A displacement measurement arrangement comprising a cylinder 34 that rotates about its axis in synchronicity with a component, such as a rotor blade of a gas turbine, and translates longitudinally in accordance with a change in a characteristic of the component, such as a change in the blade pitch, the cylinder having signal generating features comprising at least one longitudinally extending feature 50, 46 and at least one feature at least partially angled 44, 48 relative to the longitudinal feature; and at least one sensor (shown as arrows in the figure) arranged to receive a signal from the signal generating features. The change in the characteristic of the component, eg the blade pitch, is detected by sensing the change in the time delay in the generated signals due to the longitudinal displacement of the cylinder. The cylinder may be coupled to an actuator having fingers that are received in recesses 74, 76 in the cylinder. The signal generating features may include an indexing mark, for instance two closely spaced parallel longitudinal features 50, fig 4. In addition to blade pitch the invention may also be used to measure other characteristics of components that may be expressed as the longitudinal movement of the cylinder.
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