METHODS TO MEASURE THERMAL DEPOLARIZATION EFFECTS IN PIEZOELECTRIC RING ELEMENTS FOR KNOCK SENSORS

Abstract

Knock sensors use piezoelectric ceramics as the active element, whose parameters can change over time due to ambient conditions. One of the major environmental impacts on the sensor – its internal active element in particular – is that of the temperature. The knock sensor is placed directly on the device to be examined, such as the automobile engine block or the combustion engines in biogas cogeneration (CHP) units. In these units, above all, the temperature of the engine block often exceeds 100 °C. Prolonged exposure of the sensor to such conditions can damage the protective housing; more important, however, is the partial depolarization of the piezomaterial inside the sensor. We employed the frequency and the direct charge coefficient methods to determine how the temperature affects the parameters of the ring piezoelectric ce-ramics. The frequency approach allows us to establish the resonance frequency and the electrome-chanical coefficient, facilitating partial evaluation of quality in the piezoceramics. The tech-nique nevertheless cannot be used in deriving the relevant piezoelectric charge coefficient. If paired with the direct method, the procedure will characterize the stability of the resonance frequency and the depolarization level in the ring being tested. In the experiment, the CeramTec's Sonox P502 material was inserted directly into the knock sensors. The stability of the resonance frequency was assessed by using an Agilent 4294a im-pedance analyser. Subsequently, the samples were measured via the direct method to quantify the piezoelectric charge coefficient. The experiment yielded a comprehensive description of the impact exerted by a thermal wave on the active elements of a knock sensor; moreover, we identified convenient methods to de-termine the parameters of a set of samples in terms of the operational quality and suitability.Knock sensors use piezoelectric ceramics as the active element, whose parameters can change over time due to ambient conditions. One of the major environmental impacts on the sensor – its internal active element in particular – is that of the temperature. The knock sensor is placed directly on the device to be examined, such as the automobile engine block or the combustion engines in biogas cogeneration (CHP) units. In these units, above all, the temperature of the engine block often exceeds 100 °C. Prolonged exposure of the sensor to such conditions can damage the protective housing; more important, however, is the partial depolarization of the piezomaterial inside the sensor. We employed the frequency and the direct charge coefficient methods to determine how the temperature affects the parameters of the ring piezoelectric ce-ramics. The frequency approach allows us to establish the resonance frequency and the electrome-chanical coefficient, facilitating partial evaluation of quality in the piezoceramics. The tech-nique nevertheless cannot be used in deriving the relevant piezoelectric charge coefficient. If paired with the direct method, the procedure will characterize the stability of the resonance frequency and the depolarization level in the ring being tested. In the experiment, the CeramTec's Sonox P502 material was inserted directly into the knock sensors. The stability of the resonance frequency was assessed by using an Agilent 4294a im-pedance analyser. Subsequently, the samples were measured via the direct method to quantify the piezoelectric charge coefficient. The experiment yielded a comprehensive description of the impact exerted by a thermal wave on the active elements of a knock sensor; moreover, we identified convenient methods to de-termine the parameters of a set of samples in terms of the operational quality and suitability.