The evolution of microprocessors and new construction materials significantly optimized LVDT performance, range, and cost of ownership, making them the choice of technology over other displacement technologies. Today, LVDT sensors are offered in high temperature versions, extended ranges, smaller strokes, radiation resistance and other characteristics to meet the requirements of a broader range of industries.
They even operate as part of telemetry systems in measuring parameters and providing feedback to a remote monitoring system that stores information in the cloud for operator access or further processing.
An external signal conditioner supplies the excitation signal and measures output. It demodulates the low-amplitude AC output and produces a DC voltage, current, or digital output for use by meters, PLCs and other control systems. (See diagram 2.)
Users can supply a DC input and measure a DC output corresponding to the core position. They possess many of the benefits of LVDT technology while offering the convenience of directly supplying and reading a DC signal.
Both varieties remain on the market as they offer distinct benefits based on application. Below are distinct characteristics of each.
The AC-LVDT offers frictionless operation as the coil and core structure never make contact so parts do not rub together. Void of friction, the sensor offers infinite mechanical life to sustain the long-term reliability of the systems in which it is installed as well as infinite resolution in measuring small changes in core position. In fact, once installed, AC-LVDTs can offer decades of reliable operation over millions of cycles without the need for replacement. This feature is important when sensors are installed in inaccessible locations or embedded into structures as replacement would be time-consuming and cause downtime in operations.
Shock and Vibration Resistance
AC-operated LVDTs do not have electronic components that are affected by shock and vibration. In addition, vented designs that expose the coil assembly and equalize pressure inside and outside the sensor provide greater resistance to shock and vibration.
Wide Temperature Ranges
With electronics separated from the LVDT coils, AC-operated linear position sensors 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. The temperature range of a DC-operated LVDT is limited by the properties of its internal electronic signal conditioning module.
Smaller Package
Without requirements for internal electronics, AC-LVDTs are packaged in smaller sizes to fit into hard-to-reach installation. Miniaturized versions are lightweight and offer a compact 3/8” diameter for use in applications with tight space and weight restrictions.
Fast Dynamic Response
The dynamic response of an AC-operated sensor is only limited by the inertia of its core. As it is small, AC units offer a fast response that is ideal for high response dynamic measurement such as plastic injection molding machines, automation inspection equipment as well as robotic equipment requiring displacement feedback to ensure proper machinery operation
Operation in Corrosive, Pressurized Environments
By adding non-magnetic barrier to separate the LVDT coil from its core, the sensor core is only exposed to media while the coils are separated by a sleeve or tube construction of glass, metal or other non-magnetic material. This enables the sensor to operate in highly corrosive, pressurized environments such as underwater.
These LVDTs provide DC output corresponding to the core position directly compatible with computer-based systems and digital buses. As a result, DC versions of the LVDT are more popularly used in a wider variety of quality control, inspection equipment, and industrial metrology applications.
Eliminates Need for External Signal Conditioning
With internal electronics, DC-operated LVDTs eliminate the need to purchase and install external AC excitation, demodulation, and amplification equipment.
Reduced Configuration Time
Pre-calibrated DC-operated LVDTs incorporate electronics so signal conditioning capabilities are built right into the housing, allowing for easy and fast installation.
Higher Accuracy
With internal electronics including microprocessors, DC-operated sensors can achieve superior accuracy thanks to factory-set linearity compensation.
Lower Cost
Without the need for external amplification equipment to complete the processing system, overall costs are often reduced. DC units also can operate using as few as two conductors, reducing cabling costs and conductor penetrations for chambers and ovens.
4-20 mA Designs
DC-operated LVDTs are available with loop-powered 4-20mA current output that is less sensitive to background noise and reduces voltage drops associated with analog voltage signals. Users can use these units with longer cable runs with minimal effects of EMI or noise on the signal. Recalibration is not needed to correct for voltage drop.
For environments with extreme or fluctuating temperatures, an AC-operated can be specified to operate below -20°C or above 85°C. AC LVDTs can achieve much wider operating temperature ranges than DC LVDTs.
Without electronics, an AC-operated unit is more compact in size. Miniatured versions meet restrictive space requirements. AC LVDTs, in general, have a longer MTBF than DC LVDTs, making them suitable for areas with restricted access.
Applications with harsh environments such as high temperatures, vibration, and shock should consider an AC-operated LVDT as it does not contain internal electronics affected by these conditions. Applications in a light industrial setting or laboratory environment may be perfectly suitable for DC LVDTs.
An AC LVDT generally requires calibration on-site. Factory calibrated with a calibration sheet included, DC LVDTs are ideal for users who do not have calibration equipment on-site.
A DC-operated LVDT offers outputs that can be directly used by different control systems and offers “plug-and-play” functionality.
In situations where the distance between the LVDT position sensor and its associated electronics will require a cable run in excess of 100 ft, a 4-20 mA LVDT should be considered.
When using a signal conditioner, users can set endpoints over the scale of the output variables. Particularly relevant in valve monitoring applications, users can set the total output range over the valve’s stroke, with a linear output at each valve position in between.
With attributes such as frictionless measurement, infinite mechanical life, excellent repeatability, and good temperature stability over extended temperature ranges, NewTek AC-operated LVDT position sensors are the ideal linear position technology for valve applications, while providing both short and long stroke measurements, typically from 0.050 inches to 20 inches (1.2 mm to 50 cm) or more.
Diagram 3 – Mechanics of how a spring-loaded gage head measures a surface and provides 4-20mA output to a control system.