Linearly Variable Differential
Transformers (LVDTs)
Beth Dassler
Typical LVDTs1
USES?
To convert positional information into an electrical
signal.
-pressure
-displacement
-force
(a) Section view
(b) circuit diagram 2
•
• Made of a primary coil, two secondary coils,
and a magnetic core
• Primary coil excited by an AC signal (generally
a sine wave)
• Secondary coils are set up in series opposition
so when the core is centered the output voltages
cancel out
• Operates on the principle of mutual inductance
• As the core moves toward one secondary coil,
the induced voltage increases while is decreases
in the other secondary coil
M = mutual inductance in
k = coefficient of coupling between two inductances
L1 and L2 = values of the two inductances
L=n2Gµ
n is the number of turns
G is the geometric form factor µ is the effective
permeability of the medium3
Mutual Inductance
The induction of a voltage in one coil in response
to a change in current in the other coil.3
Advantages
•
•
•
•
•
•
Sensitive
Reliable
Robust
Low friction
High resolution
Ability to operate at high temperatures.
Disadvantages
• Requires complex signal processing
instrumentaion3
• Requires frequent calibration5
Example: catheter tip pressure transducer
Diaphragm
Slug
Catheter
LVDT
Example: measuring muscle contraction
Force
Muscle
Balance
Isotonic contraction
(same force)
Isometric contraction
Weight
(same length)
LVDT
Muscle sounds
References
1.
Schaevitz Sensors. LVDT Functional Advantages and Operating Principles.
http://www.sal.wisc.edu/PFIS/docs/archive/Product%20Manuals/schaevitz/LVDT_
Intro.pdf-4.pdf
2.
Longoria, Prof. R.G. Notes on LVDTs. Department of Mechanical Engineering,
University of Texas at Austin, Dynamic Systems and Controls Lab. 30 March
2000. http://www.me.utexas.edu/~lotario/me244L/labs/forcemotion/lvdt.html
3.
Webster, J.G. (ed.) Medical Instrumentatino, Application and Design 3rd Edition.
John Wiley & Sons Inc. New York, NY, 1998.
4.
Heiserman, D.L. Basic Electronics Part 2, Elements of AC Electricity. The
SweetHaven Online Interactive Tour. Unit 2: inductance; Module 3: mutual
inductance. 1997. http://www.sweethaven.com/acee/forms/frm0203.htm
5.
Anter Corporation. The Advantages of Digital Displacement Transducers over
LVDTs. http://www.anter.com/Technotes/TN64.pdf