PLC Analog Outputs (DAC): Controlling Valves, Drives, and Heaters via 4–20 mA
Content:
Аналоговые выходы ЦАП в ПЛК_ управление клапанами, приводами и нагревателями через 4–20 мА.pdf (4.31 MB)
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04.08.2026
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Description
- PLC analog outputs are channels through which the controller transmits a continuous control signal to the outside world. Unlike a discrete output that can only turn a load on or off, an analog output generates a smoothly varying current or voltage proportional to the digital value from the program. In practice, this means the PLC can not just "open a valve" but hold it at 37%, accelerate a drive to the exact required speed, or maintain a precise heater power level.
- This practical guide is dedicated to the complete mastery of PLC analog outputs using Structured Text (ST) according to IEC 61131-3. It systematically covers all aspects: from DAC physics and current loops to programming, diagnostics, and laboratory exercises.
- The book covers in detail:
- DAC physics — operating principle, resolution, non-linearity parameters (INL, DNL), numerical codes in registers of different PLC platforms.
- UNSCALE mathematics — converting engineering values to DAC code, normalisation, formulas, output anti-windup.
- ST implementation — complete UNSCALE function block with division-by-zero protection, clamping, and rounding.
- Valve control — setpoint scaling, PID flow control, writing to AO register.
- Variable frequency drive (VFD) control — analog speed reference, ramp (smooth acceleration/deceleration), discrete Start/Stop commands, VFD fault handling.
- Heater control — PID loop with anti-windup, thermal inertia, thyristor power regulator (TPM) control.
- Accuracy and errors — error budget calculation, quantisation, INL, temperature drift, compliance voltage monitoring.
- Analog output diagnostics — wire break detection, short circuit, overtemperature, power loss, expected vs measured current comparison.
- Safety and reliability — protective wrapper block (FB_SafeAO) with limiting, ramping, safe state, fault acknowledgement.
- Laboratory exercises — UNSCALE 5-point verification, valve positioner linearity test, heater plant identification via step response (Ziegler-Nichols), current loop fault diagnostics.
- Appendices — 4–20 mA correspondence table, AO commissioning checklist, quick fault diagnosis guide.
- All algorithms are accompanied by ready-to-use ST code with detailed comments. The book contains numerous practical examples: from a simple UNSCALE function to a complete safety wrapper for critical analog outputs.
- The manual is intended for automation engineers, PLC programmers, commissioning engineers, students and teachers. It will help you master continuous process control via 4–20 mA and 0–10 V analog signals, correctly configure scaling, diagnose faults, and ensure safe operation of actuators.
Additional information
- Introduction — analog outputs in industry, standard ranges (4–20 mA, 0–10 V), SCALE/UNSCALE symmetry logic.
- PLC analog output module structure — functional blocks, output types (active/passive), DAC ICs.
- Digital-to-analog conversion — DAC physics, resolution, INL, DNL, drift, numerical codes in different PLC registers.
- UNSCALE mathematics — general formulas, normalisation, DAC code conversion, combined formula.
- UNSCALE ST implementation — function block, division-by-zero protection, clamping, rounding.
- Valve control — flow control valve example, PID controller, input and output scaling.
- Variable frequency drive control — analog speed reference, ramp, discrete commands, fault handling.
- Heater control — PID loop with anti-windup, thermal inertia, thyristor power regulator control.
- Accuracy and errors — error budget calculation, error sources, compliance voltage monitoring.
- Analog output diagnostics — wire break, short circuit, overtemperature detection, expected vs measured current comparison.
- Safety and reliability — FB_SafeAO protective block, limiting, ramping, safe state, fault acknowledgement.
- Laboratory exercises — UNSCALE verification, valve linearity, heater identification, fault diagnostics.
- Appendices — 4–20 mA table, commissioning checklist, quick diagnostics.
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