In civil engineering, choosing between these two versions depends on the geometry of the structure, the expected direction of the movement, and how permanently you can alter the mounting surfaces.
1. Ball Joint Displacement Sensor (The Standard for SHM)
How it works: This version features rod-end bearings (ball joints) on both ends of the sensor. Both ends must be physically anchored—one to the stable reference side and the other across the fault or joint.When to use it:
•Monitoring Cracks & Expansion Joints:
This is the standard choice for tracking the widening of structural cracks in concrete buildings, expansion joints on bridge decks, or concrete block movements on dams.
•Multi-Directional Movement (Torsion/Shear):
Structures rarely move in a perfectly straight line. Bridges and dams experience thermal expansion, settling, and vibration that can cause slight pivoting or vertical shifting. The ball joints accommodate these angular misalignments, preventing the internal shaft of the sensor from bending or breaking.
•Long-Term/Permanent Installations:
Because both ends are securely bolted or anchored into the concrete/steel, the sensor is highly resistant to extreme weather, wind, and vibrations.
•Large Structural Shifts:
With a range of up to 400mm, it is necessary for monitoring large expansion joints on suspension bridges or significant geological shifts near dams.
2. Spring Return Displacement Sensor (For Single-Point Mounting)
How it works: This version features an internal spring that continuously pushes the probe tip outward against a target surface. The main body is anchored to a stable point, but the probe tip simply rests against the opposite surface without being bolted to it.When to use it:
•Restricted Mounting Options:
Use this when you can only drill into or attach a bracket to one side of the structure. For example, monitoring the deflection of a bridge girder where you can mount the sensor to a fixed pier, and simply let the probe tip rest against the moving girder.
•Heritage or Protected Buildings:
When monitoring historical structures, you may be prohibited from drilling anchors into specific delicate stones or facades. The spring return allows you to measure displacement while only mounting to a secondary, non-historical reference frame.
•Strictly Linear Compression/Expansion:
Best used in scenarios where the movement is purely pushing or pulling in a straight line relative to the sensor, without severe side-to-side shear forces that could cause the spring-loaded tip to slip off its target.
•Temporary Load Testing:
Ideal for short-term static load tests on bridge decks or floor slabs, where sensors need to be set up and taken down rapidly without leaving permanent anchors on both sides of the movement point.