LVDT

1. LVDT Linear Variable Differential Transformers Jason Hart CEE 498

2. This presentation covers: • How LVDT’s work • Types of LVDT’s • AC vs. DC LVDT’s • Applications to Structural Engineering • Costs

3. How LVDT’s Work

4. Types of LVDT’s • Unguided Armature • Captive Armature • Spring-extended Armature

5. Unguided Armature Measured Object Armature must be attached to the specimen Armature fits loosely in the bore Body must be separately supported & properly aligned Elevation/Cross-Section View

6. Unguided Armature • There is no wear on the LVDT because no contact is made between armature and bore. • LVDT does not restrict the resolution of measured data (“infinite resolution”). Measured Motion Elevation/Cross-Section View

7. Unguided Armature • Well-suited for short-range, high-speed applications (vibration) Measured Motion time displ. Elevation/Cross-Section View

8. Captive Armature Measured Object Armature must be attached to the specimen Armature is both guided and restrained by a low friction assembly Body must be separately supported

9. Captive Armature Advantages compared to unguided armature: • Better for longer working ranges • Preferred when misalignment may occur

10. Spring-Extended Armature Measured Object Like the captive armature, it has a low-friction bearing assembly Internal spring to continuously push the armature to its fullest possible extension Elevation/Cross-Section View

11. Spring-Extended Armature • The spring-extended armature is best suited for slow-moving applications. • Attachment between armature and specimen is not required.

12. AC vs. DC

13. AC LVDT’s • Smaller size • More accurate • Can operate within a wide temperature range • Lower unit cost. Advantages compared to DC LVDT’s:

14. DC LVDT’s • Ease of installation and signal conditioning. Signal conditioning equipment is part of the LVDT. • Can operate from dry cell batteries • Usually lower system cost Advantages compared to AC LVDT’s:

15. Applications to Structural Engineering • LVDT’s are reliable for measuring member deflection in many structural engineering experiments. • LVDT’s can measure displacement response directly in dynamic experiments. • Mounting the LVDT to a stationary location is critical.

16. Costs – Unguided Armature LVDT An LVDT with a 3” range purchased from Macrosensors, Inc. in quantities of one or two: • Costs $330 for AC. • Costs $485 for DC.

17. Costs – Spring Extended Armature LVDT An LVDT with a +/- 2” range purchased from Macrosensors, Inc. in quantities of one or two: • Costs $1156 for AC. • Costs $1359 for DC. In general, costs of LVDT’s range from $50 to $1600, without considering specialty LVDT’s such as hermetically sealed.

18. Summary • LVDT’s are robust equipment for measuring deflection. • AC LVDT’s require separate signal conditioning equipment, while DC LVDT’s include signal conditioning equipment on the device. • There are three types of LVDT: unguided armature, captive armature, and spring-extended armature. • AC LVDT’s cost less than DC, but the entire measurement system must be considered.

19. References Figure on slide 3 (How LVDT’s work): http://www.daytronic.com/products/trans/lvdt/default.htm Figure on slide 12 (AC vs. DC): http://www.macrosensors.com/ms-lvdt_faq-tutorial.html Cost information obtained from Microsensors, Inc. Links to LVDT manufacturers found in report by Tjen Nung Tjhin updated on April 24, 2001.