Since its commercial introduction more than 60 years ago, the LVDT linear position sensor has evolved from the original use as a laboratory tool to the industry, military, aerospace, subsea, downhole drilling, and the key technology for critical and reliable linear displacement measurement of nuclear energy. Power and process control applications.

With microcontroller-based electronics and innovative building materials and technologies, modern LVDT technology is competitive with other displacement sensing technologies in terms of price, performance and durability.

What is LVDT? 
A linear variable differential transformer (LVDT) is an electromechanical sensor that converts the linear motion of a object - mechanically coupled - into a corresponding electrical signal. The LVDT linear position sensor has a wide range of measurements for motions as small as a few millionths of an inch to ±20 inches.

In operation, the internal structure of the LVDT includes a primary winding centered on a pair of identically wound secondary windings that are symmetrically spaced about the primary winding. The coil is wound on a one-piece hollow form of thermally stable glass reinforced polymer, encapsulated in moisture, wrapped in a high permeability magnetic shield, and then secured in a cylindrical stainless steel housing. The coil assembly is typically a stationary component of the position sensor. It is driven by an AC signal called the primary excitation, usually between 0.4 and 5 kHz. When the core moves, the secondary coil will pick up the signal by magnetic induction ( Figure 1 ). The electrical output signal of the LVDT is the differential AC voltage between the two secondary windings that varies with the axial position of the core within the LVDT coil. Typically, the AC output voltage is converted to a more convenient high level DC voltage or current through a suitable electronic circuit.

Until recently, the total cost of using LVDT linear position sensors still prohibited widespread industrial use. In particular, the electronics required to properly operate the LVDT are complex and expensive. High-density microelectronics now integrates signal conditioning and processing into the LVDT housing without the need for an external box. Errors caused by LVDT characteristics or the environment can be corrected such that the cells are more accurate than uncorrected LVDTs. LVDT can now generate digital outputs that are directly compatible with computer-based systems and standardized buses.

New building technologies and materials also enable LVDTs to operate in harsh environments, including extreme high and low temperatures, radiation exposure, and environments under seabed or vacuum conditions. Today, LVDT linear position sensors can be found underwater, on the ground or even in the air, providing incremental feedback for applications in different environments.

Proper stroke LVDT for hydraulic applications improves positional efficiency in hydraulic applications 
by monitoring the performance accuracy of actuators and cylinders for improved operational efficiency. In most hydraulic applications, the role of the cylinder is to move something on an off-road vehicle, such as a valve, an airplane rudder or a boom or shovel. In these applications, the control system requires a feedback device that tells it how far the cylinder or actuator has moved. For example, if the pilot wants to turn the airplane, he will move the joystick. The aircraft's control system detected that he had moved the joystick and sent a signal to the rudder actuator to move the rudder. If the system cannot know how far the actuator has moved, the airplane may have turned too much or not enough.

Another example is the robot arm picking up a piece of glass. If the control system does not know when to stop the arm, the hydraulic cylinder can drive the arm directly through the glass plate based on position feedback from the LVDT.

Although linear position sensors have been considered too long for hydraulic applications, new winding techniques (such as computerized layer winding) greatly reduce the length of the linear position sensor body compared to its measurable stroke length. A favorable stroke length ratio means that the sensor can now be mounted inside the cylinder, and in the past the length of the sensor body would be too long. In other applications where the LVDT cannot be mounted inside the cylinder, its reduced length makes it easier to mount the sensor on the side of the cylinder. Many of these cylinders and actuators are small and sporty;  magnetostrictive sensors used to be the preferred solution for short-stroke actuators and cylinders <6 inches in length, and LVDTs can now be much shorter than magnetostrictive sensors.

Position measurement of the steam control valve. Due to its exceptional reliability and ability to withstand high ambient temperatures, LVDT linear position sensors are used in power plant repairs to better monitor the position of steam control valves for increased efficiency and lower operating costs.

A typical two-stage steam turbine has a regulating valve, a throttle valve, an intercepting valve and a reheat shut-off valve. Each of these valves has two or three LVDTs that are redundantly mounted on the outside of the valve, and the LVDT's connecting rod is connected directly or indirectly to the valve's door.

The plant operator must always know the position of the governor and throttle and the starting and closing positions of the reheat stop and intercept valve. Typically, these valves are fully open or fully closed, depending on the start/stop status of the turbine. As a regulating valve, the regulating valve and the throttle valve can be opened by 25% or 40% depending on the power generation requirements.

Typically, the plant has a very precise control scheme for valve position to increase operating efficiency and save fuel. Due to incorrect valve settings, the cost of improper operation and inefficiency of plant operators can be as high as millions of dollars per year.

The LVDT feedback is sent to the turbine control system so that it knows the distance the valve is open or closed. The LVDTs are very precise and the operator can rely on them to determine if the 15-inch stroke valve is actually closed, in the range of one thousandth of an inch. The combination of LVDT and modern computerized turbine control systems has saved power companies millions of dollars annually.

Oil extraction. LVDT is used in the petroleum industry for position feedback control of downhole drilling equipment, such as measuring the borehole range of the borehole bore. The LVDT is used in borehole cutters to ensure near-perfect holes during cutting. This quality is called borehole ellipticity . The LVDT provides continuous position information for the cutting pliers at temperatures up to 200 ° C and pressures up to 20,000 psi.

The LVDT for these applications is custom designed to withstand high temperatures and high pressures. In a non-conductive, chemically benign medium, the unit is rated at 20,000 psi at continuous temperatures up to 400 °F. High temperature ratings are achieved by using special structural materials such as special high melting point solders.

The LVDT coil assembly and the detachable core can withstand extremely high pressures because the mechanical structure of the coil assembly is vented (pressure balanced) to the pressure of the non-conductive medium. The coil assembly can withstand high pressure, high temperature, shock and vibration combinations.

Due to its superior performance and low cost of ownership, LVDT is used in deep-sea oil drilling applications where the immersed LVDT has an estimated MTBF (mean time between failures) of more than 1 million hours. For example, in an offshore drilling application, a linear position sensor based on LVDT is connected to pipes and pipes of offshore platforms (including production risers, catenary risers, tendons and platform legs and supports) to monitor structural movement . When a pipe or pipe strain occurs, the LVDT will report a change in position through a 2-wire, loop-powered 4-20 mA I/O, allowing action to be taken against motion. For large loads on these structures, the total motion measured is typically <2 mm.

LVDT's ever-expanding practicality 
In daily life, LVDT can be found in ATM machines to ensure proper money distribution;  flight control systems for military and commercial aircraft;  and metrology laboratories;  steel, aluminum and paper mills control thickness;  Industrial production lines to ensure the proper size of the product;  satellite actuator lenses in outer space;  on the seabed of the oil well throttle. Over the past sixty years since the earliest applications of sensors with electrical outputs, LVDT has solved countless position measurement problems. In the next few years, it will solve more.