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Electronic Design

Sixense Sensor Provides Real 3D Positioning

The Analog Devices Sharc gets a workout inside Sixense Entertainment’s 3D sensor system, which will be at the heart of a new class of game controllers. Nintendo’s Wii popularized 3D wireless controllers, but its accelerometers only provide relative motion information. Sixense’s patented TrueMotion game controller delivers absolute 3D position and orientation information (Fig. 1).

The importance of the ability to determine position and orientation cannot be stressed enough. This technology supports multiple sensors that will allow a game to determine which two players were next to each other and reaching for the same object. Alternatively, a player could use two sensors. For example, a golfing program could track the club as well as the player’s waist and hip position relative to the club. This would be impossible with an acceleration-based system.

Magnetics per form the magic. A small basestation has three small orthogonal coils that generate a weak magnetic field. Three smaller coils in each sensor detect this field. A Sharc DSP in each sensor analyzes input data from Analog Devices analog-to-digital converters attached to the coils. The initial gaming system reference platform handles up to four sensors, which is enough for two sensors for each of a pair of players.

The reference design sensors communicate using a 2.4-GHz wireless link. Each sensor delivers 60 readings/s, and they can operate for 20 hours with two AA batteries. A future move to Blackfin-based platforms will double the battery life. The sensor range is 6 to 8 feet from the USB-based base unit. Accuracy falls off outside this range, but this limitation, as well as the number of sensors and frequency, is due to cost constraints for the target market—gaming.

Higher resolution, longer range, and faster sensing are possible. Part costs for the gaming platform are on the order or $25, while a system with a 35-ft radius might be around $100. The system can handle any number of sensors because each performs its own analysis. The limiting factor is how the sensors are used. In terms of gaming, the radio connection is the limit, and different technologies can provide higher performance.

Magnetic sensing systems have a number of advantages over other sensing systems, including optical systems, because they don’t require lineof- sight operation with respect to the base unit. This enables a control unit to be moved around a player’s body without disrupting the sensor’s ability to report its position and orientation.

Magnetic materials such as iron and nickel can disrupt the sensors, but small amounts, such as a belt buckle, will not significantly affect accuracy. Likewise, even large objects that are at least a foot away will have no effect. The system requires no user calibration if these restrictions are met, though it can still operate in closer proximity if the system is calibrated. This would be significant if the technology is used in an environment such as an assembly line with large metal structures.

The magnetic field strength is 1/50th of the earth’s magnetic field. It is modulated so it is possible to use multiple base units to cover a larger area, though the initial gaming reference platform is designed for a single base unit. Sixense’s initial audience includes gamers and gaming companies, yet the technology has much more potential in applications ranging from robotics to home care. No other technology provides 3D positioning information at such a low cost.



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