Measuring Turbomachine-Blade Deflections in Two Dimensions

A report describes a method of simultaneously measuring steady-state axial and tangential turbomachine-blade deflections by use of fiber-optic probes and probe actuators designed specifically for this purpose. Each fiber-optic probe illuminates a small spot and, by measuring a change in the amount of reflected light, detects the passage of a leading or trailing edge of a blade through the spot. Each probe actuator contains a servomotor that produces controlled, small increments of rotation of an eccentric probe mount to adjust the axial and tangential position of a probe. At each rotational speed, the passage of a leading or trailing edge of interest is found by adjusting the position of a probe until the blade-passage output pulse of the probe is just discernible above noise. When this has been done for all probes and/or edges of interest, the edge-passage times are determined in relation to the time of a once-per-revolution master pulse generated by an independent optical sensor. Then the blade deflections at a given speed, relative to those at a reference speed, are determined from the known probe positions and the measured blade-passage pulse times at both speeds.