Catalogues - lvdt-rvdt-displacement-transducers - sensonics

Catalogue LVDT/RVDT Displacement Transducers

www.sensonics.co.uk

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		CHOOSING AN LVDT Questions frequently arise concerning the choice of LVDT. Our applications engineers are always willing and able to make recommendations as to the correct LVDT for your application. However the following notes are offered as a guide. There are two main categories of LVDT, namely AC input. AC output; and DC input, DC output. Within these two categories the choice of LVDT will be dependent on a number of factors and can be summarised as follows: 1) Environmental considerations; 2) Mechanical fixing; 3) Output from transducer required; 4) Linear range required. Consideration of the above will ultimately define the LVDT for the application. ENVIRONMENTAL CONSIDERATIONS The main environmental conditions encountered are elevated temperatures and contact with fluids. The LVDT upper operating temperature is determined by the components used in its construction. LVDTs with built in electronics are limited to 90°C. LVDTs with NYLATRON bushes are limited to 120°C and LVDTs with standard connector outlets are limited to 125°C. As a general rule all LVDTs with integral cable and not containing integral electronics are suitable for all but sub mersible applications. For sumbersible applications precautions are mainly taken to prevent captivated cores having a piston effect. This is easily overcome by hydraulic venting. Water ingress into cable terminations is prevented either by the provision of a gland or by attachment of integral flexible conduit. MECHANICAL FIXING LVDTs are mainly attached to the two relative moving elements by a) clamping the LVDT body and clamping the LVDT core; b) by clamping the LVDT body and spring loading the core onto the other element; c) by fixing the body and core (via an extension) by means of universally jointed rod end bearings. Many LVDTs can be configured to incorporate any of the above fixtures but .sometimes the choice of fixture will determine the LVDT to be used. ADVANTAGES OF LVDT'S FRICTIONLESS MOVEMENT The LVDT can be and often is. operated where there is no physical contact between the core and extension rod and the main body of the LVDT housing the transformer coils. This makes it ideal for measurements where friction loading cannot be tolerated but can tolerate the addition of a low mass core. Examples of this are fluid level detection with the core mounted on a float and creep tests on fibres or other elastic materials. INFINITE MECHANICAL LIFE The LVDT by virtue of its frictionless measurement has no elements that wear out. This essentially means that the LVDT has infinite mechanical life. This makes the LVDT particularly valuable in applications such as fatigue life testing of materials and structures. RUGGEDNESS OF CONSTRUCTION The principle of operation of the LVDT enables thesensing elements not only to be friction free but to be housed in a variety of housings depending on the degree of mechanical protection required. The use of rod end bearings, linear rolling element bearings and flexible conduit helps the LVDT to survive even the most severe abuse. INPUT/OUTPUT ISOLATION The fact that the LVDT is a transformer means there is complete isolation between input and output. This makes the LVDT easily isolated to prevent ground loops and obviates the need for buffer amplifiers.
						AC EXCITATION Advantages; (i) External signal conditioning permits more sophisticated electronics to be used with variable gain, zero offset, current outputs, etc. (ii) Long cable lengths (3 core screened) may be used between transducer and electronics — typically 100 metres or so. (iii) The transducer may be located in an area which would be hostile to electronics. (iv) The transducer is less expensive to replace than a DC excited one of similar characteristics. Disadvantages: (i) Initial cost of system is higher than with DC LVDT. DC EXCITATION Advantages: (i) Generally a lower initial cost for a system than with AC excitation. (ii) A simpler system to install and set up. (Hi) May be operated in the field from dry batteries. Disadvantages: (i) Transducer itself is generally more expensive to replace than a similar AC one. (ii) No control over sensitivity or zero offset without external electronics. (iii) Long cable lengths (4 core screened) may introduce volts drop problems unless external electronics is involved. (iv) Unsuitable for hostile environments due to limits imposed by internal electronics. OUTPUT FROM TRANSDUCER REQUIRED LVDTs provided with their own integral electronics produce a voltage output which goes from positive to negative corresponding to the two extremes of the LVDT core position. If other signal outputs are required then the signal has to be further conditioned or alternatively the entire signal conditioning carried out separately from the LVDT. This approach can often be simpler and more versatile if current outputs or zero and span control is required. In summary a review as to whether an AC or DC LVDT is required will often clarify all of the above points.

						INFINITE RESOLUTION The principle of operation of the LVDT, based on the mutual inductance between primary and secondary coils, gives the LVDT the characteristic of truly infinite resolution. The readability of the external display represents the only limitation. NULL REPEATABILITY The inherent symmetry of the LVDT construction provides the LVDT with the feature of an extremely stable and repeatable null. The LVDT can thus be employed in high gain closed loop control systems. LOW CROSS AXIS SENSITIVITY The output of an LVDT is predominantly dependent on the axial movement of the core within the LVDT body. Radial movement produces very little change in output. This makes the LVDT useful for measurements even when the core moves with a short arc inside the LVDT windings. CORE AND BODY SEPARATION The separation between LVDT core and coil means that a sealed non magnetic barrier can be placed between them. This enables the core to be placed inside a pressurised or corrosive environment with the LVDT body outside. Only a static seal is required to industrial seal the coil and eliminates the need for dynamic seals on the moving member. LVDTs used in this manner are actually mounted inside hydraulic actuators.


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