Overview
This proposal was concerned with the development of distributed high temperature electronics platform for integration with sensor elements to provide a high integrity serial digital data link that can be fed into FADEC (Full Authority Digital Electronic Control) and/or EHMS (Engine Health Monitoring System) units on an aircraft engine. To date most of the work undertaken and reported had been in the United States, which has concentrated on the development of high temperature electronic control systems, with high levels of functionality and more complex components, such as the Honeywell HTMOS products.
This proposal aimed to design and develop a lower cost distributed electronics platform with fit for purpose functionality that can be deployed close to the sensors and integrated into a miniature multi-chip module. The unit was designed to provide signal conditioning for a range of sensor outputs and managing the signal to enable high integrity digital signal transmission to the FADEC/EHMS units. The target long temperature of operation was 250oC for up to 5 years.
Funding
Results
Executive Summary:
This project covered the development of a distributed high temperature electronics platform for integration with sensor elements to provide digital outputs that can be used by the FADEC (Full Authority Digital Electronic Control) system or the EHMS (Engine Health Monitoring System) on an aircraft engine. This distributed electronics platform eliminates the need for the FADEC or EHMS to process the electronic signal, which will assist in making the overall system more efficient in processing only digital signals. This will offer weight savings in cables, harnesses and connector pin reduction and through less complex cooling systems.
A high level draft technical specification was provided by Turbomeca. The design concept was to take the output from several on-engine sensors (temperature probe, thermocouple, strain gauges, frequency), carry out the signal conditioning on the sensor signals, multiplexing, analogue to digital conversion and transmission of the data through a serial data bus. The unit has to meet the environmental requirements of DO-160 for a helicopter engine, with the specific need to operate at 200oC, with short term operation at temperatures up to 250oC. The system service lifetime target is 50,000 engine flight hours.
The design principle of the HIGHTECS module was based on a custom silicon on insulator (SOI) ASIC being used for the majority of the signal processing and conditioning from the range of sensors, multiplexing, analogue to digital conversion and transmission of data through an ARINC 429 databus. The ASIC was then integrated with a high temperature external clock and packaged onto a ceramic hybrid circuit. This hybrid circuit was assembled in a Kovar package, which was hermetically sealed in an inert gas atmosphere. The Kovar package was then mounted into a stainless steel enclosure containing high temperature connectors and EMI shielding.
The HIGHTECS ASIC, hybrid circuit and module have been designed and manufactured. The HIGHTECS ASIC has successfully demonstrated dual output of ARINC 429 messages; however, problems have been encountered in achieving a consistent linear output in the Analogue to Digital Conversion (ADC) transfer function. The hybrid circuit and module has also produced ARINC 429 messages, but the output has been inconsistent, which again is believed to be related to the ADC transfer function. The ADC, which was supplied to the project as an existing IP block, is sensitive to its supply voltages and does not meet its published specification. The discontinuities reduce as the analogue supply voltage is increased above the digital supply voltage and as the temperature is increased above ambient. The voltages needed to eliminate the discontinuities are above that recommended for the SOI ASIC process. A small number of devices were identified which had a functioning ADC at a digital voltage of 5V and analogue voltage of 5.5V and these devices have been assembled into the HIGHTECS hybrid circuit and module. The results show that the HIGHTECS module can function between -40oC and +225oC, with linearity of output improving as the temperature increases. A re-spin of the ASIC design was carried out to address the issues of the inconsistent ADC functionality by bringing out separate voltage references and improving the connections around and to the ADC block. The results on the 2nd version of the HIGHTECS ASIC show the analogue sensor conditioning and frequency measurements functions in line with specification on the ASIC over the temperature range -40oC up to 250oC with operation up to 275oC. However the ADC output is not linear at 5V, which is the recommended voltage for the SOI process and further work will be required outside the scope of this project to develop an improved ADC IP block which can function at 5V.
The results of long term temperature storage tests at 250oC on a SOI test device fabricated with the same semiconductor process as the HIGHTECS ASIC has indicated that the longevity exceeds the target operating lifetime of 50,000 hours based on the temperature profiles provided by Turbomeca.
Exploitation of the project results will take place through custom ASIC and Multi-Chip Module design, high temperature electronics packaging technology and knowledge of failure modes, lifetimes and reliability prediction. Dissemination of the results will be achieved through presentations to interested parties within EU Clean Sky and selected wider publication to high temperature electronics networks (e.g. HITEN).