The project aimed to design and test a MEMS based accelerometer for wing structure active control and health monitoring. The key requirements were: full scale of ±10 g; bandwidth >200 Hz; 1 year composite stability: Bias:1.5 mg and SF: 1500 ppm; vibration rectification: 25 µg/g2; noise: 100’000 accelerometers /year in the ISO9001 and 14001 certified MEMS fab, assembly and test facilities. Colibrys has a large accelerometer background IP and will manufacture the resulting accelerometers. This project contributed to establishing a European, MEMS based, ITAR free, accelerometer supply that will profit European aeronautics, defence, and instrumentation industries.
The Wingaccs project aimed to design and test an MEMS based accelerometer for wing structure active control and health monitoring. The key requirements were: full scale of ±10 g; bandwidth >200 Hz; 1 year composite stability: Bias:1.5 mg and SF: 1500 ppm; vibration rectification: 25 µg/g2; noise:
Based on previous research results in a first phase Colibrys has designed, realised and tested demonstrator sensors. In a second phase improved sensors were designed. More than 15 sensors were manufactured and tested. The initial performance targets were fully met: Noise was 2 µg/vHz, 80% of the tested sensors showed a long term stability < 1.5 mg, Vibration rectification was 10 µg/g2 in the frequency band 20 to 20 Hz and 20 µg/g2 in the frequency band 200 to 1000 Hz, and warm-up transients was
The analysis of the key components such as the analogue ASIC and the FPGA code showed that they are not yet compatible with the aeronautic quality and reliability requirements such as the norm DO 178 and DO 254 as well as the requirements for SEU and MBU immunity.
The steps towards products for wing structure active control and health monitoring are:
i. Redesign of the analogue ASIC and the digital signal processing (FPGA) according to the DO norms
- Miniaturization and performance improvement
- Digital signal processing unit assuring the performance, conformity to DO norms, immunity against SEU/ MBU and without ITARs constraints. These constraints will impact the choice of the technologies as well as the design process.
ii. Design of a custom sensor package (thermal design and miniaturisation) and an optimized MEMS sensor.
iii. Review of the sensor architecture, in particular miniaturisation of the electronics not included in the ASIC.
iv. Investigate and improve understanding of the underlying physics of the sources of long term drift including those related to charge migration.
The results of this project had a strong impact beyond the specific application targeted in this project. It laid the basis for a new generation of high performance MEMS accelerometers with a broad range of applications. These sensors met and outperformed expensive, fragile electromechanical accelerometers. They met the highest performance requirements in terms of stability, at low cost.
These sensors are also expected to open new applications for MEMS based sensors in north finding (in combined with high performance gyro's) and for even navigation, through their ability to provide precise measurements even under strong vibrations thanks to the low noise and the excellent vibration rectification.
The results of this project will be exploited by Colibrys, who is already a well known manufacturer of high performance MEMS accelerometers. The MEMS components can be manufactured in quantities on the existing Colibrys manufacturing line based on proven and well controlled processes. Throughout the development the sensors are designed taking into account the process variability avoiding the need for expensive screening or burn-in processes.
Project Context and Objectives:
The Wingaccs project aims at design and test of MEMS based accelerometer for wing structure active control and health monitoring. The key requirements are: full scale of ±10 g; bandwidth >200 Hz; 1 year composite stability: Bias:1.5 mg and SF: 1500 ppm; vibration rectification: 25 µg/g2; noise: <2 µgvHz, warm-up transients: 10 µg/min, digital output and self test. They will be achieved with a bulk micro-machined capacitive MEMS chip, Sigma Delta closed loop architecture with an analogue FE ASIC and an FPGA for digital force feed-back control and digital output. Colibrys standard process is used to manufacture the MEMS. Results from a research project show that most key requirements are within reach. In this project the challenges are to resolve open technical issues and to meet the basic performance such as the bias stability, warm-up behaviour. The goal is to lay the basis for further developments of products for specific applications, that are conform to aeronautics quality and reliability requirements such as DO 178.
On the basis of previous research a MEMS based sensor was developed.
In a first phase the system was modelled analytically and numerically to establish the basic loop characteristics such as loop stability, noise, bias stability. The requirements for the MEMS sensor, the FE ASIC and the algorithms for the digital loop control were derived.
The key result of the modelling phase is that the basic accelerometer performance could be reached.
- A stable control loop can be designed
- A noise floor close to 1 µg/vHz for a ± 10 g sensor can be reached
- The expected short term bias drift is
Based on these results a first demonstrator was designed, realised and tested. The results showed that the sensors were already meeting the specifications with respect to the bias stability, bandwidth, vibration rectification, noise and transient warm-up behaviour. However, temperature bias hysteresis and short term bias drift effects were of the order of a few 100 µg observed that would not be acceptable in many applications.
In a second phase the root cause of these effects were identified. The main contributor was the drift related to die attach. Temperature gradients within the MEMS device also contributed. To resolve the die attach issue two solutions were explored, one based on improved die attach technology and one based on mechanical decoupling of the sensitive MEMS part from the areas which are used to attach the die to a substrate (Colibrys Patent). Due to time and budget constraints the solution based on on-chip mechanical decoupling was done with a MEMS design that is based on an older design of the sensitive part that is not designed for high stability. Nevertheless, this approach showed the gain in stability that can be achieved with this approach. In a further product development phase, the implementation of the mechanical decoupling concept combined with the high stability sensitive part of the MEMS is seen as low risk, since it needs only a redesign of the MEMS but no change in the technology. Nevertheless an experimental verification of the gain in performance that can actually be achieved will be necessary.
A second potential contributor to long term drift is charge migration. Charge migration is an effect that can change the characteristics of the electrostatic force feed-back and thus lead to drift. In future work this effect will have to be characterized in more depth.
Sensors based on both approaches were realized and tested. They confirmed the initial excellent results on a larger number of samples (< 10) with respect to the key specifications. They also showed significant improvements with respect to the temperature and hysteresis issue, the mechanical decoupling being clearly the better solution.
The aspects of the compatibility of the approach with the quality requirements for aeronautics, in particular with respect to the norm DO 178 and DO254 and the immunity to SEU and MBU were analysed in collaboration with the ITD leader. The analysis showed that there are no blocking points to meet these requirements. However, the analogue ASIC and the FPGA code that were developed in a research environment need to be redesigned according to the aeronautics quality requirements.
The project showed that the initial target specifications are met with the chosen technology. Solutions for the issues with respect to hysteresis and short term drift were identified and tested.
This project has shown that the chosen technology has a large potential to meet even more stringent performance specifications that will allow using the devices in many aeronautics applications even working towards north finding and navigation applications. The key feature is not only the bias stability but also the low noise and the very good vibration rectification of the order of < 5 µg/g2 which is significantly better than what is achieved with the classical electromechanical accelerometers and other MEMS approaches.
On the basis of the results of this project, Colibrys plans to develop a family of products for various applications. The chosen approach has already reached a good level of maturity. In order to develop a mature product there are two major issues that need to be addressed. The current analogue ASIC and the FPGA code were initially developed in a university environment. They lack the development rigour and documentation that are needed for a product meeting the aeronautics standards, in particular DO178. Furthermore the current system is not immune against SEU and MBU events. Therefore the analogue ASIC and the FPGA code need to be redesigned. The development of a final product will also require a review of the sensor architecture, in particular miniaturization of the peripheral electronics such as power supplies. A custom designed sensor package and an optimized MEMS sensor implementing the findings of this project will allow to even further improve the performance of the sensor.
With these further product development steps it is expected to lay the basis for a new generation of MEMS accelerometers meeting the highest performance requirements at low cost that today can only be met by expensive, fragile electromechanical devices. These sensors are also expected to open new applications outside aeronautics thanks to its ability to provide precise measurements even under strong vibrations thanks to the low noise and the excellent vibration rectification.
The technology used in this project has also various applications outside the aeronautics area. The very high dynamic range, approaching 24 bits, opens access to many seismic applications such as earthquake monitoring, seismic intensity measurement for local evaluation of damage after an earthquake and structural monitoring of buildings, bridges, dams. The low cost of MEMS based sensors as compared to the traditionally used electromechanical devices makes dense surveillance systems technically and economically feasible.
On a European scale these new product families will provide systems companies with high performance, low cost, and ITAR free accelerometer components, improving their competiveness against competitors in the USA and Asia.
These sensors can be easily manufactured in large quantities on existing manufacturing lines, without degradation of the performance and with a high potential of cost reduction with increased quantities. This enables new applications needing high performance but also low cost. As an example in areas with high risk of earthquake it is important that the local earthquake intensity can be measured with a dense sensor network. Today such approaches are not economically feasible du to the high sensor cost.
The results of the project have been communicated at specialist conferences with a scientific and industrial audience.
Colibrys (Switzerland) Ltd