Bertec Force Plate Setup
Main Page → Bertec Force Plate Setup
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 many more. An OptiTrack motion capture system can synchronize with force plates to obtain both kinematic and kinetic measurements. Note that force plate integration is supported only with a Prime camera system using the eSync synchronization hub. This page provides quick guidelines for setting up and configuring 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.
- Version Requirement: Motive 1.10 or above.
- Supported Amplifier Models: AM6800
- For synchronization support, firmware on Bertec amplifiers must be updated to its most recent version (above June 2016 release).
- Prime series Ethernet camera system with the eSync synchronization hub for outputting Recording Gate signal.
- 1 Hardware Setup
- 2 Software Setup
- 3 Synchronization Configuration
- 4 Device Settings Profile
- 5 Force Plate Data in Motive
- 6 Data Export
Bertec Force Plate System Setup
- Refer to the respective Bertec system user documentation for detailed information on setting up the force plate system and connecting to the host PC.
Camera System Setup
- Set up an OptiTrack system. Connect the camera system to the same host PC. For more information, refer to the Quick Start Guide: Getting Started page or the Hardware Setup pages.
Wiring the eSync with the Amplifier
- Bertec AM 6800 amplifiers: Use the provided female 15-pin D-Sub connector to get the ZERO signal and the SYNC signal from the ANALOG OUTPUT port of the amplifier.
- The ZERO cable from the amplifier needs to be connected to the output port of the eSync for synchronization with the camera system.
- The SYNC cable from the amplifier needs to be interconnected between the force plate amplifiers for their internal sync.
Hot plugging is not supported with the integration. When a new device is connected to the system, you must re-start Motive to instantiate it.
Peripheral Device Module
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 added 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 may also want to install the Digital Acquire™ from Bertec to verify that the force plates are properly working. 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.
Force Plate Setup in Motive
1. Start Motive
- If the hardware and software for the force plates are configured and successfully recognized, Motive will list out 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 vertex of the wand is located at the right-hand corner of the side where the cable input is located (as 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.
Tip: To double check that the dimensions are modified properly, you can 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.
6. Set the Force Plate Dimension
- The next step is to set the force plate dimensions.
- On some force plate models, these values may get imported automatically, but if not, they will need to be manually inputted by the user. Once the force plate positions have been calibrated, enter the dimensions of the force plate in the Devices pane. Select each force plate and under its force plate properties, enter the length and width (in inches) values for the corresponding plates as reported in the specifications.
7. 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 any of 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.
8. Set Sampling Rate
- It is best to configure the sampling rate after the sync configurations have been set up which will be covered in the following section. Sampling rate of force plates is configured through the devices pane, by modifying the framerate-multiplier (e.g. 10.0x) next to the sampling rate. Configuring the multiplier ensures that sampling rate of the force plate is always a whole multiple of the camera framerate. Note that Bertec force plates support the 1000 Hz sampling rate only.
For synchronizing both the camera system and the force plate system through the eSync, follow the below steps to configure appropriate sync settings in Motive. This will allow both systems to be simultaneously triggered with reference to the master clock on the eSync.
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.
Sync Configuration Steps: eSync & Bertec Force Plate
- Basically, the Recording Gate signal must be outputted from the eSync and inputted into the Bertec force plate system for synchronization.
- Open the Devices pane and the Synchronization pane.
- In the Synchronization pane, select the Custom Synchronization mode from the dropdown menu.
- Set the Sync Input → Source to Internal Clock.
- Set the Sync Input → Clock Freq to 1000 Hz, and set the Sync Input → Input divider to 10.
- Set the Sync Output 1-4 → Type to Recording Gate triggered sync.
- Click Apply from the Synchronization pane.
- In the Devices pane, select the force plates and double check that the Record Trigger is set to Device.
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 good synchronization outcomes:
Above configurations set the sync chain to be in Free Run Mode. This means that the recording of both the mocap system and the force plate system are triggered simultaneously at the same time and each system runs at its own rate.
Two systems, however, are synchronized at the trigger point but not at every frame. For this reason, alignment of the mocap data and the force plate data may gradually drift from each other for longer captures. But this is not a problem since the sync chain will always be re-synchronized each time recording in Motive is triggered. Furthermore, Takes in general do not last too long for this drift to take effect on the data.
However, this could be an issue when live-streaming the data since recording is never initiated and two systems will be synchronized only when Motive first launches. 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.
Ethernet System Force Plate without eSync Configuration
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 exactly at the same time. 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 Devices 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.
Note: Bertec digital force plates can collect data up to a maximum of 1000 Hz sampling rate.
Device Settings Profile
Starting from Motive 2.0, configurations on external devices (e.g. Force Plates and NI-DAQ devices) are persisted between different launches of Motive.
All of the configured device settings, including the calibration, get saved on Device Profile XML files. When you exit out of Motive, updated device profiles will be saved under the program data directory (
C:\ProgramData\OptiTrack\Motive\DeviceProfiles), and this file gets loaded again when you restart Motive. The persistent settings folder can be accessed through Help → Application Folders → Persistent Setting. This XML file ensures that all of the device settings are persisted each time you close and restart Motive.
Force Plate Data in Motive
Force plate data can be monitored from the Graph View pane. You will need to configure a custom graph layouts to show force plate data. As shown in the images, make sure the desired force plate data channels (Fx, Fy, Fz, Mx, My, or Mz) are selected to be plotted. Then, when you select a force plate in Motive, and the data from the corresponding channels will be plotted on the graphs. 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. For more information about how to configure graph layouts, read through the Graph View pane page.
Live Force Plate Data
- For plotting live force plate data, the graph View Style of the corresponding graph must be configured to Live.
We recommend the following programs for analyzing exported data in biomechanics applications:
- Motive exports tracking data and force plate data into C3D files. Exported C3D files can then be imported into a biomechanics analysis and visualization software for further processing. See the Data Export or Data Export: C3D page for more information about C3D export in Motive. Note that the coordinate system used in Motive (y-up right-handed) may be different from the convention used in the biomechanics analysis software.
Common Biomechanics Convention
Since Motive uses a different coordinate system than the system used in common biomechanics applications, it is necessary to modify the coordinate axis to a compatible convention in the C3D exporter settings. For biomechanics applications using z-up right-handed convention (e.g. Visual3D), the following changes must be made under the custom axis.
- X axis in Motive should be configured to positive X
- Y axis in Motive should be configured to negative Z
- Z axis in Motive should be configured to positive Y.
This will convert the coordinate axis of the exported data so that the x-axis represents the anteroposterior axis (left/right), the y-axis represents the mediolateral axis (front/back), and the z-axis represents the longitudinal axis (up/down).
- Force plate data and the tracking data can be exported into CSV files as well. When a Take file is exported into a CSV file. Separate CSV files will be save for each force plate and it will contain the force, moment, and center of pressure data. Exported CSV file can be imported for analysis.
- To stream tracking data along with the force plate data, open the Data Streaming Pane and check the Broadcast Frame Data, and make sure that you are not streaming over the camera network. Read more about streaming from the Data Streaming workflow page.