gcary@SRC.Honeywell.COM (Greg Cary) (05/15/91)
I am constructing a compact disc jukebox for my CD collection, and I would very much appreciate some advice on location sensing. What I need to do is determine when I am passing by a known location, so that I can count off the right number of stepper motor steps so that I end up at the right CD. Two methods that come to my mind right away are 1) using a microswitch, and 2) interrupting the light between an LED emitter/detector pair. By far the most important thing is that I get repeatable results. How well do these methods work? What other methods are there? Thanks a lot in advance, Greg Cary gcary@SRC.honeywell.com office: (612)782-7683 FAX: (612)782-7438
dj@ctron.com (DJ Delorie) (05/15/91)
In article <1991May15.005403.28812@src.honeywell.com>, gcary@SRC.Honeywell.COM (Greg Cary) writes: > I am constructing a compact disc jukebox for my CD collection, and I > would very much appreciate some advice on location sensing. > What I need to do is determine when I am passing by a known location, so > that I can count off the right number of stepper motor steps so that > I end up at the right CD. Two methods that come to my mind right away > are 1) using a microswitch, and 2) interrupting the light between an > LED emitter/detector pair. By far the most important thing is that I > get repeatable results. > > How well do these methods work? What other methods are there? @set-mode(opinion,on) My printer uses optics. I've seen others with switches. If you get one of the optical modules designed for this (the ones with the slot in them), I'd expect them to be quite reliable and not succeptible to mechanical wear, damage, or tolerance changes. @set-mode(opinion,off) DJ dj@ctron.com @include(disclaimers)
smith@sctc.com (Rick Smith) (05/15/91)
gcary@SRC.Honeywell.COM (Greg Cary) writes: >What I need to do is determine when I am passing by a known location, so >that I can count off the right number of stepper motor steps so that >I end up at the right CD. Two methods that come to my mind right away >are 1) using a microswitch, and 2) interrupting the light between an >LED emitter/detector pair. By far the most important thing is that I >get repeatable results. I'd vote for 2) primarily because a set of reliable LED emitter/detector pairs will keep working under a broader range of conditions than a group of microswitches. I just imagine a microswitch getting jostled a bit so that it doesn't contact any more (tho' I guess a LED can get unreliable if it gets too dusty). Naturally, the perfect thing is to put a pair for sensing each slot in your library, thought that's probably too expensive. But don't skimp - have one at each end of your rack, and add a few in between so as to minimize the degree to which you trust positioning to the stepper motor. You might want to always find the closest LED to the slot and then step from there. Have fun. Rick. smith@sctc.com Arden Hills, Minnesota
winans@sirius.mcs.anl.gov (John Winans) (05/16/91)
In article <1991May15.005403.28812@src.honeywell.com> gcary@SRC.Honeywell.COM (Greg Cary) writes: >What I need to do is determine when I am passing by a known location, so >that I can count off the right number of stepper motor steps so that >I end up at the right CD. Two methods that come to my mind right away >are 1) using a microswitch, and 2) interrupting the light between an >LED emitter/detector pair. By far the most important thing is that I >get repeatable results. My old 8" cpm drives used leaf switches & to detect when the head was over track 0 (the head was moved via worm gear on a stepper motor) and they NEVER failed over the 8 or so times I moved to and from college in the early 80's. I havealso seen the LED version used in the positioning of a rotating disc that was part of an HO train turntable. For this to work, the positioning had to be more accurate than you would need for stepper motor use, & the turn table has been operating for more than 10 years. Either way should work fine. I would like to know how the stepper is used in this thing. Is it like a dot matrix printer (motor spins a pulley that has a string attached that runs the length of the page)? How many CD's are you going to hold in the thing? --John -- ! John Winans Advanced Computing Research Facility ! ! winans@mcs.anl.gov Argonne National Laboratory, Illinois ! ! ! !"The large print giveth, and the small print taketh away"-- Tom Waits !
jm59@prism.gatech.EDU (MILLS,JOHN M.) (05/16/91)
In article <1991May15.203301.18003@mcs.anl.gov> winans@sirius.mcs.anl.gov (John Winans) writes: >In article <1991May15.005403.28812@src.honeywell.com> gcary@SRC.Honeywell.COM (Greg Cary) writes: >>What I need to do is determine when I am passing by a known location, so >>that I can count off the right number of stepper motor steps so that >>I end up at the right CD. Two methods that come to my mind right away >>are 1) using a microswitch, and 2) interrupting the light between an >>LED emitter/detector pair. By far the most important thing is that I >>get repeatable results. If you can't trust the steppers, you can't repeat your positioning. I've seen the following method used for machine tool "home" indexing: (1) one detector is interrupted by the carriage [coarse], (2) second detector is interrupted by the rotational angle of the lead screw (if you have one) [medium], and (3) force the stepper coils to a predetermined excitation pattern [fine] (the use of fractional or partly redundant excitation patterns is sometimes used for high resolution, called "microstepping"). If the system can stop quickly enough, you may be able to logically gate the signals from (1) and (2), rather than sensing them separately. If that won't do it, you need another design. If the drive has appreciable backlash (mechanical looseness), always approach from the same direction: overstep and back up, if needed. This scheme will work with switches or optical interrupters, but the optical approach is preferred. Darlington or hysteresis-type buffers can be used with the opticals, but probably won't be needed (except for possible level shifting). For optical interrupters, I suggest Digi-Key (Thief River Falls, MN 56701-0677, or 1-800-DIGI-KEY (1-800-344-4539), or your local Hewlett-Packard electronics distributor (Newark, etc.), but have you checked with Radio Shack? (These things look like a little plastic block with two posts sticking up.) Digi-Key has a great catalog of ic's and piece-parts. Have fun! Regards --jmm-- -- MILLS,JOHN M. Georgia Institute of Technology, Atlanta Georgia, 30332 uucp: ...!{decvax,hplabs,ncar,purdue,rutgers}!gatech!prism!jm59 Internet: jm59@prism.gatech.edu
henry@zoo.toronto.edu (Henry Spencer) (05/16/91)
In article <1991May15.203301.18003@mcs.anl.gov> winans@sirius.mcs.anl.gov (John Winans) writes: >My old 8" cpm drives used leaf switches & to detect when the head was over >track 0 (the head was moved via worm gear on a stepper motor) and they NEVER >failed over the 8 or so times I moved to and from college in the early 80's. Note that the Canon laser printers -- aimed at a very cost-sensitive market by people who consistently produce high-quality equipment -- use optical interrupters for all such sensing jobs. For example, passage of paper is sensed by a little arm that moves a vane into or out of an interrupter. -- And the bean-counter replied, | Henry Spencer @ U of Toronto Zoology "beans are more important". | henry@zoo.toronto.edu utzoo!henry
wli@fornax.UUCP (William Li) (05/16/91)
In article <1991May15.005403.28812@src.honeywell.com>, gcary@SRC.Honeywell.COM (Greg Cary) writes: > I am constructing a compact disc jukebox for my CD collection, and I > would very much appreciate some advice on location sensing. > What I need to do is determine when I am passing by a known location, so > that I can count off the right number of stepper motor steps so that > I end up at the right CD. Two methods that come to my mind right away > are 1) using a microswitch, and 2) interrupting the light between an > LED emitter/detector pair. By far the most important thing is that I > get repeatable results. > How many different locations do you need to locate? If it's just one, then a single IR pair would be the best. If you need to locate several, consider using an optical rotary shaft encoder, essentially just a wheel with a lot of regularly-spaced slots and three pairs of IR tx/rx pairs -- two to get speed and direction of rotation, one to provide a sync pulse every complete rotation. This is a little expensive, though. Alternatively, you could try using a multi-turn potentiometer with its shaft slaved to the stepper somehow (eg, via a belt -- depends on what your exact setup is) and use the resistance to give you an absolute indication of position/rotation. If you're doing linear motion, you could use a pair of resistor rails. For your application, it doesn't sound as if you need ultra-high precision at all points in your travel. Steppers are generally very reliable and will miss step only if you're trying to apply too much torque to them or make them go too fast. You might be able to get away with using a counter, a spinning disk with one hole cut in it, and one IR tx/rx pair to provide a sync pulse for every rotation. Just some random thoughts. William Li School of Engineering Science Simon Fraser University Burnaby, B.C. Canada V5A 1S6 Tel: (604) 291-4451 Fax: (604) 291-4951
jack@rml.UUCP (jack hagerty) (05/17/91)
In article <1991May15.005403.28812@src.honeywell.com> gcary@SRC.Honeywell.COM (Greg Cary) writes: >I am constructing a compact disc jukebox for my CD collection, and I >would very much appreciate some advice on location sensing. >What I need to do is determine when I am passing by a known location, so >that I can count off the right number of stepper motor steps so that >I end up at the right CD. Well, let's see. Off of the top of my head I can come up with these: 1) Microswitches, which you mentioned. Cheap but clunky. 2) LED Emitter/receiver pair. Expensive since you need a lot of either emitters or receivers (whichever you put in front of the slot) 3) A better version of 2): a break beam pair mounted on the carriage with tabs in front of each stop. 4) A retro-reflective unit (emitter and receiver in one housing) with a target (a corner cube reflector or "reflector tape") at each position where you want to stop. 5) Non-optical variations on 4): a inductive proximity sensor on the carriage and a metal post at each position. Or a capacitance sensor with non-metal posts, or magnetized posts with a reed switch or Hall effect switch on the carriage, or... 6) A linear position sensor. This is a little box with a wire that plays out like a tape measure. The motion is converted to rotary and encoded internally, but you don't have to know that. They come in lengths far longer than a home CD storage would require. 7) The trickest one of all: mount a bar code reader on the carriage (the non moving type like a wand, since the carriage is doing the moving) and put a label on each position. That way you could encode each slot with a discrete name and know not just that you got there, but who's home! - Jack ============================================================================= ||Jack Hagerty, Robotic Midwives, Ltd. jack@rml.UUCP (smart mailers)|| ||Livermore, CA ...!uunet!lll-winken!rml!jack (dumb mailers)|| ||(415) 455-1143 jack%rml@lll-winken.llnl.gov (desperate mailers)|| ||-------------------------------------------------------------------------|| || "The Biblical God is a sloppy manufacturer. He's not good at design, || || He's not good at execution. He'd be out of business if there was || || any competition." - Carl Sagan, _Contact_ || =============================================================================
ee8kh@gdt.bath.ac.uk (K House) (05/17/91)
smith@sctc.com (Rick Smith) writes: STUFF DELETED >Naturally, the perfect thing is to put a pair for sensing each slot >in your library, thought that's probably too expensive. But don't >skimp - have one at each end of your rack, and add a few in between >so as to minimize the degree to which you trust positioning to the >stepper motor. You might want to always find the closest LED to the >slot and then step from there. Surely you just use one slotted opto-switch on the moving part. You have little beam breakers between each disc. Cheaper methinks >Rick. >smith@sctc.com Arden Hills, Minnesota Kevin -- "I hear the word for love, I hear the word for death, ___ But I don't hear any answers." - All About Eve / / )__ __ ^__ __ _________________________________________________________/ / ((_(( (((_((_.____