LVDT

• Linear Variable Differential Transformer(Used to measure linear displacement)

• Rotary Variable Differential Transformer (RVDT)

Cutaway view of an LVDT

• A: Input voltage excitation

• B: Ouput differential voltage

• C: Inner movable core

• D: Outer shielding C

D

Alternative arrangement of coils

LVDT Probe (used in Lab)

Flux linkage in windingsCore surrounded by coil

Flux linkage in windingsCoil with closed core

Flux linkage in windingsLVDT Coils with shielding and movable core

CASE -1 (x=0 mm, u=1e5 and 100 AT)

Flux linkage in windingsLVDT Coils with shielding and movable core

CASE -2 (x=3 mm, u=1e5 and 100 AT)

Flux linkage in windingsLVDT Coils with shielding and movable core

CASE -3 (x=6 mm, u=1e5 and 100 AT)

Specifications

• Voltage: 3 to 15 V

• Frequency: 50 Hz to 20 kHz• Power rating < 1 W

• Sensitivity: 20 mV to 200 mV/mm (for 1V excitation)• Range of displacement: ±0.05 inch to ±25 inch

• Minimum displacement measure: 0.001 inch• Sustainable temperature: -265 0C to 600 0C

Resolution and Sensitivity• Resolution: The smallest detectable incremental

change of the input parameter that can be detected in the output signal. Eg; Scale, Multi range meters

Expressed either as a proportion of the readingOR

Absolute values

• Sensitivity: For an instrument or sensor with input x and output y. Sensitivity = dy/dx

OR

Minimum input of physical parameter that will create a detectable output change. Eg; Ammeter(2A,100 div, 90o)

Practical and Theoretical Parameters

Advantages

• No contact between coil and coils (No friction)

• Infinite resolution• No hysterisis

• In some cases, core is laminated to avoid eddy currents

Mathematical Analysis

• Open secondary is assumed

1) Validity2) Amplitude and phase of secondary for given M

• Secondary is loaded by voltmeter (High Resistance)

1) Equations2) Solution for Amplitude and Phase