Linear position sensors gain acceptance

Linear variable differential transformers (LVDT) can deliver better machine operation.
By Eileen Otto, Macro Sensors April 16, 2014

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.

Smaller diameters, new materials

Figure 2: In the oil and 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. Courtesy: Macro SensorsWhile linear position sensors were once considered too long for applications with limited space, new winding techniques and computer-based winding machines allow the linear position sensor body to be reduced while maintaining or increasing stroke length. With the improved stroke-to-length ratio (now up to 80%), the LVDT linear position sensor becomes a viable position measurement device for machine tool positioning, hydraulic cylinder positioning, and valve position sensing.

Smaller, contactless linear position sensors also feature a lightweight low mass core that is ideal (see figure 2) for process control applications having high-dynamic response requirements, such as plastic injection molding machines, automatic inspection equipment, and different robotic applications requiring displacement feedback to ensure proper machinery operation.

LVDTs are also configurable in a variety of mechanical and electrical designs to meet the measurement and environmental requirements of various process control applications. New corrosion-resistant/high-temperature materials such as Monel or Inconel enable the LVDT linear position sensor to operate in more hostile environments, including those with high and low temperature extremes, radiation exposure, or vacuum pressure conditions. For applications where sensors must withstand exposure to flammable or corrosive vapors and liquids, or operate in pressurized fluid, its case and coil assembly can be hermetically sealed using a variety of welding processes.

For example, in power generation applications (see figure 3), linear position sensors designed for high temperature and mild radiation resistance can perform in power plants to provide feedback on the position of nuclear steam and gas turbine control valves for increased plant efficiency and reduced operating costs.

Figure 3: Hermetically sealed linear position sensors offer a highly accurate and long life solution for the position measurement of steam control valves in power generation plants. Courtesy: Macro SensorsIn a typical power plant, steam turbines contain a number of control valves-a reheat stop value, an interceptor valve, a governor valve, and a throttle valve. Typically, plants have very precise control schemes for valve position to increase operating efficiency and save fuel. Operating within the harsh environment of a power or steam plant, linear position sensors can determine if valves are fully opened or closed to within a thousandth of an inch, providing output to remote electronics that can be monitored by operators if something is not working properly. The combination of LVDTs with modern computerized turbine control systems saves power companies millions of dollars per year.

Sensors also play an important role in the predictive maintenance of gas turbines as part of process control systems used to monitor shell expansion and bearing vibration. When installed on turbine shells, hermetically sealed LVDT position sensors measure shell expansion, providing linear output that operators can utilize to determine proper thermal growth of a turbine shell during start-up, operation, and shutdown.

LVDTs designed to withstand shocks and heavy pounding are used in the press and dye industry for the mechanical control of machine operations as improper operation can lead to broken dyes that result in downed machines, while the ambiguous force of presses can lead to misshapen and out-of-spec parts. Spring-loaded LVDT position sensors are installed on presses so that the plunger of the sensor is compressed as the punch press comes in contact with the metal being shaped. The output of the LVDT is fed back into the machine’s control system, providing feedback on how far a press has moved and when to stop.

For more than six decades, LVDT linear position sensors have served as part of measurement and control systems, providing essential information without which many process control systems couldn’t function. From its limited use as a laboratory tool more than three decades ago, the LVDT linear position sensor has evolved into a highly reliable and cost-effective linear feedback device, making it the preferred technology for critical and reliable linear displacement measurements in an array of industrial process control applications.

Eileen Otto of Macro Sensors can be reached at eileeno@macrosensors.com.

This article appears in the Applied Automation supplement for Control Engineering and Plant Engineering