Sensor operating in adverse conditions for paper machine

Modern high speed paper making machines are fantastic, producing 1500m of paper 9m wide per minute. It's a lot of paper moving very fast. And the quality of the paper produced is superb with different finishes required for different types of end use.

The paper is produced from pulp which is fed onto a 'tape' and is then passed through a hot section, running continuously at over 90°C, which gradually dries the pulp out until it is strong enough to be handled as a continuous sheet.

The whole process is simple in nature but the machinery is extremely complex and tightly controlled to ensure the quality of the paper.The whole machine is huge taking up most of a building 300m long and is over 9m wide and about 15m high.

The machines run continuously 24 hrs a day and stoppages and breakages are very costly.

Within the machine it is important to control the position of the tape carrying the drying pulp across the rollers which are spinning fast. If the tape slides across the rollers it can be damaged resulting in complete replacement and long stoppages. The position of the tape is controlled using an optical edge recognition system combined with a mechanical servo control of the roller attitude.

At the heart of this edge control system is the Positek P101 Linear Inductive Position Sensor mounted at the end of the roller.The sensor measures the roller attitude and relays the information to the control system. The roller is operating in the section of the machine where the pulp is dried. It is very hot, 90 to 110°C, and the humidity is high in this enclosed section.

The manufacturer of the paper machines needed a sensor which could withstand this tough environment. Positek were able to offer a 2 wire 4-20mA output option on the P101 sensors which would operate over long periods at the high temperatures.

The customer also needed a sensor which was made entirely form stainless steel so that it would withstand the corrosive and damp environment. Positek were able to modify the design of some of the parts so that the sensor would satisfy the requirements of the customer.

During the early part of the project to develop the sensor for the application some difficulties were encountered with some of the mechanical parts, such as bearings, not coping with the environment over a long period. Positek is committed to working closely with customers and solving their problems and these early difficulties were resolved by co-operation between Positek and the customer. Interestingly the biggest concern at the start of the project was the ability of the electronic components to survive the harsh environment but these have performed very well.

The sensor is supplied by Positek calibrated to within ± 0.25% of the overall range both for the slope and the offset of the output. The end face of the body is used as the reference when it is calibrated. The internally-integrated Positek sensors permit simplification of the installation of the control system because the output is a loop powered two-wire 4-20mA format. The Positek sensor has just 2 connections to make, resulting in quicker installation.

A further advantage of the sensor derives from its fully linear response over the entire mechanical range. The sensor provides a linear response to within ± 0.5% of the travel.

The P101 sensor incorporates technology developed by Positek and used in its other displacement sensor products. This technology, is based on inductive coils and uses a unique custom integrated circuit to produce an accurate analogue output signal.