There is increasing demand in many high-end commercial and industrial markets for precision motion sensing for a diversity of applications. In particular, demand is growing for effective guidance and stabilization in applications with severe limitations on space and/or weight, or available budget. UK/Japanese joint venture, Silicon Sensing Systems, has developed a high-performance all-silicon micro-electro-mechanical systems (MEMS) gyro in response to this demand.
For example, in the maritime sector, growing shipping activity worldwide is leading to a new intensity in vessel movements at major ports. To ensure safe maneuvering in and out of berths in these confined environments, ship pilots need portable navigation aids that reliably deliver the highest levels of motion sensing accuracy. Maritime navigation systems manufacturer AD Navigation (Tonsberg, Norway) has used Silicon Sensing’s CRH02 MEMS gyro in its ADX XR pilot’s aid.
AD Navigation’s systems help manage the traffic in major ports like Rotterdam, Antwerp, London, and Liverpool, and are used by the US Navy for maneuvering naval vessels to a precision of only a few centimeters. Size and weight are critical parameters for these devices as they are carried onto the ship by the pilot and installed only for the duration of the pilotage into the berth.
The ADX XR pilot’s aid was used to successfully guide the world’s largest construction vessel, the Pioneering Spirit, into the close and busy confines of Maasvlakte in the port of Rotterdam. The Pioneering Spirit, a twin-hulled platform installation/decommissioning and pipelay vessel, at a massive 372 × 124 meters, is designed for the single-lift installation and removal of large oil and gas platforms and the installation of record-weight pipelines.
The CRH02 gyro was used to measure the rate of turn of the ship in the channel. This data was combined with GPS inputs and other ships’ data to predict the path of the ship through the channel, allowing optimum control at all times. Throughout the trial, CRH02 delivered precise motion data that the ADX XR device used to maintain a highly accurate and detailed 3- to 5-minutes of arc ship course prediction for the pilot throughout all fine maneuvers and docking. (Figure 1)
With 20 years of design evolution behind it, this is the latest generation of devices from a line that has produced over 30 million high-integrity MEMS inertial sensors. In the CRH02, the sensor is combined with discrete electronics to deliver high stability and low noise performance.
At the heart of the CRH02 is Silicon Sensing’s vibrating ring MEMS sensor. In this design a silicon ring vibrates, like the rim of a wine glass, when powered up. Rotation (like twisting the wine glass stem) changes the vibration mode due to Coriolis forces. These changes are detected electronically and are output as an analog or digital signal proportional to the rotation rate. This ring-resonator construction overcomes mount sensitivity problems and is less shock sensitive than other vibrating structure-type gyros such as simple beam or tuning fork-based designs. The design also avoids the rotational components of dynamically tuned gyros (DTGs), which render them sensitive to shock as well as mechanical degradation. (Figure 2)
The CRH02 represents a new generation of Silicon Sensing devices based on this MEMS sensor, with performance that is typical of much larger and more costly fiber-optic gyros (FOG). FOG is an established non-rotational gyro technology and is the usual choice for navigation solutions. FOG-based devices have historically delivered more consistent performance over time, with lower drift than MEMS devices.
The CRH02 is the first tactical-grade, non-ITAR (International Trade in Arms Regulations)-restricted, MEMS gyro. It combines the precision and low drift performance typical of a FOG or a DTG in a package that, at just 25mm square, 33mm high, and only 45 grams in weight, is far smaller and lighter than either. It is also more rugged and far cheaper to produce. (Figure 3)
The CRH02 is available fully housed or as an OEM solution, without the housing and tailored to suit specific customer needs. It, and its sister device, the CRS39, are available in rate ranges to suit any performance specification where low bias instability (BI: a measure of the lowest achievable drift characteristics) and angular random walk (ARW: a measure of gyro noise) are critical. BI and ARW are both standardized independent measures of gyro performance quality and derive from the Allan variance analysis, which aggregates real gyro test data over varying time periods.
This new generation of devices unlocks the door to many opportunities for ultra-precise guidance and motion sensing for platforms and functions where, to date, this has been neither practical, nor economically viable. Ship guidance and control is just one example. There is potential in mining, where downhole surveying requires high sensitivity gyros to detect the Earth’s rotation in order to achieve north-finding without using magnetic means.
In platform stabilization these sensors can form part of a control loop that feeds detailed platform motion data to the actuation system. In applications such as camera control, low noise performance means excellent platform stability.
In the autonomous vehicles market, yaw data is critical for accurate navigation, but yaw is the only axis where errors cannot be mitigated using complimentary accelerometers to measure gravity. The CRH02 can deliver precise yaw data for these vehicles.
In aviation, attitude and heading reference systems (AHRS) and other flight instrumentation all demand extreme reliability and ruggedness with minimal impact on available space and weight — all factors inherent in these MEMS sensors.
The Future for MEMS Gyros
MEMS gyros have come into wide use in consumer applications, primarily because of their small size and low cost. The development by Silicon Sensing of the CRH02 opens the door for many critical applications because it combines the MEMS size and cost benefits with precision, stability, and low-noise performance previously available only with fiber optic or dynamically tuned gyros.
This article was written by Andy Hughes, Marketing and Sales Manager, Silicon Sensing Systems Ltd. (Plymouth, Devon, U.K.). For more information, visit here.