ELEMENTS OF PROCESS CONTROL Pocess level control
PROCESS LEVEL CONTROL Illustrates the essential elements of a process. A level transmitter (LT), a level controller (LC), and a control valve (LV) are used to control the liquid level in a process tank. The purpose is to maintain the liquid level at some prescribed height (H) above the bottom of the tank. It is assumed that the rate of flow into the tank is random. The level transmitter is a device that measures the fluid level in the tank and converts it into a useful measurement signal, which is sent to a level controller. The level controller evaluates the measurement, compares it with a desired set point (SP), and produces a series of corrective actions that are sent to the control valve. The valve controls the flow of fluid in the outlet pipe to maintain a level in the tank.
ELEMENTS OF PROCESS CONTROL Pocess temperature control
MEASUREMENT To control a dynamic variable in a process,must have information about the entity or variable itself. This information is obtained by measuring the variable. Measurement refers to the conversion of the process variable into an analog or digital signal that can be used by the control system. The device that performs the initial measurement is called a sensor or instrument. Typical measurements are pressure, level, temperature, flow, position, and speed. The result of any measurement is the conversion of a dynamic variable into some proportional information that is required by the other elements in the process control loop or sequence.
EVALUATION In the evaluation step of the process control sequence, the measurement value is examined, compared with the desired value or set point, and the amount of corrective action needed to maintain proper control is determined. A device called a controller performs this evaluation. The controller can be a pneumatic, electronic, or mechanical device mounted in a control panel or on the process equipment. It can also be part of a computer control system, in which case the control function is performed by software.
CONTROL ELEMENT The control element in a control loop is the device that exerts a direct influence on the process or manufacturing sequence. This final control element accepts an input from the controller and transforms it into some proportional operation that is performed on the process. In most cases, this final control element will be a control valve that adjusts the flow of fluid in a process. Devices such as electrical motors, pumps, and dampers are also used as control elements.
BLOCK DIAGRAM OF BASIC ELEMENTS A process-control system to regulate flow and the corresponding block diagram. The use of current and pressure transmission signals.
THE PURPOSE AND USE OF OPEN LOOPo An open-loop control system is controlled directly, and only, by an input signal.o The basic units of this type consist only of an amplifier and a motor.o The amplifier receives a low-level input signal and amplifies it enough to drive the motor to perform the desired job.o The output of the amplifier is proportional to the amplitude of the input signal.o The phase (ac system) and polarity (dc system) of the input signal determines the direction that the motor shaft will turn.o After amplification, the input signal is fed to the motor, which moves the output shaft (load) in the direction that corresponds with the input signal.o The motor will not stop driving the output shaft until the input signal is reduced to zero or removed.
THE PURPOSE AND USE OF CLOSED LOOPo A closed-loop control system utilizes an additional measure of the actual output to compare the actual output with the desired output response.o A feedback control system is a control system that 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.o A feedback control system often uses a function of a prescribed relationship between the output and reference input to control the process.o Often the difference between the output of the process under control and the reference input is amplified and used to control the process so that the difference is continually reduced.o The feedback concept has been the foundation for control system analysis and design.o The introduction of feedback enables us to control a desired output and can improve accuracy, but it requires attention to the issue of stability of response.