Main Page → AMTI Force Plate Setup
This page provides instructions on integrating an AMTI Force plate system with an OptiTrack motion capture system.
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.
Before setting up the force plates in Motive, make sure software components required by the force plate system is installed on the computer. AMTI's software (e.g. AMTINetForce) must be able to detect and initialize the connected devices in order for the force plates to be properly initialized and used in Motive. Once this has been confirmed working, start setting up Motive. Please refer to manufacturer documentation for more information.
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.
AMTI Force Plates
AMTI force plates use the right-hand system. The long arm of CS-400 will define the Y axis, and the short arm will define the X axis of the force plate. Accordingly, Z axis is directed downwards for measuring the vertical force.
Sampling rate of force plates can be set from the Devices pane.
You can adjust the rate multiplier and set the sampling rate of the force plate to a desired multiple of the camera system framerate.
When synchronizing through the eSync, use the following steps to configure the sync settings in Motive. This will allow both systems to be triggered simultaneously with reference to the master synchronization device, the eSync.
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.
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 (fps), ex 100fps and 1000fps. Motive will alert you if the sampling rates for cameras and force plates disagree.
Without a master synchronization device, recording for the camera system and the force plates will not be triggered exactly at the same time. This can cause problems for longer takes because the sampling timing for 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.
When the force plate sampling rate is not set to an integer multiple of the camera frame rate, force plate data may record improperly; which is shown in the following screenshot.
To view and confirm the live force plate data, open the editor from the Timeline Pane. Then, open the Project Pane (or Devices 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 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.
We recommend the following programs for analyzing exported data in biomechanics applications:
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.
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).
1) Received force plate data is intermittent.