LVDT & Position / Displacement Sensors

• Draw Wire Displacement / Position Sensors

Submersible LVDTs

The standard AML/IE Series Industrial LVDT in IP68 waterproof format is suitable for submersion to depths of up to 50 metres or external pressurisation to 5bar.  Customised versions are available for applications where the LVDT is required to operate at greater depths.

Seawater & Harsh Environment LVDTs

For harsh environments where LVDTs will be subjected to aggressive media such as seawater, fuels and chemicals, the AML/IE Series Industrial LVDT can be supplied with alternative case and extension rod materials such as 316L Stainless Steel or Duplex Stainless Steel and PTFE cable to provide a highly corrorsion-resistant sensing device.

Hazardous Area Displacement & Position Measurement

If you need to measure displacement or position within a hazardous area, the AML/SGD series of strain gauge based transducers used in conjunction with a pair of Zener Barriers offers a relatively low-cost option.  Whilst it is not ATEX certified, it is a non-energy storing device and can therefore be classified as 'Simple Apparatus' and, subject to acceptance by the specifying engineer, be used within a hazardous area without certification.

We can also supply signal conditioning, amplification and display / indication equipment to accompany the AML/SGD sensor which is suitable for use in the safe area.  If hazardous area electronics are required please contact us to discuss your requirement.

The LVDT Operating Priciple

The Linear Variable Differential Transformer (LVDT) is an electro-mechanical transducer that converts the linear motion of the object to which it is coupled mechanically in a corresponding electrical signal.

The basic design consists of a cylindrical array with a primary winding centred between two identically-wound secondary windings.  The coils are wound on a hollow glass-reinforced polymer (GRP) former, which is surrounded by a high permeability magnetic shield.  This assembly is then secured into a stainless steel tube that is sealed at both ends.  This tubular element is the static part of the LVDT construction.

The moving part of an LVDT is a separate short rod of magnetically permeable material called the core.  The core is free to move within the hollow bore of the coils and and is mechanically coupled to the item being displaced (Figure A).  One of the major benefits of the LVDT design is that there is no physical contact between the core and the coils, giving the LVDT a hugely extended operating life.

The primary winding (P) is energised with a constant amplitude a.c. supply at a fixed frequency anywhere between 1kHz and 10kHz.  This produces an alternating magnetic field in the central winding which induces a signal into the secondary windings S¹ and S² depending on the position of the core (Figure B).

The output signal is the differential a.c. voltage between the two secondary windings which varies according to the position of the core within the coil
(Figure C).  When the core is positioned in the centre the output signal is zero, this is known as the null position.  As the windings are precisely wound, the signal output has a linear relationship with the physical position of the core (Figure D).

Separate instrumentation is available to externally process the resultant a.c. voltage from the secondary coils into a d.c voltage or current and can also include a digital display.  Alternatively, LVDTs can incorporate a circuit that provides oscillation, demodulation and signal conditioning.  These are referred to as DC/DC LVDTs as they require a simple DC supply and output either a d.c. voltage or current proportional and in-phase with the position of the core.

The LVDT design can be easily adapted to fulfil a broad range of applications in research and industry.  Some typical variations include:-

• Complete sealing for partial or full submersion in liquids and gases.
• Heavy duty construction for use in harsh environmental conditions.
• Miniature and low-cost models for price-sensitive OEM applications.