A control system manages, commands, directs, or regulates the behavior of other devices or systems using control loops. It can range from a single home heating controller using a thermostat controlling a domestic boiler to large industrial control systems which are used for controlling processes or machines. The control systems are designed via control engineering process.
For continuously modulated control, a feedback controller is used to automatically control a process or operation. The control system compares the value or status of the process variable (PV) being controlled with the desired value or setpoint (SP), and applies the difference as a control signal to bring the process variable output of the plant to the same value as the setpoint.
For sequential and combinational logic, software logic, such as in a programmable logic controller, is used.[clarification needed]
Open-loop and closed-loop control
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This section is an excerpt from Control loop § Open-loop and closed-loop
Fundamentally, there are two types of control loop: open-loop control (feedforward), and closed-loop control (feedback).
An electromechanical timer, normally used for open-loop control based purely on a timing sequence, with no feedback from the processIn open-loop control, the control action from the controller is independent of the "process output" (or "controlled process variable"). A good example of this is a central heating boiler controlled only by a timer, so that heat is applied for a constant time, regardless of the temperature of the building. The control action is the switching on/off of the boiler, but the controlled variable should be the building temperature, but is not because this is open-loop control of the boiler, which does not give closed-loop control of the temperature.
In closed loop control, the control action from the controller is dependent on the process output. In the case of the boiler analogy this would include a thermostat to monitor the building temperature, and thereby feed back a signal to ensure the controller maintains the building at the temperature set on the thermostat. A closed loop controller therefore has a feedback loop which ensures the controller exerts a control action to give a process output the same as the "reference input" or "set point". For this reason, closed loop controllers are also called feedback controllers.[1]
The definition of a closed loop control system according to the British Standard Institution is "a control system possessing monitoring feedback, the deviation signal formed as a result of this feedback being used to control the action of a final control element in such a way as to tend to reduce the deviation to zero."[2]
Likewise; "A Feedback Control System is a system which tends to maintain a prescribed relationship of one system variable to another by comparing functions of these variables and using the difference as a means of control."[3]Likewise; "Ais a system which tends to maintain a prescribed relationship of one system variable to another by comparing functions of these variables and using the difference as a means of control."
Feedback control systems
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Logic control
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Logic control systems for industrial and commercial machinery were historically implemented by interconnected electrical relays and cam timers using ladder logic. Today, most such systems are constructed with microcontrollers or more specialized programmable logic controllers (PLCs). The notation of ladder logic is still in use as a programming method for PLCs.[6]
Logic controllers may respond to switches and sensors and can cause the machinery to start and stop various operations through the use of actuators. Logic controllers are used to sequence mechanical operations in many applications. Examples include elevators, washing machines and other systems with interrelated operations. An automatic sequential control system may trigger a series of mechanical actuators in the correct sequence to perform a task. For example, various electric and pneumatic transducers may fold and glue a cardboard box, fill it with the product and then seal it in an automatic packaging machine.
PLC software can be written in many different ways – ladder diagrams, SFC (sequential function charts) or statement lists.[7]
On–off control
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On–off control uses a feedback controller that switches abruptly between two states. A simple bi-metallic domestic thermostat can be described as an on-off controller. When the temperature in the room (PV) goes below the user setting (SP), the heater is switched on. Another example is a pressure switch on an air compressor. When the pressure (PV) drops below the setpoint (SP) the compressor is powered. Refrigerators and vacuum pumps contain similar mechanisms. Simple on–off control systems like these can be cheap and effective.
Linear control
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Fuzzy logic
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Fuzzy logic is an attempt to apply the easy design of logic controllers to the control of complex continuously varying systems. Basically, a measurement in a fuzzy logic system can be partly true.
The rules of the system are written in natural language and translated into fuzzy logic. For example, the design for a furnace would start with: "If the temperature is too high, reduce the fuel to the furnace. If the temperature is too low, increase the fuel to the furnace."
Measurements from the real world (such as the temperature of a furnace) are fuzzified and logic is calculated arithmetic, as opposed to Boolean logic, and the outputs are de-fuzzified to control equipment.
When a robust fuzzy design is reduced to a single, quick calculation, it begins to resemble a conventional feedback loop solution and it might appear that the fuzzy design was unnecessary. However, the fuzzy logic paradigm may provide scalability for large control systems where conventional methods become unwieldy or costly to derive.[citation needed]
Fuzzy electronics is an electronic technology that uses fuzzy logic instead of the two-value logic more commonly used in digital electronics.
Physical implementation
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A DCS control room where large screens display plant information. The operators can view and control any part of the process from their computer screens, whilst retaining a plant overview on the larger screens. A control panel of a hydraulic heat press machineThe range of control system implementation is from compact controllers often with dedicated software for a particular machine or device, to distributed control systems for industrial process control for a large physical plant.
Logic systems and feedback controllers are usually implemented with programmable logic controllers.
See also
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References
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Fast Forward
By Chip McDaniel
Faced with ever-increasing cost pressures and demands for improved performance, machine builders are actively seeking new automation solutions with improved cost/performance ratios. In response to these demands, vendors must often incorporate commercial off-the-shelf components and other technologies to deliver more performance at lower costs in smaller form factors.
This article shows how machine builders and vendors can work together to deliver the automation systems demanded, and how to successfully integrate the multiple power and control subsystems and components.
A machine's automation system primarily consists of power and control components. For a smaller machine, these may be housed in one panel (figure 1); whereas larger machines may require multiple panels, often one for control and another for power. The main subsystems and components of a machine automation system are:
The power distribution subsystem feeds power to components, such as motors, drives, and controllers. The control subsystem primarily consists of safety systems, programmable controllers, discrete and analog I/O, communication systems, and HMIs. Let's look at each of these areas in more detail.
Figure 1. For smaller machines, a single panel is often used to house both the power distribution system and the control components.
The National Electric Code (NEC, also NFPA 70) has much to say about using electricity properly to safeguard persons and property. The code comes into play well before the power source connects to the machine control enclosure through a plug, disconnect, or terminal block. At the machine, the NFPA 79: Electrical Standard for Industrial Machinery is the benchmark for industrial machine safety related to fire and electrical hazards. Some of the major requirements in machine control power distribution discussed in these standards include using proper disconnect means, protecting personnel from contact with electrical hazards, and protecting equipment from overcurrent and overloads.
The disconnect—whether a switch, circuit breaker, or cord with a plug—must be provided for any control enclosure fed with voltages of 50 VAC or more. It should be properly sized, positioned, wired, labeled, and, in some cases, interlocked to the enclosure door.
Protecting personnel from contact with electrical hazards is always needed, both inside and outside a machine power or control panel. All conductors must be protected from contact by personnel. Most power distribution devices are designed to facilitate this level of protection, but live components, such as power buses, distribution blocks, and other power terminals, should be covered with a nonconductive, see-through cover.
Protecting equipment from overcurrent is critical to reduce the chance of fire. Conductors and electrical components must be protected from overcurrent related to short circuits. Overcurrent protection devices, such as fuses and circuit breakers, must be sized based on conductor current-carrying capacity, device interrupt rating, maximum fault current, system voltage, load characteristics, and other factors.
For power circuits, branch-circuit-rated devices must be used to meet current-limiting and ground fault protection requirements. Supplemental overcurrent protective devices are not suitable for use in these circuits but work well in downstream control circuits tapped from the load side of the branch circuit.
Motors have special needs in machine control. For every motor, a proper form of electrical control is required, from simple on/off to more complex variable speed applications. Motor control devices include manual motor starters, motor contactors and starters with overloads (figure 2), drives, and soft starters.
A motor circuit must include both overcurrent (short circuit) and overload protection. This typically consists of branch-circuit protection, such as properly rated fuses, and a motor starter with overload protection devices, such as thermal overloads, but additional protection may be needed.
Additional protection to consider for machine control components includes loss of cooling and abnormal temperatures. Ground fault protection is also needed, so a proper ground connection is important. Over, under, and loss of voltage must also be considered. Protection from lightning, overspeed, and loss of a voltage phase in three-phase supplies are additional considerations for proper machine control.
Some motor controllers, such as drives and combination controllers, are self-protected. If this is the case, the device's rating or manufacturer's instructions will clearly note it is suitable for output conductor protection.
Figure 2. These Fuji manual motor starters and contactors from AutomationDirect have high switching capacity and integrate the functions of a molded case circuit breaker and a thermal overload relay.
A risk assessment drives the safety system design as needed to remove motion-causing energy, including electrical and fluid power, to safely stop the equipment for protection of both personnel and machines. Many safety standards come into play for proper machine control at both a mechanical and electrical level. Proper mechanical machine guarding and access points, as well as elimination of identified hazards, is a starting point. Safety relays or safety-rated controllers must be used to monitor safety switches, safety limit switches, light curtains, and safety mats and edges.
In small machine control applications, a safety relay is probably the simplest way to integrate safety functionality for emergency stop, monitoring a guard door, or protecting an operator reaching through a light curtain. In more advanced machines, safety-rated controllers provide the same functions, but can simplify the integration of multiple safety devices. Safety-rated controllers reduce hardwired safety logic by providing a platform to program the safety functions needed for proper and safe machine control.
Available in form factors from small to large, the machine controller can be a programmable logic controller (PLC), a programmable automation controller (PAC), or a PC. The complexity of the machine control application, end-user specifications, and personal preference drive controller selection. Many vendors have families of controllers to cover a range of applications from simple to complex, allowing a machine builder to standardize to some extent. Often three or more physical configurations-small, medium, and large form factors-are available from the controller manufacturer.
Using the same software platform to program a family of controllers is becoming the norm. This allows the designer to first program the system, and then select the right controller based on its capacity to handle the number of I/O points needed, as well as special functions such as proportional, integral, derivative control and data handling. Required capabilities like extensive communications and high-speed control should be carefully evaluated, as these are often the main factors driving controller selection.
Discrete and analog inputs and outputs connect the controller to the machine sensors and actuators. These signals can originate in the main control panel through a terminal strip with wiring to field devices, but a distributed I/O architecture is often a better solution. Distributed I/O reduces wiring by moving the input or output point closer to the field device, and by multiplexing multiple I/O signals over a single cable running from the remote I/O component to the control panel.
For distributed I/O at a smaller scale, IO-Link is a point-to-point serial communication protocol where an IO-Link-enabled device connects to an IO-Link master module. This protocol communicates data from a sensor or actuator directly to a machine controller. It adds more context to the discrete or analog data by delivering diagnostics and detailed device status to the controller.
Another important part of machine control now and for the future is extensive communication capability. It is a good practice to have multiple Ethernet and serial ports available to integrate to a variety of equipment, computers, HMIs, and business and enterprise systems (figure 3).
Multiple high-speed Ethernet ports ensure responsive HMI communication, as well as peer-to-peer and business system networking. Support of industrial Ethernet protocols, including EtherNet/IP and Modbus TCP/IP, is also important for scanner/client and adapter/server connections. These Ethernet connections enable outgoing email, webserver, and remote access communication functions-all important options for machine control.
Machine control often benefits from the availability of legacy communication methods, such as serial RS-232 and RS-485. Modern controllers often also include USB and MicroSD communication and storage options.
A big part of machine control communication is cybersecurity. Consider a layered defense where protection includes remote functions that are only enabled as part of the hardware configuration. For further protection, all tags should be protected from remote access unless the tag is individually enabled for that purpose.
Figure 3. In addition to the multiple communication ports on this BRX controller, additional ports are added using a STRIDE Industrial Ethernet switch and a GS drive serial-to-Ethernet adapter.
The HMI shows vital information about machine conditions using graphical and textual views. HMIs can be in the form of touch panels, text panels, message displays, or industrial monitors. They are used for monitoring, control, status reporting, and many other functions.
The purpose of the HMI must be clearly defined. Some machines may only need a fault message display with few control functions. Other machines may demand a detailed view of machine status, access to system parameters, and recipe functionality. Clearly defining the need of the machine will help determine HMI size and capabilities, and this should be done early in the design process.
HMIs can also act as data hubs by connecting to multiple networked devices. In some machine control applications, multiple protocols may be used, and often HMIs can be used for protocol conversion. This functionality can be used to exchange data, such as status and set points, among different controllers and other smart devices.
Some HMIs can also send data to the cloud or enable remote access functionality through the Internet, given proper user name and password authentication.
Machine automation systems consist of multiple subsystems and components to provide the required power distribution, safety, and real-time control. Each of these subsystems and components must work together, and many are often networked to each other via either hardwiring, or increasingly via digital communication links. Careful design, selection, integration, and testing will ensure the automation system performs as required, both initially and throughout the life cycle of the machine.
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