Industrial Control Systems Notes

Objectives

• At the end of this section, students should be able to:
  • List the classifications of industrial control systems.
  • Describe the differences among industrial control system types with examples.
  • Define terms associated with industrial control systems:
    • Servos
    • Servomechanisms
    • Batch
    • Continuous
    • Instrumentation
  • Describe the differences between open and closed-loop systems.
  • Define terms associated with open and closed-loop systems:
    • Negative feedback
    • Controlled variable
    • Measurement device
    • Feedback signal
    • Setpoint
    • Error detector
    • Error signal
    • Controller
    • Actuator
    • Manufacturing Process
    • Disturbance
    • Measured variable
    • Manipulated variable
    • Controller output signal
  • List the factors that affect the dynamic response of a closed-loop system.
  • Describe the operation of feed-forward control.
  • List three factors that cause the controlled variable to differ from the setpoint.

Introduction

• Between WWI and WWII, feedback control was developed which allowed manually-operated machines to be replaced by automated systems.
• Manufacturing operations rely on automation to produce products efficiently, with higher productivity, quality, and reliability, and with lower waste than manual processes.
• An Industrial Controls System automatically monitors a process and takes appropriate corrective actions as needed.

Industrial Control Classifications

• Industrial control systems are classified by what they control:
  • Motion control
  • Process control

Motion Control

• A motion control system controls the physical position or motion of an object.
• Examples:
  • Industrial robotic arm for assembly or welding
  • Computer Numeric Controlled (CNC) machine tool equipment
  • Printers, copiers, and packaging machines
• Motion control systems have three common characteristics:
  • Motion control devices to control position, velocity, or acceleration/deceleration of a mechanical object.
  • Measurement devices to measure the motion/position of the mechanical object.
  • General response time is less than a second, faster than process control systems.
• Motion control systems are also called servomechanisms, or servos.

Process Control

• In a process control system, variables are monitored and regulated to ensure successful manufacturing.
• Variables may include:
  • Temperature
  • Pressure
  • Flow rate
  • Liquid/solid level
  • pH
  • Humidity
• The purpose of process control is to account and correct for outside disturbances to the system.
• Emphasis is placed on maintaining system parameters at a constant rate; the setpoint is generally not changing.
• Process control system response time is generally slower than motion control, ranging from seconds to minutes.
• Process control is more often encountered in manufacturing.
• Two types of process control systems:
  • Batch processes
  • Continuous processes
• Process control is also referred to as instrumentation.

Batch Processes

• Batch processing is a sequence of timed operations executed on the product being manufactured.
• Example: Cookie-making machine
  • Ingredients are added in correct proportions, mixed, dough is deposited, oven is turned on, cookies are sent through at a prescribed rate until done, then cooled and packaged.
• This process is applied to other foods, petroleum products, soap and cleaning products, and even medicines.
• Batch processing is useful when the same equipment can be used to make a variety of similar products.
  • Example: Oatmeal-Raisin, Peanut Butter, and Chocolate Chip cookies can all be made by the same system by modifying the ingredients.
• Batch processing is also called sequence or sequential processing.

Continuous Processes

• In continuous process systems, operations are performed on the product as it passes through the system.
• Raw materials enter continuously, and the intermediate product leaves at each process step.
• A continuous process can last for hours, days, even weeks without interruption.
• Timing, speed, temperature, feedrate, etc. are crucial to maintaining the stable production system.
• Example: Paper production
  • Pulp is produced, placed on screens, fed through rollers, and eventually comes out as paper.
  • Water, temperature, and speed are constantly monitored.
• Wire, textiles, and plastic bags also use continuous manufacturing.

Open-Loop and Closed-Loop Control Systems

• Whether a system is motion control or process control, the purpose of any control system is to maintain one or more variables at a desired value.
• Industrial control systems are also classified by how they control variables:
  • Open-loop control – Manually tuned control
  • Closed-loop control – Automated control

Open-Loop Systems

• Simplest way to control a system— “Set it and forget it!”
• Often sufficient for manufacturing applications. Example: setting the mixer to run for a specified time when mixing cookie dough.
• No feedback—no way to tell if the desired values are actually being achieved; the human performs this function.
  *Example: A driver controlling the speed of a car will need to adjust the position of the accelerator pedal to increase or decrease throttle to maintain speed when going uphill, downhill, or on flat ground, accordingly.
• Example: Valve is set so that the flowrate of the water entering the tank is equal to the flowrate leaving the tank, achieving a steady-state condition.
• Disturbance of the system will upset this balance.
  • Heavy rainfall causes more flow into the tank—without correction (closing the valve somewhat) it will eventually overflow.
  • During a drought, evaporation lowers the water level—without correction (opening the valve somewhat) the water level could become too low.

Open-Loop Control Diagram

• Environment disturbances affect the system/process plant, influencing the controlled variable (output/actual state).
• Input setpoint goes to the controller, producing a control signal that affects the actuator.
• The actuator influences the manipulated variable, which in turn affects the system/process plant and its output.

Elements of a Control System

• Setpoint: the desired condition of the controlled variable; also called command or reference.
• Controlled variable: the system parameter that we are desiring to control.
• Controller: the “brain” of the system; takes the error signal as an input and produces an output corrective action to return the controlled variable to the desired condition. Controllers are usually electronically-operated, though older devices may be pneumatic. Generally take the form of hardwired circuitry, computer software, programmable logic controllers (PLCs), or panel-mounted microprocessor controllers.
• Disturbance: a factor that upsets the manufacturing process causing a change in the controlled variable. Example: For the reservoir, rainfall and evaporation are disturbances.
• Manufacturing process: the operation performed by the actuator to control a physical variable, such as the motion of a machine or the processing of a liquid.

Manufacturing Process (The “Plant”)

• The manufacturing process is composed of mechanical, electrical, fluid, and thermal elements – the physical make-up of the system.
• These physical elements are properties of the system and cannot be changed without redesigning the system – which is costly!
• The physical elements in a controlled system cause the system to exhibit certain response dynamics.
• The physics of a system may exhibit strong inertia – it is not easily affected by disturbances, or rather, disturbances have to be quite large to cause unacceptable response.
• Many systems are highly susceptible to disturbances, or rather their inertia is relatively small with respect to the magnitude of the disturbance.
• We assume that these elements are fixed parameters of the system.

Actuator and Manipulated Variable

• Actuator: the “muscle” of the system; receives the signal from the controller and performs some action (alters some type of energy or fuel supply) to cause the controlled variable to meet the setpoint. Examples are louvers, hydraulic cylinders, pumps, and motors; also called the final control element or final correcting device.
• Manipulated variable: the amount of fuel or energy that is altered by the actuator. Example: In the reservoir, the flow rate is the manipulated variable, the valve is the actuator, and the flowrate affects the height of the water (controlled variable).

Open-Loop “Controller”

• The open loop controller is generally a simple function.
  • Timed operations
  • Generalized tuning
  • Look-up table
• Could be as simple as a worker manually adjusting a flow control valve periodically.

Feedback Control

• Industrial automated control is often performed using closed-loop systems.
• The term loop comes from the measured value of the measured variable feeding back to the comparator to determine the system error, forming a “closed loop”.
• The basic concept of feedback control is that an error must exist at the output before some corrective action can be made. Only if error is present is corrective action needed.

Closed-Loop Systems

• It is often necessary for a system to monitor the level of certain variables and perform self-correction to maintain those variables at their desired value.
• Such systems are referred to as “closed-loop” systems.
• The automated systems use a feedback loop to keep track of how closely the system is maintaining the commanded values of the variables.
• Most automated manufacturing systems use closed-loop control, designed to produce a continuous balance.

Closed-Loop Systems Example

• Reservoir with an adjustable valve connected to a float.
• As the water level drops, the float sinks, opening the valve.
• As the water level increases, the float rises, causing the valve to close.
• The desired water level is adjusted by changing the length of rod A.

Closed-Loop (Feedback) Control Diagram

• Comparator compares the input setpoint with the feedback signal.
• The controller receives the error signal and sends a control signal to the actuator.
• The actuator manipulates the variable within the system/process plant.
• The feedback sensor measures the output (controlled variable/actual state) and sends a feedback signal to the comparator.
• Disturbances and measurement noise affect the system.

Controlled Variable vs Measured Variable

• Controlled variable: the actual variable (also called parameter or factor) which is to be maintained at a specified value (level). Example: Water level in the tank.
• Measured variable: the physical variable which is measured using some device to produce a value which may be related back to the level of the controlled variable at the moment in time of the measurement. Example: Pressure measured at the bottom of the tank can be measured can the level of water in the tank at this moment in time can be calculated from this measured pressure.

Elements of Closed-Loop Systems

• Comparator: compares the setpoint value to the feedback signal and produces and output signal proportional to the difference between them; also called error detector or summing junction.
• Error signal: output of the error detector; this value represents the difference between the state of the measured value and the setpoint.
• Feedback Sensor: the “eye” of the system; some device which produces an output signal that represents the status of the controlled variable; also called the measurement device, sensor, transducer, or detector.
• Feedback signal: the output of the measurement device; also called measured value or measurement signal. This is the value which will be fed back to determine the system error.

Reasons for Error

• Three ways error occurs:
  • The setpoint is changed. Example: For the reservoir, the setpoint is changed by moving the float along the linkage.
  • A disturbance appears. Example: Rainfall and evaporation cause an external disturbance.
  • The load demand varies. If the water level in the irrigation system lowers, the decreased backpressure on the outlet will increase the flowrate through it, causing an increased demand at the inlet to maintain the water level.
• Changes in the setpoint and load demand are normal. Disturbances are unwanted.

Positive Feedback Control

• Feedback signals are either positive or negative.
• Positive, or regenerative, feedback adds to the setpoint, and aids the command input signal. This type of feedback is used in radios to amplify weak signals and improve weak reception.
• The trouble with positive feedback is that it is prone to instability. Consider the screeching sound in a loud speaker system, caused by the microphone picking up the output of the speaker, amplifying it again, and sending it back through (in a loop).
• When positive feedback is used for industrial control applications, the input usually loses control over the output.

Negative Feedback Control

• In negative feedback, the measurement is subtracted from the setpoint, producing a signal which represents the difference between the actual an desired state of the controlled variable.
• Example: The central heating system in a house uses negative feedback. The thermostat monitors the room temperature and compares it to the reference setting.

Negative Feedback Example: Cruise Control

• Desired speed is set by an electronic mechanism, usually on the steering wheel.
• A Hall-effect sensor (measures rotation) generates a signal proportional to the actual speed of the vehicle.
• An electronic error detector (comparator) determines the difference between the setpoint and the measurement, and sends an error signal to the controller.
• The controller sends a signal to the actuator, which controls fuel flow to the engine.
• If the car is ascending a hill, the car will begin to slow down and more fuel is needed to increase the speed of the car.

Practical Feedback Applications

• A heat exchanger is a commonly occurring application of feedback control.
• Cold water enters the lower pipe, passes over a series of coils, and exits through the pump at the top.
• Steam passes through the coils. As the water (or other liquid) passes over the coils, heat energy is transferred to the water.

Dynamic Response of a Closed-Loop System

• The system response is not immediate.
• A series of factors contribute to the time delay for a system to reestablish a balanced condition.
• The measure of a loop’s corrective action as a function of time is referred to as the system’s dynamic response.
• Response time of the instruments in the control loop, including the sensor, controller, and actuator.
• Time lag: The time delay between when an instrument receives a change in the input and when it produces a modified output.
• The time duration as the signal is passed from one instrument in the loop to another.
• The static inertia of the controlled variable; the variable opposes being changed. Eventually, this resistance is overcome. Thus, the controlled variable arrives at the desired state gradually. This delayed reaction is called pure lag.
• The amount of lag is a property of the type and quantity of material being changed. For instance, a gas will change temperature more quickly than a liquid.
• Dead time is the time that elapses between when the controlled variable deviating from the setpoint and corrective action beginning.

Feed-Forward Control

• Feedback control loses effectiveness in the presence of:
  • Large magnitude disturbances
  • Long time delays
• Whereas feedback control tries to correct errors after they occur, feed-forward control seeks to minimize errors before they occur.
• To perform feed-forward control, the sensor is placed at the inlet to detect disturbances, rather than detecting fluctuations in the output.
• Feed-forward control is not capable of compensating for unknown disturbances.
• Feed-forward control is usually used with feedback control, instead of alone.
• Feed-Forward is proactive; Feedback is reactive.

Objective Recap

• Obj 1: List the classifications of industrial control systems.
  • Motion control and Process control
  • Open-loop control and Closed-loop control
• Obj 2: Describe the difference between industrial control system types.
  • Motion control is faster than process control.
  • Motion control has constantly changing setpoints, while the setpoint is generally fixed for process control.
  • Motion control controls the position, speed, acceleration, or deceleration. A robotic pick’n’place arm for placing components on a circuit board is a motion control system. Process control maintains the levels of process parameters such at temperature, pressure, flowrate, and pH at constant levels. Petroleum or natural gas refining is a process control system.
  • Open-loop control does not measure the output of the system to determine the error of the controlled variable from the setpoint. Closed-loop control does measure the output to provide automatic error correction of the output from the setpoint.
• Obj 3: Define the following terms related to industrial processes:
  • Servos
    • Another name for a motion control system
  • Servomechanisms
    • Another name for a motion control system
  • Batch
    • A batch process is a process control system which performs a sequence of timed operations on the product being manufactured. Also called sequence or sequential control.
  • Continuous
    • A continuous process in a process control system in which operations are performed to the product as it passes through the system. Raw materials are constantly entering the system, and finished products are continuously exiting the system.
  • Instrumentation
    • Another name for process control
• Obj 4: Describe the differences between open- and closed-loop systems.
  • In an open-loop system, there is no feedback. The system is adjusted manually by the user, or it is set in such a way that it does not needed monitors, such as a timed process. Sequential, or batch processing, systems make use of this.
  • In a closed-loop system, the error between the desired and actual level of a variable is used to automatically provide compensation and return the variable to the desired value. Most industrial control systems use closed-loop control.
• Obj 5: Define the following terms associated with open- and closed-loop systems:
  • Negative feedback
    • In a negative feedback loop, the value of the measured variable is subtracted from the setpoint to produce a value representing the error between the desired and actual states of the controlled variable.
  • Controlled variable the variable which is monitored and maintained at a desired level
  • Measurement device the “eye” of the system, which is capable of producing some output signal related to the level of the controlled variable. (Ex: thermocouple to measure temperature).
  • Feedback signal the value of the measured variable at a moment in time, as output by the measurement device
  • Setpoint the input value applied to the loop that indicates the desired position of the controlled variable
  • Error detector also called the comparator or summing junction, it compared the setpoint with the feedback signal and produces an output proportional to the difference between them
  • Error signal the output of the error detector, proportional to the difference between the feedback signal and the setpoint
  • Controller the “brain” of the system; receives the setpoint error as input and sends a signal to the actuator as an output, of the proper value to drive the controlled variable back to the setpoint.
  • Actuator the “muscle” of the system; alters some type of fuel or energy to cause the controlled variable to return to the setpoint
  • Manufacturing Process the operation performed by the actuator to control a physical variable, such as the motion of a machine or the processing of a liquid
  • Disturbance an unwanted factor that upsets the system and causes a change in the controlled variable
  • Measured variable the condition, or level, of the controlled variable at a specific point in time
  • Manipulated variable The amount of fuel or energy that is altered by the actuator
  • Controller output signal this output signal is sent to the actuator and computed by the controller based on the error, to have a value needed to drive the controlled variable back to the setpoint
• Obj 6: List the factors that affect the dynamic response of a closed-loop system.
  • Response time of the instruments (lag time)
  • Time duration for signal to pass from one instrument to another
  • Static inertia of the system which resists change of controlled variable back to the setpoint (pure lag)
  • Dead time between when the controlled variable deviates from the desired value and when the corrective action begins
• Obj 7: Describe the operation of feed-forward control.
  • Feed-forward control tries to prevent errors from occurring by placing sensors at the inputs to check for disturbances before they cause an error at the output.
  • Feed-forward control can only correct known errors. Therefore it is usually used in combination with feedback control, which accounts for unknown errors.
• Obj 8: List three factors that cause the controlled variable to differ from the setpoint.
  • Change in the setpoint
  • Change in load demand
  • Disturbance