Bertec Force Plate Setup
When a motion capture system is used in conjunction with force plates, they work together as an efficient 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.
Analog force plate devices can only be implemented via DAQ devices. Incoming voltage signals can be detected through the data acquisition channels, but force plate related software features (vectors, position calibration, etc.) will not be supported in Motive for the analog platforms. Refer to the NI-DAQ Setup page for detailed instructions on integrating analog devices.
Motive 3.0 Update
- Starting from Motive 3.0, reference clock synchronization while in Live mode is supported.
- Supported Amplifier Models: AM6800
- Firmware Version: For synchronization support, the Bertec amplifiers must be installed with firmware version 4.5.2. The current firmware version gets displayed on the amplifier display when first powering up the amplifier. Please check this and make sure the firmware is updated to the supported versions.
- Prime series Ethernet camera system with the eSync synchronization hub.
- Motive 2.2 or 2.3.
- Bertec amplifiers currently only support a fixed sampling rate of 1000 Hz
Refer to the respective Bertec system user documentation for detailed information on setting up the force plate system and connecting to the host PC.
Wiring the eSync with the Amplifier
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.
Reference Clock Sync
Reference Clock Sync
- The SYNC cable from the amplifier needs to be connected to the output port of the eSync for synchronization with the camera system.
Click image to enlarge.
Triggered Sync Cabling
- 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. When using more than one plates, a BNC connector or a BNC splitter will need to be used to interconnect the SYNC cables between multiple amplifiers.
Click image to enlarge.
Zero cable from the amplifier
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, 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.
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 successfully instantiated in Motive, the Log pane will indicate that the device has been created and loaded.
Motive force plate representation in Perspective view.
Status Log pane in Motive.
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.
Motive with force plates and camera calibration.
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.
CS-400 calibration square with force plate force plate parts.
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 plate with CS-400 aligned properly.
Calibrated force plate position and orientation. X and Y axis is shown.
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.
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.
Setting the position of a force plate in Motive. The number label on the force plate is inverted because the force plate position and orientation has not been calibrated yet.
If the force plate dimensions are not automatically configured, you need to enter the dimensions of the force plate in the force plate properties after calibrating its positions. Go to the Devices pane and select the force plate, and its properties will get listed under the Properties pane. Enter the length and width (in meters) values for the corresponding plates as reported in the specifications.
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.
Set the force plate data to zero for more accurate data.
8. Set sampling rate
Sampling rate of force plates is configured through the synchronization setup which will be covered in the following section. You can sync the force plates either through the reference clock sync or through the triggered sync. Please note that only specific sampling rates may be supported depending on the amplifier models.
Supported Force Plate Sampling Rates
- Reference Clock Signal Sync: When using the reference clock signal from the eSync to synchronize the force plates, the force plate will basically run at the same rate as the received clock signal. You can also apply either the multiplier or the divider to the outputted clock signal to make additional adjustments.
- Recording Trigger Sync: When using the recording signal from the eSync to trigger-sync the force plates, the force plates will be running at their own free run sampling rate. In this case, only 1000 Hz sampling rate is supported, and you will need to adjust the camera frame rate in the Devices pane and apply framerate-multipliers to set the 1000 Hz sampling rate on the force plates.
There are two synchronization approaches you could take: Synchronization through clock signal or through recording trigger signal.
Synchronization via clock signal utilizes the internal clock signal of the eSync 2 to synchronize the sampling of the force plates on per-frame basis. However, when there is another device (e.g. NI-DAQ) being synchronized to the clock signal frequency, the sampling rate cannot be set for each individual device. In that case, triggered sync must be used for synchronizing the initial recording trigger. Synchronization via trigger signal utilizes the recording trigger in Motive to align the initial samples from both systems. After the initial sync, both systems run freely at their own sampling rate. If the force plates are running at whole multiples of the camera system, the collected samples will be aligned. However, since the sampling clocks are not perfectly accurate, alignment of the samples may slowly drift over time. Thus, when synchronizing via recording trigger, it is better to keep the record times short.
When synchronizing through the eSync 2, use the following steps to configure the sync settings in Motive. This will allow both systems to be triggered simultaneously with reference to the parent synchronization device, the eSync 2.
IMPORTANT NOTE: For this synchronization setup to work properly, the Bertec amplifier firmware must be updated to firmware version 4.5.2 or above. Currently installed firmware version gets displayed on the 7-segment display when first powering up the amplifier. Please check this and make sure the firmware is updated to the supported versions. If an older version is installed, please contact Bertec for instructions on updating the firmware.
Reference Clock Sync
Reference Clock Sync Setup Steps
- 4.Next, to the Clock Frequency section, input the sampling rate that you wish the run the force plates in. This clock signal will be eventually outputted to the force plate system to control the sampling rate. For this guide, let's set this to 1000 Hz.
- 5.Once the clock frequency is set, apply the Input Divider/Multiplier to the clock frequency to set the framerate of the camera system. For example, if you input 10 to the Input Divider section with internal clock frequency running at 1000 Hz, the camera system will be running at 100FPS. The resulting frame rate of the camera system will be displayed in the Camera Rate section.
- 6.Next step is to configure the output signal so that the clock signal can be sent to the force plate system. Under the Outputs section, enable the corresponding output port of the eSync 2 which the force plate system is connected to.
- 8.Now that the eSync 2 has been configured, you need to configure the force plate properties in Motive. While the force plate(s) is selected in Motive, access the Properties pane to view the force plate properties. Here, set the following properties:
- 9.Triggered Sync → None
- 10.Reference Clock Sync → True
- 11.eSync 2 Output Channel → output port used on the eSync 2.
eSync Settings Tip:
In Motive 3.0 and above, you can quickly configure eSync into biomech sync settings by right-clicking on the eSync from the Devices pane and select one of the presets from the context menu. This will enable and set all of the eSync outputs to the Internal Clock and set the clock frequency.
Applying Biomech preset settings.
Starting from Motive 3.0, clock synchronization in Live mode is supported, and the force vector visualization will be available both in Live and Edit modes.
Triggered Sync Setup Steps
- 4.Set up the output signal so that the recording trigger signal can be sent to the force plate system. In the Outputs section, enable and configure the corresponding output port of the eSync which the force plate system is connected to.
- 6.Now that the eSync has been configured, you need to configure the properties of the force plates. While the force plate(s) is selected in Motive, access the Properties pane to view the force plate properties. Here, set the following properties:
- 7.Record Trigger → Device
- 8.Reference Clock Sync → False
- 9.eSync Output Channel → output port used on the eSync.
Once this is done, the force plate system will synchronize to the recording trigger signal when Motive starts collecting data, and the force plates will free-run after the initial sync trigger. You can configure the sampling rate of the force plates by modifying the Multiplier values in Devices pane to sample at a whole multiple of the camera system frame rate.
Example eSync 2 properties for triggered sync.
Example force plate properties for triggered sync.
- Supported Frame Rate: When synchronizing two systems via recording trigger, the force plates will be running at their own free-run frame rate. In this case, only 1000 Hz sampling rate is supported for Bertec force plates. If you wish to sample at a different rate, please use the reference clock sync approach.
- Setting Framerate Multiplier: For free run sync setups, sampling rates of force plates can be set from the Devices pane, but the sampling rate of force plates must be configured to a whole multiple of the camera system's framerate. By adjusting the Rate Multiplier values in the Devices pane, sampling rates of the force plates can be modified. First, pick a frame rate of the camera system and then adjust the rate multiplier values to set force plates to the desired sampling rate.
When two systems are synchronized by recording trigger signals (Recording Gate or Recording Pulse), both systems are 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 recording trigger but not by per frame basis. 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.
Re-aligning initial sampling timing of the force plate.
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:
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. You can have this file backed up to persist configured eSync and device settings. Also, if you wish to reset the device settings, you can remove XML files other than the default one from the folder, and Motive will load from the default settings.
Force plate data can be monitored from the Graph View pane. You will need to either use a provided Force Plate Forces layout or configure a custom graph layouts to show force plate data. To view the force plate data, make sure the corresponding force plates are selected, or selection-locked, in Motive.
If you are configuring your own force plate graph layout, 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.
- The force and moment data reflects the coordinate system defined by the force plate manufacturer, which is typically the Z-down right-handed coordinate system. Note: This convention is independent of the global coordinate system used in Motive. Thus, the Fz components represent the vertical force. For more in-depth information, refer to the force plate specifications.
Graph of live force plate data.
Graph view pane layout configuration.
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.
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).
C3D export setting for applications using z-up right-handed coordinate systems.
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 saved 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.
Number of Force Plates
At the time of writing, there is a hard limit on the maximum number of force plate data that can be streamed out from Motive. Please note that only up to 8 force plate data can be streamed out from Motive and received by a NatNet SDK 4.0 application.