Derived once per rev signal generation for Blade Tip Timing systems

The majority of Blade Tip Timing (BTT) acquisition and analysis systems require a once per revolution (OPR) timing reference that is vibration free and reliable throughout the whole speed range of the rotor under investigation. Typically this consists of a probe and a target mounted somewhere close to the rotor centre line. In many applications the installation of the OPR sensor is expensive, and in the case of turbines, in a non-optimal position due to access and environmental constraints. OPR systems are usually duplicated for redundancy and failure rates are high. The loss of the OPR signal usually results in the loss of all analysis capability for a BTT application. Numerous attempts have been made to derive the OPR signal from the blade arrival time, however, the blades themselves undergo vibration (the exact same phenomena that we are trying to identify and quantify) leading to significant uncertainties in the resulting analysis. This presentation details an analytical method for reproducing the reference time for each revolution from one or many blade arrival time sensors without the need for a separate measurement channel. Validation will be presented for a number of real engine data sets covering a range of blade sizes and engine speeds. Utilising this method allows the elimination of the OPR measurement system and opens up the opportunity to reanalyse previously obtained data where the OPR system may have been compromised during the test. In a flight EHM system where weight and performance are critical the elimination of the OPR system is highly desirable.