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A motion capture system used in conjunction with force plates provide a very powerful tool for various research applications including biomechanical analysis, clinical gait analysis, physiology research, sports performance research, and more. An OptiTrack motion capture system can synchronize with force plates to obtain both kinematic and kinetic measurements. This page provides a quick guideline to setting up force plates — with digital outputs — along with the OptiTrack motion capture system: hardware setup, software setup, synchronization configurations, and post-processing of the recorded data.
For detailed information on specifications and configurations of the force plates, refer to the documentation provided by the force plate manufacturer.
As of Motive 1.9.0 we only support digital output force plate systems with AMTI Gen 5 amplifiers
When using the Ethernet camera system, connect the eSync 2 to the force plate system for more accurate synchronization.
The eSync 2 has signal output ports which can be used to send synchronization signals to input ports of the force plate system. The eSync 2 has total four output ports for integration of multiple force plates and external devices if needed. Consult our Engineers for multiple force plate synchronizations.
When force plate systems have RCA sync ports, use the 50 Ohm BNC Male to 75 Ohm RCA Jack Adapters (included with the eSync 2) and RCA cables to connect the eSync 2 and the amplifiers (e.g. AMTI Gen 5 amplifiers).
The following wiring diagram shows how the force plates are integrated into Prime series Ethernet camera system through the eSync 2. ↑
When using a USB based camera system, connect the force plate system to the OptiHub(s) within the camera system.
There is one synchronization output port for each OptiHub, and an OptiHub can connect to one force plate. When integrating multiple force plates, you may alternately use a BNC splitter to use one OptiHub as a master sync for multiple force plates. (Note: When incorporating more than two force plates into one OptiHub, you may need to use an active BNC splitter, because the signal power will be reduced each time the signal is split.)
When force plate systems have RCA sync ports, connect a 50 Ohm BNC Male to 75 Ohm RCA Jack Adapter into the output port on the OptiHub(s) and connect each OptiHub to each amplifier using a RCA cable.
The following wiring diagram shows how the force plates are integrated into the Flex series USB system. ↑
1. Start Motive. If the hardware and software for the force plates are setup correctly, Motive will display the detected force plates with number labels (1, 2, etc..). Motive will notify you if it detects an incorrect or nonexistent force plate calibration file.
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. So 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. Setup CS-400. Pull the force plate alignment tabs out and put the force plate leveling jigs on the bottom of your CS-400 calibration square. 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 the wand vertex is located at the right 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 sets the local coordinate axis used by the platform. The coordinate system for force plates are independent of the system used Motive.
AMTI Force Plates
AMTI force plates use 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 vertical force data.
5. Set force plate position in Motive. After placing the calibration square, select the CS-400 markers in Motive. Right click on the force plate you want to locate, and click Set Position. You may need to step on the force plate to figure out which platform the calibration square is on. In Motive, The uncalibrated force plates will light up in green as you step on the plate, and a force vector will appear. Repeat step 4 and 5 for other force plates as well.
Referencing to the markers on the calibration square, Motive defines the location of the force plate coordinate system — depends on the force plate manufacturer — 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.
6. Zero force plates. After you have set all of your force plate, remove the CS-400. Right click one of your force plates and click Zero (all). This will 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 the force plates.
To view 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, and a list of the force plate data will appear at the bottom of the Project Pane. Select one of Fx, Fy, Fz, Mx, My, or Mz to view the live force plate data from the timeline. This is a good way to confirm that your force plates are working.
Notes
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 that this convention is independent from the global coordinate system used in Motive. Thus, the Fz components represent the vertical force. For more information, refer to the force plate specifications.
The supported force plate sampling rates will also depend on the manufacturer. For the most up-to-date information, consult their documentation. For AMTI force plates, supported frame rates are 2000, 1800, 1500, 1200, 1000, 900, 800, 600, 500, 450, 400, 360, 300, etc...
After these configurations have been applied, the force plate data and the motion capture data will be synchronized via triggering. The settings should look something like the following:
After these configurations have been applied, the force plate data and the motion capture data will be synchronized via triggering. It should look something like the following:
If you are using the Ethernet system without the eSync, ignore the synchronization configurations. From the Cameras Pane, just 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.
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. 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 may record improperly; which is shown in the following screenshot. ↑
Before you start recording, you may want to validate that the camera data and the force plate data are in sync. There are some tests you can do to examine this.
First method is to drop a retroreflective ball/marker on the force plate few times. You will need to prepare a ball covered in retroreflective tape in order to test with the ball method. The bouncing ball produces a sharp transition when it hits the surface of the platform, and it make the data more obvious to validate the synchronization. Alternately, you can attach a marker on tip of the foot and step on/off the force plate. Make sure that your toe — closest to the marker — strikes 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 are examples of good synchronization using these methods:
The following are examples of bad synchronization using these methods:
From Motive, you can export the tracking data and the force plate data into C3D files. Exported C3D files can then be imported into a biomechanics analysis software for further processing. See Data Export page or Data Export: C3D page for more information about C3D export in Motive.
Visual3D which is our recommended biomechanics analysis software. Visual3D offers extensive biomechanics analysis pipelines including biomech skeleton modeling, joint and segment kinematics analysis, and reporting functions. Using the precise marker positional data tracked by the OptiTrack system along with the force plate data, Visual3D can perform the most accurate biomechanical analysis.
To stream the 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. Then you are good to stream into another program. Read more about streaming from the Data Streaming workflow page.
We recommend Visual3D for data streaming for biomechanics applications as well. When streaming into Visual3D, set the Visual3D Compatible setting to true from the Data Streaming pane. Real-time streaming into Visual3D uses Visual3DServer plugin provided by C-Motion.
For more information on the Visual3DServer, refer to the C-Motion Documentation Wiki: http://www.c-motion.com/v3dwiki/index.php/Visual3DServer_Overview
