F or several decades now, both the aerospace and power generation industries have put considerable effort into developing high temperature electronics, largely due to the fact that the early detection of faults in turbine systems allows for components to be repaired or replaced before a catastrophic failure event. In addition to the cost savings they provide by helping to avoid catastrophic failure, harsh environment electronics can enable the switch from schedule-based to condition-based maintenance.
This can limit turbine downtime for unnecessary maintenance, allowing for increased electricity generation revenue for terrestrial power generation turbines and increased mission availability for aerospace systems. Electronic systems designed for use in extreme environments also allow for active engine control in turbine systems, enabling more efficient systems that both provide further cost savings and reduce the environmental impacts of inefficient combustion.
Terrestrial power generation turbines are highly complex and technically advanced machines that require large-scale sensor systems to control and maintain the turbine and supporting components. High temperature sensing inside turbines has been well established for decades. However, measuring temperature, strain, and other information on turbine blades is still a very challenging task, and current sensor technology only allows for rudimentary on-blade monitoring. Performing precise measurements on rotating turbine components requires extremely sophisticated, complex, and heavy data retrieval systems and wiring harnesses that have thus far only proven useful in test labs. Harsh conditions on terrestrial power generation turbine blades can include very high temperatures and centrifugal loads.Current advanced turbines have inlet temperatures well over 1,000°C (1,832°F). On-blade sensing requires sensors to be routed off of the blade through a slip ring, which requires drilling the rotors and disks, and results in a significant reduction of component life. Consequently, such systems are not suitable for field deployment, and are only used for testing. However, a harsh environment wireless telemetry system can be utilized as an alternative to the slip ring approach.
Wide bandgap semiconductors, such as silicon carbide (SiC) and gallium nitride (GaN), can be made to operate at temperatures exceeding 500°C (932°F), and can thus be utilized at the base of turbine blades where temperatures are significantly lower. Thermocouple and strain gage sensors can be wired from anywhere on the blade back to the electronics on the blade base, where the sensor signals are acquired and encoded onto a radio frequency carrier signal. This carrier is then transmitted across the gap between the rotor and the stator and is cabled out of the turbine into the control room. Figure 1 shows the schematic of a high temperature Colpitts oscillator built aroundaccording to “Wireless Telemetry Electronic Circuitry for Measuring Strain in High Temperature Environments.”
This is a voltage controlled oscillator, which generates the radio frequency (RF) carrier and modulates the sensor information onto the carrier for wireless transmission. In addition to eliminating the penalties associated with the slip ring approach, a wireless telemetry system based on wide bandgap semiconductors would also provide highly increased signal to noise ratios (SNR) of the measured parameters, since the signals are conditioned very close to the sensors themselves.
Figure 2 illustrates the concept of a smart turbine blade (STB) developed for wireless temperature and strain monitoring, as established in the articles “Development and Testing of Harsh Environment, Wireless Sensor Systems for Industrial Gas Turbines” and “High Temperature SiC Wireless Telemetry Systems.” The STB consists of a blade that has had a thermocouple or dynamic strain gage thermally sprayed onto the blade’s thermal barrier coating (TBC) via a conformal thermal spray process that allows the sensor to be located at any position on the blade.In Figure 2, it is located at the blade tip. The connections to the sensor are thermally sprayed from the junction at the blade tip all the way to the base of the blade, where it is electrically connected to a harsh environment wireless telemetry electronic system. SiC junction gate field-effect transistors (JFETs) are utilized to both amplify the low-level sensor output, as well as to encode the local temperature of the telemetry system. The output is sent to a GaN-based voltage controlled oscillator, which frequency modulates the information signal onto a wireless carrier and transmits this information from the rotating blade.
Aerospace turbines also present incredibly harsh conditions for sensors to operate within, and the aerospace systems they drive come with their own unique operating challenges. Parameters including the temperature and vibration of turbine components can be utilized in predictive health maintenance (PHM) systems, wherein the occurrence of a turbine fault can be determined by comparing the temperature and vibration signatures to a known baseline performance. Conservative preventative maintenance schedules create a safe operating environment on aircraft the vast majority of the time; however, preventative maintenance and replacement schedules are based on statistical models and operational history, according to “High Temperature, Self-Powered Autonomous Wireless Sensor for Bearing Monitoring System for Turbine Engine PHM.”
These models are either based upon the lowest common denominator for component lifetime, which drives up operating costs, or on ignoring the statistical outliers that result from undetected material or manufacturing defects, which reduces operational safety. Currently, commercial operators must spend at least 10 labor hours to maintain an aircraft for every one hour of flight time, as established by the “Tenets of MRO Strategy for Airlines,” while military aircraft can require up to 80 man-hours of maintenance for every hour of flight time, according to “B-2 Bomber: Cost and Operations Issues.” As such, current trends in the aerospace market are pushing for the integration of diagnostic and prognostic systems to reduce the long-term cost of ownership. Figure 3 depicts a harsh environment wireless telemetry system designed for PHM of aerospace turbine bearings. The system utilizes SiC and GaN semiconductors to acquire vibration information from a piezoelectric accelerometer, as well as temperature information from a thermocouple. The information is then wirelessly transmitted to a low temperature receiver, which can decode the information and compare it to baseline vibration and temperature levels to determine whether a bearing fault is imminent. In future aerospace systems, this component health information will directly interface with a full authority digital engine controller (FADEC), enabling both safer and higher performance engine operation.
Since the early detection of faults in both power generation and aerospace turbines allows component problems to be addressed long before a catastrophic failure event, significant resources have been dedicated to developing advanced early detection technology solutions. Current solutions for rotating components require sensors to be cabled out of the turbine using a slip-ring approach. These solutions involve heavy and complex wiring harnesses, as well as turbine modification to accommodate the slip ring, and thus primarily only prove useful in laboratory environments. As such, high temperature electronics, which can withstand the harsh operating conditions inside these turbines, provide an exciting alternative to the slip-ring approach. Advanced, high temperature electronics can enable the in-turbine measurement of parameters such as temperature, vibration, and strain, and then wirelessly transmit that information out of the turbine. Wide bandgap SiC and GaN semiconductors, in particular, can function effectively at temperatures in excess of 500°C and have been used in wireless telemetry systems designed for both terrestrial power generation and aerospace turbine condition monitoring. Upon design-in, these SiC- and GaN-enabled harsh environment telemetry systems will lead to reduced operating costs and higher efficiencies in land-based power generation systems and both safer and higher performance aerospace systems. Harsh environment wireless telemetry systems are currently being adopted in these systems and will eventually become the norm due to the significant system-level benefits SiC and GaN can provide.
Filed Under: Aerospace + defense, Slip rings + rotary unions