ELECTRONIC SENSORS
Considerations When Choosing Between AC- and DC-operated LVDT Position Sensors
A
n LVDT (Linear Variable Differential Transformer) is a type of linear position sensor used in a variety of applications to measure the displacement of moving objects within a range of a few millionths of an inch up to +20 inches. Powered by either an AC or DC electrical input, the sensor converts the measured motion into a proportional electrical output that is read by operators and/or entered into a control system.
Measurement ranges vary from ±0.010 inches (±0.254 mm) to ±10 inches (±254 mm). Due to its frictionless operation as a result of no moving parts, the LVDT is very reliable and offers excellent accuracy and repeatability. Initially, LVDTs were AC-operated and did not contain internal electronics. They required external excitation from a signal conditioner that also produces a DC voltage, current or digital output. See Figure 1:
electronics. Miniaturised versions of AC-LVDTs offer a compact 3/8” diameter. As AC LVDTs typically have a longer Mean Time Between Failure (MTBF) than DC versions, they offer decades of reliable operation over millions of cycles without the need for replacement, access.
Application Operating Conditions AC-operated LVDTs are suitable for applications with harsh environments with temperature, vibration and shock extremes, as they are not limited by the properties of internal electronics. They can continuously operate without failure at high temperatures from as low as -238°F (-150°C) to as high as 1000°F (537°C) and even higher, depending on construction.
Costs
DC units have lower overall costs as they do Additionally, they can operate with as few as two conductors, reducing cabling costs and conductor penetrations for chambers and ovens.
DC-operated LVDTs are available with loop-powered 4-20mA current output, offering a more robust measurement signal that is less sensitive to background noise and reduces voltage drops in analog voltage signals. Sustaining accuracy with no errors over long-distance transmissions, the 4–20 mA current does not degrade over long 4-20 mA output position sensors offer plug-and-play compatibility with most PLCs and controllers. Units require less wiring and connections than other signals to reduce startup costs. See Figure 2:
Michael Marciante, applications engineer, NewTek Sensor Solutions
pre-calibrated, so they do not require users to have on-site calibration equipment and support an easy and fast installation. AC-LVDTs typically require on-site calibration.
AC-LVDT operates with a signal conditioner to monitor the position of an object.
Modern microprocessors enabled the creation of DC-operated LVDTs with internal signal conditioning that generate outputs directly compatible with computer-based the modern digital world, AC-operated LVDTs can offer advantages in applications. When considering which type of LVDT is best for an application, the following factors should be considered.
Size Restrictions for Sensor Installation
Applications with space and weight restrictions should consider using an AC-LVDT that has a smaller package due to its lack of internal
Simplicity
Keeping things simple is best so that personnel do not require extensive knowledge to install and calibrate sensors. With signal conditioning capabilities built into the housing, DC-operated LVDTs have no requirements to install external equipment. Outputs are directly compatible with computer-based systems, so signals from a DC-LVDT can directly download into computer software or other network programs. This makes the linear position sensor popular for use in quality control, inspection equipment and industrial metrology applications. In addition, DC-LVDTs are factory
Figure 2 shows NewTek Spring-loaded DC-LVDT providing a 4–20 mA output for use in a PLC, control- ler or other display system.
NewTek Sensor Solutions offers its Hermetically Sealed Position Sensors with a 4–20 mA current loop output that has become the data transmission standard in the process control industry.
In addition to AC and DC versions, LVDTs are available in various construction materials, mechanical interfaces and performance
SEPTEMBER 2024 | ELECTRONICS FOR ENGINEERS 33
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