Online monitoring and control system for paper production line: The core of online monitoring and automatic control in paper machine production lines lies in the QCS (Quality Control System) and DCS (Distributed Control System), which, in conjunction with various specialised sensors and actuators, enable closed-loop control throughout the entire process from pulp to finished paper, ensuring high-speed, stable and high-quality production. Detailed introduction to core systems, key monitoring instruments, automatic control equipment and typical application scenarios:
Core Control Systems
1. QCS (Quality Control System) Functions: Online monitoring of key paper parameters such as basis weight, moisture content, thickness, ash content and gloss. The scanning frame performs transverse scanning, which, when combined with longitudinal data, generates a two-dimensional quality distribution map. Core Equipment: Scanning Frame: Spans the paper web, equipped with multiple sensors, and scans transversely at a constant speed. Detection Sensors: Beta-ray basis weight sensor, infrared moisture sensor, X-ray ash content sensor, laser thickness sensor. Control Logic: Basis weight → adjust stock flow rate; moisture content → adjust steam pressure in the drying cylinders; ash content → adjust filler dosage.
2. DCS (Distributed Control System) Functions: Controls process sections including stock flow, wire section, press section, drying section and winding section; interfaces with QCS, PLC and variable frequency drives to achieve stable control of speed, tension, pressure and liquid levels. Core Equipment: Engineer station, operator station, control station, I/O modules, communication network.
Online Monitoring Instruments (Sensors)
1. Stock Monitoring (Wet End) Concentration Meter: Microwave / Ultrasonic / Optical, measures stock concentration to control stock feed rate, accuracy ±0.5%. Flowmeters: Electromagnetic flowmeters (for conductive pulp) and Coriolis mass flowmeters (high precision) measure pulp, water and chemical flow rates to ensure accurate proportioning. pH Meters / Conductivity Meters: Monitor pulp pH to control sizing and bleaching effects. Free-flow meter: Measures pulp drainage performance (CSF/SR values) online to optimise refining.
2. Web Quality Monitoring (Dry End) Basis weight sensor: Beta-ray transmission method, measures g/m², accuracy ±0.8 g/m², installed on the scanning frame. Moisture sensor: Infrared absorption method, measures moisture content to control drying intensity in the drying cylinders, accuracy ±0.5%. Thickness (Compactness) Sensor: Laser triangulation method, micrometre-level accuracy, non-contact thickness measurement to prevent surface damage. Ash Content Sensor: X-ray fluorescence method, measures filler content to control costs and strength. Surface Defect Detection: High-speed line-scan camera + intelligent lighting (e.g. ISRA PAPER MASTER), detects surface defects such as holes, ragged edges, creases and dust. Web Monitoring System (WMS): High-definition cameras, real-time monitoring of web condition in the wire section, press section and drying section, automatic capture and traceability of web breaks.
3. Process Parameter Monitoring Pressure Transmitters: Monitoring headbox pressure, press roll pressure and steam pressure, accuracy ±0.2% FS. Temperature sensors: Thermocouples / RTDs, measuring dryer cylinder surface temperature, pulp temperature and ambient temperature. Tension sensors: Magnetostrictive / fibre-optic, measuring web tension to control winding / unwinding speed and prevent paper breaks and creases. Level gauges: Radar / ultrasonic / differential pressure, measuring headbox level, pulp tank level and white water tank level.
Automatic Adjustment Actuators (Hands and Feet)
Pulp Flow Control
Headbox: Lip fine-tuning actuator (electric/pneumatic), which adjusts the lip opening to control lateral consistency.
Variable-frequency pulp pump: PID speed control based on concentration and flow rate feedback to stabilise the pulp feed rate to the wire.
Dilution water control: High-precision flow valve + dilution water branch line to adjust local concentration and correct lateral basis weight deviations.
Wire Section and Press Control
Breast roll / pressure roll drive: Servo motor + closed-loop tension control, matching wire speed with stock flow to improve fibre orientation.
Press roll pressure control: Hydraulic / pneumatic pressurisation system, automatically adjusting line pressure according to paper grade to control dewatering rate.
Dryer Section Control
Steam control valves: Pneumatic / electric control valves, adjusting cylinder steam pressure based on moisture feedback to control drying rate.
Cylinder temperature zone control: Multi-zone steam circuits + temperature sensors, optimising the drying curve to prevent over-drying or under-drying.
Web re-moistening system: Steam spray / water mist system, regulating transverse moisture distribution to reduce warping.
Winding and Drive Control
Winding Tension Control: Variable frequency drive + tension sensor, constant tension winding to ensure compact rolls and flat ends.
Speed Synchronisation System: Electronic cam + encoder, achieving precise synchronisation of wire section, press, drying and winding speeds to reduce paper breaks.
Typical Closed-Loop Control Process (Using Basis Weight as an Example)
Detection
The scanning frame's beta-ray sensors perform transverse scanning to collect real-time basis weight data.
Comparison
The QCS system compares the measured values with the setpoint to calculate the deviation.
Adjustment
The DCS outputs a signal to fine-tune the headbox lip opening or the speed of the variable-frequency pulp pump, thereby altering the pulp flow to the wire.
Feedback
Sensors continue to monitor, forming a closed loop until the deviation is eliminated.
Special Configurations for Speciality Paper Machines
High-precision QCS: Basis weight deviation ≤ ±0.5 g/m², thickness coefficient of variation ≤ 2%.
Specialised Sensors:
Medical paper: Microbial detection and air permeability sensors.
Electrical paper: Dielectric constant and breakdown voltage sensors.
Filter paper: Pore size distribution and air permeability sensors.
Clean-grade actuators: Stainless steel construction and oil-free lubrication, meeting food/medical-grade requirements.
1300 inclined net 26 small cylinder 800x1500 cylinder paper pad machine
1575 long net three pressure 12+6 cylinder paper machine
2400 Double Mesh Double Large Cylinder Paper Machine
Summary
Online Monitoring: Centred on the QCS scanning frame and specialised sensors, covering all key parameters including basis weight, moisture content, thickness and defects.
Automatic adjustment: Centred on the DCS + actuators, this system achieves closed-loop control of coating volume, pressure, temperature and tension;
it reduces basis weight fluctuations from ±5% to ±1%, moisture fluctuations from ±1.5% to ±0.5%, and reduces paper breaks by more than 50%, significantly improving yield and production efficiency.
Video Presentation
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