In the wave of digitalization sweeping the manufacturing industry today, when people are talking
about the Industrial Internet of Things, artificial intelligence and big data, a seemingly “traditional”
but crucial technology cornerstone - analog equipment - still plays a silent role in every corner of the
factory. -still play an irreplaceable role in every corner of the factory. They are the bridge between the
physical world and the digital world, and the core of the control system that senses the environment and
drives execution. In-depth understanding of the role and value of analog devices in industrial automation,
is to build a stable, efficient, intelligent production system of the underlying logic.
The essence of analog signal: the real pulse of the physical world
The industrial site is full of continuously changing physical quantities: the precise temperature fluctuations in
the reaction tank, the real-time flow of liquid in the pipeline, the subtle vibration of the motor bearings, the
precise level of the material in the tank, the force changes at the end of the mechanical arm... The essence of
these parameters is continuous, uninterrupted analog signals. They faithfully reflect the real state of equipment
operation and the dynamic characteristics of the process, and cannot be completely replaced by simple “0” and
“1” for their rich information content.
Core value: Capture the continuity, subtle changes and instantaneous state of the physical world, and provide
the most original and direct process information. This is the source of digital systems for precise control,
intelligent analysis and optimization of decision-making.
Analog Devices: The Key Hub for Sensing and Driving
In the industrial automation architecture, analog equipment is mainly responsible for two core functions:
Sense of the world's “senses”: analog input devices
Core device: sensor/transmitter
Function: Converts physical quantities (temperature, pressure, flow, level, displacement, vibration, composition, pH,
weight, torque...) directly into proportional, continuous electrical signals. Converts physical quantities (temperature,
pressure, flow, displacement, vibration, composition, pH, weight, torque, etc.) directly into proportional continuous
electrical signals (voltage, current, resistance, capacitance, etc.) Example:
Thermocouples/Resistors: Convert temperature changes to weak millivolt signals or resistance changes.
Pressure transmitters: convert pressure values to standard 4-20mA current signals or 0-10V voltage signals.
Flow meter (e.g. electromagnetic type, Koch force type): outputs an analog signal proportional to the flow rate/flow.
Liquid level meter (such as differential pressure type, radar type): output represents the liquid level height of the continuous signal.
Vibration sensors: output a continuous waveform that reflects the amplitude and frequency characteristics of equipment vibration.
Key Challenges and Considerations:
Accuracy: Accuracy of measurement is the foundation of control.
Sensitivity: The ability to detect small changes.
Linearity: The linear relationship between the output signal and the measurement.
Stability: The degree of drift over a long period of time.
Environmental adaptability: withstand high temperature, high pressure, corrosion, dust, electromagnetic interference and
other harsh conditions.
Response speed: the ability to quickly track changes in the measured (especially in dynamic control).
The “arms and legs” that changed the world: analog output devices
Core Components: Actuators/Drivers
Function: Receive continuous analog control signals (usually 4-20mA or 0-10V) from the control system and accurately
convert them into corresponding physical actions or state changes that directly affect the process. For example:
Control Valve: Continuously and accurately adjust the valve opening according to the input signal to control the flow,
pressure or level of the medium in the pipeline (e.g. pneumatic/electrical control valve).
Frequency converter: Receive the analog speed/torque signal, continuously adjust the frequency and voltage of the motor
power supply, and realize the smooth and stepless control of the motor speed/torque.
Proportional / servo valves: In hydraulic / pneumatic systems, according to the input signal to accurately control the flow,
pressure or direction of the oil / gas, drive cylinders / cylinders to achieve high-precision position, speed or force control.
Linear actuators: directly receive analog signals to control the linear displacement of the output axis.
Heater Power Controller: Continuously adjusts the power output of the heating element according to the analog input signal.
Key Challenges and Considerations:
Control Accuracy: The degree to which the actuation matches the command signal.
Resolution: The smallest amount of change that can be controlled.
Speed of Response: How quickly it follows the command signal.
Linearity and Deadband: The degree of linearity of the input/output relationship and the area of insensitivity.
Load Adaptability: Ability to maintain stable performance under different loads.
Reliability and Durability: Life and stability under frequent operation.
Signal Conditioning: The “Translator” and “Guardian” of the Analog World
Raw sensor signals are often extremely weak (millivolts), susceptible to interference or non-standardized. Actuators may also
require specific drive signals. Signal conditioning circuits/modules are critical in this regard:
Amplification: Amplifies the weak sensor signal to a standardized range suitable for subsequent processing (e.g.,
analog-to-digital conversion).
Filtering: Filtering out high frequency noise, industrial frequency interference, etc. mixed in with the useful signal to improve
the signal-to-noise ratio.
Isolation: use of optocouplers, transformers and other isolation techniques, cut off the ground loop, inhibit common-mode
interference, to protect the control system side of the circuit safety.
Linearization: Compensate for the output of sensors with non-linear characteristics (such as thermocouples).
Cold End Compensation: Ambient temperature compensation for thermocouple measurements.
Signal conversion: e.g. converts resistance changes to voltage/current signals (Wheatstone bridge) and voltage signals to
standard current signals (4-20mA transmitter) for long distance transmission and anti-interference requirements.
Enhanced driving ability: Provide enough power to drive the actuator.
The perfect dance between analog and digital: the bridge between
analog-to-digital/digital-to-analog conversion
The continuous signals generated by analog devices ultimately need to be understood by the digital control system
(PLC/DCS/controller); similarly, the digital control instructions calculated by the controller need to be converted into
analog signals to drive the actuators. Analog-to-digital converters and digital-to-analog converters are the core bridge
between the two worlds:
Analog-to-digital converters: discretize continuous analog input signals (from sensors/transmitters) into binary digital
quantities that can be processed by a digital control system. The resolution (e.g. 12-bit, 16-bit, 24-bit) determines the
fineness of the conversion (the smallest signal change that can be resolved), and the sampling rate determines the speed
at which the signal change is captured.
Digital-to-analog converter: Reduces the binary digital instructions output from a digital control system to a continuous
analog output signal (used to drive actuators). Its resolution and build-up time (the time required for the output to
stabilize) is the key to control accuracy.
Continuous Evolution and Intelligent Integration of Analog Devices
Even in the digital era, analog equipment has not stagnated, but in the continuous evolution and integration with the
depth of intelligent technology:
Intelligent sensors / transmitters: integrated microprocessor, with self-diagnostics, self-calibration, temperature compensation,
digital communication (such as superimposed HART protocol, or direct output of digital signals, such as IO-Link) and other
functions, to enhance the reliability and richness of information.
High accuracy and reliability: Advances in material science and microelectronics technology have driven the continuous
improvement of sensor accuracy, stability and life.
New Sensing Principle: MEMS (Micro-Electro-Mechanical Systems) technology has given rise to smaller, cheaper and smarter sensors.
Integration with IIoT: Intelligent analog devices have become the sensing nodes of the Industrial Internet of Things (IIoT),
and their rich raw data is a valuable resource for edge computing and cloud-based big data analysis.
Condition monitoring and predictive maintenance: Highly accurate analog signals (e.g., vibration, temperature, current
harmonics) are the core data source for equipment health status analysis and predictive maintenance.
Conclusion: Embrace Intelligence by Holding on to the Cornerstone
Analog devices are the physical layer core of industrial automation systems that perceive the physical world and
perform control actions. The continuous, real signals they provide are the cornerstone of the control system for making
accurate judgments and actions. Neglecting the precision, reliability and anti-interference ability of the analog link, even
the most advanced digital algorithms and network architectures will not be able to play their role.
In the journey towards intelligent manufacturing, we should actively embrace the cutting-edge technologies of digitalization,
networking and intelligence, and we must not neglect the continuous investment and optimization of analog equipment as
the underlying foundation. Selecting high-precision, high-reliability sensors and actuators, paying attention to the design
and implementation of signal conditioning, and ensuring the quality of analog-to-digital/digital-to-analog conversion are
the indispensable and critical steps in building a stable, efficient and intelligent modern industrial automation system. The
perfect synergy of analog and digital can release the maximum potential of industrial automation.