Sea Technology

DEC 2015

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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22 st / December 2015 www.sea-technology.com true North to be calculated. This North-fnding capability has been put to very effective use in the drilling industry to determine precisely where a drill has wandered hundreds of meters below the surface. Other precision applications include ship positioning systems where the MEMS IMUs are used to help keep a vessel on station while un- tethered in a dynamic environment. Silicon Sensing started out pro- ducing MEMS gyros for use in automotive braking systems. For this application, what matters is reliability, integrity and es- pecially cost. More recently however, the company has been focusing its R&D; on de- livering higher-and-higher per- formance MEMS inertial sensors and integrated systems. Emerging from this engineering activity is the frst of a new family of high-performance MEMS IMUs: HPIMU. The frst-generation of HPIMU is DMU30, a high-grade, all-MEMS IMU with a specifcation and price tag that challenges the established mechanical, ring laser (RLG) and fber-optic gyro (FOG) based IMUs and opens up new possibilities for subsea ve- hicle system designers due to the step change in size, cost and reliability that only MEMS can deliver. Another com- mercial advantage is that most of the high-end IMUs, often referred to as tactical grade, are sourced from the U.S., with International Traffc in Arms Restrictions (ITAR) export limi- tations. DMU30 has been developed and is manufactured in the U.K. and Japan to address the demand from marine and other industrial and commercial market applications. Being ITAR-free, it offers to extend operational coverage without export restriction-related overheads. Finding North The absolute "goodness" of any type of gyroscope or ac- celerometer in terms of its accuracy as a free inertial motion Combining three pairs of gyros and accelerometers in an orthogonal confguration, with electronics and software, and then calibrating the whole assembly over the host ap- plication's expected operating temperature range, you then have a six-degree-of-freedom, or 6-DOF, MEMS inertial measurement unit (IMU), which can be used to measure the complete 3D angular and linear motion of whatever the IMU is fxed to. From this information, you can determine a vehicle's attitude, speed and heading, and close the loop to stabilize, guide or navigate it. High-Performance MEMS We have to thank frst the automotive industry and then the handheld consumer markets for the rapid rise of the MEMS sensors. Accel- erometers in airbags and smartphones and gyroscopes in car braking systems and computer game consoles, have created a burgeoning industry leading to massive cost and size reductions and increased choice. While solving one problem—cost—the penalty has been a degradation of measurement sensitivity, accuracy and re- peatability. The bias instability (a key performance param- eter) of consumer-grade MEMS gyros, for example, is mea- sured in tens or hundreds of degrees per hour but, in order to determine attitude and heading to allow accurate guidance and navigation of underwater vehicles for any useful period of time, you need a gyro with bias instability measured in tenths or hundredths of a degree per hour. The MEMS inertial sensor industry has seen not only mas- sive growth from almost nothing to more than 100 million axes per year in the last 15 years, but also great diversity. State-of-the-art MEMS gyroscopes today range from devices the size and cost of "a dot for a dollar" to those with sen- sitivity capable of detecting the Earth's rotational rate (15° per hour) and thereby enabling an estimate of bearing from (Top) HM Submarine E1 on sea trials—the frst sub- marine to use a gyrocompass. (Bottom) DMU30 high- performance IMU.

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