How A Web Tension Controller Works

Today’s web processing industries (e.g., printing, flexible packaging, textiles, wire and cable, paper, and hygiene products) rely on consistent material tension for uniformity. Inconsistent tension creates problems such as wrinkles, stretching, misalignment, web breaks, and inconsistent roll quality; thus, web tension controllers are essential to production.

A web tension controller maintains the running material (film, foil, paper, fabric, or wire) at a controlled, consistent tension throughout its entire processing cycle (unwound to rewound). As a result, manufacturers benefit by improving product quality, increasing production speeds, decreasing materials waste, and increasing equipment efficiency.

What Is Tension On A Web?

Web tension is just the pulling force that a constant strip of material (i.e., paper, plastic) experiences while going through each of the 3 primary processing areas of a web-based system:

• Unwind area
• Process area
• Rewind area

Each of these areas needs to maintain a specific level of web tension. If the web tension is too high for the web material being processed, the web will deform either by stretching and/or by breaking. If the web tension is too low, the web will wrinkle, wander, or cause loose rolls. A Web Tension Controller (WTC) continuously controls the web tension in the processing areas where web tension is required.

Key Elements of A Web Tension Control System

A commonly used type of web tension controller exists in a closed-loop control system and consists of three main components:

1. Tension Sensors (Load Cells/Tension Rolls)

These sensors measure actual web tension (in real-time). Tension sensors passively measure the amount of mechanical force exerted on them by converting it into an electrical signal (voltage or current) using strain-gauge technology, and thus will provide you with a representation of the actual amount of web tension at each point along the length of the web.

2. Controller Component

The controller acts as the “brain” of the entire system. A controller will:

• Receive input from a sensor in the form of a tension signal,
• Compare that signal to a pre-programmed value, or preset tension value,
• Calculate the deviation between the signal received from the sensor and the preset tension value,
• Send corrective commands to an actuator so that the deviation noted between the present signal and the preset signal can be reduced or eliminated.

Modern web tension controllers utilize microprocessor-based proportional-integral-derivative (PID) control for highly accurate and responsive dynamic output.

3. Actuating Devices (Cylinders, Drive Units, & Similar Devices)

Devices that are controlled by an input signal from a controller have the ability to operate according to the orders given by the controller. 

Common examples of such devices include:

• Electromagnetic brakes 
• Pneumatic cylinders
• Powder actuators (such as dampers)
• Servo drive systems

These devices will either create or decrease tension depending on the signal that is received from the control system.

How Does a Web Tension Controller Work?

The web tension controller operates in a cycle of detecting, comparing, and correcting the tension within the web (sheet) material being processed. Here are the five steps involved in operating a web tension controller:

Step 1: Tension Detection

This first step of the cycle begins when the material passes over the tension roller or load cell. At this time, the sensor measures the force used to pull the web and produces an electrical signal.

Step 2: Signal Processing

The controller receives the signal from the sensor and compares the measured actual tension to the tension setpoint value that has been entered by the operator.

Step 3: Error Calculation

Once the controller has compared the measured tension to the setpoint tension, it calculated the error.

Step 4: Corrective Action

Based on the amount of error determined in step three, the controller produces an output signal to the actuator (e.g., brake or motor drive) to increase or decrease the torque applied to the web.

Step 5: Continuous Feedback

As the controller transforms between the five steps continuously, stable and uniform tension in the web material (sheet) will be maintained throughout the entire process. The detect-compare-correct loop is the system's fundamental working mechanism for all web tension controller systems.

Types of Web Tension Control

Open Loop Control

With open-loop systems, tension control is based on roll diameter calculations rather than actual measurements of tension. Therefore, the controller will adjust brake torque depending on the current roll diameter as it changes during unwinding or rewinding.

This is suitable for:

• Less critical applications
• Material with stable properties
• Cost-sensitive applications

Closed Loop Control

Closed-loop controls measure actual tension in real time using either a load cell or dancer roller configuration to determine the amount of force being applied to a web. The Web Tension Controller uses PID control to adjust for any deviation from the desired system setpoint continually.

Closed-loop control offers:

• Superior Accuracy
• Faster Response
• Improved Quality Control
• Less Material Waste

Closed loop is the most precise solution for high-precision industries.

Different Process Zones Require Various Methods Of Controlling Tension

Unwind

There is either a brake or a drive controlling the supply roll. As the supply roll diameter decreases, the controller adjusts the applied torque so that the web tension remains constant.

Process

The NIP or powered elements on the web are rotating at the same rate so that the web tension remains uniform during applying coatings, printing, slitting, or laminating operations.

Rewind

The motor torque applied during rewinding is adjusted so that there is a proper winding density and to avoid loose or telescoping rolls of product.

Properly maintaining web tension in each zone will result in consistent handling of web materials and produce consistent quality.

Role of PID Control in Web Tension Controller

PID controller algorithms (Proportional–Integral–Derivative) are the algorithm of choice for today's web tension controller systems because they:

• Allow for rapid response to web tension variations (fluctuations)
• Provide elimination of steady-state errors 
• Help to prevent excessive oscillation and overshoot response from tension changes 

PID Controllers enable a smooth transition for accelerating, decelerating, and speed transitions (changing speeds) while maintaining the stability of the web.

Linkage to Automation Solutions

Web Tension Controllers can be effectively linked to automate production lines through:

• An output analog signal (0-10V, 4-20 mA)
• Communicating digitally (RS485, Modbus)
• PLCs & HMIs

Allows for:

• Monitoring in real time
• Alarm creation
• Data loggings
• Remote Control function

Integration promotes process visibility and supports the 4th Industrial Revolution.

Advantages Of Using A Web Tension Controller

• Improved quality of the end product 
• Fewer breaks, resulting in reduced production waste 
• Increased production rates 
• Better rolls are being produced 
• Less machine downtime 
• Improved stability of the entire production process

Puretronics India enables manufacturers to enjoy less variation in product characteristics by maintaining a uniform tension, as well as an overall improvement in machine performance from the use of an efficient web tension control system.

Web Tension Controllers Are Used In Many Industries

Web Tension Controllers are applied in the following industries: 

• Printing and packing
• Plastic films and extrusion
• Textile processing
• Paper and foil conversion
• Hygiene product manufacturing
• Wire and cable processing 

Any industry that uses Puretronics' continuous web materials will require Web Tension Controllers to maintain correct tension levels throughout the entire process to ensure both high product quality and efficient operations.

A Web Tension Controller is pivotal in achieving consistent, quality web processing, as well as improving manufacturing efficiency in multiple sectors. By measuring, comparing, and continually adjusting tension in real-time,  Puretronics’ Web Tension Controllers remove production variability and increase machine performance levels.

When installed on lines used for unwinding, processing, or rewinding applications, effective Web Tension Controllers will deliver increased reliability, accuracy, and automation to a state-of-the-art manufacturing environment. Therefore, implementing an advanced Web Tension Controller is not optional for organisations. So, Puretronics wishes to minimise waste, optimise throughput, and produce consistent quality, rather, it is essential.