This does sound like a good ideaInstead of counting the reel, a digital encoder could be used on the string , these come in various resolutions (pulses/rev) and also an option of a one/rev marker pulse to replace the hall sensors.
The encoder would monitor the string as it traveled over a couple of rollers that had the encoder attached, the resolution could be tailored by way of roller dia etc.
The (high resolution) counting pulses could be used to detect any stoppage of the encoder, as their resolution would be much high than the marker counter.
Without knowing exact details, just an option to work with.
Max.
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