Automation in Hot Strip Mill (HSM) – Concept, Components, and Working Principle

Hot strip mills (HSM) are highly automated to ensure precision, efficiency, and quality in rolling operations. The automation system in an HSM is responsible for controlling strip thickness, width, profile, flatness, temperature, and rolling forces. The automation concept integrates sensors, actuators, controllers, and advanced algorithms to regulate the rolling process in real-time.

1. Automation Concept in Hot Strip Mill (HSM)

The primary goal of automation in HSM is to maintain consistency, accuracy, and high throughput while reducing human intervention. It involves three levels of control:

1. Level 1 Automation - Process Control & Actuation
   • Real-time process control using PLCs (Programmable Logic Controllers) and PID controllers.
   • Direct control of actuators, sensors, and drives.
   • Functions: Speed control, interstand tension control, gap control, and force control.
2. Level 2 Automation - Supervisory Control & Optimization
   • Advanced process models and algorithms for optimizing rolling parameters.
   • Uses feedback from sensors and predictive models for adjustments.
   • Functions: Automatic Gauge Control (AGC), Automatic Width Control (AWC), Automatic Flatness Control (AFC), and roll force prediction.
3. Level 3 Automation - Production Management & Scheduling
   • Planning, scheduling, and production tracking.
   • Integration with MES (Manufacturing Execution Systems) and ERP (Enterprise Resource Planning).

2. Major Components and Their Working Principle in Automation of HSM

2.1 Sensors & Measurement Devices

Sensors provide critical feedback for real-time control and process optimization.

2.2 Actuators & Control Devices

Actuators execute commands from control systems to adjust rolling parameters.

2.3 Control Systems

The control systems process sensor feedback and send corrective commands to actuators.

A. Automatic Gauge Control (AGC)

AGC maintains strip thickness by dynamically adjusting roll gap and rolling force.

• Types of AGC:
  1. Hydraulic AGC (HAGC) – Uses hydraulic cylinders for fine gap adjustments.
  2. Mass Flow AGC – Adjusts roll speed to maintain volume consistency.
  3. Tension AGC – Uses interstand tension measurements to adjust rolling force.

B. Automatic Width Control (AWC)

• Adjusts side guides and edger rolls to control width.

C. Automatic Flatness Control (AFC)

• Uses roll bending, shifting, and cooling to ensure flatness.
• Includes Work Roll Bending (WRB) and Work Roll Shifting (WRS).

D. Temperature Control System

• Controls reheating furnace and cooling sprays based on temperature feedback.

E. Loopers & Interstand Tension Control

• Loopers adjust strip tension between stands.
• Interstand tension sensors optimize mill speed synchronization.

3. Working Principle of Automation in Hot Strip Mill

The automation system functions through a combination of sensor feedback, advanced process models, and real-time control loops.

Step 1: Slab Reheating and Entry to Roughing Mill

• Automation monitors furnace temperature to ensure uniform heating.
• Slab enters the roughing mill, where thickness is reduced.
• Width control (AWC) adjusts slab edges.

Step 2: Roughing Mill to Finishing Mill Transfer

• A looper system controls strip tension.
• Mass Flow AGC ensures material volume consistency.

Step 3: Finishing Mill Operation

• HAGC dynamically adjusts roll gap for precise thickness control.
• AFC controls roll bending/shifting to correct shape defects.
• Interstand tension control optimizes strip elongation.

Step 4: Cooling and Coiling

• Laminar cooling system adjusts cooling intensity.
• Strip enters the coiler, where tension control ensures proper winding.

4. Advanced Automation Technologies in HSM

Newer HSMs integrate AI, Machine Learning, and Digital Twins for enhanced performance.

5. LVDT in Automation of a Strip Mill: Working, Design, and Principle

1. Introduction to LVDT

LVDT (Linear Variable Differential Transformer) is an electromechanical transducer used in automation and control systems of strip mills for precise position and displacement measurement. In strip mills, LVDTs play a crucial role in monitoring and controlling the thickness, tension, and alignment of steel strips during rolling.

2. Applications of LVDT in a Strip Mill

In the automation of a strip mill, LVDT is primarily used for:

• Roll Gap Positioning: Measuring the displacement of hydraulic cylinders controlling roll separation.
• Thickness Control: Providing feedback to Automatic Gauge Control (AGC) systems.
• Tension Measurement: Ensuring uniform strip tension across rolling stands.
• Roll Bending Control: Adjusting roll bending to correct strip flatness.
• Looper Position Control: Maintaining loop stability between rolling stands to prevent tension fluctuations.

3. Design of LVDT

An LVDT consists of three key components:

1. Primary Coil (Excitation Coil): Located at the center of the LVDT, supplied with an alternating current (AC) voltage.
2. Secondary Coils (S1 and S2): Two identical coils symmetrically wound on either side of the primary coil.
3. Movable Core (Ferromagnetic Rod): A cylindrical core made of permeable material that moves inside the hollow coil assembly.

LVDT Design Schematic:

 --------------------------------------
|       Secondary Coil S1              |
|       -----------                    |
|                                      |  
|       Primary Coil                    |
|       -----------                    |
|                                      |
|       Secondary Coil S2              |
 --------------------------------------
         |   Movable Core   |
         |  (Ferromagnetic) |
         --------------------


4. Working Principle of LVDT

The operation of an LVDT is based on mutual inductance between the primary and secondary coils.

Step-by-Step Working:

1. Excitation: An alternating current (AC) voltage (typically 1–10 kHz) is applied to the primary coil.
2. Induced Voltages: Due to electromagnetic induction, voltage is induced in both secondary coils (S1 and S2).
3. Core Position Effects:
   • Neutral or Null Position (Zero Displacement):
     • The core is symmetrically positioned.
     • Equal voltage is induced in both S1 and S2.
     • The net output voltage (difference between S1 and S2) is zero.
   • Core Moves Towards S1 (Positive Displacement):
     • More voltage is induced in S1 than S2.
     • The output voltage increases in phase with the excitation.
   • Core Moves Towards S2 (Negative Displacement):
     • More voltage is induced in S2 than S1.
     • The output voltage increases but is 180° out of phase with the excitation.

Mathematical Representation:

The output voltage V out is given by:

V out = VS1 − VS2

Where:

• VS1 and VS2 are the voltages of secondary coils.
• The magnitude of VoutV_{out} indicates displacement.
• The phase of VoutV_{out} indicates the direction of displacement.

5. Advantages of LVDT in Strip Mill Automation

• High Precision & Sensitivity: Provides accurate position feedback.
• Non-contact Measurement: No mechanical wear, ensuring longevity.
• Infinite Resolution: Can detect even the smallest displacement.
• Robust Construction: Suitable for harsh industrial environments.
• Temperature Stability: Works efficiently under varying temperature conditions.

6. Integration in Strip Mill Control Systems

In a strip mill, the LVDT is integrated with:

• Hydraulic AGC Systems for roll gap control.
• PLC (Programmable Logic Controller) & DCS (Distributed Control System) for real-time position feedback.
• SCADA (Supervisory Control and Data Acquisition) for monitoring and automation.

The LVDT continuously provides feedback to these control systems, allowing automated adjustments to maintain product quality.

6. AGC & HAGC Tuning in Hot Strip Mills

Automatic Gauge Control (AGC) and Hydraulic Automatic Gauge Control (HAGC) are critical for maintaining precise strip thickness in hot strip mills. Below is a detailed exploration of AGC and HAGC tuning in HSMs.

1. Automatic Gauge Control (AGC) in Hot Strip Mills

AGC ensures consistent strip thickness by dynamically adjusting the roll gap based on sensor feedback. Advanced AGC combines real-time measurements with predictive control techniques.

1.1 Types of AGC Algorithms

A. Hydraulic AGC (HAGC) Algorithm

• Uses hydraulic cylinders to control roll gap.
• Adjusts rolling force and compensates for mill stretch.
• Works in milliseconds to maintain target thickness.

B. Feedforward AGC (FF-AGC)

• Predicts rolling force changes based on incoming slab thickness, temperature, and hardness.
• Adjusts roll gap before the strip enters the stand.
• Reduces response delay and improves thickness accuracy.

C. Feedback AGC (FB-AGC)

• Uses thickness gauge measurements at the exit of the rolling stand.
• Adjusts roll gap based on deviation from target thickness.
• Works in combination with feedforward AGC for improved control.

D. Mass Flow AGC

• Maintains constant volume flow between mill stands.
• Uses speed control to compensate for thickness variations.
• Works well for compensating for material property changes.

E. Tension AGC

• Uses strip tension sensors to regulate roll gap adjustments.
• Controls interstand thickness variations.
• Prevents excessive elongation or breakage.

F. BISRA AGC

• Named after the British Iron and Steel Research Association (BISRA).
• Uses a combination of feedback, feedforward, and model-based control.
• Provides adaptive tuning based on mill conditions.

2. Hydraulic Automatic Gauge Control (HAGC) Tuning

Tuning HAGC is critical for stability, response speed, and accuracy. It involves optimizing control parameters like PID settings, hydraulic response, and compensation techniques.

2.1 Key Parameters for HAGC Tuning

2.2 Advanced HAGC Tuning Techniques

A. Adaptive Gain Control

• Adjusts PID parameters dynamically based on rolling conditions.
• Uses fuzzy logic or AI-based optimization.

B. Mill Stretch Compensation

• Predicts mill frame elongation under different loads.
• Uses force sensors and mathematical models for real-time correction.

C. Model Predictive Control (MPC) for HAGC

• Predicts future thickness deviations and adjusts roll gap proactively.
• Uses real-time physics-based simulations.

D. Nonlinear Compensation for HAGC

• Adjusts for nonlinearities in hydraulic systems.
• Includes dead-band compensation for improved accuracy.

3. Practical Implementation: AGC + HAGC Integration

1. Real-time Thickness Measurement
   • X-ray or Gamma-ray thickness gauges send feedback.
   • LVDTs measure roll position.
2. Force & Pressure Feedback
   • Load cells detect rolling force.
   • Hydraulic pressure sensors optimize roll movement.
3. Control Algorithm Execution
   • Feedforward AGC adjusts roll gap based on slab conditions.
   • Feedback AGC fine-tunes based on exit thickness.
   • HAGC tuning ensures fast, stable corrections.
4. Optimization & AI Integration
   • AI-driven adaptive tuning for real-time optimization.
   • Predictive analytics for force and roll gap settings.

AGC algorithms (feedback, feedforward, mass flow, tension control) work together to ensure precise strip thickness. HAGC tuning optimizes hydraulic response and compensation techniques for high-speed, accurate rolling.