APPLICATION NOTES

Marine Instrumentation From Willow

Description

There are many different applications for Willow sensor products in the marine world. Willow has been servicing the marine market with a variety of Crossbow products and is well versed in the diverse requirements for marine measurements.

Many applications require dynamic accuracy and resolution while the vessel may be in a high sea-state. Crossbow's inertial systems deliver performance under most marine conditions. The products have been deployed on buoys, remote underwater vehicles, racing yachts, and the US fleet of aircraft carriers. In fact, the US Navy has selected a customized version of Crossbow's VG700 Fiber Optic Gyro for its long-range landing system. The long-range landing systems consists of powerful lasers that assist night time carrier landings. The pointing system is stabilized with the VG700 Fiber Optic Gyro. Another interesting application is performance measurement and optimization on racing yachts. Several of the America's cup racing teams are using Crossbow's inertial systems for dynamic Roll, Pitch, and Heading measurements.

Capabilities

· Buoy Data Collection
· Underwater ROV Guidance
· Yacht Racing Performance Measurement
· Ship Stabilization Systems
· Aircraft Landing Systems on Large Ships
· Antenna Pointing

Product Benefits

· Dynamic Roll, Pitch, Heading Data
· Vessel Inertial Data
· Low-Power Operation
· Low-cost

Recommended Products
· VG700
· AHRS400

VG700:

The VG700 is a Fiber Optic Gyro system. The VG700 and derivatives are high performance units suitable for critical marine measurements. The unit outputs Roll, Pitch as well as body frame Roll,Pitch, and Yaw rates. This product is also customizable to meet specific application requirements.

AHRS400:

The AHRS400 is a dynamic roll, pitch, and heading sensor. It combines three MEMS rate gyro sensors, a 3-axis acceleration reference and a 3-axis magnetic reference. The AHRS400 features fast turn-on and low-power operation. An embedded Kalman Filter runs on an internal DSP processor to measure roll, pitch, and heading continuously. Absolute heading calibration for magnetic North is done with an internal hard iron calibration procedure that consists of turning the boat in several 360 degree circles. Analog and digital outputs are available.

Accuracy Performance vs. Cost

There are many different inertial measurement choices for marine applications. Crossbow's unique sensor products leverage MEMS technology and DSP software to deliver the highest level of performance at the best cost. Many high performance marine inertial sensors cost £15K and up. The performance of these sensors is not superior to the Crossbow product. Some of the products are specified differently, and hence on paper may appear to deliver more accuracy. However, these solutions tend to view accuracy in RMS (not min/max terms) and do not account for all errors. The accuracy specifications of many competing solutions are for benign marine environment. In addition several supplier specify dynamic accuracy, excluding static or initial errors. For example, if the roll, pitch sensor was placed on a level table and read a constant 0.2 Degree offset, this offset would be excluded in dynamic accuracy specification. However, removing static errors on a moving ship is difficult at best, and we specify our dynamic errors with all effects. Our dynamic accuracy is measured using aggressive sustained acceleration maneuvers that are far more challenging than most marine conditions.

The following performance graphs were plotted on a small sailing vessel in order to illustrate of the kind of dynamic tracking Crossbow's inertial sensors deliver. The plots compare the MEMS-based AHRS400 with a high performance ring laser gyro system.

Sailing Test: South Beach Marina to Golden Gate Bridge.




	Sailing Test: South Beach Marina to Golden Gate




	Pitch: During hard turn into wind




	Roll: During hard turn into wind

Heading(*): During hard turn into wind

(*) Note on Heading Performance: The AHRS400 measures north with respect to Magnetic North. It uses a 3-axis flux-gate combined with a MEMS gyro to do so. However, the reference ring-laser gyro, runs a gyro compassing algorithm and hence measures True North. This fixed offset is shown in the Heading measurement plot.