AMTI Force Plate Setup

Main PageAMTI Force Plate Setup


This page provides instructions on integrating a AMTI Force plate system in Motive.

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.

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Analog Platforms

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.


Hardware Setup


  1. Connect each force plate into the host PC. For force plates with external amplifiers, the platform must be connected to a separate amplifier that uplinks to the host computer.
  2. Setup the OptiTrack camera system and place the force plate(s) near the center of the volume. See Quick Start Guide or Hardware Setup page for details.
  3. For accurate synchronizations, connect the camera system and the force plate(s) using the synchronization hubs (OptiHub 2 and eSync 2). Connect the output ports of the hub to the sync input ports using the BNC cables or RCA cables with matching BNC-RCA adapters.
    • Note: Force plates can also be used without wiring to the synchronization hub. However, without the synchronization signals, motion capture data and force plate data may be off (~ 5 frames). Learn more in the sync configuration section.

Prime System + Force Plate

The eSync 2 output and input ports descriptions

When using an 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.


Diagram for Prime Camera System with integrated force plates.
Diagram for Prime Camera System with integrated force plates. (*) Some force plates don't have external amplifiers, but instead, have their amplifiers integrated within the platform. In this case, connect sync cables and the USB cables directly to the host PC.


Software Setup


Install Plugin

OptiTrack Peripheral Module

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.

ND OPMinstall.png DAQmxInstall.gif

Motive Setup


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 of incorrect or nonexistent force plate calibration files.

Initial view in Motive after setting up force plates.
Motive with force plates.


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.

Motive after setting up force plates and calibration.
Motive with force plates and camera calibration.


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.

CS-400 calibration square with force plate force plate parts.
CS-400 calibration square with force plate force plate parts.


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.

Force plate with CS-400 aligned properly.
Force plate with CS-400 aligned properly.


Calibrated force plate position and orientation. X and Y axis is shown.

5. Set force plate position in Motive. 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.

Setting the position of a force plate in Motive.
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.


6. Zero force plates. After you have calibrated each of your force plate, remove the CS-400 from the volume. Right click one of your force plates in Motive 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.

Set the force plate data to zero for more accurate data.
Set the force plate data to zero for more accurate data.


Force Plate Data in Motive

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.

  • Graph of live force plate data.
    Graph of live force plate data.
  • A resultant force vector displayed in Motive.


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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: 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.
  • 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...


Synchronization Configuration


Ethernet System Force Plate Sync Configuration

Steps

  1. Open the Cameras Pane and the Synchronization pane.
  2. In the Synchronization pane, select the Custom Synchronization mode from the dropdown menu.
  3. Set the Sync Input → Source to Internal Clock.
  4. Set the Sync Input → Clock Freq to 1000 Hz, and set the Sync Input → Input divider to 10.
  5. Set the Sync Output 1-4 → Type to Record Start/Stop Pulse or Recording Gate triggered sync.
  6. Click Apply from the Synchronization pane.
  7. In the Cameras Pane, select the force plates group and double check that the Sync Source is set to Free Run for the force plates.


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:

Example settings in the cameras and synchronization panes for eSync.
Example settings in the cameras and synchronization panes for eSync.

Ethernet System Force Plate without eSync Configuration

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.

Graph of bad data.
The force plate stops recording when its sampling rate disagrees with the frame rate of the camera system.

How to Validate your Synchronization

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:

Graphs of good data.
Example of good sync using tests described


The following is a example of validating bad synchronizations using these methods:

Graphs of bad data.
Example of bad sync using tests described

Exporting/Streaming Force Plate Data


Visual3D biomechanics analysis software provided by C-Motion
The MotionMonitor biomechanics analysis software.

We recommend the following programs for analyzing exported data in biomechanics applications:


C3D Export

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.

Common Conventions

C3D export setting for applications using z-up right-handed coordinate systems.

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).


CSV Export

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 save for each force plate and it will contain the force, moment, and center of pressure data. Exported CSV file can be imported for analysis.


Data Streaming

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.
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 Streaming

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