Is it time to consider using LVDTs?

Lee Hudson, application engineer at Macro Sensors, shows how developments in LVDT linear position sensor technology has resulted in its increasing use in process applications

Today’s industrial process control applications increasingly use automated systems to optimise operations and ensure a safer, more productive process. Linear position sensors used in these systems provide accurate feedback on product parameters, control states and outputs to machine controllers.

Whether implemented as a standalone component or as part of a control or safety system, the linear position sensor, also often known as an LVDT, is capable of providing linear displacement measurements from micro inches to several feet, under various operating and environmental conditions with high accuracy and reliability. Essentially, the LVDT plays an important role in machine control by providing feedback about product location. To some extent, it is the LVDT that ensures proper machine operation.

In basic terms, an LVDT linear position sensor is an electromechanical device that converts linear position or motion to a proportional electrical output (see Figure 1). 

Because there is normally no contact between the LVDT's core and coil structure, no parts can rub together or wear out. This means an LVDT features unlimited mechanical life, useful in industrial process control and factory automation systems.

Recent innovations in construction materials, manufacturing techniques, and low-cost microelectronics mean LVDTs are now a more reliable and cost-effective technology for process control applications.  In the past, the electronics necessary to operate LVDT linear position sensors were complicated and expensive, prohibiting their wide use in process control applications for displacement measurement.

Modern ASIC and microprocessors give LVDT technology more complex processing functions and enable signal conditioning within the sensor housing, so LVDTs generate digital outputs directly compatible with computer–based systems and standardised digital buses. As a result, today’s linear position sensors can provide more accurate and precise measurement of dimensions in a wider variety of quality control, inspection equipment and industrial metrology applications including online parts inspection, servo-loop positioning systems, and manufacturing process control. 

For applications where sensors must operate in extreme environments, the sensing element can be segregated from the electronic circuitry, unlike capacitive, magnetostrictive and other high frequency technologies.  Connected by long cables up to 31m, AC-operated LVDTs can work with remotely-located electronics that power the sensors, amplify and demodulate their output. Output is then displayed on a suitable readout and/or inputted into a computer-based data acquisition system for statistical process control.

While linear position sensors were once considered too long for applications with limited space, new winding techniques and computer-based winding machines allow the linear position sensor body to be reduced while maintaining or increasing stroke length. With the improved stroke to length ratio (now up to 80%), the LVDT becomes a viable position measurement device for valve position sensing.

Smaller, contactless linear position sensors also feature a lightweight low mass core suited to process control applications with high dynamic response requirements such as plastic injection moulding machines and automatic inspection equipment.

New corrosion-resistance/high-temperature materials such as Monel or Inconel enable the LVDT to operate in more hostile environments including those with high and low temperature extremes, radiation exposure or vacuum pressure conditions. For applications where sensors must withstand exposure to flammable or corrosive vapors and liquids, or operate in pressurized fluid, its case and coil assembly can be hermetically sealed using a variety of welding processes.

For example, in power generation applications (see photo below), linear position sensors designed for high temperature and mild radiation resistance offer feedback on the position of nuclear steam and gas turbine control valves for increased plant efficiency and reduced operating costs.

In a typical power plant, steam turbines contain a number of control valves. Typically, plants have very precise control schemes for valve position to increase operating efficiency and save fuel.  Operating within the harsh environment of a power or steam plant, linear position sensors can determine if valves are fully opened or closed to within a thousandth of an inch, providing output to remote electronics that can be monitored by operators if something is not working properly.

Sensors also play an important role in the predictive maintenance of gas turbines as part of process control systems used to monitor shell expansion and bearing vibration. Installed on turbine shells, hermetically-sealed LVDTs measure shell expansion, providing linear output that operators can utilise to determine proper thermal growth of a turbine shell during startup, operation and shutdown.

Related Articles