There are several issues/questions here. The Good News is you can probably "come close", but need to be a bit more specific about what you want and what you can tolerate.
You say your sinusoid is "about 3 Hz", which means that to get camera images at the peak and trough, you'd need to take 6 images per second. Yet your camera has a maximum speed of 14 fps. I'm assuming that you want a single image at the peak and trough, so you want an image (approximately) every 167 milliseconds. Assuming that the camera can do this, and that you can reliably "detect the peak", you still need to determine the (unknown, to me, at least) delay between the time you say "Snap" and the camera takes the image.
For the moment, assume you have already solved the problem of "detecting the peak", so you know when you'd like the image to be acquired. You can experimentally determine the delay by, say, feeding your 3 Hz waveform into an oscilloscope, synch the scope to the waveform (so you know, and can put a "dot on the screen" where the peak really is, and set the camera up to take an image at the Peak, at the Peak + 10 msec, at the Peak + 20 msec, etc. Now examine the images and see which is really closest to the Peak. [Yes, I know I said take the image after the Peak, but since your waveform is cyclic, a "delay" of 10 msec is equivalent to a "lead" of 167-10 msec ...]
So what's the easiest and potentially most accurate way to detect the peak? Again, if the signal is a sinusoid, don't detect the peak, instead detect the "zero-crossing" (which occurs before the peak, and hence if you need to take a picture before the peak, you have a little time). Look at a few cycles and determine the Max and Min (you might be lucky, or have designed things such that the mean is 0). Determine the Mean. Now you want to determine when you cross the mean. Note that the sinusoid will be changing at its fastest rate going through the midpoint, so you will get the most accurate time reading here.
You should now have all you need to accomplish your task (assuming that your Camera can "Snap" at a rate of at least 6 Hz). Note that if you didn't have such a narrow range between the max Camera rate (14 Hz) and the required acquisition rate (6 Hz), there's an even easier method to save the optimum image.
Suppose you needed pictures at the peak and trough of, say, an 0.1 Hz sinusoid, which has peaks and troughs occurring every 5 seconds. Set up your camera in video mode, give it a buffer of, say, 200 frames (probably overkill, but so what?) and start recording. Use your Peak Detector to tell you when the Peak occurs, determine the Buffer that corresponds to that Peak (or maybe the Buffer or two before that -- experiment with Delays), and save those Images.
Bob Schor
