Process Control & Loop Tuning Glossary
PID control, loop tuning, cascade and feedforward strategies, and the signal-processing concepts behind stable control loops. 119 terms in this topic - each defined in plain English on its own page.
A control loop is a decision made continuously: measure, compare to setpoint, act. This topic defines the theory and craft of that loop - PID and its tuning, the strategies layered on top (cascade, feedforward, ratio, override), the failure modes (windup, oscillation, interaction), and the signal-processing tools (filters, scaling, characterization) that keep loops honest. If you tune loops or design control strategies, this is your vocabulary.
119 Terms
How Do You Scale a 1-5 VDC Signal Versus 4-20 mA?
The 250-ohm resistor that makes 1 V the live zero, why the scaling math matches, and resistor-tolerance offsets.
Read → GlossaryWhat Is a Meter Factor Curve?
Multi-rate proving that linearizes a turbine or Coriolis meter across its flow range, applied live.
Read → GlossaryWhat Is Burner Air-Fuel Ratio Control With Oxygen Trim?
Setting combustion air to fuel and trimming to a target flue-gas O2 for efficient firing without going fuel-rich.
Read → GlossaryWhat is alarm dead-time filtering?
Independent on-delay and off-delay timers: the on-delay ignores transients, the off-delay holds through brief recoveries to stop flicker.
Read → GlossaryHow do you tune alarm on-delay and off-delay times?
Choosing delay values against nuisance alarms and missed events, a chatter diagnosis workflow, and what to log before changes.
Read → GlossaryWhat Is Anti-Windup?
How integral windup makes a saturated PID loop overshoot, and how clamping and back-calculation prevent it.
Read → GlossaryWhat Is Antisurge Control?
The surge control line and fast recycle valve logic that keeps a centrifugal compressor safely off its surge limit.
Read → GlossaryWhat is an auto/manual station?
The faceplate control that lets an operator take a loop off automatic and drive the output by hand.
Read → GlossaryWhat is backlash compensation in motion control?
A controller offset added on direction reversal to take up mechanical lost motion and restore accuracy.
Read → GlossaryWhat Is Fire-Tube Bath Temperature Control?
How the water or glycol bath in an indirect heater is regulated to hold a steady process outlet temperature.
Read → GlossaryWhat Is Bipolar Analog Signal Scaling?
How plus-minus 10 V inputs carry signals that swing negative, signed counts, a mid-range zero, and the symmetric equation.
Read → GlossaryWhat Is Tank Blanket Gas Pressure Control?
A slight positive pad of inert or fuel gas over stored oil that excludes air and prevents vacuum collapse via split-range control.
Read → GlossaryWhat Is Analog Input Block Averaging?
Hardware oversampling and averaging in an input module, often over a mains cycle to reject 50/60 Hz hum.
Read → GlossaryWhat Is Bumpless Transfer?
How a loop switches between manual and auto without jolting the valve, using output and setpoint tracking.
Read → GlossaryWhat is canal pool level control?
Hold each pool at a level setpoint with its check gate; upstream control follows supply, downstream control follows demand.
Read → GlossaryWhat Is Cascade Control?
How a primary loop sets the setpoint of a secondary loop to reject disturbances faster, with oil and gas examples.
Read → GlossaryWhat Is Remote/Cascade Setpoint Initialization?
The handshake that preloads a primary to the secondary's setpoint so closing a cascade causes no bump.
Read → GlossaryHow do you choose a deadband value per signal?
Measure each point's real noise band, then set flow widest and temperature tightest so noise stays silent and real moves report.
Read → GlossaryWhat is Cohen-Coon tuning?
An open-loop PID method for dead-time-dominant loops, tuned aggressively toward quarter-amplitude decay.
Read → GlossaryHow Does a Compressor Recycle Control Loop Work?
How recycle flow is modulated for turndown and surge protection, and why open percent flags waste.
Read → GlossaryWhat Is a Control Loop?
The sensor, controller, and final element that hold a process at setpoint, open vs closed loop, and how loops relate to SCADA.
Read → GlossaryWhat Is a Control Loop Audit?
A snapshot survey that ranks a site's worst loops, classifies faults, and builds a remediation backlog.
Read → GlossaryWhat Is Control Loop Interaction?
How loops sharing equipment fight into oscillation, the RGA pairing idea, and the detune-the-faster-loop fix.
Read → GlossaryWhat Is Control Loop Performance Monitoring (CLPM)?
The continuous, automated discipline of scoring every regulatory loop in a plant from historian data.
Read → GlossaryWhat Is a Control Loop Service Factor?
The fraction of time a loop runs in its intended mode with output unsaturated, the bluntest loop-health KPI.
Read → GlossaryWhat Is Control Loop Variability?
The spread of control error, and why shrinking it lets operators push setpoints toward profitable limits.
Read → GlossaryHow does air cooler fan and louver temperature control work?
Holds process outlet temperature by staging fans, modulating louvers, and recirculating warm air for freeze protection.
Read → GlossaryWhat are auto-recloser dead time and reclaim time?
The 79 cycle timing: arc-deionization dead time, the reset reclaim window, and lockout.
Read → GlossaryWhat Is Deadband?
The range where a change produces no action, why it stops valve hunting and nuisance alarms, and how it is used in SCADA.
Read → GlossaryWhat Is Decay Ratio (Quarter-Amplitude Damping)?
The ratio of successive overshoot peaks, the classic quarter-amplitude target, and why smoother is now preferred.
Read → GlossaryWhat Is Decoupling Control?
Compensators that cancel one loop's effect on another so interacting loops can be tuned as if independent.
Read → GlossaryWhat Is Derivative Time?
Rate action, derivative kick, and why most oil and gas flow and pressure loops set it to zero.
Read → GlossaryHow does differential pressure control work in district heating?
How network pumps hold a dP setpoint at the critical consumer, and how floating setpoints cut pump energy.
Read → GlossaryWhat Is Direct-Acting vs Reverse-Acting Control?
The setup choice that decides whether a loop stabilizes or runs away, tied to valve fail position.
Read → GlossaryWhat Is a Double-Scaled Tag in SCADA?
When a value already in engineering units is scaled again, producing a wildly wrong but sometimes plausible number.
Read → GlossaryWhat Is Engineering Units Scaling in SCADA?
How a raw 4-20 mA or ADC count becomes real units like psi or barrels, and where mis-scaling hides.
Read → GlossaryWhat Is Error-Squared Control?
Nonlinear control whose gain rises with error size, so the loop ignores small deviations but reacts hard to large ones.
Read → GlossaryWhat Is an Exponentially Weighted Moving Average?
A recursive average that weights recent samples more heavily and stores just one value per tag.
Read → GlossaryWhat Is External Reset Feedback?
Back-calculation that feeds the real downstream value into a controller's integral to stop cascade and selector windup.
Read → GlossaryWhat Is Feedback Control?
How closed-loop control corrects error after it appears, how it differs from feedforward, and its role in process control.
Read → GlossaryWhat Is Feedforward Control?
How it corrects for a measured disturbance before the process variable moves, and why it pairs with feedback.
Read → GlossaryWhat Is a First-Order Low-Pass Filter in a PLC?
A one-line recursive filter that smooths noisy analog inputs, trading noise rejection against added lag.
Read → GlossaryHow do you fix a Modbus scaling mismatch?
Confirm the raw integer, recover the documented scale factor, apply the gain and offset once, and verify against a known reading.
Read → GlossaryWhat Is Floating Control?
The raise, lower, and hold mode for motor-operated valves and dampers - integrating control distinct from PID.
Read → GlossaryHow does flow time and flow-dependent linearization work in a flow computer?
How a flow computer accumulates flow time and interpolates a multi-point curve so meter factor varies with rate, versus a fixed K-factor.
Read → GlossaryWhat is flow-paced wet well level control?
Modulating a variable-speed pump to match inflow and hold a near-constant level, smoothing discharge to the plant.
Read → GlossaryWhat Is a First-Order-Plus-Dead-Time (FOPDT) Model?
The three-number model, gain, time constant, and dead time, that every tuning rule fits to a step test.
Read → GlossaryHow does compressor fuel gas pressure control work?
Closed-loop and mechanical control that holds a stable fuel supply pressure across load swings with regulators and trips.
Read → GlossaryWhat Is Gain Scheduling?
Switching between multiple tuning sets across operating regions to control nonlinear loops like tanks and pH.
Read → GlossaryWhat Is Gap Control?
A strategy that takes no action while error stays inside a band around setpoint, cutting valve travel on surge tanks and noisy loops.
Read → GlossaryWhat Is a High/Low Signal Selector in Override Control?
Auctioneering logic that passes the highest or lowest of several controller outputs to the final element to build override control.
Read → GlossaryWhat is IMC tuning?
The model-based framework behind lambda tuning, where one knob sets speed and trades off robustness.
Read → GlossaryWhat Is Integral Time?
Reset in repeats/minute vs minutes/repeat vs seconds, and the unit conversion that trips up field tuning.
Read → GlossaryWhat Is an Integrating vs Self-Regulating Process?
The settle-versus-ramp distinction that decides how a loop must be tuned, and why level loops break the rules.
Read → GlossaryHow do you tune an integrating process?
Why ramping loops like tank level need integrator-plus-dead-time models and integrator lambda and IMC rules.
Read → GlossaryHow Does an Irrigation Pump VFD Pressure Control Loop Work?
The PID loop that holds mainline pressure, its energy savings, and the dry-run and deadhead protections.
Read → GlossaryWhat Is Lambda Tuning?
A model-based method that targets a chosen closed-loop response for smooth, non-oscillatory control.
Read → GlossaryWhat Is a Lead-Lag Dynamic Compensator in Feedforward Control?
A transfer-function block that times a feedforward signal so its correction arrives with the right dynamics to cancel a disturbance.
Read → GlossaryWhat Is Lead-Lag Control?
A lead unit modulates first and lag units stage in as demand rises, with run-hour rotation and anti-short-cycle logic.
Read → GlossaryWhat Is a Limit Cycle in a Control Loop?
The self-sustained oscillation a sticky valve forces on a stable loop, and why tuning never cures it.
Read → GlossaryWhat Is Loop Tuning?
How controller settings are adjusted for a stable, responsive loop, the common methods, and why tuning depends on the process.
Read → GlossaryWhat Is a Manipulated Variable?
The controller output - valve, VFD, or pump - the loop moves to drive the PV to setpoint.
Read → GlossaryWhat Is a Median Filter for Spike Rejection?
Taking the median of the last N samples removes isolated spikes a moving average would smear, and suits impulse noise.
Read → GlossaryWhat Is Model Predictive Control (MPC)?
A multivariable controller that predicts future outputs over a horizon and optimizes a constrained move plan each scan.
Read → GlossaryWhat is nonlinear gain in a PID?
How gain that varies with error size and an error deadband cut valve wear and cycling on noisy loops.
Read → GlossaryWhat Is Override Control?
How a high or low selector lets a constraint loop take over the valve to protect equipment without a full shutdown.
Read → GlossaryWhat Is a Percent-of-Scale Value?
The dimensionless 0-100 percent view of a signal in its span, why HMIs use it, and span versus reading accuracy.
Read → GlossaryWhat Is a Photoionization Detector (PID)?
A UV-lamp gas instrument that reads low-ppm VOCs and benzene, distinct from a PID control loop.
Read → GlossaryWhat Is PID Auto-Tuning?
How a controller runs a test to identify process dynamics and calculate its own gain, reset, and rate - and its limits.
Read → GlossaryWhat Is PID Control?
How proportional, integral, and derivative terms combine to hold a process at setpoint, plus tuning basics for oil and gas loops.
Read → GlossaryWhat Is a PID Health Index?
A composite 0 to 100 loop grade rolling up service factor, variability, oscillation, and valve travel.
Read → GlossaryHow does sample time affect a discrete PID?
Why the execution interval adds dead time, coarsens derivative action, and how to pick a scan rate.
Read → GlossaryPID Tuning: Gain, Reset, Rate
What proportional gain, integral reset, and derivative rate each do, how they interact, and practical tuning trade-offs.
Read → GlossaryWhat Is a Piecewise Linear Scaling Table?
How X-Y breakpoints and interpolation handle a nonlinear sensor or tank strapping table, versus one linear scale.
Read → GlossaryWhat Is Sensor Linearization With a Polynomial?
Fitting a curve to a nonlinear sensor so a PLC converts raw signal to engineering units, and the overfitting trap.
Read → GlossaryWhat Is the Difference Between Position and Velocity PID Algorithms?
Absolute-output versus change-per-scan PID forms, and why velocity eases bumpless transfer and windup.
Read → GlossaryHow Do You Set a PV Vent Setpoint?
Placing the vent set pressure and vacuum in the roof hierarchy above blanketing and below the tank design limits.
Read → GlossaryWhat Is Process Dead Time?
Pure transport and measurement delay - the biggest limit on loop performance in pipelines and long lines.
Read → GlossaryWhat Is Process Gain?
How far the PV moves per unit of output - the process side of gain and why it forces gain scheduling.
Read → GlossaryWhat Is a Process Time Constant?
The 63.2 percent response time (tau) read from a bump test - the input to Lambda and IMC tuning.
Read → GlossaryWhat Is a Process Variable?
The live measured signal a control loop reads back, and how it differs from setpoint and output.
Read → GlossaryWhat is proportional and derivative kick?
Why a setpoint step spikes the output, and how acting on the measurement instead of error removes it.
Read → GlossaryWhat Is Proportional Band?
The inverse of gain (PB percent equals 100 over gain) and the DCS-versus-PLC unit confusion it causes.
Read → GlossaryWhat Is a Pump-Off Setpoint on a Rod Pump?
The fillage or load threshold on the dynamometer card that tells a pump-off controller to shut a well in.
Read → GlossaryWhat Is Ratio Control?
How it holds two flows in a fixed proportion using a wild and controlled stream, with combustion and blending examples.
Read → GlossaryWhat Is Glycol Reboiler Temperature Control?
The loop that sets lean-glycol purity and gas dew point while holding TEG below its ~400F thermal-degradation limit.
Read → GlossaryWhat is recipe scaling?
Adjusting formula quantities when a control recipe is made for a batch size different from the master's nominal.
Read → GlossaryWhat is relay feedback auto-tuning?
An on-off relay test that induces a bounded oscillation to identify ultimate gain and period automatically.
Read → GlossaryWhat Is Reverse Scaling With a Negative Slope?
When 4 mA is the high value and 20 mA the low, the negative slope in the equation, and the backward-reading symptoms.
Read → GlossaryWhat Is a Scaled Frequency Input Conversion?
Turning a raw pulse frequency in Hz into an engineering rate like flow, speed, or RPM with a K-factor.
Read → GlossaryWhat Is a Scaling Factor and Offset in SCADA?
The gain and bias that convert a raw signal to a usable value, and how to compute them from endpoints.
Read → GlossaryWhat Is Separator Level Control (Throttling vs Snap-Acting)?
Snap-acting dump valves versus modulating throttling control, when each fits, and how proportional band shapes cycling.
Read → GlossaryWhat is servo loop tuning and stiffness?
Raising loop gains for stiffness and bandwidth, limited by inertia ratio and resonance, aided by notch filters.
Read → GlossaryHow do you set up engineering unit scaling?
Match the transmitter range, respect the 4-20 mA live zero, apply square root once, and verify at both endpoints.
Read → GlossaryWhat Is a Setpoint?
The target value a control loop drives toward, how it differs from the process variable, and how remote setpoints work in SCADA.
Read → GlossaryWhat Is a Setpoint Marker on a Trend?
Horizontal reference lines for setpoints and limits that show margin to a limit at a glance on a trend.
Read → GlossaryWhat is setpoint weighting in a PID?
How beta and gamma factors decouple setpoint tracking from disturbance rejection and tame proportional kick.
Read → GlossaryWhat Is a Slew-Rate Limiter on a Signal?
Logic that caps how fast a signal or setpoint changes per scan, the max-change-per-dt build, versus a low-pass filter.
Read → GlossaryWhat Is Split-Range Control?
How one controller drives two valves in sequence across its output, with heating, cooling, and pressure examples.
Read → GlossaryWhat Is Square Root Extraction (Flow)?
Why DP-based flow follows the square root of pressure, where to extract, and how to avoid double-rooting.
Read → GlossaryWhat Is Suction Pressure Override Control on a Compressor?
The protective loop that backs a compressor off when suction falls, and the low-select logic behind it.
Read → GlossaryWhat Is a Surge Control Line?
How the surge control line is offset from the surge limit and how proximity to it opens the recycle valve.
Read → GlossaryWhat is a tag scaling engine in SCADA?
The centralized subsystem converting raw registers to engineering units by per-tag rules, with quality flags on bad data.
Read → GlossaryWhat Is Tank Vapor Pressure Control?
Holding a tank's vapor space in a tight band with blanket gas and recovery draw to prevent emissions and collapse.
Read → GlossaryWhat is TEG hot shoe and cold side temperature control?
A TEG's watts follow delta-T across its thermopile, so hot-shoe and finned cold-side temperatures set the output.
Read → GlossaryWhat Is Three-Element Control?
The boiler feedwater scheme that combines feedforward, cascade, and feedback to beat drum-level shrink and swell.
Read → GlossaryWhat Is Three-Position Floating Control?
Raise, hold, and lower commands that drive a motorized valve with no positioner, using a neutral zone and pulse timing.
Read → GlossaryWhat Is Heater Treater Treating Temperature Control?
How the treating temperature drives emulsion breaking and BS&W, the fuel and shrinkage trade-off, and the SCADA loop.
Read → GlossaryWhat is Tyreus-Luyben tuning?
A conservative closed-loop rule from ultimate gain and period that detunes to keep tricky loops stable.
Read → GlossaryWhat Is Valve Backlash (Deadband) in Control Loops?
Mechanical lost motion that stalls the valve on every reversal, and how to tell it apart from stiction.
Read → GlossaryWhat is velocity feedforward in servo tuning?
Injecting commanded speed ahead of the feedback loop to slash following error without raising loop gain.
Read → GlossaryHow do you verify a blower-to-DO cascade loop?
Prove the inner airflow loop is fast and clean first, then confirm the slower outer DO loop trims airflow to hold DO without fighting it.
Read → GlossaryHow do you verify lift-station level control bands?
Confirm pump-off, lead-on, lag-on, and high-level setpoints are ordered and spaced correctly, then inject a level to watch each fire.
Read → GlossaryHow do you verify a split-range calibration?
Step one output across the range, hold each valve to its window, scrutinize the crossover, and confirm the fail actions agree.
Read → GlossaryHow do you verify a VFD analog speed reference?
Check the min and max endpoints, the linearity and direction, the minimum-speed clamp, and the lost-signal behavior.
Read → GlossaryWhere should square-root extraction be configured?
Exactly one device linearizes DP flow - transmitter, flow computer, or SCADA - and a mid-range check catches double or missing extraction.
Read → GlossaryWhy are DNP3 analog values wrong?
Ceiling means variation clipping, clean factor means double scaling, sign flips mean signed-unsigned mismatch - isolate by hops.
Read → GlossaryWhy Does a Tag Read 10x Off or Negative After a Scaling Error?
How a swapped span/zero, wrong raw range, or reversed direction produces impossible values with Good quality, and how to back-check the conversion.
Read → GlossaryWhat Is Ziegler-Nichols Tuning?
The classic ultimate-gain method and its quarter-amplitude-decay result, often too aggressive for field loops.
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