Linear Variable Differential Transformers - Operation, Handling and Selection

Linear Variable Differential Transformers (LVDTs) are linear position/displacement transducers that are exceptionally rugged, frictionless and provide a long cycle life with proper use. AC-operated LVDTs do not feature any electronics and hence can be used at cryogenic temperatures as well as at high temperatures up to 650°C under high shock and vibration levels.

Therefore, LVDTs are widely utilized in demanding applications such as in hydraulics, satellites, automation, power turbines, commercial and military aircraft, nuclear reactors, etc. Two types of LVDTs are available and they are AC-operated and DC-operated LVDTs.

Criteria for LVDT Selection

Figure 1. LVDT

Choosing an LVDT for a particular application is a complex task and involves a number of factors, such as electrical stroke, electrical stroke, package, environment, AC or DC operation, accuracy, linearity, precision, loads, proximity of multiple LVDTS in application, and certifications.

Handling and Installation of LVDT Core

Customized LVDTs are needed to match the special requirements of OEM applications. MEAS has new design capabilities to meet such requirements. An LVDT should not be used without the core because the input impedance will reduce significantly and the surplus current flowing via the primary coil can damage the windings. This is true for short stroke LVDTs that have low input impedances.

Figure 2. MEAS LVDT cores

Figure 3. DC-SE LVDT with Guided and Captive core

For optimum performance of an LVDT, the core is annealed in accordance to a unique MEAS process so as to mitigate stresses caused by machining. This annealing process offers core homogeneity with better permeability to magnetic fields. Therefore, the core becomes susceptible to dropping, bending, filing, clamping, machining, grinding and other mechanical stresses.

This core is separately packed in a rigid plastic tube to prevent damage during transportation. However, once the core is taken out of the tube, extreme care should be taken to prevent performance degradation of the LVDT.

The cores offered by MEAS are threaded each side because the core needs to be mechanically symmetrical for optimum performance of LVDT. MEAS provides both metric and imperial threads.

Conversely, ferromagnetic metals should not be utilized because they tend to hinder the LVDT operation. Likewise, brass, aluminum and other low resistivity metals should not be used since they tend to draw more energy from the LVDT and considerably degrade LVDT’s performance.

In addition, there are certain non-magnetic stainless steels that have permanent magnetization and should be demagnetized prior to linking them to the core. Even though composites and plastics can be used, they tend to have larger coefficients of expansion when compared to metals and hence generate LVDT output zero/offset shifts with temperature.

In order to ensure frictionless operation and extended cycle life, the core should not contact directly with the LVDT boreliner. Thus, proper alignment is extremely important. However, since LVDTs are not sensitive to transverse position, the core does not have to be centered within the boreliner.

LVDTs with longer strokes tend to sag because of the flexibility of the connecting rod and the weight of the core. In such cases, MEAS offers LVDTs that come with the Guided Core option, wherein the core is shielded by PTFE sleeve or self lubricating bushings. MEAS also provides the Captive Core option for applications where the core must not be taken out of the LVDT.

LVDT Installation

Clamping blocks can be used to install LVDTs since most LVDT housings are available in cylindrical versions. Certain versions are developed for bulkhead mounting. To this end, set screws tend to press onto the housing’s surface and hence should not be used for installation as they could damage the internal parts. As stated above, the core and the boreliner should be aligned correctly to achieve frictionless operation and extended cycle life.

Figure 4. Mounting blocks

LVDTs with Electrical Connection Features

MEAS provides LVDTs with a variety of electrical connection features, such as connectors, lead-wires, screw terminals and cables. Cable assemblies, mating connector kits, and connector options for LVDTs with cables are also available. Shielded cables are recommended for all installations. MEAS also offers a wide range of signal conditioners with "sync" input and output.

Figure 5. Signal conditioners

The length of the cable influences both AC operated and DC operated LVDTs. For AC LVDTs, there are no stringent guidelines since a number of variables exist. But in case of DC LVDTs, the cable length affects the amount of noise applied onto the DC output voltage and produced by the EMI waves. However, the noise level can be cut down by using proper cable shielding methods, or a DC current output LVDT can be used to overcome this issue.

Conclusion

MEAS develops innovative signal conditioning solutions using analog and digital electronics. The company has been utilizing these solutions to design LVDTs that set a new benchmark in terms of functionality and performance. MEAS provides customized sensors at affordable costs.

About Measurement Specialties

Measurement Specialties is a global designer and manufacturer of sensors and sensor-based systems which measure pressure/force, position, vibration, temperature, humidity, and fluid properties. Measurement Specialties' products are used as embedded devices by original equipment manufacturers (OEMs) or as stand alone sensors for test and measurement to provide critical monitoring, feedback and control input.

This company is at the heart of many everyday products and provide a vital link to the physical world.

Measurement Specialties is an applications company and this company understands that embedded solutions often require customized designs. The portfolio for Measurement Specialties includes technologies capable of measuring most physical characteristics and allows them to design the right sensor for the application.

Physical property, electrical input/output and package configuration are all important considerations when developing products to meet their customers’ needs.

This company continues to expand its technology portfolio and geographic reach through the acquisition of strategically complementary companies. Measurement Specialties' operations in the US, Europe and China provide resources close to their customers. This global footprint allows this company to offer the lowest cost of ownership to OEMs.

Measurement Specialties value proposition is to understand the customer's sensing needs and develop a solution that meets their performance and cost objectives. At Measurement Specialties, Measurement Specialties are Sensing Your World.

This information has been sourced, reviewed and adapted from materials provided by Measurement Specialties.

For more information on this source, please visit Measurement Specialties.

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