Polhemus Uses SHARC® for 3D Motion Tracking System for Medicine, Film, Virtual Reality

Polhemus is the number one global provider of electromagnetic 3D motion tracking systems for motion capture, scanning, and overall tracking applications for medicine, film, virtual reality, sports, university research, military training and simulation, and computer-aided design. The company promotes some interesting examples of 3D tracking, including full-body character animation and 3D digitizing of some characters and objects in many Hollywood films.

"The SHARC ADSP-21161   Processor is intuitive and easy   to work with and highly cost-  effective....definitely a slam   dunk choice"
Bob Higgins            
Lead Designer         

Founded in 1969, the Colchester, Vermont-based company, which has been developing 3D tracking systems for more than 30 years, turned to Analog Devices, Inc. (ADI) for processing power for its latest 3D motion tracking system dubbed LIBERTY™, which comprises up to 16 sensors.

Polhemus chose an ADSP-21161 SHARC® processor from ADI to drive LIBERTY. Capable of 600 million math operations per second (MFLOPs), the ADSP-21161 processor sets a new level of performance for low-cost processors with more than three times the performance of comparable models for about the same price. With ADSP-21161 processors on board, LIBERTY operates at an unprecedented speed of 240 updates per second per sensor, and an accuracy of .03 inches and 0.15 degrees (root mean square error of 51 points in a typical operating space) with all sensors simultaneously tracking objects in real-time.

How 3D Tracking Works
3D tracking works by determining location, orientation, and/or positioning information relative to some coordinate system; it requires XYZ coordinates for the origin and XYZ coordinates for the receiver. Electromagnetic trackers use the attenuation of oriented electromagnetic signals to determine the absolute position and orientation of a tracker relative to a source. The source contains three orthogonal coils that are pulsed in rotation, one after another. Each pulse transmits a radio frequency electromagnetic signal that is detected by a sensor. The sensor also contains three orthogonal coils, which measure the strength of the signal from the current source coil.

By using the known pulse strength at the source and the known attenuation of the strength with distance, these nine values can be used to calculate position and orientation of the sensor coils. The source and the sensor are connected to a box, which contains a microcomputer and electronics associated with the pulses.

Uses of 3D Tracking
The uses of 3D tracking are many, but a few examples include film animation, 3D image analysis for surgery, surgical training, and biomechanics. The ability to use 3D tracking systems helps moviemakers bring filmed images into the digital realm, making them look more lifelike. A 3D tracking solution captures minute details, allowing animators to scan models and then translate the data in real-time into a full 3D graphical image that instantly appears on the computer screen for animation. This process can also be used for computer games.

Advances in 3D imaging means images once produced only by academic centers with dedicated 3D labs can now be produced on workstations at busy hospitals. Images can be created and evaluated quickly enough to be useful for trauma assessment and even guided surgery. For surgical training, ultra-lightweight headgear that incorporates a field-of-view scene camera transmits images to a large format video display, making it ideal for an instructor to explain surgical techniques to students. For biofeedback, physical therapy, and occupational-therapy studies, a 3D real-time movement analysis system utilizes magnetic field technology to capture changes in an electromagnetic field as an individual moves through the field, and then converts that information into a real-time, skeletal representation of that movement.

Polhemus LIBERTY System
Many 3D tracking systems use the human body as an input device. To that end, the Polhemus LIBERTY system comprises a number of components, including a system electronics unit (SEU), which is the hardware and software necessary to generate and sense the magnetic fields. The SEU also computes position and orientation and interfaces with the host computer via an RS-232 or USB interface. Another component of LIBERTY is a source. It contains electromagnetic coils enclosed in a molded plastic shell and emits magnetic fields. LIBERTY also includes sensors, which also contain electromagnetic coils that are also enclosed in a molded plastic shell, to detect the magnetic fields emitted by the source. The source and the sensors are connected to the SEU via cables. The position and orientation of the sensors are precisely measured as they are moved.

3D Tracker Performance Meaurements
Accuracy or registration is the difference between an object's actual position and the position reported by the tracker.
Resolution is the fineness with which the tracking system can distinguish individual points or orientations in space.
Jitter is a change in the reported position of a stationary object.
Drift is a steady increase in tracker error with time.
Lag is the difference between when a sensor first arrives at a point and when the tracking system first reports it (sometimes also called latency).
Update rate is the number of tracker position/orientation samples per second that are transmitted to the receiving computer.
Range is the position range or working volume and orientation range, which is a set of orientations that the tracking system can report with a given resolution.
Interference and noise is the action of some external phenomenon on the tracking system that causes the performance to degrade.

The LIBERTY employs one ADSP-21161 processor for every four sensors, which fit on a single circuit board. Bob Higgins, a lead designer for the Polhemus LIBERTY system, noted that the ADSP-21161 processor's general-purpose I/O pins greatly simplified the interface to the sensors. But Higgins said he chose the ADI SHARC ADSP-21161 processor for a variety of reasons. He lauded the ADSP-21161 processor's high degree of math precision and accuracy, and support for IEEE-compatible 32-bit floating-point, 40-bit floating-point, and 32-bit fixed-point math. "The number crunching is great," said Higgins. "The SHARC ADSP-21161 processor is intuitive and easy to work with and highly cost-effective. It was definitely a slam dunk choice."

Higgins also said that ADI's libraries of highly optimized assembly language routines were very useful and, in combination with ADI's tools, helped speed the time to market for LIBERTY. "Plus ADI's support, which includes people with years of experience, is fabulous."

LIBERTY features an update rate of 240 Hz per sensor, latency of 3.5 milliseconds, a range of 36 in. (90 cm), and resolution of 0.00015 in. (0.0038 mm) at 12 in. (30 cm) range and 0.0012° orientation. The system comes in two forms, one with 1 - 8 sensors and the other with 1 - 16 sensors. Thanks to ADI's SHARC ADSP-21161 processor, LIBERTY is the fastest, most accurate, scalable electromagnetic tracker available today. It offers the speed, ease-of-use, scalability, distortion sensing, and improved signal-to-noise ratios necessary for high-quality 3D tracking data. For applications from films to sports medicine to virtual reality, LIBERTY and ADI's SHARC ADSP-21161 processor are right on track. Polhemus provides the absolute best in time proven "6 Degree of Freedom" systems, eye tracking, and handheld 3D scanners.

Please contact Polhemus for additional information or visit www.polhemus.com.

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