FEATURE SENSING TECHNOLOGY DELIVERING AVIONICS SYSTEMS OF THE FUTURE
Bob Scannell, business Development Manager for ADI’s inertial MEMs products investigates MEMs devices in the avionics industry
MEMs IP with its signal processing, ADI has been able to distinguish itself in the high performance MEMs community. ADI IMUs (inertial measurement units) also address the challenge of implementing inertial sensors into complex avionics systems, that have to depend on multiple sensor types in multiple dimensions to appropriately distinguish the complex motion they experience. iSensor IMUs incorporate up to ten degrees of freedom sensing, with all the necessary alignment, calibration and first order fusion. All factory integrated and tested. For example, the ADIS16485/8 IMUs are a
M
aintaining performance, while also improving size, weight, power and cost,
is one of the biggest challenges facing avionics equipment manufacturers in today’s market. Traditional solutions based on legacy MEMs and other inertial technologies have a proven track record of meeting the performance objectives of the avionics industry. But those same approaches have tended to fall far short of more recent cost and size goals. Without more optimised technology choices, the increasing pressure on newer generation avionics systems to meet these requirements is making the challenge more difficult for equipment manufacturers. If we explore the range of inertial MEMs
components available across the entire electronics industry today, the solutions fall into three main categories or application focuses: military, automotive and consumer. As you would expect, there are significant
differences in the pedigree of these technologies. For example: • Technologies with a military origin are usually highly robust but inflexible with size, weight, power (SWAP) and cost
• Consumer devices are great at meeting cost goals but performance and ruggedness can be quite limited
• Technology originally developed for the automotive industry is usually optimised to meet rigid targets on a range of important parameters, including performance, ruggedness, cost, size, weight and power The roadmap and potential of these technologies for further development also differ quite significantly. ADI’s MEMs technology for automotive requirements is
32 SEPTEMBER 2015 | ELECTRONICS Figure 2:
ADI IMUs address the challenge of
implementing inertial sensors into complex avionics systems
capable of roadmap advances in performance, while also improving SWAP and cost. The highly robust quad-core gyro sensing
structure in ADI MEMs is an important element that enables it to meet these requirements. The quad-core gyro sensing structure rejects shock and vibration influences on the angular sensing mechanism. It also has a proven track record in avionics, automotive, medical and smart munitions programs. The symmetry of the two pairs of anti- phase resonators enable a high level of common mode rejection for non-rotational inputs and the high resonator and demodulation frequency (approximately 18KHz) is used to facilitate superior rejection of out-of-band signals. Vigorous linear- acceleration/vibration analysis has been carried out on the core sensor, as well as sweeps above its resonance frequency, showing its capacity to reject this influence.
SENSING TECHNOLOGY The well matched and optimised sensor conditioning also plays an equally important part along with the robust sensor core design. Basically, the sensor element records a real-life motion (i.e. structure rotation) and translates it to a quantifiable electronic signal (i.e. voltage). Without proper attention to bandwidth timing, phase, sampling rates, resolution, and other drift characteristics such as temperature and voltage stability, the translation and subsequent processing might have opportunity for inaccuracies. All of these functions depend on an advanced and robust sensor conditioning. By successfully marrying its proprietary
combination of 6 and 10 degree-of-freedom sensors which are positioned in avionics systems today, meeting all performance and reliability goals, delivering up to an order-of- magnitude SWAP benefit. The company’s MEMs technology has proven itself against fibre optic gyro (FOG) inertial technology. In a recent1 comparison of an ADI IMU against a $30k FOG IMU, the significant advances in high-performance MEMs technology were clearly demonstrated, as well as its ability to achieve similar levels of performance to legacy FOG systems, with better than an order-of-magnitude advantage in critical SWAP/cost factors.
MAINTAINING CRITICAL PERFORMANCE UNDER COMPLEX AND RUGGED CONDITIONS There are three important features of the IMU design that ensure the rejection of erroneous motion artifacts related to vibration or other extraneous signal input. For the core sensor element, the sub-system design, and the signal processing circuits, the design requirements were precisely targeted at maintaining performance under complex motion, through the rejection of all unwanted motion. To improve the performance, the iSensor sub-system application uses multiple (of the quad resonator) sensors for each axis of measurement, with two sensors mechanically reoriented from a second pair, making first order cancelation available for systemic common non-rotational signals and sensitivities (thermal, supply, and residual acceleration sensitivity). Processing happens at high data rates (adequately oversampled), to guarantee the preservation of high performance created through the core sensor elements and sub-system design. These attributes have been carefully tuned in order for the device to survive and perform in hostile avionics, automotive and military environments, leveraging several man-years of sensor, signal processing and application.
ADI
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