Feature Transducers, transmitters & sensors


oday’s industrial process control applications increasingly use automated systems to optimise operations and ensure a safer, more productive process. Linear posi- tion sensors used in these systems pro- vide accurate feedback on product parameters, control states and outputs to machine controllers.


Is it time to consider using LVDTs? T


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


Whether implemented as a stand- alone 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 displace- ment measurements from micro inches to several feet, under various operating and environmental conditions with high accuracy and reliability. Essentially, the LVDT plays an impor- tant role in machine control by provid- ing feedback about product location. To some extent, it is the LVDT that ensures proper machine operation. In basic terms, an LVDT linear posi- tion 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 struc- ture, 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 compli- cated and expensive, prohibiting their wide use in process control applica- tions for displacement measurement. Modern ASIC and microprocessors


give LVDT technology more complex processing functions and enable signal conditioning within the sensor hous- ing, so LVDTs generate digital outputs directly compatible with computer–based systems and standard- ised digital buses. As a result, today’s linear position sensors can provide more accurate and precise measure- ment of dimensions in a wider variety of quality control, inspection equip- ment and industrial metrology applica- tions including online parts inspection, servo-loop positioning systems, and manufacturing process control.


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For applications where sensors must operate in extreme environ- ments, the sensing element can be seg- regated from the electronic circuitry, unlike capacitive, magnetostrictive and other high frequency technolo- gies. 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


Figure 1


In the oil and gas industry, compact LVDTs are used in the position feed- back control of down hole drilling equipment such as bore scopes that measure the ID of the drilled hole


and/or inputted into a computer-based data acquisition system for statistical process control.


While linear position sensors were once considered too long for applica- tions with limited space, new winding techniques and computer-based wind- ing machines allow the linear position sensor body to be reduced while main- taining 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


Figure 1 (top): The basic LVDT design consists of three elements: one pri- mary winding; two identical secondary windings; a mov- able magnetic armature or ‘core’


automatic inspection equipment. New corrosion-resistance/high-tem- perature 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 appli- cations where sensors must withstand exposure to flammable or corrosive vapors and liquids, or operate in pres- surized fluid, its case and coil assembly can be hermetically sealed using a vari- ety of welding processes.


Hermetically sealed linear posi- tion sensors offer an accurate and long life solution for the position measurement of steam control valves in power generation plants


For example, in power generation applications (see photo below), linear position sensors designed for high tem- perature 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 tur- bines contain a number of control valves. Typically, plants have very pre- cise 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, provid- ing output to remote electronics that can be monitored by operators if some- thing is not working properly. Sensors also play an important role in the predictive maintenance of gas turbines as part of process control sys- tems used to monitor shell expansion and bearing vibration. Installed on tur- bine 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, opera- tion and shutdown.


Macro Sensors www.macrosensors.com SEPTEMBER 2012 Process & Control Enter 218


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