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I have been asked to help with the design of a small (tiny) winch system intended for use in very harsh automotive road conditions.

This system is required to raise or lower an assembly with a total travel of about 18 inches. My task is to come up with a method of setting the upper and lower limits.

One of the options that we have come up with is purely mechanical: the winch motor turns a threaded rod (all-thread) which has a traveler nut. The nut hits a micro-switch at each end of the travel. Upper and lower limits are set by sliding the micro-switches to the appropriate position at each end of the threaded rod.

Although this system should be reliable, it is both large (long) and hard to seal. I'm trying to come up with something better.

Counting motor rotation is dead easy. This gives me a way to control the total distance that the winch cable travels. What I need is some method of coming up with an index point.

Suggestions gratefully accepted.

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  • \$\begingroup\$ How about an absolute position encoder? \$\endgroup\$ – Tyler Mar 17 '17 at 15:22
  • \$\begingroup\$ Absolute position encoder on the motor shaft would tell me where I am within a single turn of the motor shaft. However, the motor shaft will turn several revolutions to reach from end to end of the travel. \$\endgroup\$ – Dwayne Reid Mar 17 '17 at 18:46
  • \$\begingroup\$ You can get multi-turn absolute position encoders. \$\endgroup\$ – Tyler Mar 17 '17 at 18:50
  • \$\begingroup\$ @Tyler: Link, please? \$\endgroup\$ – Dwayne Reid Mar 17 '17 at 18:57
  • \$\begingroup\$ This is just the first one I happened to click on... beisensors.com/pdfs/hmt25-optical-absolute-encoder.pdf "4096 counts per turn and can count up to 4096 turns" \$\endgroup\$ – Tyler Mar 17 '17 at 19:00
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My suggestion is to use a speed reductor, mounted on a shaft of the winch, if possible. The reduction ratio then determines the total length of the cable. This method is used in industry with cranes, lifting table. You then have to put the limit switches on the stroke end.

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  • \$\begingroup\$ If this is how its done on a crane (with all the associated safely implications) this must be the optimum solution. \$\endgroup\$ – Paul Uszak May 23 '17 at 20:58
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Issue is winches do not always wind evenly so just a measure of how much turn on the winch may be inaccurate for multiple turns.

If so, you need to feed the cable through a metering roller that has some form of encoder on it to accurately measure the amount of cable fed out or in.

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If, because of other design considerations, you can guarantee the cable will always be in good contact with the metering roller, you may be able to skip the idler arm and roller parts.

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With a wide cleat to block light on either side of the cable, an IR optical interrupter with a narrow beam and daylight blocking is effective with recessed emitter detectors in a plastic housing can block direct sunlight.

schematic

simulate this circuit – Schematic created using CircuitLab

One can use a single emitter or dual for redundancy and dual detector to measure a difference in voltage from one side blocked or logical OR from either side blocked with calibrated levels determined by selected load resistors in PD such as SFH 229FA detectors.

I would try the differential method using a comparator with hysteresis then variations of 2:1 in emitter level or more would have minimal effect.

Optical design is key in compact mechanical cases.

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    \$\begingroup\$ Issue with any optical design though is keeping it clean... nothing wrong with the answer though... just a reminder. OP did say "very harsh road conditions".. that would include slush, snow, ice, dirt, water etc on the cable I am assuming. \$\endgroup\$ – Trevor_G Mar 17 '17 at 16:33

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