Process control instrumentation refers to the various devices and systems that are used to measure and control process variables in industrial processes. These process variables could include temperature, pressure, flow rate, pH, humidity, etc. The goal of process control instrumentation is to maintain the process variables at desired levels in order to optimize the production process and produce high-quality products.
An example of process control instrumentation is a temperature control system for a chemical reactor. The temperature of the reactor is an important process variable that needs to be kept within a certain range in order to ensure the chemical reaction taking place inside the reactor is occurring at the desired rate. A temperature sensor is used to measure the temperature of the reactor, and this information is used to control a heating or cooling system that adjusts the temperature as needed to maintain it within the desired range.
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Self-Regulatory Control Process
A self-regulatory control system is a type of control system that is able to maintain a process variable at a particular level without the need for external input or reference. This type of system is able to adjust its behavior based on the current conditions of the process, and it does not require a setpoint or reference value to function properly.
The output flow rate of a liquid in a tank is related to the height of the liquid, with the rate increasing as the height increases. If the output flow rate is higher than the input rate, the liquid level will decrease. If the output flow rate is lower than the input rate, the liquid level will increase. This process has a self-regulating property, meaning that the liquid will reach a level where the input and output flow rates are equal and stay there. However, if the input flow rate changes, the liquid level will also change and the system will not be regulated to a specific reference value. For instance, the liquid level will adopt a value where the input and output flow rates are equal and remain at that level unless the input flow rate changes.
Human-Aided Control Process
Human-aided control is a method of process control in which a human operator is responsible for making decisions and taking actions to keep a process operating at a desired level. In a human-aided control system, the operator monitors the process variables and adjusts them as needed to maintain the process within a specific range. This type of control system can be used in various applications, such as manufacturing, transportation, and utilities. In these systems, the human operator usually monitors the process variables and decides how to adjust the process to maintain it at a desired level. The operator may use a range of tools and technologies, such as sensors, gauges, and control panels, to monitor the process, and may adjust the process manually or with automated devices.
To maintain the level of a liquid in a tank at a particular value, H, regardless of changes in the input flow rate, we need to use a control system that is more advanced than a self-regulating system. To do this, we can use a sensor to measure the liquid level and a valve to adjust the outflow flow rate. The actual liquid level, h, is known as the controlled variable, while the output flow rate is called the manipulated variable or controlling variable. By using these tools, we can control the liquid level independently of the input flow rate.
Automatic Control Process
Automatic control is a method of process control in which a control system makes decisions and takes actions to maintain a process at a desired level without the need for human intervention. To do this, the control system uses sensors and other input devices to monitor the process variables and adjust them as needed to keep the process operating within a specific range.
To provide automatic control, the system is modified so that machines, electronics, or computers can replace the role of the human operator. In the above example, a sensor is added to measure the value of the level and convert it into a proportional signal, s, which is input to a machine, electronic circuit, or computer called the controller. The controller performs the function of the human operator in evaluating the measurement and providing an output signal, u, to change the valve setting through an actuator connected to the valve by a mechanical linkage. When automatic control is used in systems like this, which are designed to regulate the value of a variable to a setpoint, it is referred to as “process control.”
Basic Element of the Process Control System
- Process
In a process control system, the term “process” refers to the industrial process that is being controlled. This could be a chemical reaction, a manufacturing process, a power generation process, or any other type of process that occurs in an industrial setting. The basic elements of a process control system are
- Process variables: These are the parameters of the process that are being measured and controlled. Examples include temperature, pressure, flow rate, pH, humidity, etc.
- Sensors: These are devices that are used to measure process variables. For example, a temperature sensor might be used to measure the temperature of a chemical reactor.
- Control equipment: This is the equipment that is used to control the process variables. For example, a heating or cooling system might be used to control the temperature of a chemical reactor.
- Actuators: These are devices that are used to manipulate the process in some way. For example, a valve might be used to control the flow of fluid through a pipe.
- Control logic: This is the algorithm that is used to determine how the control equipment should be used to manipulate the process in order to maintain the process variables at desired levels.
- Control interface: This is the means by which the operator interacts with the control system, such as a control panel or a computer interface.
In short, a process is a series of steps or stages involved in a manufacturing sequence and may involve various variables that need to be controlled. These variables can be controlled individually in single-variable processes or simultaneously in multi-variable processes, which involve multiple variables that may be interconnected. The process, also known as the “plant,” can be complex and may consist of multiple phenomena.
- Measurement
In the context of a process control system, measurement refers to the act of determining the value of a process variable. This often entails employing a sensor to collect data about the operation and then processing that data to determine the value of the variable.
For example, if the process variable being measured is temperature, the measuring procedure could include employing a temperature sensor to collect data on the temperature of a chemical reactor. The sensor would create an electrical signal proportionate to the temperature, which the control system would process to compute the reactor’s actual temperature.
Measurement is an important component of a process control system because it enables the system to determine the present status of the process and make any necessary adjustments to keep the process variables at the desired values. It would be impossible to control the process properly without accurate measurement.
In another way, measurement also refers to the process of converting a variable, such as a temperature or pressure, into a corresponding analog form, such as an electrical voltage or current, or a digitally encoded signal. This process typically involves the use of a sensor, which is a device that performs the initial measurement and energy conversion. The sensor generates an analog signal that represents the value of the variable being measured. This signal may then need to be transformed or conditioned in some way in order to make it usable by the other elements of the process control system. The result of the measurement is a representation of the variable value that can be used by the system to control the process. For example, in a process control system that monitors the level of liquid in a tank, a sensor might be used to measure the level of the liquid and generate a signal that represents this measurement. This signal would then be used by the control system to control the level of the liquid in the tank.
Sensor vs. Transducer
A sensor is a device that is used to measure a physical quantity, such as temperature, pressure, or humidity. It converts the physical quantity into a corresponding electrical signal, which can be used to represent the value of the physical quantity.
A transducer is a device that is used to convert energy from one form to another. In the context of process control systems, transducers are often used to convert a physical quantity, such as pressure or temperature, into an electrical signal that can be used to represent the value of the physical quantity. Transducers are similar to sensors in that they both perform measurements, but sensors are typically more specialized and are designed to measure a specific physical quantity, whereas transducers are more general-purpose and can be used to measure a wide range of physical quantities.
In summary, a sensor is a specialized device that is used to measure a specific physical quantity, whereas a transducer is a more general-purpose device that is used to convert energy from one form to another and can be used to measure a wide range of physical quantities.
Imagine a process control system that is used to monitor the temperature of a chemical reactor. The system needs to measure the temperature of the reactor in order to control it and maintain it at the desired level.
A sensor that could be used in this system might be a thermocouple, which is a device that is specifically designed to measure temperature. It works by measuring the voltage generated by the thermocouple junction, which is a function of the temperature of the junction. The thermocouple generates an electrical signal that is proportional to the temperature of the reactor, and this signal is used by the control system to determine the temperature of the reactor.
A transducer that could be used in this system might be a pressure transducer, which is a device that is used to convert pressure into an electrical signal. The pressure transducer works by measuring the strain on a diaphragm caused by the pressure of the fluid being measured. This strain is converted into an electrical signal using a strain gauge, which is a resistor that changes resistance in response to strain. The pressure transducer generates an electrical signal that is proportional to the pressure of the fluid, and this signal could be used by the control system to monitor the pressure of the fluid in the reactor.
So in this example, the thermocouple is a sensor that is specifically designed to measure temperature, whereas the pressure transducer is a transducer that is used to convert pressure into an electrical signal.
- Error Detector
An error detector is a device or system that is used to detect deviations from a desired value or range in a process control system. It is used to identify when a process variable has strayed from its desired value, indicating that there may be a problem with the process or the control system.
In a human-aided control system, the human operator looks at the difference between the actual level of a process variable and the desired setpoint level and determines an error based on the magnitude and polarity of this difference. In an automatic control system, the same kind of error determination must be made before any control action can be taken by the controller.
For example, consider a process control system that is used to control the temperature of a chemical reactor. The system is designed to maintain the temperature of the reactor within a certain range in order to optimize the chemical reaction taking place inside the reactor. An error detector in this system might be a temperature sensor that is used to measure the temperature of the reactor. If the temperature falls outside of the desired range, the error detector will detect this deviation and send a signal to the control system indicating that there is a problem. The control system can then take appropriate action to correct the problem and bring the temperature back within the desired range.
- Controller
After an error has been identified in the process control system, the next step is to determine the appropriate action to take in order to correct the error. This process, which is carried out by a compensator, filter, or controller, involves evaluating the error and deciding how to manipulate the process in order to bring the controlled variable back to the desired value. The evaluation can be performed by a human operator or by a computer, and in modern systems, this task is often carried out by a microprocessor-based computer. To make this determination, the controller needs to have both the current value of the controlled variable and the desired value of the variable, expressed in the same units.
The controller is an important part of a process control system that is responsible for determining the necessary actions to take in order to maintain the process variables at the desired levels. It does this by comparing the measured value of the controlled variable to the desired setpoint value and deciding how to manipulate the process to correct any error. For example, in a system that controls the water level in a tank, the controller might be a computer or other microprocessor-based device that is programmed to evaluate the error between the measured water level and the setpoint level. If the water level is too high, the controller might open a valve to drain some of the water out of the tank. If the water level is too low, the controller might activate a pump to add more water to the tank.
- Control Element
The final element in a process control system is the device that directly affects the process and adjusts the controlled variable to the setpoint value. This element, known as the final control element, receives input from the controller and uses it to perform an action on the process. For instance, in a system that regulates the water level in a tank, the control element could be a valve that adjusts the outflow of fluid from the tank. The controller in the system would send a signal to the valve indicating whether the water level is too high or too low, and the valve would open or close in response to this signal in order to adjust the outflow of water and bring the water level back to the desired setpoint. Sometimes, an intermediate operation called an actuator is needed between the controller output and the final control element to convert the small energy signal from the controller into a larger energy action in the process.
