Configure force plates

As part of setting up your Vicon system, you manage the connection and configuration settings for supported force plates in your Vicon system with the appropriate force plate node. You can select the required force plate node under the Devices node in the System Resources pane when Vicon Nexus is connected to a Vicon system with at least one force plate unit and is in Live mode. When Nexus is in Offline mode, the force plate node is displayed under the Devices node of the Vicon Data node.

The Devices node lists each force plate connected to your Vicon system. For each device, the node name includes any display name specified in the Name property, its sample rate in brackets, and the force plate type in parentheses, for example, Name [1000Hz] (AMTI AccuGait Force Plate). If no analog source is selected, [No Source] is displayed after the device type. Channel - # sub nodes are displayed for each channel. Predefined configurations for some supported force plates are supplied with Vicon Nexus.

Force plates data can be acquired through the analog capture functionality of a Vicon connectivity device (Vicon Lock+ or MX Giganet). This topic describes the analog connection only. If your force plate has a digital output, it is possible that this data stream can also be captured, but this depends on the data stream's format and the equipment's manufacturer. For information on the digital data streams that can be used with Vicon systems, contact Vicon Support.

To configure force plates for analog data capture:

1. In the System Resources pane, click the Go Live button.
   The first time you use Nexus, the Devices node is empty. You must add and configure a new force plate device before it will be displayed in the System Resources tree.
2. In the System Resources tree, right-click the Devices node, point to Add Analog Device and from the context menu select the type of force plate that is integrated in your Vicon system.
   The selected force plate node automatically expands to display the newly created device. (For Motekforce Link treadmills, two new device nodes are displayed, one for each plate.) If the appropriate type is not displayed, contact Vicon Support.
   The colored icon beside a force plate node identifies the status of the device and of any connected analog source:

   • Green play button: Component OK (active or connected); if an analog device is connected, the analog source is selected and all channels are configured.

   • Yellow pause button: Component is not fully set up (e.g., not all channels have been assigned a pin or the calibration matrix has not been configured).

   • Gray play button: Component connected but not contributing any data.

   • Red stop button: Component down (unavailable or disconnected).

   
   These colored icons correspond to those used for the device summary in the Status Communications pane.
3. In the Properties section at the bottom of the System Resources pane, view or change settings for the following properties:
   
   1. In the General section, enter a Name.
   2. In the General section, go to the Calibration File field, and load the manufacturer's calibration file:
      
      • If the calibration file is not listed in the drop-down list, click the ellipsis (...) next to the Calibration File field to browse to the relevant location, and select it. The file becomes available in the drop-down list.
      • Select the calibration file. (For Motekforce Link treadmills, note that a single calibration file is used for both plates.)
      • If not supplied by the manufacturer's calibration file, you can set the device Dimensions, Position, Orientation, and Origin in those Properties sections.
   3. If there is no calibration file, click Show Advanced at the top right of the Properties section and enter the Calibration Matrix 6x6 Matrix values manually.
      Important: For the force plate to become active, you must enter the Matrix values, either via a calibration file or by manual entry.
   4. In the Source section, select a Source (the Vicon connectivity device to which the device is attached) from the drop-down list.
      The Source drop-down list contains all connected Vicon connectivity devices; a USB force plate will have its Source drop-down list populated with connected USB devices of the required type.
   5. In the Source section, use the Fill button to populate the input connections sequentially (if these are consecutive on the Vicon connectivity device).
   6. In the Source section, select the Gain for the Source from the choice of gains available for the Vicon connectivity device.
      
4. In the System Resources tree, if necessary, expand the force plate node to expose the Force, Moment and CoP (Center of Pressure) channels.
   The colored icon beside the output node identifies the analog channel status:
   
   • Green arrow: Analog channel connected to source device.
   • Yellow arrow: Analog channel has not been assigned a pin.
5. To tare the force plate at zero load, in the General section, click the Zero Level browse button and enter the matrix properties.
   Tip: You can also tare the force plate by right-clicking on the force plate name in the System Resources tree and selecting Zero Level.
6. In a 3D Perspective view, ensure that a gray or colored rectangle with the number 1 on it, representing the force plate, is displayed in the capture volume where you have positioned it.
   If you have multiple force plates configured, they will be numbered in the order they appear in the System Resources tree.
7. In the capture volume, have someone step onto the force plate. You should see the force vector being displayed in real time.
8. At the top of the System Resources pane, click the Save button to save your system configuration settings to a .system file in the Systems configurations folder (see Manage configurations in Vicon Nexus).
9. From the System Resources tree, expand the force plate node and select the Force output.
10. Switch to a Graph view.
11. If necessary, select Components from the Graph Type drop-down list. A real-time graph of the Force output is displayed.
12. Verify that the vertical (Fz) force component is equal to [known mass * 9.81].

For more information, see the following topics.

See also Force plate properties in the Vicon Nexus Reference Guide.

Force plate minimum setup requirements

For a force plate to appear in a 3D Perspective view, you must configure at least the following properties:

• Source
• Calibration Matrix
• Position (X, Y, Z)
• Name: Assigning a force plate name is recommended, especially if you are setting up more than one force plate.

For a force plate to appear in Live mode in the 3D Perspective view, you must fully configure:

• Source
• Calibration Matrix
• Dimensions (X, Y)
• Origin

The Position and Orientation are necessary for the CoP (Center of Pressure) and Force vector to be represented correctly, but they can be changed according to the setup. In subsequent sessions, you may wish to configure additional properties to suit the needs of your motion capture application.

Change force plate display options

You can configure colors and other force plate display options in the Options dialog box (F7).

You turn a display option on or off by selecting or clearing the corresponding check box.

To view force plates in a 3D Perspective view, ensure the Force Plates check box on the left side of the Options dialog box is selected. You can change the following options:

Force Plate section

Foot Contact Colors section

If you have designated a force plate to be a left or right foot contact, by default Nexus displays a left-foot contact force plate in red in the 3D Perspective view, a right-foot contact force plate in green. An auto-detect force plate is displayed in light gray, and an invalid force plate (neither left, right nor auto-detect) is displayed in dark gray.

To change the default colors for the force plate display, in the Options dialog box, select the Force Plates option, and in the Properties pane on the right, click on a color in the Foot Contact Colors section to activate the color picker.

Force Vector section

You can visualize the force vector by selecting the Draw and Draw Butterfly check boxes. Then you can set the quality of the vector display, as well as the radius and scaling factor (length), the butterfly style (whether the butterfly is represented as transparent, as a wireframe or both), and color.

Setting a Scaling Factor value causes the vector to be displayed as an arrow originating from the force plate in the direction of the force.

Values below the Force Threshold (N) are not visualized as force vectors on the plate. This setting does not affect the value for Force Threshold that is written into a recorded trial (see Visualize and record Force Threshold).

Moment Vector section

You can select whether the moment vector is visually represented in the 3D Perspective view, as well as the quality, radius and color of the moment vector display.

You can see the results of your settings in the 3D Perspective view as you configure an option.

Visualize and record Force Threshold

The Force Threshold of a force plate specifies the noise floor value in Newtons. Forces below this value are assumed to be noise and are clamped to zero.

An Options setting for force plates enables you to specify a different value for the Force Threshold depending on whether you are recording data or displaying a trial.

• The value for the Force Threshold that is set in the System Resources pane for the force plate affects the values that are written into a recorded trial. For example, if the Force Threshold is set to 25 N, all values below this are considered noise and values of zero are recorded.

• The value for the Force Threshold that is set in the Options dialog box ensures that values below this threshold are not visualized as force vectors on the plate. This prevents very small values, which are simply noise, from creating a distracting ‘flickering’ force vector on the plate. If you are distracted by flickering while viewing a trial, raise this value to remove the flickering. This does not affect the value for Force Threshold that is written into a recorded trial.

To specify the Force Threshold that is recorded for a force plate:

1. Ensure the system is in Live mode.
2. In the System Resources tree, select the force plate.
3. In the Properties pane, ensure the Advanced properties are displayed.
4. In the General section, change Force Threshold (N) from the default (25 N) to the required value.
   Values below this magnitude are ignored and are recorded as zero.

To specify a value for visualizing the force vector:

1. Press F7 to open the Options dialog box.
2. On the left click ForcePlates.
3. In the Properties pane on the right, in the Force Vector section, change the Force Threshold (N) to the required value (the default is 25 N).
   Regardless of the setting of the Force Threshold in the System Resources pane, the force vector that is displayed in the view pane is below the threshold that is set in the Options dialog box. For example, if you set the value to 10 in the System Resources pane, but left it at 25 in in the Options dialog box, a value of 15 would be prevented from causing flickering, but would be recorded in saved trials.

View combined output from multiple force plates

You can combine selected outputs from multiple force plates, both in Live mode and when reviewing captured trials in Offline mode. For example, in trials where the subject's feet land on separate force plates, you can examine the combined landing force or overall CoP from both feet.

To view combined output from multiple force plates:

1. In the Options dialog box (press F7), on the left, click ForcePlates and in the Properties pane on the right, in the Force Vector section, ensure Draw Combined Force Vector is selected.
2. In the System Resources tree, CTRL+click to select the required outputs from each force plate. Ensure you select identical outputs from each force plate.

   

3. In a Graph view, in the Graph type list, select Combined Forceplates.

   

4. In the Graph view, the combined output you selected is displayed.

   

Usage recommendations for Motekforce Link treadmill

Note the following recommendations for setting up and using a Motekforce Link treadmill with Nexus:

• For ease of use, align the Vicon world Y-axis with the direction of walking on the treadmill. To achieve this, you can either move the treadmill in the laboratory to align with the Vicon world axes in Nexus, or change the Vicon world axes to align with the position of the treadmill. You can then set the incline of both plates of the treadmill by adjusting the X Orientation property of the force plate.

  

• Assuming you follow this advice so that the direction of walking on the force plates aligns with the Vicon Y-axis, and the left-right (medial-lateral) direction aligns with the Vicon X-axis, inclination of the force plate will correspond to a rotation of the plate about its X-axis. For example, to set a 10 degree inclination of the treadmill, you would set a 10 degree X (deg) rotation in the plate's Orientation properties.
• Each capture must use only one inclination of the treadmill. If you need to capture multiple inclinations, to enable you to switch easily between different inclinations, create a number of separate system files: one for each required inclination.