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<tr><td class="contentnopad">This paper describes a testing methodology and resultant set of four variables that can be used to quickly and easily document the correct installation, configuration, and combined working status of force platform (FP) and three-dimensional (3D) motion capture components of a clinical movement analysis (CMA) laboratory. Using a rigid, rod-shaped testing device, CMA laboratory data are collected simultaneously from the FP and motion capture components (typically, video-based kinematic measurements) as the device is manually loaded while being pivoted broadly about a point on the FP. Using a computational method based on static equilibrium, it is possible to independently measure the rod's orientation and tip position during the moving trial, using FP derived data exclusively, and to compare these estimates to rod orientation and tip position estimates derived exclusively from the motion capture component. The motion laboratory accreditation test (MLAT) variables include: the difference (angle) between the orientation of the long axis of the testing device as independently determined from kinematic measures (motion capture component) and the FP derived data; and the difference (x, y, z) between the center of pressure position (FP derived) and the position of the testing device tip (motion capture derived) that loads the FP. A numerical dynamics model was explored to evaluate the appropriateness of the static equilibrium-based FP data model and to determine guidelines for testing device movement frequency and FP loading. The MLAT technique provides a simple means of detecting the combined presence of errors from many sources, several of which are explored in this paper. The MLAT has been developed to help meet one criteria of the CMA laboratory accreditation process, and to serve as a routine quality assessment tool.</td></tr> | <tr><td class="contentnopad">This paper describes a testing methodology and resultant set of four variables that can be used to quickly and easily document the correct installation, configuration, and combined working status of force platform (FP) and three-dimensional (3D) motion capture components of a clinical movement analysis (CMA) laboratory. Using a rigid, rod-shaped testing device, CMA laboratory data are collected simultaneously from the FP and motion capture components (typically, video-based kinematic measurements) as the device is manually loaded while being pivoted broadly about a point on the FP. Using a computational method based on static equilibrium, it is possible to independently measure the rod's orientation and tip position during the moving trial, using FP derived data exclusively, and to compare these estimates to rod orientation and tip position estimates derived exclusively from the motion capture component. The motion laboratory accreditation test (MLAT) variables include: the difference (angle) between the orientation of the long axis of the testing device as independently determined from kinematic measures (motion capture component) and the FP derived data; and the difference (x, y, z) between the center of pressure position (FP derived) and the position of the testing device tip (motion capture derived) that loads the FP. A numerical dynamics model was explored to evaluate the appropriateness of the static equilibrium-based FP data model and to determine guidelines for testing device movement frequency and FP loading. The MLAT technique provides a simple means of detecting the combined presence of errors from many sources, several of which are explored in this paper. The MLAT has been developed to help meet one criteria of the CMA laboratory accreditation process, and to serve as a routine quality assessment tool.</td></tr> | ||
<tr><td class="contentnopad">[[ | <tr><td class="contentnopad">[[https://www.c-motion.com/download/CalTester/CalTesterArticle2003.pdf CalTesterReference]]<!--./files/CalTesterArticle2003.pdf --></td></tr> | ||
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<tr><td class="contentnopad">We increased the accuracy of an instrumented treadmill’s measurement of center of pressure and force data by calibrating in situ and optimizing the transformation between the motion capture and treadmill force plate coordinate systems. We calibrated the device in situ by applying known vertical and shear loads at known locations across the tread surface and calculating a 6 × 6 calibration matrix for the 6 output forces and moments. To optimize the transformation, we first estimated the transformation based on a locating jig and then measured center-of-pressure error across the treadmill force plate using the CalTester tool. We input these data into an optimization scheme to find the transformation between the motion capture and treadmill force plate coordinate systems that minimized the error in the center-of-pressure measurements derived from force plate and motion capture sources. When the calibration and transformation optimizations were made, the average measured error in the center of pressure was reduced to approximately 1 mm when the treadmill was stationary and to less than 3 mm when moving. Using bilateral gait data, we show the importance of calibrating these devices in situ and performing transformation optimizations.</td></tr> | <tr><td class="contentnopad">We increased the accuracy of an instrumented treadmill’s measurement of center of pressure and force data by calibrating in situ and optimizing the transformation between the motion capture and treadmill force plate coordinate systems. We calibrated the device in situ by applying known vertical and shear loads at known locations across the tread surface and calculating a 6 × 6 calibration matrix for the 6 output forces and moments. To optimize the transformation, we first estimated the transformation based on a locating jig and then measured center-of-pressure error across the treadmill force plate using the CalTester tool. We input these data into an optimization scheme to find the transformation between the motion capture and treadmill force plate coordinate systems that minimized the error in the center-of-pressure measurements derived from force plate and motion capture sources. When the calibration and transformation optimizations were made, the average measured error in the center of pressure was reduced to approximately 1 mm when the treadmill was stationary and to less than 3 mm when moving. Using bilateral gait data, we show the importance of calibrating these devices in situ and performing transformation optimizations.</td></tr> | ||
<tr><td class="contentnopad">[[ | <tr><td class="contentnopad">[[https://www.c-motion.com/download/CalTester/FP_in_situ_calibration2009.pdf FPLoc Reference]]</td></tr> | ||
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The [[MTD2|MTD-2 rod]] is a rigid, machined rod with a conical (pointed) tip at each end is used together with a handle and a test plate, each with machined conical depressions. Five tracking targets are attached to the testing rod using rigid posts. Data are sampled simultaneously from the force platform (FP) and the cameras, as forces are applied through the rod to the force platform. The Mechanical Testing Device (MTD-2) is manufactured and supported by [http://www.motion-labs.com/ Motion Lab Systems, Inc.] The MTD-2 is a precision-machined calibration-testing tool that can be assembled in less than a minute to create a calibration-testing object suitable for a number of 3D biomechanics laboratory tests. (NOTE: There is an MTD-3 rod that supports adding a load cell on it, and the rods are the same size and work equally well. The MTD-3 may replace the MTD-2.) | The [[MTD2|MTD-2 rod]] is a rigid, machined rod with a conical (pointed) tip at each end is used together with a handle and a test plate, each with machined conical depressions. Five tracking targets are attached to the testing rod using rigid posts. Data are sampled simultaneously from the force platform (FP) and the cameras, as forces are applied through the rod to the force platform. The Mechanical Testing Device (MTD-2) is manufactured and supported by [http://www.motion-labs.com/ Motion Lab Systems, Inc.] The MTD-2 is a precision-machined calibration-testing tool that can be assembled in less than a minute to create a calibration-testing object suitable for a number of 3D biomechanics laboratory tests. (NOTE: There is an MTD-3 rod that supports adding a load cell on it, and the rods are the same size and work equally well. The MTD-3 may replace the MTD-2.) | ||
The following graphics are from the [ | The following graphics are from the [https://www.c-motion.com/download/CalTester/CalTesterArticle2003.pdf CalTester Paper] Holden JP, Selbie WS, Stanhope SJ, "A proposed test to support the clinical movement analysis laboratory". | ||
[[Image:Fig1CalTesterArticle.png]] | [[Image:Fig1CalTesterArticle.png]] | ||
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Estimate the errors between the force platform recordings and the Motion Capture System. This simple test takes only a few minutes, could be performed prior to any data collection, and provides reassurance that your data collection is sound. | Estimate the errors between the force platform recordings and the Motion Capture System. This simple test takes only a few minutes, could be performed prior to any data collection, and provides reassurance that your data collection is sound. | ||
The following calculations and explanations are from the [ | The following calculations and explanations are from the [https://www.c-motion.com/download/CalTester/CalTesterArticle2003.pdf CalTester Paper] Holden JP, Selbie WS, Stanhope SJ, "A proposed test to support the clinical movement analysis laboratory".<br> | ||
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<b>Section I : Calculating the rod orientation variable | <b>Section I : Calculating the rod orientation variable |
Revision as of 15:41, 22 August 2022
Language: | English • français • italiano • português • español |
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Using CalTester
The functionality is now available in Visual3D when a CalTester license key is provided. There is also a stand-alone version of Visual3D that ONLY supports the CalTester, and these options now replace the older software. Users of the old software should have access to the new software automatically when they log into our web downloads page.
UPDATE: The standalone CalTester-Plus software application has been discontinued and replaced with a newer CalTester stand-alone application. The new application is simply the CalTester tab in Visual3D all by itself.
As a reminder, always go to the CalTester tab first, before opening any CalTester related data files.
CalTester Background
Accurate and reliable kinematics and kinetics data are essential to the appropriate application of movement analysis data for clinical and research purposes. Proper laboratory calibration includes the accurate determination of the positions of the force platform(s) and cameras in the laboratory coordinate system, as well as correct setting of force platform parameters. Any errors in the parameter settings or calibration measurements will lead to incorrect values of kinetic calculations that rely on the force data.
CalTester is an essential tool for laboratories that:
- Have instrumented Treadmills
- Have instrumented Stairs
- Move cameras and/or Move force platforms regularly
- Have amplifier switches that can be easily bumped, or are regularly changed
- Have settings that are regularly changed
- Have students or unsupervised visitors in the laboratory
- Are required to have regulatory oversight of their laboratory
Functionality is based on recording the position and orientation of a standard commercially available precision mechanical testing device MTD-2 CalTester Rod via the motion capture system.
Implementation of the CalTester functionality was based on the following articles. | ||||||||
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The design of the MTD2 device allows a force to be applied to the surface of the force platform without any applied moment. |
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The MTD-2 rod is a rigid, machined rod with a conical (pointed) tip at each end is used together with a handle and a test plate, each with machined conical depressions. Five tracking targets are attached to the testing rod using rigid posts. Data are sampled simultaneously from the force platform (FP) and the cameras, as forces are applied through the rod to the force platform. The Mechanical Testing Device (MTD-2) is manufactured and supported by Motion Lab Systems, Inc. The MTD-2 is a precision-machined calibration-testing tool that can be assembled in less than a minute to create a calibration-testing object suitable for a number of 3D biomechanics laboratory tests. (NOTE: There is an MTD-3 rod that supports adding a load cell on it, and the rods are the same size and work equally well. The MTD-3 may replace the MTD-2.) The following graphics are from the CalTester Paper Holden JP, Selbie WS, Stanhope SJ, "A proposed test to support the clinical movement analysis laboratory". |
Within the CalTester mode there are two classes of functionality:
Estimate errors in the Center of Pressure and Orientation of the Force Vector. |
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Estimate the errors between the force platform recordings and the Motion Capture System. This simple test takes only a few minutes, could be performed prior to any data collection, and provides reassurance that your data collection is sound. The following calculations and explanations are from the CalTester Paper Holden JP, Selbie WS, Stanhope SJ, "A proposed test to support the clinical movement analysis laboratory". Free-body diagram of testing device: Fp, ground reaction force; Fg, gravitational force (weight); Fa, applied force; r, position vector between tips (p to a) of testing device rod: The rod orientation variable (
Under 2D dynamic conditions, the following holds:
CalTesterPlus does not calculate |
Estimate the position and orientation of a force platform, instrumented treadmill, or instrumented stair that minimizes these errors.. |
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Force Platform signals are computed in compliance with [the C3D File Format]
About Force Platform Parameters. |
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Force Platforms, Instrumented Treadmills, Instrumented Stairs are examples of external force measuring devices. Each of these devices generates signals that are recorded by the Motion Capture System. These signals are used in conjunction with a set of parameters to compute a force signal comprising a Force Vector, a Center of Pressure, and a Free Moment applied to the platform. The set of parameters, and how they are used in the computations are unique to each manufacturer. The user should refer to the force platform documentation to identify the correct parameters. These parameters are typically stored in the c3d file alongside the signals. For calculations involving the interaction of an object/person in the motion capture volume and the force platform, it is necessary to establish the location of the force platform in the laboratory, so that the [can be transformed into the motion capture volume]. In most cases the errors identified in the CalTester report are a result of determining the position and orientation of the platform in the motion capture volume, and not in errors from the platform sensors directly. |
Experimental Data Collection
CalTesterPlus requires consistent data in order to make the correct calculations.
Assumptions About Data Being Used. |
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The program makes the following assumptions about the data that is being used:
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In order to use the CalTesterPlus program you need a set of properly collected data for each force platform you wish to calibrate.
Data Collection for Error Report. |
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In order to use the CalTesterPlus program you need a set of properly collected data for each force platform you wish to calibrate. To collect a useful motion trial for the report mode follow the following process.
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Data Collection for Force Platform Location. (FP Loc Tab) |
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To collect a useful static trial for CalTester follow the following process.
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CalTester Toolbar
Tool Bar![]() |
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Error Report
Creating the CalTester Error Report |
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The report capabilities of the CalTester tab creates an error report based on collection trials. 1. Add C3D files In order to open a C3d file for use in the CalTester tab you must first navigate to the CalTester tab and leave it active. The C3D files are then opened by clicking on the open button. 2. Check Report Once the files are loaded the right side of the CalTester tab will show the first page of the CalTester report. You can navigate between the three report pages by using the page buttons in the tool bar. 3. Print Report The report that has been generated on the right side of the screen can be saved as a pdf document by using the file drop down menu and selecting Print or by using the Ctrl + P short cut. 4. Understanding the Report A detailed explanation of how to understand the report that CalTester produces can be found here. Note: If there appears to be errors or discrepancies in the report or the 3D-view check that the markers have been identified correctly and that the rod dimensions are correct using the Modify CalTester Rod Dimensions button. |
Understanding Report Page 1 |
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The Laboratory Calibration Test Report includes the mean, standard deviation, and range (minimum and maximum) of the four report variables. These are provided on graphs and with numerical values. (1) The background on the file: the top of the report contains the summary of the c3d file being used. This includes the number of frames used, the minimum force settings (for CalTester and Visual3D), and the dates the c3d file was collected and when the report was created. (2) The report format: the second section of the report provides an overview of how the information on the rest of the page will be organized. (3) The Difference in Force Orientation Error: this is the angle (in degrees) between the applied force reaction vector (i.e., the ground reaction force minus the weight of the calibration-testing rod) and the orientation of the long axis of the rod, as determined from the target data. Note: The force orientation variable is calculated from the dot product of this predicted unit vector and the vector along the axis of the rod as determined with the motion capture system. (4) The differences in CoP (for x, y, and z): these are the components of the displacement vector between the CoP location measured by the force platform and the endpoint of the calibration-testing rod (adjusted for the specified vertical height above the force platform). Differences can be due to many different factors, including errors in the force platform configuration (analog scale factors, force platform origin specification), the force platform interface (channel connections and assignments), or the force platform alignment (corner locations, which are used to transform force platform data into the laboratory coordinate system). Differences due to the kinematic system are related to uncorrected camera non-linearities, poor 3D calibration, and target image distortions.
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Understanding Report Page 2 |
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The magnitude of the force vector and the error signals are displayed in the Graphs tab of the Report. The CalTester Center of Pressure error is measured and graphed for the X, Y and Z directions for each of the frames of data. The orientation of the rod with regards to the vertical is graphed as well as the error of the orientation. From this view the user can select the range of data to be processed. This will allow the user to eliminate anomalous data from the calibration calculations. To select the range, left click and drag the mouse on the Ground Reaction Force Magnitude graph. A green highlighted section will appear on the graph and all of the graphs will be limited to this range. To return to the original range, left click on the Ground Reaction Force Magnitude graph and the full range will return to all of the graphs. |
Understanding Report Page 3 |
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The third page of the report has six graphs. The first graph is the CalTester Segment Residual Graph. This graph shows the residual of the whole CalTester Segment during the trials that were used. The final five graphs show the residuals for the individual markers. For the sample shown below the markers that are used are: C1, C2, C5, C3, and C4. |
Estimating the Force Platform Location
Estimating the Force Platform Location Using a Jig
When using movable force platforms (ex. incline treadmill, etc.) you will want to use a jig to specify the force platform location. The jig is a cluster of targets which are fixed to the force platforms. First the jig must be defined by locating the force platform relative to the jig. Then the jig can be used to identify the location of the force platform.
When using the jig to identify the location of the force platform, there are two options. If you are using the jig template (Visual3Dv6.00.28 & older), you must follow the instructions for 2a. If you are using the Visual3D pipeline command (Visual3Dv6.00.29 & newer), you can follow the instructions for 2b.
(2b) Specifying the Force Platform Location using Platform_Corners_From_Jig Command |
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If you output the jig results as a pipeline command (available in Visual3Dv6.00.29 and newer), you can use the Platform_Corners_From_Jig Command to update the force platform corners.
The force platform corners will be updated based on the location of the jig. |
Tutorials
Videos to describe using CalTester can be found on YouTube:
If you prefer a course format, this information can also be viewed in the links below (same videos, different format with additional content):
Acknowledgement
C-Motion, Inc. acknowledges that the development of CalTester software was funded in part by an STTR grant (R43 HD37286) from the National Institute of Child Health and Human Development (NICHD). C-Motion also gratefully acknowledges assistance provided by the Physical Disabilities Branch in the Warren Grant Magnuson Clinical Center at the National Institutes of Health.