Programmable Logic Controllers Third Edition Frank D. Petruzella Mc. Graw-Hill
Chapter 14 Process Control and Data Acquisition Systems
Process Control Process control involves the automatic regulation of a control system. A variety of approaches can be used for process control, depending on the complexity of the process being controlled. Commonly controlled variables in a process include: Øtemperature Øspeed Øposition Øflow Ørate Øpressure Ølevel Incoming Product Packaged Product
Continuous Process A continuous process is one in which raw materials enter one of the system and the finished product comes out the other end of the system; the process itself runs continuously. In many cases parts are mounted sequentially, in an assembly line fashion, through a series of stations. Units being assembled are moved from station to station using a transporter mechanism, such as a conveyor. A specific assembly may utilize only manual operations, or it may include machine operations.
Continuous Process
Batch Process The steps followed in baking a cake are a good example of a batch process. In this case we may follow a recipe that involves adding ingredients, stirring the mixture, pouring into baking pans and baking them for a specific time at a specific temperature. Industrial batch processes are similar but done on a larger scale. Products produced using the batch process include food, beverages, pharmaceutical products, paint, and fertilizer. In batch processing there is no flow of product material from one section of the process to another. Instead, a set amount of each of the inputs to the process is received in a batch, and then some operation is performed on the batch to produce a finished product, or an intermediate product that needs additional processing.
Batch Process
Batch Process ØTwo ingredients are added together, mixed, and heated. Ø A third ingredient is added. ØAll three are processed and then stored.
Individual Production The individual, or discrete, product control production process is the most common of all processing systems. With this manufacturing process, a series of operations produces a useful output product. The workpiece is normally a discrete part that must be handled on an individual basis.
Individual Production The item produced may be bent, drilled, welded, and so on, at different steps in the process. Robot Controller
Control Process In the modern automated industrial plant, the operator merely sets up the operation and initiates a start, and the operations of the machine are accomplished automatically. These automatic machines and processes were developed to massproduce products, control very complex operations, or operate machines accurately for long periods. They replaced much human decision, intervention, and observation.
Individual Control The individual control configuration is used to control a single machine. This type of control does not normally require communications with other controllers. The operator enters the feed length and batch count via the interface control panel and then presses the start button to initiate the process. Rail lengths vary widely. The operator needs to select the rail length and number of rails to cut.
Centralized Control Centralized control is used when several machines or processes are controlled by one central controller. The control layout uses a single, large control system to control many diverse manufacturing processes and operations. Each individual step in the manufacturing process is handled by a central control system controller. No exchange of controller status or data is sent to other controllers. One disadvantage of centralized control is that, if the main controller fails, the whole process stops. A central control system is especially useful in a large, interdependent process plant where many different process must be control for efficient use of facilities and raw materials.
Distributive Control The distributive control system (DCS) differs from the centralized system in that each machine is handled by a dedicated control system. Each dedicated control is totally independent and could be removed from the overall control scheme if it were not for the manufacturing functions it performs. Distributive control involves two or more computers communicating with each other to accomplish the complete control task. This type of control typically employs local area networks (LANs), in which several computers control different stages or processes locally and are constantly exchanging information and reporting the status on the process
Distributive Control Communications among computers is done through single coaxial cables or fiberoptics at very high speed. Distributive control drastically reduces field wiring and heightens performance because it places the controller and I/O close to the machine process being controlled.
Distributive Control Because of their flexibility, distributive control systems have emerged as the system of choice for numerous batch and continuous process automation requirements. Distributive control permits the distribution of the processing tasks among several control elements. Instead of just one computer located at a central control point doing all the processing, each local loop controller, placed very close to the point being controlled, has processing capability. Some distributed controllers are housed in enclosures that can be field-mounted without control cabinets.
1. Which of the following is a commonly controlled variable of a process control system? (a) temperature (b) speed (c) flow (d) all of these 2. In the modern automated industrial plant, the operator merely sets up the operation and initiates a start, and the operations of the machine are accomplished automatically. (True/False)
3. An individual control process does not normally require communications with other controllers. ( True / False ) 4. An engine assembly line is an example of a batch process. ( True / False ) 5. A continuous process involves the flow of product material from one section of the process to another. ( True / False )
6. One disadvantage of the____ control configuration is that if the main controller fails the whole process is stopped. a. centralized b. distributive c. batch d. continuous process
7. The steps followed in baking a cake are a good example of a _______ process. a. centralized b. distributive c. batch d. continuous process
8. Distributive control involves several machines or processes controlled by one central controller. (True/False) 9. Distributive control drastically increases the amount of field wiring required. (True/False) 10. Distributive control involves two or more computers communicating with each other to accomplish the complete control task. (True/False)
Structure Of Control Systems A process control system can be defined as the functions and operations necessary to change a material either physically or chemically. Process control normally refers to the manufacturing or processing of products in industry. In the case of a programmable controller, the process or machine is operated and supervised under control of the user program.
Components Of A Process Control System
Components Of A Process Control System Provide inputs from the process and from the external environment Are related to a physical variable so that their electrical signal can be used to monitor and control a process Convert physical information such as pressure, temperature, flow rate, and position into electrical signals Sensors
Components Of A Process Control System Operator-machine interface Allows human inputs through various types of switches, controls, and keypads Allows inputs from a human to set up the starting conditions, or alter the control of a process Operates using supplied input information that may include; emergency shutdown, or changing the speed, the type of process to be run, the number of pieces to be made, or the recipe for a batch mixer
Components Of A Process Control System Signal Conditioning Involves converting input and output signals to a usable form Includes signal-conditioning techniques such as amplification, attenuation, filtering, scaling, A/D and D/A converters
Components Of A Process Control System Convert system output into physical action Actuators Have process actuators that include flow control valves, pumps, positioning drives, variable speed drives, clutches, brakes, solenoids, stepping motors, and power relays Solenoid Valve Can send outputs directly from the controller to a computer for storage of data and analysis of results Indicate the state of the process variables through external actuators such as meters, monitors, printers, alarms, and pilot lights
Components Of A Process Control System Controller Makes the system’s decisions based on the input signals Generates output signals that operate actuators to carry out the decisions
Open-Loop Control System Control systems are broadly classified as either open-loop or closed-loop. The open-loop control system is controlled by inputting to the controller the desired set-point necessary to achieve the ideal operating point for the process and accepting whatever output results. The controller receives no information concerning the present status of the process or the need for any corrective action. Open-loop control reduces system complexity and costs less when compared to closed-loop control. Open-loop control systems are not as commonly used as closed-loop control systems because they are less accurate.
Open-Loop PLC Stepper Motor Control System Stepper motors are often used to control position in low-power, low-speed applications. A stepper motor is basically a permanent magnet motor with several sets of coils, termed phases (A and B), located around the rotor. The phases are wired to the PLC output assembly and are energized, in turn, under the control of the user program. The PLC does not receive feedback from the motor to indicate that rotation has occurred, but it is assumed that the motor has responded correctly. A stepper motor converts electrical pulses into specific rotational movements. The movement created by each pulse is precise and repeatable, which is why stepper motors are so effective for positioning applications.
Open-Loop PLC Stepper Motor Control System Stepper Motor PLC Output Stepper Motor Output Module Step Pulses Open-loop, or nonfeedback, control is only as stable as the load and the individual components of the system.
Closed-Loop Control System A closed-loop control system is one in which the output of a process affects the input control signal. The system measures the actual output of the process and compares it to the desired output. Adjustments are made continuously by the control system until the difference between the desired and actual output is as small as is practical.
Closed-Loop Control System - Determines whether the process operation - The component that directly affects a - Produces the appropriate corrective output - The signal that contains information about the -The input that determines the desired operating matches the set point process change signal based on the error signal input current process status point for the process - Referred to as the error signal or the system - Has motors, heaters, fans, and solenoids that - Refers to the feedback signal - Normally provided by a human operator, deviation signal are all examples of output actuators - Ideally, matches the set point although it may also be supplied by another - The magnitude and polarity of the error signal electronic circuit will determine how the process will be brought back under control
Container-Filling Closed-Loop Process An empty box is moved into position and filling begins. The weight of the box and contents is monitored. When the actual weight equals the desired weight, filling is halted.
Container-Filling Closed-Loop Process A sensor attached to the scale weighing the container generates the voltage signal or digital code that represents the weight of the container and contents. The sensor signal is subtracted from the voltage signal or digital code that has been input to represent the desired weight. As long as the difference between the input signal and feedback signal is greater than 0, the controller keeps the solenoid gate open. When the difference becomes 0, the controller outputs a signal that closes the gate.
Container-Filling Closed-Loop Process Virtually all feedback controllers determine their output by observing the error between the set point and a measurement of the process variable. Errors occur when an operator changes the set point intentionally, or when a disturbance or a load on the process changes the process variable accidentally. The controller's role is to eliminate the error automatically.
Controllers may be classified according to the type of power they use. Pneumatic controllers are decisionmaking devices that operate on air pressure. Electric (or electronic) controllers operate on electric signals. Controllers are also classified according to the type of control they provide as follows: Ø On/Off Ø Proportional (P) Ø Integral (I) Ø Derivative (D)
On/Off Control With on/off control (also known as two-position and bang-bang control), the final control element is either on or off – one for the occasion when the value of the measured variable is above the set point, and the other for the occasion when the value is below the set point. The controller will never keep the final control element in an intermediate position. Controlling is achieved by the period of on/off cycling action. The following slide shows a system using on/off control, in which a liquid is heated by steam. If the temperature goes below the set point, the steam valve opens and the steam is turned off. When the liquid’s temperature goes above the set point, the steam valve closes and the steam is shut off.
On/Off Liquid Heating System The measured variable will oscillate around the set point at an amplitude and frequency that depends on the capacity and time response of the process.
On/Off Control A deadband is usually established around the set point of an on/off controller. The deadband of the controller is usually a selectable value that determines the error range above and below the set point that will not produce an output as long as the process variable is within the set limits. The inclusion of a deadband eliminates any hunting by the control device around the set point. Hunting occurs when minor adjustments of the controlled position are continuously made due to minor fluctuations.
Proportional Control (P-Controller) Proportional controls are designed to eliminate the hunting or cycling associated with on/off control. Proportional controls allows the final control element to take intermediate positions between on and off. This permits analog control of the final control element to vary the amount of energy to the process, depending on how much the value of the measured variable has shifted from the desired value. The proportional controller allows tighter control of the process variable because its output can take on any value between fully on and fully off, depending on the magnitude of the error signal.
Motor-Driven Analog Proportional Control Valve A proportional control valve is used as the final control element. The valve receives an input current between 4 m. A and 20 m. A from the controller; in response it provides a linear movement of the valve. A value of 4 m. A corresponds to minimum value (often 0) and 20 m. A corresponds to maximum value (full scale). The 4 m. A lower limit allows the system to detect opens. If the circuit is open, 0 m. A would result, and the system can issue an alarm.
Time Proportioning a 200 W Heater Element Proportioning action can also be accomplished by turning the final control element on and off for short intervals. This time proportioning (also known as proportional pulsewidth modulation) varies the ratio of on time to off.
Proportional Band The proportioning action occurs within a "proportional band" around the set as illustrated in the chart. Outside this band the controller functions as an on/off unit. Within the band the output is turned on and off in the ratio of measurement difference from the set point.
Proportional Control Offset The operation of a proportional controller leads to a process deviation known as offset or droop. This steady-state error is the difference between the attained value of the controller and the required value. It may require an operator to make a small adjustment (manual reset) to bring the controlled variable to the set point on initial start-up, or when the process conditions change significantly.
Proportional Control Steady State Error When valve B opens, liquid flows out and the level in the tank drops. This causes the float to lower opening valve A, allowing more liquid in. This process continues until the level drops to a point where the float is low enough to open valve A, thus allowing the same input as is flowing out. The level will stabilize at a new lower level, not at the desired set point.
Integral Control (I-Controller) The integral action, sometimes termed reset action, responds to the size and time duration of the error signal. With integral action, the controller output is proportional to the amount of time the error is present. Integral action eliminates offset. Integral controllers tend to respond slowly at first, but over a long period of time they tend to eliminate errors. The integral controller eliminates the steady-state error, but may make the transient response worse.
Derivative Control (D-Controller) The derivative controller responds to the speed at which the error signal is changing – that is, the greater the error change, the greater the correcting output. The derivative action is measured in terms of time. With derivative action, the controller output is proportional to the rate of change of the measurement or error. The derivative mode controller is never used alone. With sudden changes in the system the derivative controller will compensate the output fast.
Proportional Plus Integral (PI) Control Action Proportional plus integral (PI) control combines the characteristics of both types of control. A change in the measurement causes the controller to respond proportionally, followed by the integral response, which is added to the proportional response. Because the integral mode determines the output changes as a function of time, the more integral action found in the control, the faster the output changes.
Proportional Plus Derivative (PD) Control Action Proportional plus derivative (PD) control is used in process-control systems with errors that change very rapidly. By adding derivative control to proportional control, we obtain a controller that responds to the measurement's rate of change as well as to its size.
Proportional-Integral-Derivative Control (PID Controller) The functions of the individual proportional, integral and derivative controllers complements each other. If they are combined its possible to make a system that responds quickly to changes (derivative), tracks required positions (proportional), and reduces steady state errors (integral).
PID Control A PID controller can reduce the system error to zero faster than any other controller. It is the most sophisticated and widely used type of process controller. Because it has an integrator and a differentiator, this controller must be custom-tuned to each process controlled.
PID Control Using A PLC PID is essentially an equation that the controller uses to evaluate the controlled variable. Either programmable controllers can be fitted with input/output assemblies that produce PID control, or they will already have sufficient math functions to allow PID control to be carried out.
PLC Control Of A PID Loop The controlled variable (pressure) is measured and feedback is generated The controller then issues a command (control output) to correct for any measured error by adjustment of the position of the outlet valve The PLC program compares the feedback to the set point and generates an error signal The error is examined in 3 -ways: with proportional, integral, and derivative methodology
SLC 500 PID Output Instruction The SLC 500 PID programmable controller output instruction uses closed-loop control to automatically control physical properties such as temperature, pressure, liquid level, or flow rate of process loops. This instruction normally reads data from an analog input module, processes this information through an algorithm, and provides an output to an analog output module as a response to effectively hold a process variable at a desired set point. This task makes the instruction one of the most complex available for PLCs.
SLC 500 PID Output Instruction A file that stores the data required to operate the instruction The element address that stores the process input value The element address that stores the output of the PID instruction Double click setup screen on the instruction to bring up a display that prompts you for other parameters you must enter to fully program the PID instruction.
Data Acquisition Systems Data acquisition is the collection, analysis, and storage of information by a computer-based system. A data acquisition or computer interface system allows you to feed data from the real world to your computer. It takes the signals produced by temperature sensors, pressure transducers, flow meters, and so on, and converts them into a form your processor can understand. With an acquisition system, you can use your computer to gather, monitor, display, and analyze your data.
SCADA System Supervisory control and data acquisition (SCADA) systems have additional control output capabilities you can also use to control your processes accurately for maximum efficiency.
Components Of A Data Acquisition System
Components Of A Data Acquisition System In addition to producing products, PLCs also produce data. The information contained in this data can often be used to improve the efficiency of the process. The great advantage of data acquisition is that data are stored automatically in a form that can be retrieved for later analysis without error or additional work.
Modular Data Acquisition System Most sensors cannot be wired directly to a data acquisition board without preconditioning. Special signal conditioning modules are needed.
11. Which of the following provides inputs from the process and from the external environment? (a) controller (b) process actuators (c) process sensors (d) all of these 12. A stepper motor converts electrical pulses into specific rotational movements. (True/False)
13. Programmer A says the process variable refers to the signal that determines the operating point for the process. Programmer B says the process variable refers to the signal that contains information about the current process status. Who is correct? a. Programmer A only b. Programmer B only c. both Programmer A and Programmer B d. neither Programmer A nor Programmer B
14. Programmer A says the error signal in a closed-loop control system determines whether the process operation matches the set point. Programmer B says the magnitude and polarity of the error signal will determine how the process will be brought back under control. Who is correct? a. Programmer A only b. Programmer B only c. both Programmer A and Programmer B d. neither Programmer A nor Programmer B
15. Compared to open-loop control, closed-loop control: a. is more accurate b. is more complex c. requires feedback concerning the status of the process d. all of these
16. ON/OFF control allows the final control element to take intermediate positions between on and off. (True / False) 17. A proportional controller is designed to eliminate the hunting or cycling associated with ON/OFF control. (True / False)
18. The integral action responds to: a. the size and time duration of the error signal b. the speed at which the error signal is changing c. proportional bandwidth d. proportional gain
19. A PID controller: a. is tuned using a signal generator b. is factory-tuned for optimum performance c. must be custom-tuned to each process d. both a and b
20. Programmer A says data acquisition systems can not usually be programmed to save the maximum value of all measurements made in a certain interval. Programmer B says they cannot usually be programmed to save the minimum value. Who is correct? a. Programmer A only b. Programmer B only c. both Programmer A and Programmer B d. neither Programmer A and Programmer B
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