Linear position sensors gain acceptance

Linear variable differential transformers (LVDT) can deliver better machine operation.


Figure 1: The basic LVDT design. Courtesy: Macro SensorsToday's industrial process control applications increasingly use automated systems to optimize operations and ensure a safer, more productive process. Linear position sensors used in these automated systems provide highly accurate feedback on product parameters, control states, and outputs to machine controllers.

Whether implemented as a stand-alone component or as part of a control or safety system, the linear position sensor, also often known as an LVDT, is capable of providing linear displacement measurements from micro inches to several feet, under various operating and environmental conditions with high accuracy and reliability. Essentially, the LVDT linear position sensor plays an important role in machine control by providing feedback about product location. To some extent, it is the LVDT linear position sensor that ensures proper machine operation.

Mechanics of an LVDT

In basic terms, an LVDT linear position sensor is an electromechanical device that converts linear position or motion to a proportional electrical output (see figure 1). More specifically, the LVDT position sensor produces an electrical output signal directly proportional to the displacement of a separate movable core. Typically, the ferrous core within the LVDT is attached to the moving element on the piece of equipment requiring position feedback.

The basic LVDT design consists of three elements:

  1. One primary winding
  2. Two identical secondary windings
  3. A movable magnetic armature or "core."

The primary winding is excited with an ac supply generating a magnetic field which, when the core is placed in the central or "null" position, includes equal voltages in both of the secondaries. The secondaries are wired series opposed so that their combined output represents the difference in voltage indicated in them, which in this case is zero. As the core is moved left or right, the difference in inducted voltages produces an output that is linearly proportional in magnitude to the displacement of the core. Its phase changes 180-deg from one side of the null position to the other.

In the oil & gas industry, compact LVDTs are used in the position feedback control of down-hole drilling equipment such as bore scopes that measure the ID of the drilled hole. The sensor coil assembly and separable core inherent to the technology can withstand extremely high pressures of the environment as the mechanical configuration of the coil assembly is vented (pressure balanced) to the pressure of the nonconductive mediums. As the sensor coil assembly can withstand a combination of high pressure, elevated temperatures, shock, and vibration, the LVDT is able to make measurements in down-hole drilling equipment possible where space is at a premium and the environment is hostile.

In operation, the LVDT's primary winding is energized by alternating current of appropriate amplitude and frequency, known as the primary excitation. The LVDT linear position sensors' 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 high-level dc voltage or current for convenient use by a computer or other digital output device.

Because there is normally no contact between the LVDT's core and coil structure, no parts can rub together or wear out. This means that an LVDT linear position sensor features unlimited mechanical life. This factor is highly desirable in many industrial process control and factory automation systems.

Enhanced use in process control

Recent innovations in construction materials, manufacturing techniques, and low-cost microelectronics have revolutionized the LVDT linear position sensors into a more reliable and cost-effective technology for process control applications. In the past, electronics necessary to operate LVDT linear position sensors properly were complicated and expensive, prohibiting their wide use in process control applications for displacement measurement.

Modern ASIC and microprocessors give LVDT technology more complex processing functions and enable signal conditioning within the sensor housing so LVDTs generate digital outputs directly compatible with computer-based systems and standardized digital buses. As a result, today's linear position sensors can provide more accurate and precise measurement of dimensions in a wider variety of quality control, inspection equipment, and industrial metrology applications including online parts inspection, servo-loop positioning systems, and manufacturing process control.

For applications where sensors must operate in extreme environments, the sensing element can be segregated from the electronic circuitry, unlike capacitive, magnetostrictive, and other high-frequency technologies. Connected by long cables up to 31 meters (100 ft), ac-operated LVDTs can work with remotely located electronics that power the sensors, and amplify and demodulate their output. Output is, then, displayed on a suitable readout and/or inputted into a computer-based data acquisition system for statistical process control. This ability to transmit data to a remote computer has made linear position sensors popular in quality assurance schemes.

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