How to ensure the position of this pattern?

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Thanks for your answers.However,it is not easy to detect red circles. The fans have two diffrent color:red and blue,I don't know the reason why "houghcircles" can't work it well. Pictures of red fans have been added to the question.

You are correct, red does seem to be much more sensitive to settings. I suspect the more complicated background is to blame, in part. Both of the blue images are against white, mostly.

Ok, here's a thought.

• Filter your image, and binarize it with Threshold. (red or blue, whichever, do the other color after)
• Find the Centroid (or centroids, if you have multiple fans in image, you can segment)
• Convert the image to log-polar around the centroid. Each column of the image is a different radius circle from the centroid.
• At the largest radii, where the circle touches the three blades, you'll have three peaks.
• Take one of those, or average a section of columns together. If you're centered, they're evenly spaced, 120 degrees apart.
• You probably aren't, so use the inscribed angle theorem (Not sure how, can't be bothered to do the math more than enough to show it works.) to find the direction and distance to move to the center.

Does all that make sense?

Upon further reflection, I have decided that that is a "clever" solution, and is not to be trusted.

If you have lots of time, give it a shot.

Otherwise, I would focus on using a centroid method to generally localize until you're close, then tune the Hough Circles for the close up case. Perhaps do something to enhance the edges on the circles so they stand out more against the blades.

On the other hand, if you do try it, let me know, I'm interested to see if it works.

Okay,Thank your for your help. I will tell you the progress.