Temposonics®

Magnetostrictive, Absolute, Non-contact

Linear-Position Sensors

Linear Position Sensing in Steel Mills and Metal Forming Applications

White Paper

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Document Part Number 551427 Revision A

Model RD4 Detached electronics position sensor

By Matt Hankinson, Ph.D., Technical Market-ing Manager of MTS Sensors

INTRODUCTION

Magnetostrictive linear position sensors have been used for decades in steel mills in a variety of applications to provide precise position feedback for control. Machinery ranging from mold oscillators, coke oven doors, hot and cold rolling mills, hot-dip galvanizing and coil levelers are just a few of the areas utilizing magnetostrictive linear transducers to improving equipment performance and uptime. In high speed casting and rolling mills, a millimeter discrepancy can quickly add up to tons of wasted steel or poor product quality.

UNIQUE ENVIRONMENTAL CHALLENGES FOR STEEL MILLS

There are numerous challenges with harsh environments in the steel mills ranging from high temperature to shock and vibration. With the enormous cost of equipment downtime, finding a reliable solution that can stand the environmental factors is critical.

In continuous casting, extreme heat and moisture, combined with continual vibration, electromagnetic interference and abrasive grit, will eventually ruin even the most durable equipment. With sensitive electronics, the problem is even more pronounced. To protect sensors from the elements, they are often inserted within the hydraulic cylin-ders used on the machine. The sensor head, however, often remains outside of the enclosed cylinder environment.

A NEW SOLUTION

In early 2012, MTS Sensors, a division of MTS Systems, introduced the RD4 Model Temposonics® magnetostrictive linear positioning sensors. Based on the R-Series Temposonics Sensor line, the RD4 incorporates a well shielded and reliable detached electronics setup.In the Model RD4 Sensor, the sensing element is connected to the electronics via a cable that enables the electronics housing to be mounted away from the measurement location.

Currently, the electronics housing, along with its mounting block, can be configured with either a side cable connection or a bottom cable connection. Side cable connections work with many different threaded rod styles. This flexibility of design allows for easy sensor installation into standard threaded port opening on the top of the cylinder end cap. The bottom cable connection is for use with the pressure-fit rod style.

Temposonics® Linear-Position Sensors - White PaperLinear Position Sensing / Steel Mills, Document Part No.: 551427, Revision A 05-13 MTS Sensors

Proper design and careful sensor installation is required to assure the correct fit and o-ring sealing, resulting in more resilient electronics that provide the benefits (i.e., precision and reliability) of traditional magnetostrictive systems with significantly lower risk of product dam-age.

To minimize electrical noise, the cable length is limited between the sensing element and electronics, but the distance is typically more than enough to accommodate the equipment layout and provide the performance needed in these applications.

Another option for handling high temperature is to fully embed the sensor inside a welded cylinder. This provides the protection of the hydraulic cylinder and cooling of the hydraulic fluid with the tradeoff of not being able to easily replace the sensor module.

THE DEMAND FOR ACCURACY

Advanced rolling mills utilize advanced Hydraulic Gap Control (HGC) systems to keep steel traveling on the hot roll lines at the proper thickness. The key to these systems resides in HGC cylinders mounted on the sides of the roll stand. Advanced sensors bolted onto the cylinders monitor any deviation, with resolution down to 1 micron, and communicate that data to the control room. Any discrepancy will be immediately evident and allow workers to compensate in the most efficient method possible.

To meet customer demand, the equipment in the steel plant runs nearly continuously, moving, shaping and cooling tons of metal every hour. It falls to a team of operators and managers to ensure that each machine not only continues to operate, but that there is no deviation whatsoever. Banks of computers monitor every machine, alerting the team to deviations as small as a few microns.

Even a small discrepancy in the thickness of a slab on the line can quickly add up to thousands of pounds when you consider the speed at which processing lines move. Consistency and quality control are critical to ensuring that our customers get the exact product they are looking for in the most efficient method possible.

Traditionally, these systems have incorporated advanced incremental linear encoders due to their precise measurement capabilities. The downside is these encoders are expensive and rely on delicate elec-tronics to measure and communicate signals. They are susceptible to heat, pressure and vibration and usually require continual maintenance and replacements.

These sensors operate in hostile environments where temperatures rarely drop and pressure, vibration and exposure to water are essen-tially constant. Maintaining and replacing failing linear encoders can be a major source of downtime on the hot roller lines.

One of the initial concerns might be whether the magnetostrictive posi-tion sensors could match the accuracy of the digital linear encoders. However, the difference between the two technologies is negligible upon installation. The accuracy of the MTS R-Series meets the per-formance needs and also offers the benefit of improved reliability and lower price.

R-Series sensors can report with linearity as low as ± 20 microns with a fast update rate. For sub-millisecond update times, there are options such as digital SSI and Ethercat. Recent advances in testing protocols have provided the data necessary to specify these sensors for even the most advanced and demanding rolling mill applications.

MAINTENANCE AND INSTALLATION BENEFITS

Magnetostrictive sensors are also easier to use once installed. The advanced electronics involved with operating linear encoders are prone to erratic deviations based on a phenomenon known as temperature drift. Temperature drift causes small changes in the data as it is be-ing collected and transferred back to the computer console. While magnetostrictive sensors are also affected by temperature drift, the deviations tend to be more predictable and, thus, easy to overcome.

In addition to better reliability and robustness in rougher environments, ease of installation as another major advantage. Because the electron-ics are less sensitive and complex, removing and reinstalling these cylinders is a fairly easy process. Even if a sensor needs to be replaced, the associated downtime is considerably less than it was with the en-coders, which had to be hardwired and required more care during in-stallation.

Additionally, the magnetostrictive sensors are available in a redundant package with spare output channels not available on linear encoders. These channels provide additional redundancy and further reduce downtime associated with these applications.

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MTS and Temposonics are registered trademarks of MTS Systems Corporation.All other trademarks are the property of their respective owners.

Printed in USA. Copyright © 2013 MTS Systems Corporation. All Rights Reserved in all media.

Document Part Number: 551427, Revision A 05/13

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In numerous steel mills that have replaced linear encoders with magnetostrictive sensors, customers have improved productivity and significantly reduced downtime and manpower associated with replacements and maintenance.

METAL SHAPING AND PROCESSING

Precision measurement is important in any industry. In metal shaping and processing, where material defects are difficult to remedy, it is absolutely vital. When a coil of metal leaves the truck, before it can be cut or shaped, it must first be flattened into workable sheets. This goal is typically achieved using one of several different leveler technologies. Options include roller levelers, tension levelers, temper mills, precision levelers and stretch levelers.

Of these, stretch leveling, which involves stretching the metal beyond the elastic yield point, offers a low level of defects, such as wavy edges and buckling. The elastic yield point is the point at which the shape of a material is permanently altered. Accomplishing this task requires ex-treme force and precision control, and therefore a precision feedback device. Ideally, a stretch leveler will pull to just beyond the yield point and then stop. The larger the stretch leveler, the more important this precision becomes.

Levellers, as well as other types of machinery, may also require mul-tiple feedback devices on the equipment. There are multiple options for linear position sensors to provide the networking capability for the electrical interface. Increasingly, Industrial Ethernet protocols such as Ethernet/IP, EtherCAT, Profinet, or POWERLINK are employed to reliably network multiple sensors. There are also Fieldbus protocols such as Profibus, Devicenet, and CANbus. Finally, there is always the option of analog outputs from the sensor such as current or voltage which supports a wide range of controllers.

(Photo courtesy of Butech Bliss)

HOW MAGNETOSTRICTION WORKS

Magnetostrictive-based sensors work by inducing a sonic strain pulse in a specially designed magnetostrictive waveguide by the momentary interaction of two magnetic fields. One field comes from a movable permanent magnet which passes along the outside of the sensor tube, the other field comes from a current pulse or interrogation pulse ap-plied along the waveguide. This interaction produces a strain pulse, which travels at sonic speed along the waveguide until the pulse is detected at the head of the sensor.

The magnet’s position is determined with high precision by measuring the elapsed time between the application of the interrogation pulse and the arrival of the resulting strain pulse. Consequently, accurate non-contact position is achieved with absolutely no wear to the sensing components.

ABOUT MTS SENSORS:

MTS Sensors, a division of MTS Systems Corp., is the global leader in the development and production of magnetostrictive linear-position and liquid-level sensors.

MTS Sensors Division is continually developing new ways to apply Temposonics® magnetostrictive sensing technology to solve critical applications in a variety of markets worldwide. With facilities in the U.S., Germany, Japan, and China, MTS Sensors Division is an ISO 9001 certified supplier committed to providing customers with innova-tive sensing products that deliver reliable position sensing solutions.