Accuracy of Video Gauge for displacement measurement
Precise measurement of displacement is traditionally achieved using devices such as a Dial Test Indicator (DTI) or a linear variable differential transformer (LVDT). The aim of this experiment is to validate the Video Gauge as a method for measuring displacement by comparing it directly with a calibrated DTI.
This experiment was carried out independently by Airbus UK.
The approach taken was to mount a target speckle pattern on a micromanipulator and traverse the target, in steps, across the field of view of a fixed camera. A calibrated DTI was used to measure the actual displacement of the target. The Video Gauge was used to measure the displacement of the target at 9 different points on the speckle pattern (the displacement of all 9 points should be identical). The displacements measured by the Video Gauge could then be compared directly to the DTI measurements. The set up is shown below.
The Video Gauge is a real-time system and in this case was making measurements at 15Hz. At each displacement step, 20 readings for each of the 9 targets (giving 180 samples) were used to calculate an average displacement and the standard deviation of the displacement measurements.
The graph below shows the output of the Video Gauge plotted against the output of the DTI. The blue error bars show the standard deviation of the Video Gauge measurements. A linear tend-line, shown in red, was fitted to this data. The gradient of the trend-line shows that the DTI and Video Gauge outputs are identical to within 0.1%. The R2 of the trend-line demonstrates that the relationship between the Video Gauge and DTI outputs is perfectly linear. Over a 10mm displacement range the error bars were less than 0.5μm.
Advantages of the Video Gauge
For DTIs and LVDTs one device is required per measurement whereas the Video Gauge is able to make multiple measurements simultaneously.
Another advantage of the Video Gauge is that it is a 2D system. This enables it to measure displacement in both the X & Y axis simultaneously, whereas DTIs and LVDTs are only able to measure displacement along a single axis.
The Video Gauge is also able make other measurements such as position, rotation and strain at the same time as measuring displacement.
DTIs and LVDTs have a fixed measurement range. The measurement range of the Video Gauge can be varied simply by changing the lens. See this case study where the Video Gauge was used to measure displacements in the order of 20nm and this case study for displacement in the order of 1m.