Protecting linear positioning sensors in harsh environments

October 2nd, 2014, Published in Articles: EngineerIT

 

All industrial manufacturing, ranging from refining raw materials to plastic bottles to aerospace vehicles, is driven by a balance of economics and performance. Companies that can increase the performance of their operations and products without unrealistic costs can create a true value differentiation from their competitors and drive innovation in the industry.

One of the most reliable ways to improve operations in a manufacturing facility is implementing higher precision measurement and control of the machines throughout an operation. In a fast pace, competitive environment, every micron matters. High speed precision manufacturing is the now the standard.

This simple need has driven the innovation and development of linear positioning sensors for almost half a century. Today, there are few production processes that do not rely on some type of positioning measurement methods, whether it is a simple encoder or a more advanced solution such as magnetostriction. Better equipment control and quality assurance means less waste and greater productivity and has been one of the primary drivers of advanced automation techniques.

There are still areas, however, where the installation and use of linear positioning sensors is problematic. Due to the mechanical and environmental sensitivity of many positioning technologies, many manufacturers struggle to find a good solution in applications where the sensor is proximal or exposed to corrosive materials, high temperatures, electromagnetic interference (EMI), continual shock and other factors that could damage the sensor or compromise the performance.

Fig. 1: Harsh environment in a steel mill.

As a result, these manufacturers must often consider suboptimal or less precise measurement techniques, which can result in slower production and less efficient operations. In many instances, it can even impact safety and environmental compliance.

What needs protecting?

Before discussing ways the industry can address these issues, it is first important to understand the basic mechanical components of a sensor and how it fits into a machine. A linear positioning sensor’s primary role is to measure the displacement between points on a straight (or linear) path. That relationship is communicated, either via analog or digital signal, to a device where it can be interpreted by either a human operator or automation software for a variety of purposes.

The components inherent in almost all linear positioning applications are:

  • The sensing element, usually a rod or extrusion that resides along the linear path that is being measured.
  • The electronics, where the displacement between multiple points on the element is converted into a linear measurement,
  • The controller or software, where that linear measurement is interpreted into usable data, and
  • The actuator, a device for converting a source of energy into linear motion

Compromise or failure of any of these components could result in a system failure. Since the controller is often housed away from the machine in a controlled environment such as a control center, the solutions defined in this article will focus on the design and protection of the sensing element and electronics.

Protecting the sensor through product and cylinder design

MTS Systems, sensors division, has developed and introduced several new sensor products designed to directly address the issue of harsh environments. These solutions have centered around three areas – embeddable products, detachable electronics and redundant design for greater safety standard compliance.

Embeddable products – where design matters

The simplest, and often most effective, way to ensure the integrity of a sensor is to embed it inside of a hydraulic or pneumatic cylinder. This has become the standard in mobile hydraulics, where moving equipment on construction and mining equipment is continually exposed to the elements- and is also seen in some industrial applications.

In industrial manufacturing applications, it is often preferable to install the sensor externally where it can be easily replaced if necessary. How- ever, there are sometimes machine designs that require an embedded solution either because of space constraints, or for the protection of the sensor in harsh environments such as high temperature.

To address this issue, MTS developed the Temposonics model EE sensor:  a fully embeddable sensor, including the electronics, which is compact enough to be integrated directly into a hydraulic cylinder. It is also designed to withstand higher temperature ratings, making it ideal in applications such as steel mills or other harsh application such as rock crushers, presses or outdoor applications.

Detached electronics – where space matters

Embedded solutions do have another drawback, however. They have to enclose both the sensing element and the electronics to perform the measurements. This is not a concern in applications this model was designed to address that use simple interfaces such as an analogue signal. However, when more advanced controller interfaces are needed, such as Industrial Ethernet, a different approach is required.

A solution involves remotely mounting the electronics away from the equipment to avoid the harsh environment.

Fig. 2: Detached electroics with R-series model RD4 sensor.

The  R-series model RD4 sensor incorporates a detached electronics design (the sensing element is connected to the interface electronics via a cable), allowing manufacturers and operators to place sensitive electronics in areas where interference or damage is less likely. This is useful for a variety of harsh environments such as high shock and vibration or high temperature.

Redundancy – where compliance matters

Even the best designs and protective housings can experience failures, however. Many machine designers address this by installing multiple redundant sensors across the equipment. While effective, it is also the bulkiest and most expensive solution possible.

To help address this goal, the G-series model GTE embedded magnetostrictive sensor was developed. Embedded into a hydraulic cylinder or other equipment like the model EE sensor, the model GTE features a dual channel output and two sensor elements in the same housing. This allows extremely compact design with the same redundancy offered in the multiple sensor scenario described above. This sensor is also designed to withstand temperatures up to 850C and features an IECEx Zone 2 approval for use in hazardous areas.

The redundant sensor design also makes integration within a cylinder a more attractive concept by addressing the reliability concern. Machine engineers, operators and maintenance crew can have greater confidence that the sensor will continue to perform even in the event of a failure. The improved reliability reduces the costly downtime associated with machine repair, especially when an embedded solution is used.

Contact Wim Annandale, ATI Systems, Tel 011 383-8300, technical@atisystems.co.za

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