Work with Vicon IMUs

If you are using Vicon IMUs (Inertial Measurement Units) with Nexus, first ensure you have set up the IMUs and Nexus as described in Configure Vicon IMUs.

The following are brief notes on using Vicon IMUs with Nexus.

For an introduction to the latest Vicon IMUs, watch the Vicon video Capture.U Tutorial - Unboxing Blue Trident, available on YouTube.

Attach IMU sensors to a subject

Stream live data from Vicon IMUs

Nexus 2.10 and later enables you to stream live data from your Vicon IMUs so that you can visualize the outputs in real time.

You can use any of the data channels from the IMU:

• HighG accelerometer
• LowG accelerometer
• Gyroscope
• Magnetometer
• Global angle (angle-axis from the onboard 9-axis quaternions)

To stream live data from Vicon IMUs:

1. Ensure the required IMUs are connected (see Connect Vicon IMUs to Nexus).
   All the available axes automatically stream data:
   • LowG accelerometer (16 G) (x, y, z)
   • Gyroscope (x, y, z)
   • Magnetometer (x, y, z)
   • Global Angle (x, y, z)
   • HighG accelerometer (200 G) (x, y, z)

     

     The default stream rate is 50 Hz, but can be 30 Hz, 50 Hz or 100 Hz.While the IMUs are connected to Nexus, a real-time preview of data streaming from the IMUs is displayed (when Nexus is in Live mode). This preview data stream is supplied to Nexus at the selected stream rate, regardless of the trial collection rate (225 Hz, 1125 Hz or 1600 Hz).
     The preview stream is optimized for low latency data visibility and may therefore have small frame gaps where data does not appear.

2. To change the stream rate, select an IMU on the System tab, and in the General section of the Properties, click in the Stream Rate field to change its value.
   Note that the stream rate that you specify, together with the number of connected IMUs, affects performance.
   
   

Select the required axes

By default, all axes are enabled.

To select the required output, you choose from a list of presets.

To select the output axes:

1. In the System tree, select the required IMU.
2. In the Properties pane, click the Output Preset menu.
3. From the list, select the required option:

   

In addition to selecting the output from the Output Preset list, you can select or clear Enabled in the Properties for the axes, to customize the output.

To deactivate an axis:

1. On the System tab, select the required IMU and expand its node.

   

2. To deactivate an axis, click on the required axis and in its Properties, clear the Enabled check box.

   

   On the System tab, the color of the cleared axis changes from green to orange and the Graph displays null results for that axis.

   

   If Global Angle is deactivated, the coordinate system for the relevant sensor is no longer displayed in the 3D Perspective view.

Use IMU streamed data with the Vicon DataStream SDK

All Vicon IMU data can be passed to the Vicon DataStream.

For example (using C++):

Download the latest version of the datastream SDK from:
https://www.vicon.com/software/datastream-sdk/?section=downloads

Calibrate Vicon IMUs

If you plan to capture global angles, to ensure that the IMU sensor data is stable and to minimize drift, you must calibrate the IMUs.

To calibrate Vicon IMUs:

1. Ensure you are streaming global angles (see Stream live data from Vicon IMUs).
2. To calibrate an IMU sensor, wave the sensor in a figure-of-eight movement several times for at least 10 seconds, ensuring that it rotates through all three axes during the motion.

   

3. Put the sensor on a flat surface such as the floor or a table and spin it through two full rotations around the z axis.
   
   

   Tip: To avoid any unwanted influence on the sensor, do not place it in proximity to anything metallic.

4. Wait at least five seconds until the data has stabilized.

If you want to align the global angle outputs from the IMUs to the Vicon world, you can then perform wand alignment and segment alignment, as described in Align IMU data with the Vicon world.

Capture Vicon IMU data

Nexus enables you to capture IMU data so that you can perform offline processing on it.

1. Before capturing data, ensure all required axis channels are enabled and streaming (see Stream live data from Blue Trident sensors).
2. To check the axis channels, on the System tab, expand a Vicon IMU node.All enabled channels are displayed with a green icon.

   

   In the Graph view, the selected axes for each selected IMU are displayed.

   

3. To capture IMU data, in the Capture Tools pane, enter a trial name and other details and in the Data Source Setup section, ensure Device Data is selected.

   

   The default sample rate is 225 Hz. However, the rate changes to:
   • 1125 Hz, if the Global Angle axis is deactivated (see Select the required axes)
   • 1600 Hz, if only HighG accelerometer is enabled
4. Click Start to begin capturing and capture your subject's movement in the usual way (see Capture the required movement).
5. When you've finished capturing, click Stop.
   The streamed data is saved into the current Session folder and is displayed as an .x1d file in Data Management.

   

   To use the onboard IMU data, you must transfer it (see Download Vicon IMU data).
6. To view the streamed data, load the X1D.
7. In the 3D Perspective view, the IMU coordinate system is displayed.
   Note that the IMU coordinate system is displayed only if the Global Angle axis is enabled (see Select the required axes).

   

   To display the streamed data in a Graph view, in the System tree, select the IMUs.

   

Download Vicon IMU data

After you've captured the required data, you can download the recorded data from your Vicon IMUs so that you can load it into Nexus.

1. To download IMU onboard data to the current Session folder, in the Communications pane, click the IMeasureU tab.

   

   In the Transfer pane, a list of all the IMUs and trials available to transfer is displayed.

   

   For details of the options, see Transfer pane options.

2. To transfer the IMU onboard data, connect the IMU to a USB adapter and using the micro-USB cable, connect it to the PC.

   

   In the Transfer pane, the icons to the left of the IMUs that you plugged in are displayed in orange.

3. In the Transfer pane, ensure the required IMU sensor and trials are selected and to start the data transfer, select Transfer Files.

   

   The progress status is displayed when the data transfer starts.
   During data transfer, the sensor is temporarily disconnected from the Bluetooth^®.
   After a trial is transferred, the progress status changes to Transfer Succeeded.

   

   After the transfer operation, for each trial, you can find the following files in the Session folder:

   • An .x1d containing the preview data (30 Hz, 50 Hz, or 100 Hz)
   • An .imu file containing the higher quality data (225 Hz, 1125 Hz, or 1600 Hz) from the IMU
4. You can now view and process the IMU data along with the rest of the trial data.
   

Transfer pane options

On the IMeasureU tab of the Communications pane, in the Transfer pane, you can select or remove the required trials and devices for transfer. The buttons and lists in the Transfer pane enable you to filter the displayed items so that you transfer only the items that are required

This table describes the available options.

The connection status of each sensor to the PC (via USB) is indicated by the color of the IMU symbol to the left of the sensor name in the Transfer pane:

• Orange: Connected
• Gray: Disconnected

Note that:

• If you have filtered an item from the list, it is automatically de-selected. This is to prevent accidental transfer of items that are not displayed in the list.
• Select All and Select None operate on all items that are visible in the list: an item must be visible in the list for its selection status to be updated.
• The top level sensor item is never filtered out of the list.
• The filtering is dynamic and the list refreshes when:
  • Sensors are connected or disconnected
  • Trials are captured
  • Sensors are erased

Erase data from Vicon IMUs

After you've downloaded data from Vicon IMUs, you can delete the data in Nexus so that the IMUs are ready for re-use.

In addition, if you have erased data from an IMU sensor using Capture.U Desktop or IMeasureU Step, you can update the information displayed on the IMeasureU tab in Nexus so that it does not display the erased data.

• Erase onboard data
• Remove erased trials from the IMeasureU tab

Erase onboard data

To erase the onboard data:

1. In the Transfer pane, select the IMU(s) and then click Erase Selected Devices .

   

   A warning prompts you to confirm that you want to erase the onboard IMU data.

   

2. If you want to erase the onboard data, click Yes.When the data has been erased, the connected IMUs remain on the Transfer list, but their trials are removed from the list and a caret (>) is no longer displayed to the left of the sensor name.

   

Remove erased trials from the IMeasureU tab

If you have erased data from an IMU sensor using Capture.U Desktop or IMeasureU Step, you can update the information displayed on the IMeasureU tab so that it does not display the erased data.

To remove erased trials from the IMeasureU tab:

1. With the IMU from which you have erased data (using Capture.U Desktop or Step) connected to the PC, open Nexus.

2. In the Communications panel, select the IMeasureU tab and click Show Advanced.
   The Sync trials with sensor button is displayed.

   

3. On the IMeasureU tab, select the IMU sensor.
   Erased trials are displayed in red.

   

4. Click Sync trials with sensor.
   Nexus prompts you to confirm that you want to continue to remove the erased trials from the list.

   

5. Click Yes to continue.
   In the Trial list on the IMeasureU tab, the red (erased) trials are no longer displayed.

   

Restore missing data to a Vicon IMU sensor

You can restore accidentally deleted or corrupted information that is needed to download the information from the sensor and store it with the trial it was captured with.

To do this, you can copy the information back from a trial-specific file that is saved at the end of a capture.

To restore missing information to a sensor:

1. Plug in the sensors that have data on them that is deleted or corrupted and that you want to restore.
2. In the Communications panel, select the IMeasureU tab and click Show Advanced.
3. In the Trial list, select the required sensor(s).

4. Click Rebuild Transfer List.

   

5. In the Rebuild Transfer List dialog, click in the Folder to process box, enter or browse to the folder to process and if required select Look in child folders to process all of its subfolders.

   

   Note that the folder you select does not have to contain the trials, for example, the folder can be the parent of multiple session folders. In this case, you would select Look in child folders.

6. Click Continue.
   The trial-specific data is copied and you can now download the restored information from the sensor.

   

Align IMU data with the Vicon world

If required, you can align the global angle outputs from IMUs to the Vicon world. This enables global angles from the IMUs to correspond to specific segments within Nexus.

Before you begin, ensure you have calibrated your system in the usual way (see Calibrate a Vicon system) and that the IMUs have Global Angle output selected (see Select the required axes).

To align IMU data with the Vicon world, complete these steps.

• Attach a Vicon IMU to a Vicon Active wand
• Align the IMU and Vicon worlds
• Align Vicon IMUs with segments

Attach a Vicon IMU to a Vicon Active wand

The Vicon IMU alignment clip enables you to attach a Vicon IMU to an Active Wand.

To attach a Vicon IMU to a wand:

1. Locate the hole above the wand handle as indicated in the following image.

   

2. With the IMU figure facing up, attach the clip to the wand.

   

3. Slide the clip down, so that the clip attachment is inserted securely in the hole.

   

4. Insert a Vicon IMU into the IMU clip.

   

Align the IMU and Vicon worlds

1. In the Nexus System tree, under Devices, select the sensor you inserted into the clip (see Attach a Vicon IMU to a Vicon Active wand).

   

2. In the Communications pane, click the IMeasureU tab and in the Alignment pane, under World Alignment, click Start.
   
   

   Tip: If the Start button is not enabled, make sure:

   • You've selected a sensor in the System tree (see Step 1).
   • The sensor has Global Angle output enabled (ie, Global Angle is selected – see Select the required axes).
   • Your Nexus system includes at least two calibrated cameras.
3. In your capture volume, wave the wand for several seconds.
   In the Alignment pane, Alignment Residual is displayed beneath the Stop button.
4. Under World Alignment, click Stop. The IMU world for the selected sensor is aligned to the Vicon world for the L-frame object.

   

Align Vicon IMUs with segments

1. In the System tree, select the sensor that is now aligned with the Vicon world, and on the IMeasureU tab, in the Alignment pane, under Update Alignments, click Copy.

   

   This copies the world alignment values from the aligned IMU to all the others.You can now attach the selected IMU to the required segment. In this example the IMU is associated with a Subject segment.
2. In the Subjects Resources pane, select the subject segment (in this case, Forearm) that you want to associate with the IMU.
3. In the Alignment pane, under Segment Alignment, click Start.
   The Start button changes to display Align to Segment.

   

4. Click Align to Segment.
   The selected IMU is now aligned with the selected Subject segment (in this case, Forearm).
   
   
   
   

5. For each IMU that you want to align to a segment, select the IMU in the System tree and then repeat Steps 2–4 above.

Export IMU data

You can export data captured using IMUs by running the appropriate pipeline.

To export IMU data:

1. If you want to select data from only some devices, in the System Resources pane, ensure the required IMUs are selected.
2. In the Pipeline Tools pane, expand File Export and double-click Export ASCII to add it to the current pipeline.
3. In the Properties pane, ensure that the settings are as required. In the Devices section, if necessary, change the Devices for Export list to specify your selection.

4. Run the Export ASCII operation.
   To open the exported file in Microsoft^® Excel^®, on the Data Management tab, click the relevant C icon and then click the filename.

   

   The data is displayed in an Excel spreadsheet.