**8. Conclusion**

*Acoustics of Materials*

**Figures 12** and **13**.

weighted values to each specified range.

F = |*IC*|+

would be to define specific ranges of values for each sensor parameter and assign

Single sensor data can be used to detect faulty behavior but cannot readily differentiate or identify failures in the trigger sources, microwave tube, or other modulator electronics. This virtual sensor method can be applied to the monitoring of anomalous acoustic and cathode current pulses which are characteristic of a failed RF pulse. More specifically, this method can be used to count the number of anomalous pulses from each one of the sensor interfaces described in Section 4 and

To illustrate the advantages of a virtual sensor, consider the combined failure function, F, for a particular placement of an acoustic emission sensor whose parameter is represented by EAE and a current sensor whose parameter is represented by Ic, where

∫

*EAE*(*t*)*dt*. (5)

0 *t*

A virtual sensor for this function which represents the magnitude of the current pulse and the integrated AE energy can be used to add the number of faulty counts from the two real sensors producing a virtual sensor output. Long-term trends and analysis can be used to characterize the behavior and identify trend signatures for different types of microwave tubes. This synergistic effect of virtual sensing adds diagnostic and, more importantly, prognostic capabilities to the ICAS or any other monitoring system. The failure function, F, can be adapted to the needs and complexity of any system and can be defined to extract specific information required from that system. This technique was demonstrated on a magnetron tube (2J55). The experiment was conducted at the Microwave Tubes Built-In Test Project laboratory at SPAWAR Systems Center, San Diego. The upper section of **Figure 14** shows an ICAS screen capture of the cathode current (green) and acoustic emission (yellow) faulty pulse counts. The lower section shows the virtual sensor outputs where two failure functions, F1 and F2, have been defined and measured. Function F1 = x−y (yellow), where the difference between the outputs of the two real sensors represents virtual sensor 1, with x = cathode current and y = acoustic emission; and function F2 = x\*y (green), where the product of the outputs of the

**54**

**Figure 14.**

*Cathode current and acoustic emission faulty pulses and failure functions.*

The experimental results presented in this paper demonstrate the use of advanced acoustic emission techniques as a nondestructive testing method for insitu performance monitoring of high-power radar tubes such as pulsed magnetrons, TWTs, and klystrons. It was shown experimentally that changes in the amplitude and frequency content of the cathode current pulses are strongly correlated to changes in acoustic emission pulse energy both under normal and stressed operational conditions. The necessary electronics were developed to successfully interface the outputs of the current and acoustic emission sensors with the integrated condition assessment system (ICAS) used by the U.S. Navy. ICAS was used to demonstrate the use of virtual sensors where data from real sensors are combined into a failure functions F and captured in trend and single sensor format. In summary, this technique has demonstrated the unique ability to monitor, detect, and identify microwave tube performance. It has also demonstrated the ability to be utilized as a diagnostic tool by looking at long-term performance trends. More experimental research is needed to identify particular trends and signatures for the behavior of different types of microwave tubes under different circumstances to provide a fully prognostic capability for microwave tube systems.
