ASME MC85.1M-1981 — Terminology for Automatic Control

28 10 月, 2026
ASME Standards

⚙️ ANSI MC85.1M-1981 (Terminology for Automatic Control) is the American National Standard for automatic control terminology, prepared by the ANSI Committee MC85 under the sponsorship of the American Society of Mechanical Engineers (ASME), and approved by the ANSI Board of Standards Review on February 24, 1981. The Committee was organized in 1955, with a scope designated as: terminology pertaining to systems such as automatic process control, feedback control, regulating, and other related systems not requiring human intervention as part of the regulating procedure. This edition integrates the basic material first published in 1963 and augmented in 1966 and 1972, introduces new concepts, terms, and definitions arising recently in the technology of automatic control, and applies minor editorial revisions to improve clarity and usefulness. It provides an authoritative listing of system terms and their definitions for use in technical literature, specifications, and catalogs.

💡 Division of labor: MC85.1M covers “system-level” control terminology; specialized terms of the industrial process instrumentation field (instrument performance, operating influences, hardware, and product qualification) are covered by ANSI/ISA S51.1, Process Instrumentation Terminology. The two complement each other.

1. Scope and Organization of the Standard 📌

The body of the Standard has four sections plus an appendix: Section 1 Introduction, Section 2 General Block Diagrams, Section 3 Functional System Diagrams, Section 4 Terms and Definitions, with a supplementary Appendix. Terms are arranged in alphabetical order, with these editorial conventions:

Where more than one term may apply to the same concept, the preferred term appears with the definition;
Synonymous terms are identified by parentheses;
Deprecated terms are indicated by an asterisk (*);
Defined terms used as part of other definitions are set in italics for ready cross-reference.

The Standard supports its definitions with a set of diagrams: a block diagram of a closed loop, a simplified block diagram of the essential elements of an automatic control system, a more complex expanded block diagram, a block diagram in state variable notation, a partial expansion of forward elements, the Bode diagram of frequency response for a typical loop transfer function B/E, a typical Nyquist diagram, diagrams of dead zone and apparent dead time, and typical time responses to a step increase of input and of load.

⚠️ The preferred/synonym/deprecated convention is highly practical: when reading specifications, if you encounter an asterisked deprecated term or a parenthesized synonym, return to the preferred term for the definition to avoid ambiguity from mixed terminology of different eras.

2. Block Diagrams and System Structure Terms 🔧

Sections 2 and 3 define the basic structural concepts of systems in block-diagram language:

Closed Loop: a closed signal path formed by the forward path and the feedback path — the basic topology of a feedback control system.
Open Loop: a control structure without a feedback closing path, where the controlled variable does not participate in comparison.
Feedback: the process of returning a portion of the output (or a function of the output) to the input to be compared with the set point.
Forward Elements: the elements between the error signal and the controlled variable.
Controlled Variable: the quantity controlled to remain at the desired value; Manipulated Variable: the quantity manipulated to affect the controlled variable.
State Variable Notation: a modern representation of systems by first-order differential equations of a set of state variables, facilitating multivariable analysis and computer simulation.

💡 A block diagram is not merely a drawing tool — it is the “graphical definition” of terminology: every system-structure term in MC85.1M has a definite position on the corresponding block diagram. Reading the diagram is reading the definition.

3. Control Action and Controller Terms 🎯

Section 4 defines the various control actions precisely — the heart of automatic control terminology:

Control Action	Definition Essentials	Typical Application
Proportional Action	Output proportional to the error (P action)	Simple level, pressure regulation
Integral Action	Output proportional to the time integral of the error (I action); removes steady-state offset	Flow, temperature offset-free regulation
Derivative Action	Output proportional to the rate of change of the error (D action); lead compensation	Large-inertia temperature, composition processes
Proportional-plus-Integral (PI)	P plus I: speed and offset-free performance	Most common in process industries
Proportional-plus-Derivative (PD)	P plus D: reduces overshoot, improves stability	Speed, tension control
Proportional-plus-Integral-plus-Derivative (PID)	Three-action controller combining P, I, D	General high-performance process control
On-Off (Two-Position) Action	Non-continuous control switching between two states	Domestic temperature control, relay loops

⚠️ PID parameters must match the process characteristics: blindly increasing proportional gain for a process with large dead time causes sustained oscillation, and derivative action is sensitive to noise. The MC85.1M diagrams (Bode plot with s = jω) are exactly the tools used to analyze such stability issues.

4. Process Dynamics and Performance Terms 📊

The Standard defines common terms describing process dynamics and control system performance:

Dead Time (Transportation Lag): a delay between a change in input and the beginning of the output response, common in material transport and pipeline flow.
Dead Zone: the range of input within which the output does not change; related to the Neutral Zone.
First-Order Lag and First-Order Lead: inertial and lead elements described by a single time constant.
Time Constant: the time for a first-order step response to reach about 63.2% of its final value.
Overshoot and Decay Ratio: characteristic parameters of an underdamped step response.
Frequency Response: the steady-state amplitude and phase response of a system to sinusoidal input, shown by Bode and Nyquist diagrams for stability analysis.
Control Interaction: the phenomenon whereby adjustment of one loop in a multi-loop system affects another loop.

🚨 Dead time and dead zone are the two process characteristics most likely to cause control instability: dead time makes feedback “late,” and dead zone leaves small errors unresponsive. In engineering, identify these characteristics first, then decide the control action combination and tuning strategy.

5. Engineering Practice Essentials 🛠️

🔍 Terminology first: in multi-discipline control discussions (mechanical, electrical, instrument, process), standardize terms per this Standard to avoid slang such as interchanging “controller,” “regulator,” and “actuator.”
🔧 Design from the block diagram: draw the closed-loop block diagram, define the controlled and manipulated variables, then choose the control action, and finally tune parameters — the methodology implicit in the Standard.
📊 Simulation validation: modern control design can build simulation models in state variable notation, verifying performance with step response and frequency response.
📋 Specification references: procurement and acceptance documents for control valves, controllers, and DCS should cite the Standard’s term definitions so both parties agree on “proportional band,” “integral time,” etc.

✅ In practice, use the terminology as “contract language”: writing in the technical agreement that “proportional action means output proportional to the error … per ANSI MC85.1M-1981” significantly reduces interface disputes over equipment.

6. Frequently Asked Questions ❓

❓ Q: How do MC85.1M and the ISA process instrumentation terminology divide the field?
A: MC85.1M covers system-level control terminology (closed loop, feedback, control actions, dynamics); ANSI/ISA S51.1 covers process instrumentation terms (transmitters, control valves, performance and operating influences). Use them together.

❓ Q: Why are deprecated terms marked with an asterisk?
A: Control technology evolves rapidly, and the meaning of terms in older literature may differ from current definitions. The asterisk warns readers that a term is no longer recommended; use the preferred term so documents from different eras are understood consistently.

❓ Q: What role do Bode and Nyquist diagrams play in the Standard?
A: They are the standard graphical representations of the frequency response of the open-loop transfer function B/E, used to define and illustrate concepts such as stability and phase margin — the graphical support of frequency-domain terminology.

🔍 Q: Are the terms of this Standard still applicable in the DCS/PLC era?
A: Yes. Digital control changes the implementation, but system-level concepts such as closed loop, feedback, PID control action, dead time, and time constant remain the language of modern control systems, and the Standard’s definitions remain valid.