Shear Sensing Technology

Shear Sensing is a patented Blue Line Engineering differential sensing technique that achieves very high accuracy over a wide dynamic range. Shear Sensing is applied in our Edge Sensor technologies, which measure the edge-to-edge displacement between two flush surfaces. The concept employs two physically isolated pairs of electromagnetically coupled coils, which are positioned on opposite edges of the segments or surfaces to be aligned. One pair is designated as the passive pair, since it requires no power. The other pair, referred to as the active pair, is driven by a frequency-stabilized source--it is the only sensor pair that is wired to the electronics. The pairs are positioned so that as one coil in the passive sensor moves away from one coil in the active pair, the passive and active counterparts move closer together. (See figure at left.)

The two coils of the active sensor pair comprise opposing sides of a balanced bridge circuit, so that as impedance increases on one side, it decreases on the other. This differential placement of sensors can be used to detect shifts in motion along any segment edge, resulting in a wide variety of directional measurements. Our edge sensor are particularly suited to large adaptive optic telescopes, where they can be used to measure phase difference between opposing mirror segments.

Many benefits of Blue Line's inductive sensor arrangement stem from its ease of use. This can be seen by comparing Blue Line’s geometric approach (illustrated below) vs. the cantilevered paddle arrangement employed on the Keck Telescopes, which greatly complicates the installation and removal of mirror segments.

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