On occasion, anomalies in the highly dynamic test data obtained during rocket engine tests may appear. These issues must be investigated, and corrective action is generally mandated before testing resumes; usually only three days are allotted to identify and correct the issues. Spurious signals in piezoelectric accelerometer data channels, of unknown cause, are a typical challenge encountered. Commercial test equipment suitable for testing piezoelectric sensors is expensive and requires that the sensor be removed from the test article for evaluation. The testing process that was developed provides an economic means for the monitoring of piezoelectric sensors while they are installed or after they have been removed from test articles. The in situ health monitoring apparatus developed includes: (1) an exciter circuit that applies a pulse to a piezoelectric transducer and (2) a data processing system that determines the piezoelectric transducer’s dynamic response to the excitation pulse.
With this technology, characteristics such as resonant frequency, response, cable status, connectivity, bonding, and linear range can be determined. Sensors can be tested in a very wide frequency range, extending to 200 MHz and beyond, without requiring physical contact with or removing them from their mounted locations, and without requiring specially constructed transducers or special wiring. Monitoring can be done as far away as 250 ft (≈75 m), or longer if certain provisions are made. These assessments are performed in situ, and can be conducted with hand-held test equipment or integrated into instrumentation systems. The system also provides the ability to monitor piezoelectric transducers between propulsion tests to detect any trend indicative of transducer failure or detachment. The testing system is not limited to identifying degraded performance in the sensor’s piezoelectric elements; it can detect changes within the entire sensor, and sensor housing. With slight modifications, this monitoring system can be used with all common transducer instrumentation.