'Astronomical' application of LVDT sensors

Ed Herceg -- Control Engineering, 10/1/2006

Because of its design, the core of an LVDT can be moved, and a position measurement can be made, without any friction being introduced into the measurement process.

Linear variable differential transformer (LVDT) position sensors are used for linear position measurement and feedback in factory automation, motion control, metal fabricating, power plant, and gas/steam turbine systems. These sensors are used for position measurement on governor valves, throttle valves, reheat/stop valves, interceptor valves, and many other control valves. An unusual application for LVDTs demonstrates their capabilities.

Today's newer reflecting telescopes used in astronomical observatories are composed of a series of computer-aligned mirror segments, instead of a one-piece cast mirror as in older telescopes. LVDT position sensors are used to precisely locate the mirror segments in the contour of the mirror. Other types of position sensors were tried in these telescopes but, with their friction-free operation, LVDTs offer superb repeatability with sufficient robustness to meet the application requirements.

This application uses two small-size, relatively short-range LVDTs per mirror segment. The core of each LVDT is attached to a small-diameter ball-screw-type mechanical drive system that is driven by its own microstepper motor, which gets its input from the computer. Analog-to-digital converters in the motor take the feedback voltage outputs from the signal conditioner for each LVDT, digitize them, compare the positions to a mirror contour template stored in memory, and close the servo loop by sending the appropriate control pulses to the microstepper.

The LVDTs used in the telescopes have full ranges of about +/- 1 mm, with resolutions of better than 100nm. LVDTs are an excellent choice in this application because they can be mounted in a manner that permits the movable core to be positioned anywhere within its linear range without touching or contacting the internal bore of the LVDT's coil. This means the core can be moved, and a position measurement can be made, without any friction being introduced into the measurement process.

The transformer'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, which is usually the stationary element.

The moving element of an LVDT is a separate tubular armature of magnetically permeable material called the core, which is free to move axially within the coil's hollow bore. The core is mechanically coupled to the object whose position is being measured--in this case, the mirror drive. Because the bore is large enough to provide substantial radial clearance to the core, there is no mechanical contact and no rubbing, dragging, or other source of friction. This friction-free feature is not only useful in this positioning application, but also for materials creep testing, vibration measurements, and dimensional gauging systems.

In operation, the LVDT's primary winding is energized by ac of appropriate amplitude and frequency, typically 3 V rms at 3 kHz. The LVDT's output signal is the differential ac voltage between the two secondary windings connected in series opposing, which varies with the axial position of the core within the LVDT coil. This AC output is converted by suitable electronic circuitry in the signal conditioner to high level analog dc voltage or current that is more convenient to use.

Because an LVDT operates on electromagnetic coupling principles in a friction-free structure, it can measure infinitesimally small changes in core position. This infinite resolution capability is limited only by the signal-to-noise ratio in the LVDT's signal conditioner. These same factors also give an LVDT outstanding repeatability.

Two other features of an LVDT impact its use in this application. First, an LVDT responds to motion of the core along the coil's axis, but is generally insensitive to cross-axis motion of the core or to its radial position. Thus, an LVDT functions without adverse effect in applications involving misaligned moving members or in cases where the core doesn't travel in a precisely straight line. The second important feature is that an LVDT is an absolute output device. This means that in the event of loss of power, the LVDT's output data will not be lost. When power is restored, the LVDT's output will be the same as before the power failure occurred.

When telescopes are located on top of mountains or on mobile carriers in desert locations, the LVDTs have not exhibited any adverse effects from harsh environmental conditions. Linear variable differential transformer position sensors are also the technology of choice for more complex telescopic systems, as well as current and legacy turbine applications and power plants.

For more information: www.macrosensors.com