Catalogues - lvdt-rvdt-displacement-transducers

Catalogue LVDT/RVDT Displacement Transducers

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PRINCIPLE OF OPERATION

The LVDT is an electromechanical device that produces an electrical signal whose amplitude is proportional to the displacement of the transducer core.

The LVDT consists of a primary coil and two secondary coils symmetrically spaced on a cylindrical former. Fig. 1.

METHOD OF LINEARITY MEASUREMENT

1 The core is adjusted mechanically to the electrical null, i.e. the position where the AC output from the secondary windings is a minimum.

2 The core is then displaced to the full range at one end of the stroke and a DC reading taken. The core is then moved in pre-selected steps until it reaches full range at the other end of the stroke, a DC reading being taken at each step. It is arranged by setting the excitation/demodulator circuit, that the mid position (i.e. electrical null as in (1) above) is zero.

3 The maximum and minimum readings are added and divided by the number of steps taken in order to determine the straight increments. Thus a straight line is obtained which passes through zero and which deviates from the actual curve by the same amount at each full range.

4 The actual readings are now taken from the theoretical straight line taking care of sign, i.e. reverse on passing through zero. This is because the error changes sign on passing through zero.

5 The linearity is now calculated as shown beneath. Typical Results

Stroke +/- 1.0", 10 steps of .200" = 2.0" overall Chart

|secondary| primary [secondary

|secondary| primary [secondaryI

FIG. 1

■ Vout

~VlN-»

FIG. 2


A	B	C	D
Stroke	Actual Readings	Theoretical Line	Difference
+1"	1492	1489	3
+.8"	1195	1191	4
+.6"	898	893	5
+.4"	598	595	3
+.2"	300	298	2
0	0	0	0
-.2"	299	298	-1
-.4"	597	595	-2
-.6"	894	893	-1
-.8"	1188	1191	3
-1.0"	1485	1489	4

Core position

D = B-C to zero then C-B Maximum difference is between 5 and —2 i.e. 7 Therefore percentage error is 7 x 100 = 0.235%

1489 x 2

As this may be either side of line it is normal to quote it as plus or minus.

Thus it becomes percentage error = +/— 0.117% or +/- 0.12%.

If a more careful calibration were carried out and notice taken of decimal points column D could become 6.0 instead of 7.0.

Percentage error in this case +/— 0.10%.

From hundreds of observations using different measuring techniques and different operators it has been realised that percentage errors can be made to vary by up to 2% with the same instrument. Since the majority of LVDTs fall well inside our specific limit of +/— 0.5% FS the actual recorded figure is taken as a genuine approximation although strictly speaking the result should be recorded: —

Percentage error = +/- 0.12% +/- 0.10%.

It should be noted that these linearities take no account of any errors in position measurements, electronic circuit or recording instrument linearities, i.e. it is total system linearity.

Other techniques such as root mean square deviations will give results up to factors of 4 to 6 better but we do not consider that these methods are as practical for the installation engineer.

Core at-100% Core at 0 Core at+100% (NULL POSITION) CORE DISPLACEMENT FIGURE 3. LVDT OUTPUT VS CORE POSITION.

A magnetic core inside the coil assembly provides a path for the magnetic flux linking the coils. The electrical circuit is configured as in Fig. 2 with the secondary coils in series opposition. When an alternating voltage is introduced into the primary coil and the core is centrally located then an alternating voltage is mutually induced in both secondary coils. The resultant output is zero as the voltages are equal in amplitude and in 180° opposition to each other. When the core is moved away from the null position the voltage in the coil, toward which the core is moved, increases due to the greater flux linkage and the voltage in the other primary coil decreases due to the lesser flux linkage. The net result is that a differential voltage is produced across the secondary tappings which varies linearly with change in core position. An equal effect is produced when the core is moved from null in the other direction but the voltage is 180° different in phase.