ODIN provides a routine to apply an offset and rotational transformation to the co-ordinates measured by each CODA unit, so as to reference them to a common room co-ordinate frame. When two or more CODA units are used together, both the origin offset and rotational transform must be applied to ensure that the co-ordinates measured by each unit are referenced to a common co-ordinate frame, and to force platforms if present. This process is called Alignment. The alignment data (transform matrix) is stored by the RTNet Server and is automatically re-loaded and applied whenever the system is re-booted. Since even a very small movement of a CODA unit will miss-align the system. It is best to re-align at the beginning of each measurement session. ODIN does not automatically warn you if the system is miss-aligned.
Standard Alignment Routine
The alignment routine consists in defining the room X-axis and the X-Y plane using a set of 3 markers or more. This section will explain how to perform an alignment with 3 and 5 markers. whatever the number of markers used during the alignment routine, some precautions are required:
- The markers defining the X-axis must be at least 30cm away from from each other but we recommend a distance of 50cm when using more than one CX1 or CXS units.
- The markers defining the XY-plane must be at least 30cm away from from each other but we recommend a distance of 50cm when using more than one CX1 or CXS units
- the vectore defining the X-axis and XY plane must relatively perpendicular
- All markers must be visible to all active CODA units, and there should be no reflecting surfaces nearby.
The first things to do is to place the markers on the floor. Figure 7 and 9 show how to place the marker on the floor when using 3 and 5 markers, respectively. Once the markers are on the floor, please ensure that all markers used for the alignment are all visible by all active CODA units by using the “Marker text display”. Each marker must been seen by each individual sensor of each active Coda unit. Picture 5 and 6 show two examples of marker visibility when using 3 markers. Two active units were used in these examples. Figure 5 shows an example of good visibility, where each marker is visible by each sensor of each unit. On the other hand, picture 6 shows an example of bad marker visibility. Marker 2 is not visible by the sensor C of the Coda unit 1. In this situation the alignment won’t work as marker 2 is not seen by unit 1. This might be because the marker is occluded by an object or because the unit is too low.
Once the visibility is good, open the Align Coda Co-ordinates dialogue by selecting Hardware > CODA alignment (see below).
The “Codamotion System Alignment” window is composed of five panels. The first panel is dedicated to the definition of the origin. The second and the third allow you to define the X-axis orientation and the last two allow you to define the X-Y plane. Note: each panel has a visibility indicator for each CODA unit in the system. The number of vertical bars in each panel indicates the number of active units that the selected configuration has. This bar will be used later as an indicator of marker visibility during the alignment.
Pictures 7 and 9 show the marker configuration (floor and Alignment dialogue box) when using three markers whereas pictures 8 and 10 show the marker configuration (floor and Alignment dialogue box) when using five markers.
In this example example, when using 3 markers, the parameters must be set as follow:
- Origin: Marker 1
- X-axis 1: Marker 1
- X-axis 2: Maker 2
- XY-plane 1: Marker 1
- XY-plane 2: Marker 3
When using 3 markers, the parameters must be set as follow:
- Origin: Marker 5
- X-axis 1: Marker 1
- X-axis 2: Maker 2
- XY-plane 1: Marker 3
- XY-plane 2: Marker 4
Enter the marker numbers in the appropriate boxes and click on “align”. When clicking on “Align”, the system will record 15 frames of data. All markers used during the alignment need to be in view during these 15 frames and thus by each individual Coda unit.
2.2 Successful Alignment
If the process works correctly then the message “Alignment created and stored. Successfully aligned at: xxx” will appear above the “Align” button and highlighted in green. Each vertical bar of each panel will become green and will indicate the degree of visibility of the markers seen by each unit: the brighter the intensity of the marker is, the brighter the green appears.
Click on “close” to close the alignment dialogue box. The system will automatically restart the current connection in order to load the new transformation matrix. This might take a few seconds. The alignment produced is stored as part of the configuration set up in the hardware server (part of RTNet). It will persist even if ODIN is restarted, or if ODIN is run from a different computer, provided the same hardware server is used with the same CODA units. If a hardware configuration is selected with different CODA units from that used for the original alignment, the user should perform a new alignment. Remember that the transformation matrix will become invalid if any CODA units are moved, even by a very small amount.
There is no automatic warning of misalignment of CODA units, but the “marker text” display will show a large error value (in the fourth column) for markers visible to at least two CODA units. For markers visible to more than one CODA unit simultaneously, an error value is displayed in the “marker text” display. This is the Euclidean distance between the average co-ordinates position of the marker and the one measured by each CODA unit.
Performing An Alignment With The Codamotion Alignment Jig
Codamotion can provide an alignment jig to be used when performing an alignment. It consists in a marker drivebox and three markers embedded in a special structured frame. It is composed of two arms that can be folded when unused. It is especially designed for aligning the system when using force plate(s). The frame has specially design slots that fit the force plate in order to perfectly aligned the markers to the plate.
Marker identities are generally set from 53 to 55. The alignment dialogue box should therefore be set as follow:
- Origin: Marker 53
- X-axis 1: Marker 53
- X-axis 2: Maker 54
- XY-plane 1: Marker 53
- XY-plane 2: Marker 55