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Selecting HSV frame rates Introduction Having a high speed video (HSV) system that runs at higher frame rates is generally better than having one of the basic slower models — after all, "high speed" is what high speed video is all about. And it never hurts to have a little extra “horsepower” when tackling a challenging job. But sometimes 200 frames per second (fps) is really enough, and the higher cost of a faster machine makes the purchase much more difficult to justify. Getting enough speed to do the job without overspending can seem pretty complicated.
To make matters even more confusing, each HSV application is unique in at
least some ways. But fortunately, there are some general guidelines to make
the decision easier. In this note, we’ll focus on high speed imaging in a
factory environment, but the same type of analysis is applicable to most motion
analysis tasks.
First Approximation The first factor we want to consider in selecting a frame rate is the speed of the production line. If we multiply the frame rate by 60 and divide the result by the line speed (in pieces per minute), we get the number of frames per piece that flows down the production line. As a general rule, we’d like to have at least ten to thirty images of each piece. In other words, fewer than ten frames is probably not enough to yield adequate information, and thirty or more frames is generally sufficient. Note that this is only a general guideline, and other factors (as discussed below) may affect the final decision.
The table below shows the number of frames per piece for a variety of frame
rates and line speeds. The marginal zone (ten to thirty
frames per piece) is shown in green and the
“inadequate” zone (less than ten frames per piece)is shown in bright blue.
Additional Considerations Although staying in the unshaded area of the table usually gives us enough information, other factors sometimes affect the frame rate requirements. One such factor is the duration of certain single events (e.g., explosions, impacts, ballistics). In these cases, multiplying the frame rate by the event duration yields the relevant number of frames and gives an indication of the information detail that will be captured. Another factor—one that is much more common, and therefore, much more important to consider—is acceleration. For instance, to study the wing motion of a hummingbird, a frame rate of 500 fps can slow down the little hummer enough to make it look more like a soaring hawk. But if we wanted to study the specific deformation of the tips of the wing feathers as the wings changed from their upward stroke to their downward stroke, we would need the increased detail that a higher frame rate would provide.
Back in the factory, acceleration can also be a critical factor. Each
individual piece on the line goes through a variety of motion changes
during fabrication, assembly, filling, labeling, and packaging operations,
even if the overall line speed is modest. In an extreme example, an
injection molding line may have a piece rate of less than ten pieces per
minute (suggesting that even a 30 fps standard video system might be adequate),
but the critical moment when the mold is released and the part is ejected can
have a huge effect on the quality of the product and the success of the
operation, so higher capture frames can be critical to the for optimizing
the operation.
HSV Product Selection Call Time Machines if you would like more help in determining your requirements. We offer a whole family of HSV products to serve nearly any application, including monochrome products at up to 8000 fps and color systems at up to 1000 fps. If budgetary limitations are a constraining factor, lease plans are available; and we can arrange generous upgrade and trade-in policies. Contact us for further information.
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