Linear position sensors gain acceptance

04/16/2014


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(at)macrosensors.com.

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


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