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 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.
For Ethernet camera systems, connect the force plate system through the eSync 2 for more accurate synchronization.
The eSync 2 has signal output ports that are used to send out synchronization signals to slave devices. Connect the BNC output ports of the eSync into sync input ports (Genlock/Trigger Input) of force plate amplifiers.
If force plate systems have RCA sync ports (e.g. AMTI Gen 5 amplifiers), use RCA cables along with the 50 Ohm BNC Male to 75 Ohm RCA Jack Adapters included with the eSync 2 to connect the amplifiers. The above wiring diagram shows how force plate systems need to be connected to an Ethernet camera system through the eSync 2.
There are total four output ports on the eSync, and multiple force plates and external devices can be integrated if needed. Consult our Engineers for multiple force plate synchronizations.
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. 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.
Note : Even if you are not using NI-DAQ, it is still necessary to install NI-DAQmx drivers that comes 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.
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 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.
After these configurations have been applied, the force plate data and the motion capture data will be synchronized via triggering. The settings should look similar to the following:
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 (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.
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.
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 make the data more obvious for validating the synchronization. Alternately, you can attach a marker on 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 compare when the motion capture data says the ball or foot made contact with the force plate to when the force plate says it made contact. ↑
The following is a example of validating good synchronizations using these methods:
The following is a example of validating bad synchronizations using these methods:
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).