Since its commercial introduction more than 60 years ago, the LVDT linear position sensor has evolved from its initial use as a laboratory tool to becoming the preferred technology for critical and reliable linear displacement measurements in industrial, military, aerospace, subsea, downhole drilling, nuclear power and process control applications.
Through microcontroller-based electronics and innovative construction materials and techniques, modern LVDT technology is now competitive with other displacement sensing technologies in terms of price, performance, and durability.
What is an LVDT?
A linear variable differential transformer (LVDT) is an electromechanical sensor that converts the rectilinear motion of an object—to which it is mechanically coupled—into a corresponding electrical signal. Available in a variety of measurement ranges, an LVDT linear position sensor can measure movements as small as a few millionths of an inch to up to ±20 in.
In operation, the LVDT's internal structure consists of a primary winding centered between a pair of identically wound secondary windings, symmetrically spaced about the primary. The coils are wound on a one-piece hollow form of thermally stable glass reinforced polymer, encapsulated against moisture, wrapped in a high permeability magnetic shield, and then secured in a cylindrical stainless steel housing. This coil assembly is usually the stationary element of the position sensor. It is driven from an AC signal, known as the primary excitation, and normally between 3–6 kHz. The secondary coils will pick up the signal, through magnetic induction, as the core moves (Figure 1). The LVDT's electrical output signal is the differential AC voltage between two secondary windings, which varies with the axial position of the core within the LVDT coil. Usually this AC output voltage is converted by suitable electronic circuitry to a high-level DC voltage or current that is more convenient to use.
Figure 1. The components of a typical LVDT
Until recently, the total cost of using an LVDT linear position sensor prohibited broad industrial use. Specifically, the electronics necessary to operate the LVDT properly were complicated and expensive. High-density microelectronics now enable signal conditioning and processing functions to be incorporated within the LVDT housing rather than requiring an external box. Errors caused by the LVDT characteristics or the environment can be corrected, making units a magnitude more accurate than uncorrected LVDTs. And LVDTs can now produce digital outputs directly compatible with computer-based systems and standardized buses.
New construction techniques and materials also enable LVDTs to operate in hostile environments, including those with high and low temperature extremes, radiation exposure, and subsea or vacuum pressure conditions. Today, the LVDT linear position sensor can be found underwater, in the ground, and even in the air, providing incremental feedback for applications in widely different environments.
The Right Stroke for Hydraulic Applications
LVDTs provide position feedback in hydraulic applications by monitoring the performance accuracy of actuators and cylinders to improve operational efficiencies. The role of the cylinder in most hydraulic applications is to move something, e.g., a valve, airplane tail rudder, or a boom or shovel on an off-road vehicle. In these applications, the control system needs a feedback device that tells it how far the cylinder or actuator moved. For example, if a pilot wants to turn the plane, he moves the joystick. The plane's control system senses that he has moved the joystick and sends a signal to the tail rudder actuator to move the tail rudder. If the system has no way of knowing how far the actuator has moved, the plane could turn too much or not enough.
Another example would be a robotic arm picking up a piece of glass. If the control system does not know when to stop the arm, based on position feedback from an LVDT, the hydraulic cylinder could drive the arm right through the piece of glass.
Although linear position sensors were once considered too long for hydraulic applications, new winding techniques, such as computerized layer winding, have considerably reduced the length of the linear position sensor body compared to its measurable stroke length. A favorable stroke-to-length ratio means the sensor can now be installed inside of a cylinder whereas in the past the sensor body length would have been much too long. In other applications where the LVDT can not be installed inside of 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 have short motions; where magnetostrictive sensors were once the favored solution for short-stroke actuators and cylinders <6 in. long, an LVDT now can be much shorter than a magnetostrictive sensor, making it a more desirable choice.
Position measurement of steam control valves. Because of their extraordinary reliability and their ability to withstand high ambient temperatures, LVDT linear position sensors are being used in the rehab of power generation plants to better monitor the position of steam control valves for increased efficiency and reduced operating costs.
A typical two-stage steam turbine has a governor valve, a throttle valve, an interceptor valve, and a reheat stop valve. Each of these valves would have two or three LVDTs mounted redundantly to the valve's exterior with the LVDTs' connecting rods attached directly or indirectly to the valve's gate.
Plant operators must always know the position of governor and throttle valves and the position at start up and shut down of reheat stop and interceptor valves. Typically, theses valves are fully open or fully closed, depending on the turbine start/stop status. As modulating valves, the governor and throttle valves might be 25% or 40% open, depending on power generation requirements.
Typically, plants have very precise control schemes for valve position to increase operating efficiency and save fuel. The cost to the plant operator of improper operation and inefficiency due to wrong valve settings can be as much as several million dollars a year.
LVDT feedback is sent to the turbine control system so that it knows how far a valve is opened or closed. LVDTs are so precise that operators can rely on them to determine, to within a thousandth of an inch, whether a 15 in.-stroke valve is really closed. The combination of LVDTs with modern computerized turbine control systems saves power companies millions of dollars per year.
Petroleum extraction. Serving the petroleum industry, LVDTs are used for position feedback control of downhole drilling equipment such as bore scopes that measure the inner diameter of the drilled hole. LVDTs are used on the drilling cutters to ensure a near-perfect hole as it is being cut, a quality called bore hole ovality. The LVDT provides continuous position information of the cutter jaw at temperatures up to 200°C and pressures up to 20,000 psi.
LVDTs for these applications are custom built to survive high temperatures and high pressures. Units are rated for pressures to 20,000 psi in electrically nonconductive, chemically benign media, at continuous temperatures as high as 400°F. The high temperature ratings are achieved by using special construction materials, including special high-melting-point solder.
The LVDT coil assembly and separable core can withstand extremely high pressures because the mechanical configuration of the coil assembly is vented (pressure balanced) to the pressure of the nonconductive media. The coil assembly can withstand a combination of high pressure, elevated temperatures, shock, and vibration.
Because of their excellent performance and lower cost of ownership, LVDTs are used in deep-sea oil drilling applications where the projected MTBF (Mean Time Between Failure) for a submerged LVDT is in excess of 1 million hours. In one offshore drilling application, for instance, LVDT-based linear position sensors are attached to the pipes and tubes (including production risers, catenary risers, tendons, and platform legs and braces) of offshore platforms to monitor structural movement (Figure 2). As the pipe or tube is strained, LVDTs report a position change, via 2-wire, loop-powered, 4–20mA I/O, to a surface control system that can take steps to counter the motion. For the huge loads on these structures, the total movement being measured is typically <2 mm.
Figure 2. High-pressure and seawater-resistant LVDTs enhance the performance of measurement systems used to monitor the structural integrity of offshore platforms and pipelines
The Ever-Expanding Utility of LVDTs
In everyday life, LVDTs can be found in ATM machines, ensuring money is correctly dispensed; in flight-control systems for military and commercial aircraft; as well as in metrology labs; steel, aluminum, and paper mills to control thickness; industrial production lines to ensure properly dimensioned products; in outer space on satellite actuator lenses; and under the sea on oil well choke valves. In the more than six decades since the earliest application of sensors with electrical outputs, the LVDT has solved myriad position measurement problems. In the years to come it will solve many more.