Chapter 2: Fundamentals of Process Control

Final element

a device that receives a control signal and regulates the amount of material or energy in the process. Examples include control valves, hydraulic pistons, variable-speed drives, relays, pumps and dampers.

Low-water fuel cutoff

a level measuring device that shuts down a boiler when the water level drops below the lowest allowed level.

Bias

is a systematic error or offset introduced into a measurement system. Bias typically shows up as an error in measurement where the measurements are all on one side of the true value.

Process variable

the dependent variable that is to be controlled in a control system. Ex. Temperature, pressure, level and flow.

Setpoint

the desired value at which the process should be controlled and is used by the controller for comparison with the process variable.

Control element or controller

a device that compares a process measurement to a setpoint and changes the control variable to bring the process variable back to the setpoint. This is accomplished by sending a signal to the final element. Signal may be air pressure, electric current, voltage or digital signal with atleast one other signal.

Measuring element

a device that establishes a scaled value for the measured process variable. i.e. temperature transmitter is a measuring element that converts a thermocouple voltage to a scaled temperature value.

Process Control

a system that combines measuring materials and controlling instruments into an arrangement capable of automatic action.

Variable

a value measured by an instrument

Fieldbus networks

are digital communication protocols that are optimized for use in the process automation industries.

Device networks

are digital protocols optimized for use in factory automation industries.

Static characteristics

are the characteristics of an element that describe the operation of an element at steady-state conditions when the process is not changing.

Dynamic characteristics

are the characteristics of an element that describe the operation of the element at unsteady-state conditions when the process is changing.

Dead zone

is a condition where there is no response to a change because the change is less than the sensitivity of an element.

Time Proportional On/Off control

is a control strategy that has a predetermined output period during which the output contact is held closed for a variable portion of the output period. It allows a process to be regulated more precisely than is possible with simple On/Off control. A modulated response to the output signal is accomplished by varying the percentage of time the final element is On and Off over a defined time interval, called cycle time. Shorter cycle time increases the precision of the control over the process. Longer cycle time reduces wear and tear on the operating components.

Proportional control

is a control strategy that uses the difference between the setpoint and the process variable to determine a control output that is sent to a final element. The cumulative amount of differences and the rate of change of the process variable can also be used to determine a control output. The predominant type of controller used in proportional control is a microprocessor based electronic controller that can accept analog or digital input signals and provide an analog or digital input signals.

Control loop

is a control system in which information is transferred from a primary element to the controller, from the controller to the final element, and from the final element to the process.

Instrument loop

is a control system in which one or more instruments are connected together to perform a task.

A closed loop

is a control system that provide feedback to the controller on the state of the process variable due to changes made by the final element. The primary element measures the process variable and sends a signal to the controller. The controller compares the value of the variable to the setpoint and sends a signal to a final element. The primary element detects the value of the process variable after the final element has made changes. This closes the loop.

An open loop

is a control system that sends a control signal to a final element but does not verify the results of that control. It exists if the feedback is absent or disabled, such as when the control element is operated manually. Open loop control can be used for relatively simple processes where the relationship between the control variable and process variable is well understood.

Drift

is a gradual change in variable overtime when the process conditions are constant. For example, the operation of the valves change over time as the valve stem gets dirty and friction affects the movement.

Hysteresis

is a property of physical systems that do not react immediately to the forces applied to them or do not return completely to their original state. Systems that exhibit hysteresis are systems whose condition depends on their immediate history. Frictional or magnetic forces may cause hysteresis. It affects valve actuators by slowing down the response to a changing control signal because of a worn linkage or overtightened packing nut.

Stability

is the ability of a measurement to exhibit only natural, random variation where there are no known identifiable external effects causing the variation. i.e. Pressure measurement has stability when external effects, such as valves closing and pumps starting, do not affect the pressure being measured.

Fidelity

is the ability of an element to follow a change in the value of an input.

Range

is the boundary of the values that identify the minimum and maximum limits of an element. Range is specified with two numbers representing the lowest and the highest values.

Reproducibility

is the closeness of agreement among repeated measured values when approached from both directions. i.e. when a thermocouple voltage is measured, a control signal can be generated to adjust a process. It is usually reported as a percentage of the reading. It includes hysteresis and drift.

Precision

is the closeness to which elements provide agreement among measured values. Precision does not describe the same thing as accuracy. It only measures agreement among the measured values. It does not compare the measured values to a standard or true value.

Linearity

is the closeness to which multiple measurements approximate a straight line on a graph. It is usually measured as nonlinearity and expressed as linearity.

Repeatability

is the degree to which an element provides the same result with successive occurrences of the same condition. It is usually reported as a percent of the average reading.

Accuracy

is the degree to which an observed value matches the actual value of a measurement over a specified range. Manufacturers usually specify control element accuracy as the worst-case accuracy over the entire range. It is often stated as a percentage of the full-scale range or as a percentage of the reading.

Nonlinearity

is the degree to which multiple measurements do not approximate a straight line on a graph.

Dynamic error

is the difference between a changing value and the momentary instrument reading or the controller action.

Span

is the difference between the highest and lowest numbers in the range.

Deadband

is the range of values where a change in measurement value does not result in a change in controller output. It is the difference between the value at which the controller activates and the value at which the controller deactivates.

On/Off control

is the simplest and most common control strategy. A controller activates or deactivates the final element depending on whether the measured variable is above or below the setpoint.

Sensitivity

is the smallest change of a value a primary element can detect, smallest change in input that can cause a control element to change its output, or the smallest change a final element can produce.

Response time

is the time it takes an element to respond to a change in the value of the measured variable or to produce a 100% change in the output signal due to a 100% change in the input signal.

Process automation

process involving batch and continuous flow of liquids, gases, and bulk solids. Common operation include chemical reactions, cooling and heating and mixing and separating products. Places priority on process safety, process uptime, high capital investment, long plant lifetimes, and data management and product traceability.

Factory automation

refers to processes usually involving the piece flow of product. Typically used in the manufacturing of discrete products. Places priority on production speed, low cost, real-time processing and flexibility.

Process Control System

requires more than measuring and controlling the physical and chemical characteristics of a process material

Overshoot

tendency for the process variable to go above the setpoint

Undershoot

tendency for the process variable to go below the setpoint

Control variable or Manipulated variable

the independent variable used to adjust the process(dependent) variable. Typical examples are flow rate of steam used to heat a process, the flow rate of a refrigerant used to control a process, and the amount of reagent required to neutralize a wastewater stream.

Primary element

the sensing device that detects the condition of the process variable. Examples include thermocouples, pressure gauges, level gauges, and flowmeters.