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When the motion capture system is used in conjunction with force plates, they become a very powerful tool for various research applications including biomechanical analysis, clinical gait analysis, physiology research, sports performance research, and many more. The OptiTrack motion capture system can synchronize with force plates to obtain both kinematic and kinetic measurements. This page provides quick guidelines to 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.
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 force plate system through the eSync 2 for more accurate synchronization.
The eSync 2 has signal output ports which can be used to send synchronization signals to the force plates. There are total four output ports, and multiple force plates and external devices can be integrated 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.
In each OptiHub, there is a synchronization output port which can be used to synchronize one force plate or another external device. When integrating multiple force plates, each OptiHub within the system can be used to wire multiple devices. Alternatively, you may use a BNC splitter to configure a single OptiHub as a sync master 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..). Incorrect or nonexistent force plate calibration file will be notified in Motive.
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, and 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. 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 the wand vertex is located at the right-hand 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 its local coordinate axis within the global system. The coordinate system for force plates are independent of the system used Motive.
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.
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. When there are multiple force plates, you may need to step on the force plate to figure out which platform the calibration square is on. In Motive, 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 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 sets 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. Here, you can confirm that your force plates are working properly.
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 similar to 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:
We recommend the following programs for analyzing exported data in biomechanics applications:
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 and visualization software for further processing. See Data Export page or Data Export: C3D page for more information about C3D export in Motive.
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.
Motive can stream the tracking data and the force plate data into various applications — including Matlab — using NatNet Streaming protocol. Find more about NatNet streaming from the User's Guide included in the download.
Visual3D
For 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
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