Middletown, RI--Combining an electronic compass and a GPS receiver with intelligent displays allows engineers to produce a navigation system capable of either dead reckoning or GPS navigation. In addition, the system can display the orientation of an armored vehicle's hull and turret simultaneously.
Called TACNAV, the system's compass is accurate to two degrees. It uses a sensor antenna that contains a toroidal-ring-core fluxgate magnetometer to detect magnetic north. Supported by a heavy helical spring, the antenna is located in a position where the magnetic field will remain as stable as possible. Helmholtz cage compensation helps the toroidal core see a uniform field.
Robert W. B. Kits van Heyningen, vice president of engineering at KVH Industries, Inc. explains that the compass was originally developed for use on racing yachts. Later, KVH adapted it for use on large naval vessels such as aircraft carriers. In the late 1980s, Picatinny Arsenal contacted KVH and asked the company to build a compass capable of functioning on a main battle tank. Success in that project created interest in a more capable system for armored military vehicles, and KVH responded with TACNAV.
Basically, the design challenge required that engineers take data from several different sensors and combine those data streams to produce navigation and directional information. Software provided the key to interfacing the sensors and generating useful outputs. TACNAV, the end product of this effort, consists of five subsystems: a sensor antenna; a driver's control box; a turret angle interface box (called TANGO); a commander's display; and a driver's display. The system can interface with the vehicle's odometer, turret angle encoder, laser rangefinder and GPS to provide interactive navigation data and target location data.
Given that KVH developed TACNAV for the military, ruggedness and reliability became important design issues. "Our entire design philosophy is based on a modular approach," says Ron Paradis, a component and military products engineer at KVH. Each subsystem in TACNAV contains an Intel 8051 eight-bit microprocessor. Internal communications employ messages output in ASCII. Kits van Heyningen explains that its low cost and the ready availability of tools and compilers drove the selection of the 8051.
Each of the intelligent subsystems carries out internal tests to determine its state of health. Placing intelligence in each subsystem makes the system highly modular and easy to service. Rated at 10,000 hours MTBF, the system's mean time to repair is 30 minutes.
Displays for the driver and vehicle commander employ heated and backlighted four-plane LCDs designed for ñ20-degree readability. Heating the displays enables them to operate in temperatures as low as 10C. Military usage required placing the displays in rugged metal housings, and also made it necessary to do tests with night vision goggles to ensure display visibility.
In an armored vehicle equipped with TACNAV, data from an absolute turret angle encoder and a GPS receiver are received by the TANGO. That information goes directly to the vehicle commander's display, along with compass and odometer data. Laser rangefinder information can also be presented to the commander. TANGO output goes to the driver's display via a pair of processor boards, along with odometer data and directional information derived from the fluxgate magnetometer's output. The processors calculate compass headings, turret position, and other parameters, and deliver that information to other subsystems.
According to Kits van Heyningen and Paradis, interfacing with the GPS system was one of the most difficult parts of the design project. They found it necessary to use a great deal of microprocessor overhead to interrogate the GPS.
By capturing the output of a laser rangefinder, TACNAV enables an armored vehicle's crew to navigate to a new location without losing the target's location. So they can illuminate a target, then move to a different spot and fire on it, without using the rangefinder a second time. This capability can reduce the danger of being shot at by hostile folks who observe the laser.
Engineers at KVH believe TACNAV is a military system with considerable potential in civil markets. Consider just one potential area of use: Because TACNAV collects data from several sources, concentrates it, and then communicates it in a digital form, it can be transmitted to a central location for analysis. This characteristic might make many types of mass transportation systems more user-friendly. A bus or rail vehicle could be interrogated, and information on its location presented to the public. Thus, although your bus might be late, you would have a fair idea of when you could expect its arrival.
Additional details...Chris Burnett, V.P., Business Development, KVH Industries, Inc., 110 Enterprise Center, Middletown, RI 02842, (401) 847-3347.