pkahn@ADS.COM (Philip Kahn) (05/30/91)
I have two axes that I am controlling with steppers. The steppers are
to be operated open loop, but I need to have some home and limit
switches. Thing is, they're a pain in the butt to mount, and it winds
up not to be so very cheap when at least 4 are required.
An easy way out of this is to find a really cheap, repeatable shaft
encoder that effectively will provide me with "multiple limit" positions.
I don't want to spend more than $10 per in quantity, they should slide
over the shaft, and I don't need good resolution (obviously), though
repeatability probably means it should be optical. HP makes some nice
inexpensive encoders (that have great resolution), though the $25 per
is too pricey. Any suggestions??
thanks,
phil...
P.S.: I don't think a pot and A/D has the repeatability I need, though
I'm eager to be proven wrong :-)mike@wang.com (Mike Sullivan) (05/31/91)
pkahn@ADS.COM (Philip Kahn) writes: >An easy way out of this is to find a really cheap, repeatable shaft >encoder that effectively will provide me with "multiple limit" positions. >I don't want to spend more than $10 per in quantity, they should slide >over the shaft, and I don't need good resolution (obviously), though >repeatability probably means it should be optical. HP makes some nice >inexpensive encoders (that have great resolution), though the $25 per >is too pricey. Any suggestions?? When I were a lad... Back when I worked for Real World Interface we built our own shaft encoders by drilling holes in a gear blank ( any number you like ) and ran it through a opto-isolator ( led-reciever pair ). This requires a little mounting hardware but you ought to able to squeak under the $10 mark. But you tell kids that these days and they won't believe ya! ;-) HTH mike -- ______________________ / __ __ \ | Michael J. Sullivan | Love all | | \ /\ |\ | / ` | | Independent | Serve all | |__/ /--\ | \| \__T | | mike@WANG.COM | Save the Planet \______________________/ | FUN FUN FUN FUN FUN FUN| - Rock Hard -
hbg6@citek.UUCP (John Schuch) (06/05/91)
>pkahn@ADS.COM (Philip Kahn) writes: > >..... HP makes some nice >inexpensive encoders (that have great resolution), though the $25 per >is too pricey. Any suggestions?? ^^^^^^ What shaft encoder is $25 and in what quantity? All the ones I have found are at least 5 times that much. In my application I'd rather the encoder have its own shaft. John
jackm@agcsun.UUCP (Jack Morrison) (06/15/91)
I missed the original posting, but assuming the Subject line covers it, you might consider the method some satellite dish positioners use. As I understand it, they have a magnet on the shaft, and a reed switch that sees it go by each revolution. Then just count the transitions. Not much resolution, but you could put the magnet on a separate shaft that was geared to spin several times for each main shaft revolution. And/or use a disk with several magnets mounted around the circumference. If this violates someone's patent, you didn't hear it from me ;-) -- "How am I typing? Call 1-303-279-1300" Jack C. Morrison Ampex Video Systems 581 Conference Place, Golden CO 80401
klaus@captain-crunch.ai.mit.edu (Klaus B. Biggers) (06/15/91)
In article <966@agcsun.UUCP> jackm@agcsun.UUCP (Jack Morrison) writes: >As I understand it, they have a magnet on the shaft, and a reed switch that >sees it go by each revolution. Then just count the transitions. ^^^^^ You will need to use some method to determine which direction to count, quadrature of some type, if your system is reversible, otherwise, when the system reverses, you will continue to count up when you should be counting down.... I hate it when that happens!! -klaus klaus@snowhite.ced.utah.edu _______________________________________________________________________________
mack@intvax.UUCP (Michael J. McDonald) (06/17/91)
It really depends on how cheap you think cheap is and what your application really looks like. For example, if your system only turns in one direction then you don't have to worry about quad detection and you can use a single switch style encoder. However, If you started using several switches, then you might quickly end up with a system that is cheaper than a standard encoder. One person suggested gearing, but often geared systems are more expensive than just using a simpler (but seemingly more expensive) optical encoder. If I were developing a home-brew encoder, I would probably use a knotched disk attached to the shaft and a radio-shack quality photo transistor/detector pair reading across the gaps. Quad detect would be accomplished by using two sensing points and spacing them so that you got on-on at one detector's transition when the motor was turning one way and on-off when turning the other way. If I were designing system (not Home-brew), I would probably contact companies like HP, and BEI for encoder information as well as companies that make magnetic devices (Red Lion) and Wiggand Effect Sensors (SensoTec) and ask them what the cheapest system would be for YOUR application.
tonyo@sagerat.CNA.TEK.COM (Tony Ozrelic) (06/18/91)
Regarding cheap shaft encoders: The Do-It-Yourself Type: Make a small plastic disk, about 2 inches in diameter. Paint one half of the disk black, leave the other half clear, or glue aluminum foil to it (for reflective sensors). Position 2 optical sensors at 90 degrees to each other. The outputs will give you 4 pulses per rev after decoding. Mount on shaft of motor by drilling hole in center just a tad (smidge, in the metric system) smaller and force-fitting the disk onto the shaft. A drop of super glue does the rest. The Cheap-Off-The-Shelf Type: Buy a Bourns Optical Encoder from Digi-Key. For $25 you can get a 128-pulses-per rev encoder. Decoding the outputs is left as an exercise for the student :-) tony "i hate it when he says that" o.
dmturne@PacBell.COM (Dave Turner) (06/19/91)
In article <3755@intvax.UUCP> mack@intvax.UUCP (Michael J. McDonald) writes: >If I were developing a home-brew encoder, I would probably use a >knotched disk attached to the shaft and a radio-shack quality photo >transistor/detector pair reading across the gaps. Quad detect would An article in the July, 1980 Ham Radio magazine told how to make a simple optical shaft encoder. The article, "Rotary-dial mechanism for digitally tuned transceivers" used a photodisk with 50 and 100 marks per revolution. It included circuitry to detect rotation and direction of rotation. It could be setup for 50, 100 or 200 pulses per revolution. The photo disk was available for $1.00 from the author. The 50 mark disk was about 2.5 inches in diameter; the 100 mark disk was 3.5 inches diameter. A query to an online HAM callbook shows that his address has changed. Anyone interested in the photodisk (and maybe a copy of the HR article) might want to write to him (include a SASE) at: Chet B. Opal, K3CU 4900 Timberline Drive Austin, TX 78746 He said in his article that he welcomed letters. -- Dave Turner 415/823-2001 {att,bellcore,sun,ames,decwrl}!pacbell!dmturne
ear@wpi.WPI.EDU (Eric A Rasmussen) (06/19/91)
In article <6185@ptsfa.PacBell.COM> dmturne@PacBell.COM (Dave Turner) writes: >The photo disk was available for $1.00 from the author. The 50 mark disk >was about 2.5 inches in diameter; the 100 mark disk was 3.5 inches diameter. Excuse me if I am missing some subtle flaw in reasoning, but I think I have just come up with a very simple and cheap method of producing the optical disk part of an optical encoder. Steps ----- 1) Use a CAD package, drawing program, or whatever to design the simple radial lines and circle pattern which will appear on the disk. 2) Make a good dark printout of said design on a laser printer. 3) Convert this to a transparency such as used for overhead projectors with one of those thermal things often found in the office closet. 4) Cut the design out of the plastic. Bingo, you have a clear plastic disk with your lines. Some reenforcement might be needed, such as using a transparent glue to attach it to a second layer of clear plastic or glass, but other than that I can't think of a good reason why it wouldn't work. Furthermore, you could easily customize the resolution to suit your needs. +---------< Eric A. Rasmussen - Mr. Neat-O (tm) >---------+ +< Email Address >+ | A real engineer never reads the instructions first. | | ear@wpi.wpi.edu | | (They figure out how it works by playing with it.) | | ear%wpi@wpi.edu | +---------------------------------------------------------+ +-----------------+ ((( In Stereo Where Available )))
ruspini@nas.nasa.gov (Diego C. Ruspini) (06/20/91)
In article <6185@ptsfa.PacBell.COM> dmturne@PacBell.COM (Dave Turner) writes: >The photo disk was available for $1.00 from the author. The 50 mark disk >was about 2.5 inches in diameter; the 100 mark disk was 3.5 inches diameter. Here is a Postscipt program I wrote a while back to make encoder disks of any size and with as many slots as you want. It probably a bit clunky but its pretty easy to change any options that you wish to make different. Edit the example calls to the subroutine "drawdisk" to fit your personal needs and print it out on transparency film ( make sure your laser printer can handle transparency film). If your printer can't handle transparency, xerox it onto a transparency, this is what I did, the quality is not great but appears to work. I then glued this to a piece clear plastic which you can find in almost any hobby store. If your realy cheap you can use the plastic from a cover of a cassette tape or CD. With this a couple of optical encoders a 7414 & a 74121 you can make yourself a good shaft encoder, dirt cheap. I also had a Postscript program that would produce disks for absolute shaft encoders in gray code, but I can't seem to locate it now. If enough people are interested I can look at some dusty old tapes I have and see if I can scrounge it up. PROGRAM FOLLOWS ----------CUT HERE----------- %! %%Creator: Diego C. Ruspini %%Title: Encoder Disk Maker %%CreationDate: 4/10/88 % Invoke drawdisk as follows % <x-loc> <y-loc> <radius> <#-slots> drawdisk % see examples below /inch { 72 mul } def /cm { 28.3465 mul } def /mm { 2.83465 mul } def /outerratio 0.95 % Outer slot radius as percent of outside diameter /gap 0.2 inch def % Size of slot /center 0.01 inch def % Radius of center hole /drawdisk { /segments exch def /outerradius exch def /yorigin exch def /xorigin exch def /osegment 0.95 outerradius mul def /isegment osegment gap sub def 0 setgray xorigin yorigin outerradius 0 360 arc fill 1 setgray xorigin yorigin osegment 0 360 arc fill 0 setgray /slice 360 segments div def 0 1 segments 1 sub { /segment exch def newpath xorigin yorigin moveto xorigin yorigin outerradius segment slice mul segment slice mul slice 2 div add arc xorigin yorigin lineto fill } for xorigin yorigin isegment 0 360 arc fill 1 setgray xorigin yorigin center 0 360 arc fill } def % Example disks 2 inch 2 inch 0.5 inch 16 drawdisk 4 inch 2 inch 0.5 inch 8 drawdisk 6 inch 2 inch 0.5 inch 32 drawdisk 2.5 inch 4 inch 3 cm 32 drawdisk 5 inch 4 inch 25 mm 64 drawdisk % Replace with disks of your own choosing showpage % dont delete this line
mk59200@cc.tut.fi (Kolkka Markku) (06/20/91)
In article <1991Jun19.044959.23200@wpi.WPI.EDU> ear@wpi.WPI.EDU (Eric A Rasmussen) writes: >Excuse me if I am missing some subtle flaw in reasoning, but I think I have >just come up with a very simple and cheap method of producing the optical disk >part of an optical encoder. > >Steps >----- >1) Use a CAD package, drawing program, or whatever to design the simple radial >lines and circle pattern which will appear on the disk. >2) Make a good dark printout of said design on a laser printer. >3) Convert this to a transparency such as used for overhead projectors with >one of those thermal things often found in the office closet. >4) Cut the design out of the plastic. Yes, this should work fine if you don't need very fine resolutions. If you have access to a Linotronic phototypesetter you could output the encoder disk directly to film. If you are a software hacker, write a PostScript program to output the encoder wheel direcly. I once made a 8-bit Graycode encoder this way. -- Markku Kolkka mk59200@cc.tut.fi