FRAGTrack™ - Determining lens focal length on face shovel

Note: The Orica FRAGTrack System has been designed for mounting on Conveyors and Face Shovels, not on Excavators.

To explain why Excavators are not supported, please compare the two diagrams below.


You will see that Face Shovels (Figure 1) are on the same level as the dig face, providing the F60 Camera Unit with a clear view of the muck rill, allowing for image capture. Excavators (Figure 2) sit on the edge of the area they are digging. The area that needs to be captured by the F60 Camera Unit is below the bottom of the Excavator, making image capture unworkable without some kind of a boom/extension system for the unit to sit out on.


Lens Focal Length

The steps to decide on the lens focal length to order for your FRAGTrack System are:

  1. Consult the document ‘Camera Specifications for Different Shovel Types’ located on the Field Installation & Support Knowledge Base (Orica internal only) page.
  2. If the Face Shovel model and F60 Camera Unit installation location is listed in that document, use the recommended lens focal length given there. Otherwise, continue with this procedure.
  3. Download the specification drawing of the Face Shovel showing the bucket in the resting position and marked with the measurement grid. Example: image-3.png
  4. Determine where on the Face Shovel the F60 Camera Unit is to be mounted. The usual locations are:
    a. Cab roof
    b. Gantry walkway image-4.png
  5. Using the provided measurements to the resting position of the bucket, and Pythagoras’ Theorem, calculate the distance to the teeth of the Face Shovel’s bucket. Example: image-5.png
  6. Using the table below (which is taken from the ‘Lens Selector’ spreadsheet on the FI&S Sharepoint page - Orica internal only) decide on which lens focal length to use. If it is on the threshold between two lengths, refer to the Minimum Bin Size section below. image-6.png For the Face Shovel in the diagram above, we calculate the distance as approximately 7m, meaning a lens focal length of 12.5mm should be ordered.
    However, specifying a lens focal length also places a limit on the minimum bin size that can be used by the FRAGTrack system. See the next section for more details.

Minimum Bin Size

The FRAGTrack system requires the smallest analysis bin to contain at least 8 pixels. Fixing the lens focal length and camera distance also determines the pixel size on the muck pile image. Hence there is a lower limit on the smallest bin size that the system can use. Equivalently, if we fix the minimum bin size (and focal length) then there is an upper limit on the camera distance above which the system will not trigger.

The relationship between focal length, camera distance and minimum bin size is given on the Min Bin Size sheet of the Lens Selector spreadsheet, a copy of which is shown below.

Minimum Bin Size

For example, a focal length of 12.5mm and a muck pile distance of 7m means that the minimum bin size is 35mm. (In fact, with this bin size, the system will trigger at distances up to 7.9m.)

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