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When a motion capture system is used in conjunction with force plates, they work together as a powerful tool for various research applications including biomechanical analysis, clinical gait analysis, physiology research, sports performance research, and much more. An OptiTrack motion capture system can synchronize with force plates to obtain both kinematic and kinetic measurements. Force plate integration is supported only with a Prime camera system using the eSync synchronization hub. For Bertec force plate integration support, you must use Motive 1.10 or above. This page provides guidelines for setting up and configuring Bertec force plates — with digital outputs — along with the OptiTrack motion capture system.
For detailed information on specifications and configurations on the force plates, refer to the documentation provided by the force plate manufacturer.
Component Wiring (Free-Run)
In order to integrate force plate systems with Motive, you will need to setup the required drivers and plugins. Motive installer is packaged with the Peripheral Device module which can be installed during the Motive installation process. This module includes all necessary drivers and plugins for integrating external devices including force plates (AMTI and Bertec) in Motive. During the Motive installation, a list of program features will be shown in the Custom Setup section. Here, change the setting for the Peripheral Device module, as shown in the below image, so that the module is installed along with Motive Files.
Note : Even if you are not using NI-DAQ, it is still necessary to install NI-DAQmx drivers that come up next in the installer.
In addition to the Peripheral Device module, you also need to install the Digital Acquire™ from Bertec. Visit the below webpage to download the software, and follow the respective instructions to install. This software installs remaining resources for connecting the Bertec force plates.
Note: You cannot run Digital Acquire and Motive together and have them connected to the plate(s) at the same time.
1. Start Motive. If the hardware and software for the force plates are configured and recognized properly, Motive will display the detected force plates with number labels (1, 2, etc..). Motive will notify you of incorrect or nonexistent force plate calibration files. When the devices are properly instantiated in Motive, the following status messages will be shown under that Status Log.
2. Calibrate cameras. Calibrate the capture volume as normal to get the orientation of the cameras (see the Quick Start Guide or Calibration page for more information). The position of the force plate is about the center of the volume, and when you recalibrate or reset the ground plane, you will need to also realign the position of your force plates for best results.
3. Setup CS-400. On the CS-400 calibration square, pull the force plate alignment tabs out and put the force plate leveling jigs at the bottom. The leveling jigs align the calibration square to the surface of your force plate. The alignment tabs allow you to put the CS-400 flush against the sides of your force plate giving the most accurate alignment.
4. Place CS-400 on force plate. Place the calibration wand on the force plate so that the wand vertex is located at the right-hand corner of the side where the cable input is located (shown in the image below). A correct placement of the calibration square is important because it determines the orientation of the force plate and its local coordinate axis within the global system. The coordinate systems for force plates are independent of the system used Motive. Bertec force plates use the right-hand system. The longer arm of CS-400 will define the Y axis, and the shorter arm will define the X axis of the force plate. Accordingly, Z axis is directed downwards for measuring the vertical force.
5. Set force plate position in Motive. After placing the calibration square on the force plate, select the CS-400 markers in Motive. Right click on the force plate you want to locate, and click Set Position. When there are multiple force plates in a volume, you may need to step on the force plate to find which platform the calibration square is on. In Motive, uncalibrated force plates will light up in green and a force vector will appear when you step on the plate. Repeat step 4 and 5 for other force plates as necessary.
Referencing to the markers on the calibration square, Motive defines the location of the force plate axis within the global coordinate system. When setting the position, the previously defined force plate dimension values and mechanical-to-electrical center offset values will be applied and reflected in the Perspective View pane.
6. Zero force plates. After you have calibrated each of your force plates, remove the CS-400 from the volume. Right click one of your force plates in Motive and click Zero (all). This will tare the scale and set the current force on the plate data to 0. This will account for a small constant amount of measurement offset from the force plate. Remember that it zeros all of the force plates at once. So make sure there are no objects on the force plates.
Note: Zeroed scales of Bertec force plates are saved within their software driver, and each time the driver restarts, these settings are refreshed. This means that the force plate zero setting will be refreshed each time you start Motive, or each time the device is disabled and enabled back again in Motive. Please be aware of this behavior and zero your plates when necessary. In Motive, there is a Zero On Enable property setting for Bertec force plates under the Devices pane, and enabling this setting will automatically zero your plate each time the device is enabled or when Motive restarts. The Zero On Enable setting is enabled by default.
When synchronizing through the eSync, use the following steps for configuring the sync settings in Motive. This will allow both systems to be triggered simultaneously with reference to the master synchronization device. The Recording Gate signal must be sent out from the eSync and inputted into the Bertec force plate system for synchronization.
IMPORTANT NOTE: For this synchronization setup to work properly, the Bertec amplifier firmware must be updated to its most recent version (above June 2016 release). Contact Bertec for instructions on updating the firmware.
Before you start recording, you may want to validate that the camera and force plate data are in sync. There are some tests you can do to examine this.
The first method is to record dropping a retroreflective ball/marker onto the platform few times. The bouncing ball produces a sharp transition when it hits the surface of the platform, and it makes the data more obvious for validating the synchronization. Alternately, you can attach a marker on a tip of the foot and step on and off the force plate. Make sure that your toe — closest to the marker — strikes the platform first, otherwise the data will seem off even when it is not. You can then monitor the precise timing of the ball or the foot impacting the force plate and compare them between the mocap data and the force plate data. ↑
The following is an example of validating both good and bad synchronizations using these methods:
First of all, it is important to note that without a master synchronization device, recording for the camera system and the force plates will not be triggered precisely at the same time. Also, for longer takes the sampling timing of mocap data and the force plate data will eventually deviate from each other. If you want your system to be timed perfectly, we recommend synchronizing through the eSync 2.
If you are using an Ethernet system without an eSync, ignore the synchronization configuration options. From the Cameras Pane, make sure that that the force plate sampling rate is a multiple of the tracking frames per second (e.g. 100 Hz and 1000 Hz). Motive will alert you if the sampling rates of cameras and force plates disagree. When the force plate sampling rate is not set to an integer multiple of the camera frame rate, force plate data may record improperly and cause an error.
Also note that this mode will introduce a gradual drift between the two data sets, and the offset will be increasingly significant for longer recordings. To zero out the drift, the ReSynch feature can be used. Right-click on force plates from either the Devices pane or the perspective view, and select Resynch from the context menu to realign the sampling timing of both systems.
Note: Bertec digital force plates can collect data up to a maximum of 1000 Hz sampling rate.
To view and confirm the live force plate data, open the editor from the Timeline Pane. Then, open the Project Pane (or Cameras Pane) and select one of the force plates. A list of available channels will appear under each force plate instance. Here, you can confirm that your force plates are working properly. Select Fx, Fy, Fz, Mx, My, or Mz channels to view the live force plate data from the timeline. Multiple channels can be displayed at once. Both reconstructed markers and force plate channels can be selected to display two different plots on the timeline. In live sampling, the force plot will be sub-sampled when plotted along with trajectory data, but all of the recorded samples will be fully resolved in the playback mode.
If you wish to double check the force plate position calibration, you may create a long trackable rigid body and use it to apply force against force plate. If the force plate location is precisely calibrated in Motive, the force vector will go right through the rigid body.
When playing back a Take with force plate data, integrated devices will appear under the assets group in the Project pane. When a force plate is selected, available channels will be listed at the bottom of the pane. You can select data channels and respective signals will be graphed in the Timeline pane, as shown in the image below.