The linear variable differential transformer (LVDT) is an electro-mechanical device whose electrical output is linearly proportional to the displacement of a moveable core. It consists of a primary coil with two secondary coils placed on either side of the primary coil. A rod-shaped soft magnetic core inside the coil assembly provides a path for the magnetic flux linking the coils.
When the primary coil is energised by an alternating current, source voltages are induced in the two secondary coils. The secondary coils are connected in series, with the start of each winding being connected together. This arrangement produces a net zero signal output from the secondary coils when the induced voltages are equal in each of them.
This condition occurs when the core is centrally disposed between the two secondary coils. A movement of the core leads to an increase in magnetic coupling to the coil in the direction of movement and a reduction in the magnetic coupling to the other coil, producing a net output signal from the connected secondary coils. Movement in the opposite direction produces an identical signal output but of opposite phase.
There are certain advantages to be gained from using LVDTs in pressure sensors. In such devices, a pressure responsive element is directly coupled to the core of the LVDT, which detects the movement of this element. The core displacement of the LVDT, prompted by the movement of a metallic pressure responsive diaphragm, produces the transducer output. Some LVDT pressure transducers are fitted with a single, precision metallic diaphragm (complete with over-range pressure protection stops) as the pressure-responsive element. This arrangement is suitable for differential, gauge and absolute transducers, which all employ a common design philosophy.
The primary advantage of using an LVDT transducer is that the moving core does not make contact with other electrical components within the sensor assembly, as is the case with alternative types of pressure sensing device. This configuration ensures both high reliability and long life. Moreover, the LVDT design lends itself to easy modification in order to fulfill a whole range of different applications in both research and process engineering.
The LVDT gauge-type pressure transducer is well protected from damage that might otherwise arise from positive over-pressure. The sensor’s safe limits are normally much greater than those specified by the manufacturer and are unmatched by alternative technologies. Often, the sensor will still operate above the specified over-pressure limit, but at a reduced accuracy – quite beyond the capability of silicon and thick-film pressure sensors.
Process containment and compatibility
And unlike silicon and thick film pressure sensors, LVDT pressure transducers provide process containment for applied static pressures of up to 400bar or higher. Special welding techniques are used to improve rupture integrity, supported by an over-pressure stop. In addition, the diaphragm material can be relatively thick, offering enhanced durability and improved resistance to corrosion induced pin-holing. LVDT pressure transducers can also be impact shock-loaded in all three axes without sacrificing sensor performance. The diaphragms are not made from brittle materials and so failures due to shock loads are rare.
Process compatibility is also a key requirement of a pressure transducer. With LVDT pressure sensors, flush diaphragms can be provided rather than fluid-filled units. This offers enhanced process compatibility and does not limit the temperature range. In addition, if the pressure sensor is required to perform in a hygienic application, such as a dairy or food processing plant, a low cost silicon-filled sensor will require a barrier of some sort to prevent contamination should the sensing element become ruptured. The LVDT pressure sensor, on the other hand, is inherently suited to hygienic, FDA-compliant applications, and there is generally a wide range of process interface and wetted material options available to the user, including Tantalum, Hastelloy, stainless steel, Monel, Inconel and PTFE sintered coatings.
Transmitters for LVDT based pressure and level transducers are also available in both analogue and digital signal processing versions. Most analogue transmitters will offer zero and span adjustment, a square root option, time constant and ±100% offset adjustment. Digital electronic types offer local configuration of zero and span, along with the ability to turn the instrument preset non-linear function either to ‘on’ or ‘off’. Moreover, digital types can normally be configured via an integral communication port.
Submersible type LVDT pressure sensors normally use digital signal processing and have the option of either a simple single wire configuration port that allows zero and span calibration together with the ability to turn the instrument preset non-linear function to on or off, or full RS485 communication that enables full transmitter configuration.
There are extensive applications of LVDT pressure transducers in the nuclear sector, where they are deployed on reactor research and development work, as well as process operations.
These include leak detection on nuclear transport flasks; detection of leakage from Magnox storage ponds; monitoring material storage pond levels; storage room pressure monitoring; level measurement in effluent treatment works; and glove box gas handling systems. LVDTs are even used in weapons de-commissioning, where the sensor must tolerate aggressive media such as hydrobromic acid, or where radiation immunity is critical. In the last category, LVDT transducers normally remain stable at exposures of up to 10 million rad, and some manufacturers even offer versions that allow up to 10,000 million rad exposure without damage to the sensor.
Yet another advantage of LVDT based sensors also is their ability to connect remotely from the signal conditioning electronics at distances of up to a kilometre or more. This enables the sensor to be located in environments subject to extreme radiation, high temperatures (up to 200oC) and high magnetic fields - conditions that would spell certain death for signal conditioning electronics assemblies!
Sam Drury is sales and marketing director at Impress Sensors & Systems