Process Control Definitions - Automation Direct DL05 User Manual

Chapter 8: PID Loop Operation

Process Control Definitions

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Manufacturing Process – the set of actions that adds value to raw materials. The process can
involve physical changes and/or chemical changes to the material. The changes render the
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material more useful for a particular purpose, ultimately used in a final product.
Process Variable – The controlled variable part of the process that you wish to control. It may
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be temperature, pressure, level, flow, composition, density, the ratio of two streams, etc. Also
known as the actual value.
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Setpoint – This is the target for the process variable. When all conditions of the process are
correct, the process variable will equal the setpoint.
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Control Output – The result of the loop calculation, which becomes a command for the
process (such as the heater level in an oven). This is sometimes referred to as control variable.
Error Term – The algebraic difference between the process variable and the setpoint. This is
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the control loop error, and is equal to zero when the process variable is equal to the setpoint
(desired) value. A well-behaved control loop is able to maintain a small error term magnitude.
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Manipulated Variable – This is what is used to effect the controlled variable. For example, the
fuel used in a furnace might be manipulated in order to control the temperature.
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Disturbance – Something in the system that changes such that corrective action is required.
For instance, when controlling a flow and the upstream pressure drops, the control valve must
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open wider in order to keep flow constant. The drop in upstream pressure is the disturbance.
Final Control Element – This is the physical device used to control the manipulated variable.
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Valves are probably the most widely used final control element.
Lag Time – The time it takes for the process to respond to a change in manipulated variable.
11 This is also known as the capacitance of the system. When you're in the shower and you turn
up the hot water a little, the time it takes before the water gets hot is the lag time.
Dead Time – The time it takes for a change in the process to be recognized. Composition
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analyzers and quality control are usually sources of significant dead time.
Loop Configuring – Operator-initiated selections which set up and optimize the performance
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of a control loop. The loop calculation function uses the configuration parameters in real time
to adjust gains, offsets, etc.
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Loop Monitoring – The function which allows an operator to observe the status and
performance of a control loop. This is used in conjunction with the loop configuring to
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optimize the performance of a loop (minimize the error term).
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C
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DL05 Micro PLC User Manual, 6th Edition, Rev. A