Glossary

Terms used by BHTune's tuning workflow, OPC DA integration, and run history.

Term	Definition
DA2 / DA3	OPC Data Access 2 and OPC Data Access 3 browse sources reported by the gateway. The da2 or da3 label identifies the source used for namespace browsing; it is not a process type or a BHTune driver.
FOPDT	First-Order Plus Dead Time, a process model described by gain, time constant, and dead time. BHTune's in-process simulator uses an FOPDT model. See the simulator model.
Gateway	The separate opcda-bridge service that exposes OPC DA servers to BHTune over a network connection. BHTune connects to the gateway rather than using DCOM directly. See OPC gateway compatibility.
Hysteresis	A switching band around the MRFT's PV threshold. It prevents small noisy PV changes near the threshold from causing repeated relay switches.
ItemID	The exact identifier for an item, or tag, in an OPC DA server. Use the ItemID returned by the server's browse results or a trusted mapping; punctuation in an ItemID does not define its hierarchy.
MRFT	Modified Relay Feedback Test. BHTune switches the MV by a configured relay amplitude as the PV crosses a threshold, measures the resulting oscillation, and uses it to calculate PID settings. See MRFT concepts.
MV	Manipulated Variable: the controller output that affects the process, such as a valve position. During a live MRFT, BHTune commands MV relay steps and verifies accepted writes by reading the value back.
Namespace index	The gateway's optional persistent index of an OPC DA server's browsable items. It enables whole-namespace search; ordinary browsing and direct ItemID entry do not depend on it.
Process type	The process category selected for a tune: flow, pressure-line, pressure-vessel, level, temperature-mixing, or temperature-heat-exchange. It selects tuning correlations and process defaults; it is separate from the DCS/PLC template.
ProgID	The registered programmatic identifier of an OPC DA server on the gateway host. bhtune opc servers lists registered ProgIDs so you can select the exact server.
PV	Process Variable: the measured value of the process that the controller regulates. BHTune samples the PV to detect the MRFT oscillation and calculate tuning results.
Quality	The OPC status attached to a value. BHTune treats exact Good as good, exact Uncertain according to the configured policy, and every other quality as bad for tuning-critical reads.
Relay amplitude	The size of each MRFT MV step, expressed as a percentage of the configured MV range. It is an input to the test, not a PID setting.
Restore	The best-effort return of applicable live loop values to their recorded pre-test targets after a tune. It is distinct from reverting PID constants: bhtune history revert restores the values recorded before a PID write, while bhtune restore-loop recovers an explicitly eligible orphaned tune.
SP	Setpoint: the target value the controller is meant to maintain. For an OPC DA tune that starts in the template-defined Auto mode with a configured setpoint tag, BHTune records the original SP for restoration.
Template	A DCS/PLC definition that maps a base tag to loop tags and describes controller-specific values such as mode codes, PID units, and rounding. Templates are not process types. See DCS/PLC templates.
Write-back	An explicitly requested write of calculated PID constants to their mapped controller tags. BHTune reads the prior values, writes and verifies each constant, and records the result. A simulator has no PID tags to write back to.