Optical Tracking Education Guide

Northern Digital Inc. (NDI)

June 2016P/N 8300349 Rev 001

www.ndigital.com

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Table of Contents

1. Introduction ..........................................................................................................................................................2

2. Primary Usage .....................................................................................................................................................2

3. Optical Tracking Explained ..............................................................................................................................3

3.1 Linear and Planar Sensors ..........................................................................................................................4

4. Equipment Calibration ....................................................................................................................................4

4.1 Advantages of Factory Calibration ...........................................................................................................5

5. Measurement Volume .......................................................................................................................................5

5.1 Volume Size by Application ........................................................................................................................6

6. Measurement Environment ...........................................................................................................................7

6.1 Flexible Setup ...................................................................................................................................................8

7. Marker Identification .........................................................................................................................................8

8. Measurement Accuracy ....................................................................................................................................9

9. Measurement Resolution .............................................................................................................................. 11

9.1 The Effects of Pixel Size and Pitch on Resolution ............................................................................ 11

10. Measurement Data Quality ...................................................................................................................... 12

11. Data Access and Control ........................................................................................................................... 13

12. Third-Party Integration .............................................................................................................................. 14

13. Data Analysis Software .............................................................................................................................. 15

14. Technology Comparison ............................................................................................................................ 15

15. Conclusion...................................................................................................................................................... 16

1. IntroductionSelecting a motion tracking system is no easy task; there are many tracking technologies to choose from, each with different tracking methods and outcomes. It’s like purchasing a vehicle. You must first choose between a car and a truck, before you can narrow it down to one model. And that’s not even considering the various trim/accessory options.

Just like you rely on a car for safe and reliable transportation, you must be able to rely on your mo-tion tracking system to provide you with accurate and repeatable measurement data. As the motion tracking system will be the technical foundation of your kinematic experiments, potentially for years to come, its advantages and limitations should be weighed carefully. The very performance of the equipment could alter measurement results, putting your experiment’s credibility at risk. And re-quirements that seem unnecessary today may become important for a different experiment several years down the road. Think of the motion tracking system as an investment towards future publica-tion of your research.

This guide focuses on motion tracking systems based on optical tracking technology. Do optical motion tracking systems meet all tracking requirements for all experiments? No. But no technology does. This guide explains the core concepts and functions of optical tracking technology, specifically how it pertains to the Optotrak Certus®, and why optical tracking is the best choice for large-vol-ume tracking applications that require exceptional measurement accuracy.

2. Primary UsageThe Optotrak Certus is an optical motion tracking system, the heart of which is an active optical tracker that provides real-time 3D and 6DOF (six degrees of freedom) tracking. The Optotrak Certus is designed for motion tracking applications that require the highest temporal and spatial measure-ment accuracy: scientific research in academia and bioengineering.

Many of the optical motion tracking systems on the market are designed primarily for animation or ‘mocap’ use in the filmmaking and entertainment industries. As a consumer, how do you know if your (future) optical motion tracking system is intended—and performs better—for research or ani-mation applications? Consider the following factors:

• Measurement Accuracy: How accurate is the measurement data? How is accuracy achieved?

• Measurement Resolution: What is the data resolution? How is resolution determined?

• Measurement Environment: Is a special dark room required? How easy is the system setup?

• Data Access: Can you access the raw measurement data? Are black-box calculations used?

• Equipment Calibration: How is the equipment calibrated? How does that affect data results?

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Each of the above factors is explained throughout this guide, as are other topics that outline the difference between optical motion tracking systems that are research-grade, and those that are not.

3. Optical Tracking ExplainedOptical Tracking Systems use light as their means of triangulating the position of a point of interest in 3D space. There are two main types of optical tracking Systems: Passive and Active.

Passive optical tracking systems emit infrared (IR) light to illuminate the tracking area; this light is reflected back to the camera by retro-reflective markers attached to the object of interest. Active optical tracking systems use LED markers that emit their own light, rather than reflecting light emit-ted by the camera.

Active optical tracking systems (or Active Marker Systems as they’re also known) will emit light in different ways: some systems (like the Optotrak Certus) will turn on the LEDs individually in se-quence, while other systems will activate the LEDs all at once. The reasons for these different illumi-nation patterns (firing sequences) are described in Section 7: Marker Identification.

In both active and passive optical tracking systems, the light is intercepted at multiple locations by the cameras. Based on the location of the cameras relative to each other and the path of the light, the X, Y and Z coordinates of the markers are triangulated, which allows the 3D position of the ob-ject of interest (or subject) to be calculated.

This triangulation can be visualized as a crossing of planes or lines; see Figure 1: Triangulation in 3D.

Figure 1: Triangulation in 3D (3 Planes)

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Each Optotrak Certus has three cameras (optical lenses) that detect infrared light emissions to triangulate marker positions at the hardware level. Compare this to a single-lens motion tracking system in which multiple cameras must be used to triangulate markers from 2D images.

Where one Optotrak Certus can triangulate all visible marker positions within a 35m3 measurement volume, a single-lens system is physically incapable of doing so. Even when multiple cameras are used, the single-lens system must still rely on its software and predefined assumptions to calculate the marker positions. Therefore the accuracy and precision of such systems are largely dependent on how well the software can identify the marker centroid from the 2D image.

3.1 Linear and Planar SensorsThe performance of an optical tracking system also depends on whether planar or linear sensors are used within the camera. Passive optical tracking systems typically have sensors that consist of a planar (square) array of pixels, much like a digital camera. A bigger array (i.e. more pixels) allows for a bigger image area to be captured in one frame for a given optical tracking system.

However, it takes longer to scan all the pixels in a large array, and consequently, the frame rate will be slower. For a given pixel array, the frame rate can be increased by ‘cropping’ the image, that is, scanning only a subset of pixels. But this will reduce the capture (measurement) volume and image resolution.

The Optotrak Certus uses linear sensors that consist of a single row of pixels. Because there is only one row of pixels, scanning is very fast, as is the frame rate. The image does not need to be cropped, which ensures measurement volume and measurement resolution are not compromised.

4. Equipment CalibrationThe performance of an optical tracking system is subject to the quality of its calibration. Calibration is the process of setting the camera’s internal and external parameters, where internal parameters specify how light enters the camera (i.e. the camera model) and external parameters specify the spatial relationship between cameras. The outcomes of calibration are determined by A) how well the camera model matches the actual camera, and B) how well parameter values are estimated.

A key component of calibration is mapping the camera’s outputs to all known inputs (3D positions) within the measurement volume. Optical tracking systems (cameras) can be calibrated by the user (in-field calibration) or during the manufacturing process (factory calibration).

User calibration is carried out with a wand of known dimensions, the calibration data of which is mapped to the camera’s parameter values. The user waves this wand to establish the relative 3D reference for each camera, and establish where each camera is relative to one another, and their respective position within the 3D space. From this information, the camera’s software sets corre-sponding target locations within the measurement volume.

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4.1 Advantages of Factory CalibrationHowever, the wand calibration method does not produce an absolute 3D reference frame. As such, scale errors cannot be completely accounted or compensated for. User calibration is also prone to variability and errors, with its results dependent on the time, thoroughness, and experience of the user completing the calibration. Consequently, camera accuracy will vary from day to day; from user to user; and from one environment to the next. This has a significant impact on the repeat-ability and reproducibility of the measurement data.

The Optotrak Certus is factory calibrated: the parameter values needed for calibration are built into the system itself. The Optotrak Certus has a large measurement volume of 35m3, which means there are thousands of 3D positions (target locations) for it to capture. All of these known 3D posi-tions are programmed into the system, and provide the basis of the camera’s internal and external parameter values. This form of calibration is considered permanent, ensuring accurate and reliable measurement performance day after day.

The NDI calibration facility fully encompasses the entire Optotrak Certus measurement volume, and system accuracy is validated using a coordinate measurement machine (CMM). NDI provides calibration certificates for all of its optical tracking systems, including the Optotrak Certus. All NDI systems are considered certified, valid measurement systems, the output of which can be traced back to standards set by the National Institute of Standards and Technology (i.e. NIST traceable).

5. Measurement VolumeThe Optotrak Certus has a measurement (tracking) volume of 35m3 per single optical tracker. This measurement volume is displayed in Figure 2: Optotrak Certus Measurement Volume.

Figure 2: Optotrak Certus Measurement Volume

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The range over which a marker can be seen by a motion tracking system depends on how much en-ergy (light) emitted by the marker reaches the camera sensor per unit of time (power). If this power falls below the sensor threshold, the marker cannot be detected.

Because passive (retro-reflective) markers must reflect light from a source proximal to the camera, the light actually has to travel twice as far as an active marker, which emits its own light. This ‘shorter travel distance’ is why an active optical tracking system can track further than passive cameras.

There is also the intensity and dispersion of the light to consider – only a fraction of the light hits the passive marker, not all of which is reflected back to the camera sensor. The consistency of light will therefore vary by passive marker, and the detection of each marker will vary as a result. By emitting its own light, the detection of each active marker remains constant and uniform within the measure-ment volume.

5.1 Volume Size by ApplicationThe measurement volume for the Optotrak Certus system can be scaled to meet different tracking requirements. A single Optotrak Certus optical tracker is optimal for Neuroscience and Orthopaedics research applications that need to capture fast, imperceptible movements within a small tracking volume: Neuromechanics, Motor Control, In-Vitro Joint, Joint Elasticity Modeling, and more.

To track localized kinematic movement from multiple angles, using two or more Optotrak Certus optical trackers is recommended to eliminate any potential gaps in the measurement volume. This is will provide a medium-sized tracking volume that is well suited to research applications such as Reaching and Grasping, Posture/Balance, and Prosthetics studies amongst others.

Large-volume tracking applications in Biomechanics and Sports Science such as Gait Analysis require three or more Optotrak Certus optical trackers to capture full-body motion through all the planes and axes of human movement.

6. Measurement EnvironmentThe Optotrak Certus is an active optical tracking system, whereby its markers emit their own light for tracking purposes. This is in contrast to a passive optical tracking system, in which light emitted by the camera is reflected back by retro-reflective markers to determine their respective 3D positions. One of the drawbacks of passive optical tracking is stray reflections – light from sources other than the markers is reflected to the camera and mistakenly included in the tracking data.

To avoid stray reflections, a passive optical tracking system must be set up inside a specially con-trolled room that has no windows and/or is painted entirely black and uses thick light-blocking cur-tains. An example of this type of setup is shown in Figure 3: Motion Capture Dark Room.

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Figure 3: Motion Capture Dark Room

As a further constraint, in the case of motion tracking systems designed for animation use, the cameras are often permanently mounted to rigging (rails) along the ceiling of the room. This makes it very difficult to physically move the system to a different lab.

6.1 Flexible SetupThe Optotrak Certus can be used in any measurement environment without fear of stray reflections – no special preparation of the room is required. And because the Optotrak Certus is factory-cali-brated, there is no need to manually calibrate the optical trackers or tracking area with a wand; the measurement volume is pre-calibrated for immediate tracking. The system will maintain calibration even if the optical tracker is moved.

The Optotrak Certus can be fix-mounted or mounted on a portable stand. Multiple Optotrak Certus optical trackers can be daisy-chained together to enlarge the measurement volume, and arranged in almost any configuration to fit room dimensions and tracking requirements. The system can be easily set up and moved from room to room by just one person. And its optional heavy-duty, weather-resistant travel case allows for secure transport to remote locales.

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7. Marker Identification Marker Identification (or marker sorting as it’s also called) is the process of accurately and consis-tently identifying individual makers from one frame to the next, and from one camera to another (if multiple cameras are used). Problems such as Stray Reflections, Phantom Markers, and Crossed/Merged Markers complicate this process, and can hurt measurement results.

Stray Reflections occur when light from objects other than the retro-reflective markers is captured in the 2D image. Crossed/Merged Markers occur when two markers that are close together appear to have a shared centroid, giving the illusion of a single marker. And Phantom Markers are the result of triangulation errors by the passive optical tracking system; see Figure 4: Phantom Markers for an example.

Figure 4: Phantom Markers (real markers in white, phantom markers in grey)

These problems are most common with passive optical tracking systems. Retro-reflective markers are identical and inert by design; there is no way to differentiate between them. Furthermore, marker identification occurs only within the passive optical tracking software (not hardware), and only after the measurement data has been collected. At which point there is no mathematical solution to dif-ferentiate between phantom and real marker positions.

The Optotrak Certus controls each active marker at the hardware level. It identifies markers through a sequential firing order, whereby each marker is fired successively at the beginning of a new frame. This allows each marker to be automatically identified and individually tracked.

Marker data collected by the Optotrak Certus is inherently sorted – each maker is continuously rec-ognized as it’s tracked throughout the measurement volume for the duration of the measurement trial. This automatic marker identification greatly decreases post-processing time, and eliminates the potential for costly processing errors.

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8. Measurement AccuracyThe Optotrak Certus records 3D measurements to an accuracy of 0.1mm RMS. Using a target as an analogy, accuracy refers to trueness, how close measurements are to the center ‘bullseye’ (the true value), and precision, how close repeated measurements are to each other – see Figure 5: Accuracy and Precision Target.

Figure 5: Accuracy and Precision Target

With respect to 3D tracking, the marker represents the bullseye, which the Optotrak Certus mea-sures in time and space (temporal and spatial measurement, respectively) to an accuracy of 0.1 mm RMS from a tracking distance of 1.5 m to 7 m. For perspective, 0.1 mm is the average height of a single strand of human hair – it’s considered the smallest measurement detectable to the naked hu-man eye.

Accuracy is decreased by random and systematic errors; the former affects measurement precision, while the latter affects trueness. Excluding errors caused by users, random errors are the result of electronic noise or precision limitations of the measurement system, as well as inconsistent mea-surements (readings) by the system. Systematic errors are due to imperfect calibration of the mea-surement system.

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The high precision of the Optotrak Certus reduces the occurrence of random errors, and by exten-sion, any variability (deviation) of readings; i.e. measurements are consistent and repeatable. Addi-tionally, as the Optotrak Certus is factory-calibrated, the occurrence of systematic errors is reduced – the correct measurement parameters are already known and remain constant, as such, the true value is the same for each collected measurement.

The stated Optotrak Certus accuracy is based on a single marker stepped through more than 1,000 positions throughout the measurement volume using the mean of 20 samples at each position at 20°C. This accuracy is validated for every Optotrak Certus optical tracker during the factory calibra-tion process.

9. Measurement ResolutionThe Optotrak Certus has a measurement resolution of 0.01 mm. In scientific terms, resolution is the ability to resolve the smallest increment/decrement difference between position values; i.e. the fin-est detail to which a measurement can be taken and read. Because resolution represents the ‘best case’ or upper limit of measurement trueness and precision, the accuracy of any system cannot exceed its resolution, and is generally a higher value.

Due to the different types of optical sensors used in optical tracking systems, some motion tracking systems report resolution in megapixels. In such systems, motion is captured as a 2D image, not 3D data points. As with a digital photo, a megapixel represents one million pixels or dots of informa-tion that comprise the image.

However, a passive optical tracking system with a high megapixel count does not necessarily pro-duce the best measurement accuracy or resolution. While more megapixels do provide more detail about the 2D image, it is still difficult for the passive camera sensor to distinguish between mark-ers. This is especially true of markers that pass close by one another – the point where the markers intersect could be captured as a single centroid. Think also of the Rayleigh Criterion, where due to diffraction of light, two distinct markers cannot be resolved and appear as one.

9.1 The Effects of Pixel Size and Pitch on ResolutionThe number of megapixels is just one factor that determines 2D image quality – the pixel pitch (the distance between pixels), and the size of each pixel, are another. If the distance is too large, indi-vidual markers cannot be resolved and will be lost. See Figure 6: Pixel Resolution, which illustrates how resolution is affected by the pixel pitch and the marker’s distance from the camera.

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Figure 6: Pixel Resolution (low resolution on the left; high on the right)

In any event, it would fall to the passive system software to determine the marker centroid, and in turn, the 3D position. As discussed in Section 11: Data Access and Control, processing the data could involve averaging, filtering or other black-box data manipulation techniques to falsely in-crease measurement accuracy.

The Optotrak Certus is able to differentiate each 3D data point, and resolve any changes in posi-tions to the smallest degree, at the hardware level. It does not rely on nor use averaging, filtering, templating or oversampling to increase data accuracy, precision, or resolution.

10. Measurement Data QualityThe Optotrak Certus has a marker frequency of 4600 Hz and a frame rate of 4600/(n+1.3), where n is the number of markers. This allows for the most subtle of movements to be captured with minimal lag. The sampling speed of the Optotrak Certus, combined with its exceptional equipment accuracy and calibration, enables a continuous signal of interest to be captured with minimal noise.

High accuracy is especially important to minimize RMS errors when reconstructing a time-varying signal from collected data points. The more information about the signal that is captured in the data points, the better the reconstruction. To increase the amount of available signal information, less accurate motion tracking systems will sample data at higher frequency rates (over-sampling), and then average that data to increase measurement accuracy.

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However, it’s not enough to rely on high sampling speeds alone; noise must also be taken into consideration. Systematic noise affects measurement trueness, while random noise compromises precision. Motion tracking systems that have a low sampling rate are prone to reconstruction errors due to noise. An example is shown in Figure 7: RMS Error as a Function of Noise.

Figure 7: RMS Error as a Function of Noise (highest sampling rate on the bottom; lowest sampling rate on the top)

11. Data Access and ControlTraditional motion capture systems are designed for animation applications, only recording mark-ers as 2D images. These 2D images are then processed via a black-box algorithm to calculate the markers’ 3D positions. However, users have no way of knowing if templating, oversampling, filter-ing, averaging or other forms of data interpolation/assumptions were used when calculating the 3D positions. These assumptions could introduce unwelcome bias to the measurement data, or worse, errors.

This is illustrated in Figure 8: Acceleration and Position Profiles. Using position measurements you can derive velocity and acceleration profiles. If the position data is measured inaccurately, or pro-cessed in any way, errors in the position signal will be magnified, and the derivatives could look very different than the true velocity or acceleration profile. As shown, two very similar position trajectories (blue, green lines) could be associated with two very different acceleration profiles.

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Figure 8: Acceleration and Position Profiles

12. Third-Party IntegrationThe ability to integrate with a variety of third-party hardware and software, and the ease of the integration, also separates research-grade and animation motion tracking systems. An experiment shouldn’t be limited by the source and/or parameters by which data is collected, integrated, and interpreted. The Optotrak Data Acquisition Unit (ODAU) is an optional 16-bit, 16 single-ended input channels unit that allows for the collection and synchronization of analog and digital signals from third-party hardware such as:

• EMG Sensors

• Strain Gauges

• Testing Frames

• Robot Actuators

• Force Plates

• Pressure Transducers

• And more

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In addition, the Optotrak Certus accepts and emits a trigger and clock signal to control events, and integrates seamlessly with tools that provide live feedback to the subject during experiments.

The Optotrak Certus also supports full integration with third-party software packages (e.g. MATLAB, LabVIEW) or custom code, which is achieved via TCP/IP streaming, and managed by the NDI First Principles™ software. Users can create customized software setups and experiment calculations. The recorded data is exported in ASCII or C3D format. And using the optional Optotrak API, users can ‘talk’ directly to the system.

13. Data Analysis SoftwareIn regard to data analysis, the Optotrak Certus system comes with the NDI First Principles™ software to collect, manage, and present 3D and 6DOF measurement data in real time, or through post-hock analysis. The system also includes the NDI 6D Architect™ software for developing 6DOF rigid bodies.

Users can also build and qualify digitizing probes for designating virtual markers or for general digitization and coordinate frame alignments. Marker data, rigid body data, and data integrated from third-party hardware and software is displayed in an intuitive graphical and text interface.

Unlike the software of some motion tracking systems, the NDI software platforms are ‘open’ – users have full access to the raw data, and complete control over how the data is manipulated, analyzed and presented. Users can create their own utilities and custom calculations, and define experiment parameters such as joint axes/alignment. They are not constrained by predefined or fixed software settings and assumptions.

14. Technology ComparisonOptical tracking technology is known for its sub-millimeter measurement accuracy and resolution, which occurs wirelessly over a large measurement volume. However, no one technology can fulfill all tracking requirements for all applications; every tracking technology has its distinct advantages and limitations. While other tracking technologies do exist, they do not provide accuracy or mea-surement volume comparable to active optical tracking:

• Electromagnetic (EM) Tracking: uses field generators in combination with electromagnetic sensors to create a measurement volume and track the position and orientation of the sensor in 3D space. While EM tracking does not require a line of sight between system components, the sensor must be wired to a power source. Update rate is anywhere from 40-225 Hz. The measurement volume is much smaller, and the tracking environment must be free of ferrous or conductive metals.

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• Inertial Tracking: uses accelerometers (motion sensors) and gyroscopes (rotation sensors) to calculate the position and orientation of a moving object within a confined measurement volume. Inertial sys-tems are prone to drift, whereby errors in acceleration and velocity cause errors in position data. Data is usually reported as the difference between RMSE (Root Mean Square Errors) position and orienta-tion values. Inertial sensors can be large and cumbersome, potentially interfering with the subject’s natural movement.

That being said, maintaining a clear line of sight between the camera and markers is a limitation of optical tracking. However, overcoming that limitation is not insurmountable. By using two or more optical trackers, users can track the subject from multiple angles. Even if one optical tracker is oc-cluded, the other will still be able to ‘see’ the subject. Moreover, by using digitized points, virtual markers can be tracked in areas that are hard to see or are susceptible to occlusions. This has the added benefit of eliminating potential gaps in the measurement volume, which is especially impor-tant in large-volume tracking applications such as biomechanics studies.

15. ConclusionOptical motion tracking systems, such as the Optotrak Certus, provide the best measurement ac-curacy over the largest measurement volume of any comparable tracking technology. However, in order to meet the stringent standards of scientific measurement, and be labelled as research-grade, the motion tracking system should:

• Provide active tracking over a large measurement volume;

• use linear sensors to achieve fast frame rates;

• be factory calibrated;

• automatically track and sort markers at the hardware level;

• deliver highly accurate and repeatable measurement results;

• and produce data without the use of black-box algorithms.

Although the other factors described in this guide are just as important in differentiating between motion tracking systems intended for research and animation applications, these are the core un-changing attributes. The measurement environment, equipment setup, even integration with third-party hardware and software can be modified by the user to improve system performance.

But if the core functionality of the optical motion tracking system isn’t designed first and foremost for scientific use, then measurement accuracy and reliability is usually sacrificed to some extent. In which case, that motion tracking system would be better suited to animation applications only.

Copyright 2016 Northern Digital Inc. All rights reserved. Due to continuous product improvement, specifications are subject to change without notice. NDI, OPTOTRAK, CERTUS are registered trade-

marks of Northern Digital Inc. NDI FIRST PRINCIPLES, NDI 6D ARCHITECT are trademarks of Northern Digital Inc. The Optotrak Certus is a general metrology instrument. Use in a particular application

must be determined by the user. All weights and dimensions are approximate. NDI products and their application or use may be covered by Northern Digital Inc. patents. Other patents pending.

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