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This page provides instructions on integrating a Bertec Force plate system in Motive.
As of Motive 1.10, only free run configuration is supported for integrating Bertec force plate systems, and devices will need to run independently at its own acquisition rate. The Bertec SDK currently does not support external sync signals, and precise synchronization with the camera system is not supported yet.
The following setup does not support synchronization signals. Synchronization supported setups will be updated in the future.
Component Wiring (Free-Run)
In order to integrate force plate systems with Motive, you will need to setup the drivers and plugins for the force plates. For Motive 1.10 and above, Motive installer is packaged with OptiTrack Peripheral Module (OPM) which installed during the Motive installation process. The OPM includes all necessary drivers and plugins for integrating external devices including force plates (AMTI and Bertec). During the Motive installation process (1.10 and above), a list of program features will be shown in the Custom Setup section. Here, change the setting for the OptiTrack Peripherals Module, as shown in the below image, so that the module is installed along with Motive Files. For integrating only the digital force plates, you don't need to install NI-DAQmx drivers that comes up next in the installer.
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 recognized and 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 relative to the center of the volume, and when you re-calibrate or reset the ground plane, you will need to also realign the position of your force plates for best results.
3. Enter Force Plate Dimensions. Under the Devices pane, click on each of the Bertec force plates and input the force plate dimensions (width and length) as well as the XYZ offset distances for the actual transducers from the physical center. Refer to the Bertec product documentation for detailed information. After defining respective values, the dimensions of the corresponding force plate assets will be modified and reflected in the Perspective View pane. Confirm that the dimensions are changed appropriately. To ensure the dimensions are modified properly, you may also place extra retroreflective markers on each corner of the platform and monitor the coincidence of the markers position with the force plate assets from the perspective view.
4. 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.
5. 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 system 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.
6. 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 coordinate system within the global coordinate system. Motive uses manufacturer defined X, Y, and Z mechanical-to-electrical center offset when calculating the force vector and the center of pressure. For digital based plates, this information is available from the SDK and also stored in the plate's on-board calibration data.
7. Zero force plates. After you have calibrated each of your force plate, remove the CS-400 from the volume. Right click one of your force plates in Motive and click Zero (all). This sets 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.
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. When both reconstructed markers and force plate channels are selected, the force plot will be sub-sampled in order to be plotted along with trajectory data.
If you wish to confirm the force plate 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 device is selected, corresponding channels will be displayed at the bottom of the pane. When the channels are selected, respective signals will be graphed in the Timeline pane, as shown in the image below.
As stated in the disclaimer at the beginning of this article, Bertec force plates can only be integrated in Free Run configuration as of Motive 1.10, and precise synchronization with the mocap system is not be supported yet. In the Free Run mode, both systems will be triggered by Motive simultaneously, but they will run at their own individual sampling rate. Thus, the force plate data must be sampled at a rate whole multiple of the camera frame rate; otherwise, the recorded data will not be aligned. Under Devices pane, set the frame rate of the camera system to a whole divisor of the force plate sampling rate, and the respective multiplier value will be indicated next to the sampling rate. A sample configuration is shown in the image.
Note that the Free Run 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.