By Eileen Otto, Sales and Marketing Director, Macro Sensors
New manufacturing techniques, high-tech electronics and emerging construction materials have transformed LVDTs into high performance, low cost alternatives to other displacement technologies.
Ever since being introduced as a lab measurement tool for military use during WWII, the linear variable differential transformer, or LVDT, has expanded its use into industrial, commercial, aerospace, subsea and process control applications. A highly reliable displacement feedback device, the LVDT linear position sensor is capable of measuring movements as small as a few millionths of an inch up to ± 20 in. (<±0.5 m). In recent years, new microprocessors and corrosion resistance/high temperature materials have expanded its capabilities, enabling the LVDT Sensor to work in new environments—while making it more competitively priced against other displacement sensing technologies.
While new construction materials and electronics may have optimized its performance and cost of ownership, the basic operating principle of LVDT technology remains the same. As an electromechanical transducer, an LVDT converts the rectilinear motion of an object to which it is coupled mechanically into a corresponding electrical output signal. Usually this AC output voltage is converted by electronic circuitry to a high-level DC voltage or current. Figure 1 shows a more detailed construction of an LVDT.
AC- and DC-operated LVDTs
The first LVDTs on the scene were AC-operated and required separate signal conditioning. The time required to calibrate these two components together, as well as the expense for additional equipment, prohibited widespread use of the technology. Modern ASIC and microprocessors enabled manufacturers to create DC-operated LVDTs that incorporated signal conditioning and embedded more complex process functionality within the sensor body, while keeping the overall package size small.
DC-operated LVDTs maintain all the desirable characteristics of their AC-operated counterparts, but offer the simplicity of DC operation. Comprised of an AC-operated LVDT as well as a carrier generator/signal conditioning module, the DC-operated LVDT eliminates the volume, weight and cost of conventional external AC excitation, demodulation and amplification equipment. As there is no need for calibration or amplification equipment, setup time is reduced, along with overall system cost.
Outputs from DC-operated LVDTs are directly compatible with computer–based systems and standardized digital buses. With greater immunity to noise than analog systems and greater capacity to control errors, digital technology provides the complete signal from an LVDT or other sensor for download into computer software or other network programs. In networked communications, digital communications also enables a daisy chain of multiple signal conditioners on one bus line, reducing wiring costs, as each sensor does not need to be wired to the signal conditioner. As many industries move to digital output, DC-operated LVDT Linear Position Sensors have become more popular.
However, there are times when an AC-operated LVDT position sensor can actually outperform its DC-operated counterpart. Without internal electronic components, AC-LVDTs can be offered in smaller packages to fit in compact locations. AC-operated units also have better shock and vibration resistance and can operate over higher temperature ranges.
For applications where sensors must operate in extreme temperatures and/or high radiation, the sensing element of an AC-operated LVDT can be segregated from the electronic circuitry. AC-operated LVDTs can, then, be connected by long cables up to 100 ft, to work with remotely located electronics that power, amplify and demodulate their output. While separate electronics are still required, push button zero and span controls have replaced digital pots and improved software has reduced the time and expense associated with calibrating these sensors.
Electronics and manufacturing improve performance
While both AC- and DC-operated linear position sensors are inherently highly reliable and repeatable devices, new manufacturing and enhanced computer winding techniques have further improved their performance by as much as a magnitude over LVDT technology of just a decade ago. A standard LVDT can possess a linearity of ± 0.25% of full scale output; a corrected LVDT with microprocessing can linearize its output to ± 0.05% of full scale output.
Today’s LVDTs are also able to reproduce the same output for repeated trials of exactly the same input under constant operating and environmental conditions. In fact, transducer repeatability is affected only by the mechanical factors of the physical members or structures to which its core and coil is attached. Many critical applications require this unit-to-unit consistency.
In addition to improving LVDT performance, new computerized layer winding techniques and smaller embedded microprocessors have considerably reduced the length of the LVDT linear position sensor body compared to its measurable stroke length. Up to 80% more compact for any given stroke, the current generation of LVDTs are now used for machine tool positioning, hydraulic cylinder positioning, valve position sensing, and automatic assembly equipment. Lightweight low mass cores also optimize dynamic response and reduce weight, making LVDTs suitable for applications having high dynamic response requirements or where weight is a premium such as on aircraft and in satellites.
In the past, LVDTs were limited by their sensitivity to the environment, which caused errors in linear feedback. While once predominantly available in stainless steel, these sensors are now constructed in a variety of materials including Inconel, Monel, titanium and Hastelloy for more resistance to the environment and to extend their use in hostile applications including those with high and low temperatures, mild radiation, subsea and vacuum pressure conditions.
For example, Monel 400 provides excellent resistance against pitting and attack by microorganisms in warm, shallow waters while titanium and Hastelloy hold back against pressure and corrosion when measurements must be obtained in seawater depths down to 7500 ft. and with external pressures of 3800 psi. Exotic alloys such as cobalt, nickel and chromium with mineral insulation can deliver even higher performance from LVDTs, where comparable technologies will not survive. For applications where sensors must withstand exposure to flammable or corrosive vapors and liquids, or operate in pressurized fluid, sensor case and coil assembly can be vented or hermetically sealed.
As a result of these construction options, LVDT position sensors are becoming popular choices when it comes to providing displacement feedback in harsh and deepwater environments, including offshore applications, downhole drilling, and power generation.
From its introduction as a laboratory tool, the LVDT linear position sensor has evolved into a highly reliable and cost-effective linear feedback device, making it the preferred technology for critical and reliable linear displacement measurements in an array of industrial, consumer, process control and military applications.
Macro Sensors
www.macrosensors.com