IF SensorDifference > 0 THEN ActuatorAdjustmentFactor <— SensorDifference/DesiredOutputLevel AdjustmentDirection <— 'up' CALL ActivateActuator(AdjustmentDirection, ActuatorAdjustmentFactor) ENDIF IF SensorDifference < o THEN ActuatorAdjustmentFactor <— ABS(SensorDifference)/DesiredOutputLevel AdjustmentDirection <— 'down' CALL ActivateActuator(AdjustmentDirection, ActuatorAdjustmentFactor) ENDIF FOR TimeFiller <—1 TO 999999 END FOR UNTIL EndReadingSensor Note the following features of the program: • A procedure is called to activate the actuator only if the sensor reading shows a significant change. • The code will only work properly if it can be guaranteed that the activation of the actuator has caused a change in the property before the sensor reading in the next iteration of the loop. Compiled By : Anjalee Rajapakse 5
