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Will this laser idea work? On topic
This may seem off topic but it's not really. Measuring is important to
machining. And I need to make a measurement without putting pressure on the item being measured. Now, for the purists, I know that light will put a little pressure on what I'm measuring, but this will have way less effect than any kind thing I can measure. I have an encoder that outputs 10,000 pulses per revolution. With a shaft measuring ..19515" (radius) each pulse equals .0001 on the circumference. So, if the radius keeps multiplying by 10 then the amount traveled by the circumference will increase by 10. Here's the plan: Put a mirror on the shaft. Shine a laser at the mirror. Adjust the mirror until the laser is visible on a target 1591.5" (132.625 feet) away. This will increase the amount traveled to .1" for every pulse. So, measuring the distance the spot travels will show how many pulses should be generated. The reason for this measurement is to rule out backlash in the encoder. It appears that there is a 9 pulse error. In other words, if the encoder shaft is turned one way and the count is noted, and then turned the other way until the count changes, it appears that the shaft turns an amount equal to 9 pulses. I need to rule this out because this is the error I'm getting is a mechanical system and it appears that all the mechanical lash has been reduced to less than ..0001". The last thing seems to be the encoder itself. Thank You, Eric R Snow, E T Precision Machine |
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Eric,
This should work fine. Note that as the mirror moves through angle A, the reflected beam moves through angle 2A, so that if the target is 159 inches from the mirror and the mirror moves 360 degrees/10,000 = 0.000628 radians, the laser spot moves 2* 0.000628 * 159 inches = 0.2 inches. Should be easy to see. Don Holly Eric R Snow wrote: This may seem off topic but it's not really. Measuring is important to machining. And I need to make a measurement without putting pressure on the item being measured. Now, for the purists, I know that light will put a little pressure on what I'm measuring, but this will have way less effect than any kind thing I can measure. I have an encoder that outputs 10,000 pulses per revolution. With a shaft measuring .19515" (radius) each pulse equals .0001 on the circumference. So, if the radius keeps multiplying by 10 then the amount traveled by the circumference will increase by 10. Here's the plan: Put a mirror on the shaft. Shine a laser at the mirror. Adjust the mirror until the laser is visible on a target 1591.5" (132.625 feet) away. This will increase the amount traveled to .1" for every pulse. So, measuring the distance the spot travels will show how many pulses should be generated. The reason for this measurement is to rule out backlash in the encoder. It appears that there is a 9 pulse error. In other words, if the encoder shaft is turned one way and the count is noted, and then turned the other way until the count changes, it appears that the shaft turns an amount equal to 9 pulses. I need to rule this out because this is the error I'm getting is a mechanical system and it appears that all the mechanical lash has been reduced to less than .0001". The last thing seems to be the encoder itself. Thank You, Eric R Snow, E T Precision Machine |
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On Mon, 3 Oct 2005 11:30:22 -0400, Ned Simmons
wrote: In article , says... This may seem off topic but it's not really. Measuring is important to machining. And I need to make a measurement without putting pressure on the item being measured. Now, for the purists, I know that light will put a little pressure on what I'm measuring, but this will have way less effect than any kind thing I can measure. I have an encoder that outputs 10,000 pulses per revolution. With a shaft measuring .19515" (radius) each pulse equals .0001 on the circumference. So, if the radius keeps multiplying by 10 then the amount traveled by the circumference will increase by 10. Here's the plan: Put a mirror on the shaft. Shine a laser at the mirror. Adjust the mirror until the laser is visible on a target 1591.5" (132.625 feet) away. This will increase the amount traveled to .1" for every pulse. So, measuring the distance the spot travels will show how many pulses should be generated. The reason for this measurement is to rule out backlash in the encoder. It appears that there is a 9 pulse error. In other words, if the encoder shaft is turned one way and the count is noted, and then turned the other way until the count changes, it appears that the shaft turns an amount equal to 9 pulses. I need to rule this out because this is the error I'm getting is a mechanical system and it appears that all the mechanical lash has been reduced to less than .0001". The last thing seems to be the encoder itself. Thank You, Eric R Snow, E T Precision Machine It sounds like it would work, but aren't you off by a factor of 10 on the distance to the target? It looks to me like 1/10000 of a rev at the shaft would move the spot 1" at 1591". Do you have access to a microscope you could fixture to look at a paper disk with a reference mark attached to the encoder shaft? Or if the guts of the encoder are accessible, you could look directly at the encoder disc. You don't need much magnification to resolve .001". All that said, unless something is very wrong with the encoder, I doubt you'll find anything other than a very small amount of hysteresis in the detector. I apologize for not responding further to your earlier requests, but really didn't have much to add other than to stress the need to pay attention to everything that could possibly go wrong when working to tenths, which I know you already know. Which method of driving the encoder did you settle on? Ned Simmons Boy, I need to proofread my posts. There were a lot of errors in the above. Especially the times ten answer. You are right, 132 feet would show 1 inch. Which is what I want. The driving method so far is a belt. I can detect no slipping at all. But I keep getting this .0009 error. I don't know if this is a display error because it's reading the encoder disc in quadrature. I thought about looking at the disc itself. It's 2.000 inches in diameter with 2500 marks on it. which makes the spacing about .0025". This is readable with a magnifier. But I thought that amplifying the error would be easier because I can see the movement at the same time as the display changes. I can very easily see 9 inches movement at 132 feet. And it turns out I have a target just about that far away. A nice clean cement wall. Thanks for reading and responding. Eric |
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SNIP
If you've got an o'scope you can look directly at the encoder channels to eliminate any possibility that it's the readout, if that's what you mean by display error. Less convenient, but workable, would be a voltmeter or two. Or even an LED on each channel, as long as you're careful to limit the current based on the encoder's drive capability. Ned Simmons Ned, I have a scope. I don't really know how to use it. I need to get a book on oscilloscopes in general. The scope is a Tektronix 465B. The only probe I have is one with a red and black wire terminating in little hook shaped clamps. If this will work can you tell me how? Thanks, Eric |
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Hey Eric,
I think you "typo-ed" two places. The first mention of the shaft radius should be 0.159154943, rather than .19something; and later on you effectively multiply 0.0001 X 10,000 and get 0.1 instead of a whole number "1". These didn't seem to affect the idea though, except what does fastening a mirror onto the shaft do for you? Why not just fasten the laser to the shaft, and measure the laser-beam movement at the 1591.54943" point as 1". If you don't have a 132 foot distant surface available, then you might consider using mirrors to multiply/divide that distance to something more practical, or better yet, make the 132 foot point right near where you are going to be working?. As his is a "counting" discovery, you don't need much "wall" space. You can count forward and backward 18 pulses in 3 feet of wall, or along a yard-stick!! Interesting. Let us know how you make out. I've had very little to do with encoders except use in determining speed and the derived acceleration and deceleration, and they were 1024 X 2 models. Any chance that two encoders on the same shaft would help you? That's how we determined direction. Take care. Brian Lawson, Bothwell, Ontario On Mon, 03 Oct 2005 07:50:45 -0700, Eric R Snow wrote: This may seem off topic but it's not really. Measuring is important to machining. And I need to make a measurement without putting pressure on the item being measured. Now, for the purists, I know that light will put a little pressure on what I'm measuring, but this will have way less effect than any kind thing I can measure. I have an encoder that outputs 10,000 pulses per revolution. With a shaft measuring .19515" (radius) each pulse equals .0001 on the circumference. So, if the radius keeps multiplying by 10 then the amount traveled by the circumference will increase by 10. Here's the plan: Put a mirror on the shaft. Shine a laser at the mirror. Adjust the mirror until the laser is visible on a target 1591.5" (132.625 feet) away. This will increase the amount traveled to .1" for every pulse. So, measuring the distance the spot travels will show how many pulses should be generated. The reason for this measurement is to rule out backlash in the encoder. It appears that there is a 9 pulse error. In other words, if the encoder shaft is turned one way and the count is noted, and then turned the other way until the count changes, it appears that the shaft turns an amount equal to 9 pulses. I need to rule this out because this is the error I'm getting is a mechanical system and it appears that all the mechanical lash has been reduced to less than .0001". The last thing seems to be the encoder itself. Thank You, Eric R Snow, E T Precision Machine |
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On Mon, 03 Oct 2005 11:21:42 -0500, don holly
wrote: Greetings Don, I don't understand exactly. Oh, yes I do. I drew a picture and it's all clear. Duh. With the mirror at 45 degrees the included angle is 90. And the mirror only has to turn 45 degrees to make the light shine directly back on itself. Thanks, Eric Eric, This should work fine. Note that as the mirror moves through angle A, the reflected beam moves through angle 2A, so that if the target is 159 inches from the mirror and the mirror moves 360 degrees/10,000 = 0.000628 radians, the laser spot moves 2* 0.000628 * 159 inches = 0.2 inches. Should be easy to see. Don Holly Eric R Snow wrote: This may seem off topic but it's not really. Measuring is important to machining. And I need to make a measurement without putting pressure on the item being measured. Now, for the purists, I know that light will put a little pressure on what I'm measuring, but this will have way less effect than any kind thing I can measure. I have an encoder that outputs 10,000 pulses per revolution. With a shaft measuring .19515" (radius) each pulse equals .0001 on the circumference. So, if the radius keeps multiplying by 10 then the amount traveled by the circumference will increase by 10. Here's the plan: Put a mirror on the shaft. Shine a laser at the mirror. Adjust the mirror until the laser is visible on a target 1591.5" (132.625 feet) away. This will increase the amount traveled to .1" for every pulse. So, measuring the distance the spot travels will show how many pulses should be generated. The reason for this measurement is to rule out backlash in the encoder. It appears that there is a 9 pulse error. In other words, if the encoder shaft is turned one way and the count is noted, and then turned the other way until the count changes, it appears that the shaft turns an amount equal to 9 pulses. I need to rule this out because this is the error I'm getting is a mechanical system and it appears that all the mechanical lash has been reduced to less than .0001". The last thing seems to be the encoder itself. Thank You, Eric R Snow, E T Precision Machine |
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On Mon, 03 Oct 2005 13:33:52 -0400, Brian Lawson
wrote: Hey Eric, I think you "typo-ed" two places. The first mention of the shaft radius should be 0.159154943, rather than .19something; and later on you effectively multiply 0.0001 X 10,000 and get 0.1 instead of a whole number "1". These didn't seem to affect the idea though, except what does fastening a mirror onto the shaft do for you? Why not just fasten the laser to the shaft, and measure the laser-beam movement at the 1591.54943" point as 1". If you don't have a 132 foot distant surface available, then you might consider using mirrors to multiply/divide that distance to something more practical, or better yet, make the 132 foot point right near where you are going to be working?. As his is a "counting" discovery, you don't need much "wall" space. You can count forward and backward 18 pulses in 3 feet of wall, or along a yard-stick!! Interesting. Let us know how you make out. I've had very little to do with encoders except use in determining speed and the derived acceleration and deceleration, and they were 1024 X 2 models. Any chance that two encoders on the same shaft would help you? That's how we determined direction. Take care. Brian Lawson, Bothwell, Ontario Brian, The mirror is about 1/2 inch on a side and .09 thick. The laser is 1.5 inches dia and 8 inches long. With wires coming out. I feel it may have more influence on shaft movement if mounted directly to the shaft than the mirror will. Direction is determined by only one encoder. It has two readheads out of phase with each other. So A leads B turning one direction and B leads A in the other. Thanks, Eric On Mon, 03 Oct 2005 07:50:45 -0700, Eric R Snow wrote: This may seem off topic but it's not really. Measuring is important to machining. And I need to make a measurement without putting pressure on the item being measured. Now, for the purists, I know that light will put a little pressure on what I'm measuring, but this will have way less effect than any kind thing I can measure. I have an encoder that outputs 10,000 pulses per revolution. With a shaft measuring .19515" (radius) each pulse equals .0001 on the circumference. So, if the radius keeps multiplying by 10 then the amount traveled by the circumference will increase by 10. Here's the plan: Put a mirror on the shaft. Shine a laser at the mirror. Adjust the mirror until the laser is visible on a target 1591.5" (132.625 feet) away. This will increase the amount traveled to .1" for every pulse. So, measuring the distance the spot travels will show how many pulses should be generated. The reason for this measurement is to rule out backlash in the encoder. It appears that there is a 9 pulse error. In other words, if the encoder shaft is turned one way and the count is noted, and then turned the other way until the count changes, it appears that the shaft turns an amount equal to 9 pulses. I need to rule this out because this is the error I'm getting is a mechanical system and it appears that all the mechanical lash has been reduced to less than .0001". The last thing seems to be the encoder itself. Thank You, Eric R Snow, E T Precision Machine |
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On Mon, 03 Oct 2005 17:56:08 GMT, xray
wrote: On Mon, 03 Oct 2005 10:33:39 -0700, Eric R Snow wrote: SNIP If you've got an o'scope you can look directly at the encoder channels to eliminate any possibility that it's the readout, if that's what you mean by display error. Less convenient, but workable, would be a voltmeter or two. Or even an LED on each channel, as long as you're careful to limit the current based on the encoder's drive capability. Ned Simmons Ned, I have a scope. I don't really know how to use it. I need to get a book on oscilloscopes in general. The scope is a Tektronix 465B. The only probe I have is one with a red and black wire terminating in little hook shaped clamps. If this will work can you tell me how? Thanks, Eric What encoder are you using, and what is it connected to? A US Digital H6S-2500-I. And I realize I made a bunch of errors in my first post. I am using the ED3 display from US Digital too. I need to eliminate the encoder as a source of error before I continue. I don't think it's the problem but so far it seems like the culprit. Eric |
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On Mon, 3 Oct 2005 14:22:21 -0400, Ned Simmons
wrote: In article , says... SNIP If you've got an o'scope you can look directly at the encoder channels to eliminate any possibility that it's the readout, if that's what you mean by display error. Less convenient, but workable, would be a voltmeter or two. Or even an LED on each channel, as long as you're careful to limit the current based on the encoder's drive capability. Ned Simmons Ned, I have a scope. I don't really know how to use it. I need to get a book on oscilloscopes in general. The scope is a Tektronix 465B. The only probe I have is one with a red and black wire terminating in little hook shaped clamps. If this will work can you tell me how? You don't really need probes for this, Eric, though you can use the one you've got on one channel. Plug it into CH1 on the scope and hook it to channel A of the encoder with the encoder connected to the depowered readout. (I'm assuming this is not a differential encoder - it seems unlikely - you'd have wires called A and A', and B and B', or the like if it is.) Connect the ground clip to the readout chassis or signal ground. All you need for encoder channel B is to make a connection to the center pin of the scope's CH2 BNC connector. If you don't have a mating connector, you can stick a pin in the center of the connector and use a jumper, just be careful not to damage that nice old scope. Set the toggle under the VOLTS/DIV knob to GND on both channels. Push in both CH1 & CH2 pushbuttons on the left of the screen, all the rest in that group should remain out. Set the TRIG MODE to AUTO. Set the TIME/DIV KNOB to around 1ms. Make sure the coaxial knobs are locked together. Now fiddle with the POSITION controls way over on the left to get the two traces separated on the screen. If you don't get the traces, try fussing with the SOURCE and LEVEL on the lower right. (I'm winging this by looking at a picture of a 465B that Google found for me.) Set the toggle under the VOLTS/DIV knob to DC on both channels. Set the VOLTS/DIV knob to 2V on both channels. If the probe is a 10:1, set that channel to .2V Turn the readout power on. You should the encoder transitions when you turn the shaft. Play with the trigger settings if you want to synch the sweep to the signal with continuous rotation. That should get you started. Don't be afraid to play with the knobs, you can't hurt anything. Hopefully someone else will chime in if I've missed anything obvious. Ned Simmons Thanks Ned. I'll let you know how it turns out. Eric |
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On Mon, 03 Oct 2005 18:19:17 GMT, xray
wrote: On Mon, 03 Oct 2005 10:33:39 -0700, Eric R Snow wrote: SNIP If you've got an o'scope you can look directly at the encoder channels to eliminate any possibility that it's the readout, if that's what you mean by display error. Less convenient, but workable, would be a voltmeter or two. Or even an LED on each channel, as long as you're careful to limit the current based on the encoder's drive capability. Ned Simmons Ned, I have a scope. I don't really know how to use it. I need to get a book on oscilloscopes in general. The scope is a Tektronix 465B. The only probe I have is one with a red and black wire terminating in little hook shaped clamps. If this will work can you tell me how? Thanks, Eric I was just thinking... Why not make a test setup to check the encoder outside of your intended system? Say you want to move the shaft 1/10000 rev with .01 inch applied. The circumference required will be .01 * 10000 = 100". The radius for that circle is about 15.9". Why not attach a light, but firm, arm to your encoder, 16" long. To remove as much load as possible, put the encoder so its shaft is horizontal with the 16" arm hanging straight down. Clamp a micrometer so it touches the rod near the end and put a light spring (rubber band?) to pull the lever against the micrometer barrel. Now, moving the micrometer .01 should move the encoder 1/10000 rev. Obviously, you could shorten the lever by some factor and still get decent measurements. Just one other approach, if you feel so inclined. -Rex What I'm trying to do is move the encoder in the system and see what happens. Since I have the laser, mirror, and some clay the setup will be easy. I don't want the measuring to influence the encoder. So if the carriage moves .0008 and the display doesn't I can't tell where the problem is. If the carraige moves .0008 and the encoder shaft turns the requisite amount, and the display doesn't change then it's the encoder. But if the carriage moves .0008 and the encoder shaft doesn't move, and the display doesn't change, then the error is in my setup and I can look for it. I don't think it's the encoder, but it seems to me that the mechanical setup is backlash free. So until I eliminate something it won't be solved. And the encoder check is the easiest so far. Eric Eric |
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In article , Eric R Snow says...
Put a mirror on the shaft. Shine a laser at the mirror. Adjust the mirror until the laser is visible on a target 1591.5" (132.625 feet) away. This will increase the amount traveled to .1" for every pulse. This should work. You can shorten the beam path by folding it multiple times - ie, multiple bounces with mirrors. Be sure to use first surface mirrors or the extra reflections will drive you nuts. Basically you just want to calibrate your encoder. Jim -- ================================================== please reply to: JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com ================================================== |
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On Mon, 03 Oct 2005 18:48:46 GMT, xray
wrote: On Mon, 03 Oct 2005 11:06:12 -0700, Eric R Snow wrote: On Mon, 03 Oct 2005 17:56:08 GMT, xray wrote: What encoder are you using, and what is it connected to? A US Digital H6S-2500-I. And I realize I made a bunch of errors in my first post. I am using the ED3 display from US Digital too. I need to eliminate the encoder as a source of error before I continue. I don't think it's the problem but so far it seems like the culprit. Eric H6S-2500-I? That's 2500 CPR (Counts Per Revolution). For a shaft with 1" circumference, that gives a resolution of .0004". I'm thinking you've got an uncertainty in the +- 1 count range, which is pretty close to what you are seeing. Had to go look to refresh my memory. As I recall the best A2 encoders, which I have used, are spec'd to 12 bits output although they give 16 bits. 12 bits is 4096, or 2.4 tenths in your application. I think you need a smaller shaft. Actually, since the encoder is being read in quatrature the resoultion is 10,000 pulses per rev. When moving the carriage one direction it counts right along with a 1/10000" .500" travel indicator. When the direction is reversed the carriage must move .0009" before the display shows a change. After the .0009" movement is satisfied everything works OK. The problem occurs no matter where in the .500" of the indicator I use. It also happens if the indicator is moved anywhere in the 8" travel of the carriage. If the problem still shows with the laser measure then I'll try it with 1X counting instead of 4X. But that should not make any difference. I think. Eric |
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On Mon, 03 Oct 2005 10:56:34 -0700, Eric R Snow
wrote: On Mon, 03 Oct 2005 13:33:52 -0400, Brian Lawson wrote: Brian, The mirror is about 1/2 inch on a side and .09 thick. The laser is 1.5 inches dia and 8 inches long. With wires coming out. I feel it may have more influence on shaft movement if mounted directly to the shaft than the mirror will. Direction is determined by only one encoder. It has two readheads out of phase with each other. So A leads B turning one direction and B leads A in the other. Thanks, Eric Hey again Eric, I must be missing something about using the mirror. Maybe I should just shut up and see what you do, or the results anyway, but that's not me!! My lack of understanding is in/with the deflection of this mirror. As I understand it, you are going to 1 - fasten a mirror to the shaft, 2 - shine a laser beam on the mirror, 3 - note the point this reflected beam contacts a wall 132 feet away, 4 - make a "mark" on the wall at that point 5 - rotate the encoder one pulse (by turning or stepping the shaft to create a single "output" pulse of the encoder) 6 - repeat step 5 for ten or more "pulses", and then reverse the .. step direction for the same number, and see if there is a .. difference between the start point and the end point. To me, that should work, but if you are going to measure the distance the beam "moves" from pulse to pulse, then the mirror will cause a "doubling" deflection type error with each pulse. That was the reason I suggested fastening the laser direct to the shaft. Am I wrong about your intent, or the error? Take care. Watch the laser in the eyes. Brian Lawson. |
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On Mon, 03 Oct 2005 15:42:08 -0400, Brian Lawson
wrote: On Mon, 03 Oct 2005 10:56:34 -0700, Eric R Snow wrote: On Mon, 03 Oct 2005 13:33:52 -0400, Brian Lawson wrote: Brian, The mirror is about 1/2 inch on a side and .09 thick. The laser is 1.5 inches dia and 8 inches long. With wires coming out. I feel it may have more influence on shaft movement if mounted directly to the shaft than the mirror will. Direction is determined by only one encoder. It has two readheads out of phase with each other. So A leads B turning one direction and B leads A in the other. Thanks, Eric Hey again Eric, I must be missing something about using the mirror. Maybe I should just shut up and see what you do, or the results anyway, but that's not me!! My lack of understanding is in/with the deflection of this mirror. As I understand it, you are going to 1 - fasten a mirror to the shaft, 2 - shine a laser beam on the mirror, 3 - note the point this reflected beam contacts a wall 132 feet away, 4 - make a "mark" on the wall at that point 5 - rotate the encoder one pulse (by turning or stepping the shaft to create a single "output" pulse of the encoder) 6 - repeat step 5 for ten or more "pulses", and then reverse the . step direction for the same number, and see if there is a . difference between the start point and the end point. To me, that should work, but if you are going to measure the distance the beam "moves" from pulse to pulse, then the mirror will cause a "doubling" deflection type error with each pulse. That was the reason I suggested fastening the laser direct to the shaft. Am I wrong about your intent, or the error? Take care. Watch the laser in the eyes. Brian Lawson. Actually Brian, I want to do both, watch for change AND measure movement. I did not realize the mirror would double the movement until it was pointed out to me. Then it clicked and I remembered. The laser is 3mw, 632 nm. Still, I'm always careful with lasers since I saw a hole burned through a wall at a lab at UW in Seattle. Eric |
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O.K., today's silly question- Maybe I'm missing something here but how does the readout know to count up or down with only the raw clocking out from the encoder? Seems to me that without the quadrature info the display won't know whether to increment or decrement- O.K, so lets say I have missed something in the discussion & the unit is counting in the right direction, if you reverse the encoder's rotation how many pulses does it have to generate before it knows it's going backwards?? Enquiring Minds want To Know- H. |
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Eric R Snow writes:
So, measuring the distance the spot travels will show how many pulses should be generated. This won't work with a simple beam. The laser spot will inevitably be many times the size of the motion of the spot you are trying to measure. We have hashed this over and over with people who speculate they can align their round-column mill drills using a laser. I'm always surprised by people who think laser pointers have near-zero angular diameter. Not the case. |
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According to Eric R Snow :
On Mon, 3 Oct 2005 14:22:21 -0400, Ned Simmons wrote: In article , says... SNIP If you've got an o'scope you can look directly at the encoder channels to eliminate any possibility that it's the readout, if that's what you mean by display error. Less convenient, but workable, would be a voltmeter or two. Or even an LED on each channel, as long as you're careful to limit the current based on the encoder's drive capability. Ned Simmons Ned, I have a scope. I don't really know how to use it. I need to get a book on oscilloscopes in general. The scope is a Tektronix 465B. A nice 'scope. The only probe I have is one with a red and black wire terminating in little hook shaped clamps. If this will work can you tell me how? [ ... ] if it is.) Connect the ground clip to the readout chassis or signal ground. The main question is whether he can identify the ground clip on the probe as described. Most Tektronix scope probes of relatively recent vintage have a groove about half-way back from the tip to the entrance point of the cable. A clip, sort of like a small version of a hairpin clips into this to connect the ground clip to the scope probe's ground. Older Tektronix scope probes, as well as ones by Hewlett Packard, had a threaded hole in the side of the probe, into which the end of the ground clip screwed. Recent Tektronix scope probes have a coaxial tip, with a sleeve surrounding a tiny point. An extension slips onto this for most work, with a spring-loaded hook which is exposed by pulling back on a skirt on the probe tip. For really high frequency work, an alternative tip slips on, which picks up ground from the sleeve, and has a very short projection for the center conductor. Your color scheme does not sound like any Tektronix scope probe which I have seen. Note that any probe needs to be tuned to the scope. This is done by connecting the probe tip to the calibrator output (a square-wave pulse), and adjusting one (or more for higher frequency probes) adjustments to maximize the squareness of the displayed trace at the corners. Adjusted too far in one direction, and the trace will curve gently from the vertical to the horizontal. Adjusted too far the other direction and the trace will overshoot beyond the horizontal, and then return to it -- sometimes with one or two ringing overshoots in both directions. If there is more than one adjustment for the probe (usually in body of the connector to the scope, but rarely on the scope probe itself), one adjustment will adjust the squareness of the corner, and the next will adjust a portion of the horizontal top or bottom of the square-wave train. I've seen some (high voltage probes, with 10KV maximum voltage) with four or five adjustments in a large box at the connector end. Some of the older Tektronix (and others) probes will have a knurled knob where the flange of the probe joins the body. Loosen this, and unscrew or screw tighter the tip end to adjust the response of the probe. Once your probe is tuned to the scope, you should not need to re-adjust it until you move it to another scope (Or for a scope with plug-ins, until you move it to another plug-in). You probably don't really need to tune it for what you are doing, but if you don't, you may see the pulse height change with speed of motion (as the pulse never really reaches the top of the waveform). All you need for encoder channel B is to make a connection to the center pin of the scope's CH2 BNC connector. If you don't have a mating connector, you can stick a pin in the center of the connector and use a jumper, just be careful not to damage that nice old scope. Indeed so. Set the toggle under the VOLTS/DIV knob to GND on both channels. Push in both CH1 & CH2 pushbuttons on the left of the screen, all the rest in that group should remain out. Set the TRIG MODE to AUTO. Set the TIME/DIV KNOB to around 1ms. Make sure the coaxial knobs are locked together. Now fiddle with the POSITION controls way over on the left to get the two traces separated on the screen. If you don't get the traces, try fussing with the SOURCE and LEVEL on the lower right. (I'm winging this by looking at a picture of a 465B that Google found for me.) Set the toggle under the VOLTS/DIV knob to DC on both channels. Set the VOLTS/DIV knob to 2V on both channels. If the probe is a 10:1, set that channel to .2V Turn the readout power on. You should the encoder transitions when you turn the shaft. Play with the trigger settings if you want to synch the sweep to the signal with continuous rotation. That should get you started. Don't be afraid to play with the knobs, you can't hurt anything. Hopefully someone else will chime in if I've missed anything obvious. Excellent scope operation at a distance course. Enjoy, DoN. -- Email: | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
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I think most of the important stuff has been covered. From your laser
dimensions I'm assuming a HeNe laser and thus a pretty good beam quality, not a diode laser pointer. Check your spot size to be sure it's okay, at your target, after reflection from the mirror, because that spot size will ultimately limit how small a motion you can detect and it will grow proportional to distance so you can't beat it by getting further away. Only point I haven't seen mentioned is that the mirror needs to be mounted on the end of the shaft so that the shaft axis lies in the reflecting plane, and the laser beam needs to be perpendicular to the shaft axis. That way you just get mirror rotation as the shaft rotates. Any distance off-center and you get a mix of translation and rotation and there goes your measurement. I'd suggest drawing two light rays some rotational angle of the shaft apart, both on center and some distance off center, to get an idea of what precision you need for this setup. -- Regards, Carl Ijames carl.ijames at verizon.net |
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In article , DoN. Nichols says...
The main question is whether he can identify the ground clip on the probe as described. Most Tektronix scope probes of relatively recent vintage have a groove about half-way back from the tip to the entrance point of the cable. A clip, sort of like a small version of a hairpin clips into this to connect the ground clip to the scope probe's ground. For low-frequency use, sometimes the ground at the probe is best omitted. On rare occasions the little ground clip has been known to detach from the ground point (in high sproing factor installations) and flip deftly through the air to land on the nearest high voltage terminal point. With predictably exciting results. Not that *I've* ever done this. Ahem. Jim -- ================================================== please reply to: JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com ================================================== |
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On Mon, 03 Oct 2005 23:12:29 GMT, Howard Eisenhauer
wrote: O.K., today's silly question- Maybe I'm missing something here but how does the readout know to count up or down with only the raw clocking out from the encoder? Seems to me that without the quadrature info the display won't know whether to increment or decrement- O.K, so lets say I have missed something in the discussion & the unit is counting in the right direction, if you reverse the encoder's rotation how many pulses does it have to generate before it knows it's going backwards?? Enquiring Minds want To Know- H. Howard, The encoders I'm using have a clear disc with a bunch of lines arranged radially and measure about .1" long from the edge toward the center. The two line detectors ( A and B) are arranged in a "phased array". This means that the detectors are placed such that first A detects the leading edge of a line, then B detects this edge, then A detects the trailing edge of the line, then B detects this line. Actually, the line A and B use can be different lines as long as the detectors are placed properly and the lines are evenly spaced (which they are). It works like this: The detector A would see the dark leading edge of the line, then as the disc turns detector B sees this edge but sees it before the disc has turned enough for A to see the trailing edge. Then the disc turns a little more and A sees the trailing edge. Then B sees this edge but sees it before A sees the leading edge of the next line. Kinda wordy explanation. Surely someone here can say it better and clearer. OK, now that we know the pattern of detection we can see that A will lead B when the disc turns one direction and B will lead A when turning the other direction. So that's how the display knows the direction. This detection scheme also allows both single and quadrature pulses to be read by the display. If the display only increments one count on the leading edge of A then the number shown will be the number of lines that have passed A. But if done the way explained above the display is able to count 4 pulses for each line passed. I don't know, but I suppose that the display still uses detectors A and B to tell which direction the disc is turning, even when it is only showing the actual number of lines that have passed one detector. Eric |
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According to Eric R Snow :
On 4 Oct 2005 00:11:27 GMT, (DoN. Nichols) wrote: [ ... ] The main question is whether he can identify the ground clip on the probe as described. Most Tektronix scope probes of relatively recent vintage have a groove about half-way back from the tip to the entrance point of the cable. A clip, sort of like a small version of a hairpin clips into this to connect the ground clip to the scope probe's ground. [ ... ] DoN, Maybe probe is the wrong word. What I have is a coaxial cable with a twist on connector that fits the input on the 'scope. The cable is about 18 inches long. At the other end two wires with clips on the ends are coming out. The red wire is connected to the center wire of the coax. The black to the outside conductor of the coax. It has molded into the strain relief: POMONA ELECTRONICS and the numbers 5155. I don't know if probe is really the correct word. But using it, along with Ned's great instructions, I was able to see all four of the transitions for each line of the encoder. Eric O.K. I know those, and I would never call them a probe. At least, you don't have to worry about adjusting the vertical V/Div for that, it is a 1:1 connection. And until you get to high frequencies, you probably don't need to tune it for the scope's input characteristics, either. I probably would just call it a BNC to clip lead cable, FWIW. The twist-on connector is a BNC (Sometimes attributed to "Berkeley Nucleonics Corporation", but I think that the "B" is for "Bayonet" and that it predates the company. The "N" would seem to be for the Type-N connector, except that is a larger connector for a larger cable. But the other features (aside from the Bayonet locking ring) are quite similar to the Type-N scaled down. There is also a "TNC" connector (Threaded instead of Bayonet), but that is less common, and less convenient for most uses. You really should get the proper 10X probes for your 'scope, as they have the advantage of not loading down the circuit under test the way the capacitance of the coax cable does. Here is an auction about to close for a nice set: 7549779574 It has gotten fairly steep already, so you might want to wait for another set at a better price. This set started at $9.99, which would be a nice price, but it is now up to $56.00 with about an hour and a quarter to go. This auction: 7551038306 has a buy-it-now, and a more affordable price, but I'm not sure who actually made these probes. I'm pretty sure that they are not by Tektronix. This one: 7551281799 is still pretty affordable, and the probes are about the right vintage for your scope, I think. (Quite similar to the ones for my Tektronix 454 scope, but a bit faster.) There are several in close sequence starting with: 7551229886 which are starting at an attractive $1.00 each, but those are 1X probes, not 10X. My ebay search was a simple: Tektronix scope probes and it turned up a lot of scopes as well. Enjoy, DoN. -- Email: | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
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On Mon, 03 Oct 2005 18:19:35 -0500, Richard J Kinch
wrote: This won't work with a simple beam. The laser spot will inevitably be many times the size of the motion of the spot you are trying to measure. (snip) True, barring exotic lasers and optics. Now having dismissed what won't work ..... let's devise what might work. Same idea, Eric .....optical leverage .... but without the laser. Tape a scale on the distant wall. View the reflection of the scale in the mirror using a powerful (spotting) scope, preferably one with a reticle. The scope can be close to the mirror because it's focussed on the distant scale via the mirror. Your line of sight via the mirror will move 0.1" at range of 159 inches, as you noted. The distant scale will appear to move that much when the mirror rotates. With a 20X scope, it will be as if you are viewing the scale from a distance of about 8 inches. You can easily discern 0.1" from that distance. At least I hope you can! Your scale may need to be marked with lines .025" thick for clear visibility in your scope -- depends on the scope. I can very easily discern .025" lines at 100 feet thru my inexpensive spotting scope, though it doesn't have a reticle. A good riflescope of 20X or more magnification, or a transit or theodolite, would work well. |
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On 3 Oct 2005 17:18:58 -0700, with neither quill nor qualm, jim rozen
quickly quoth: In article , DoN. Nichols says... The main question is whether he can identify the ground clip on the probe as described. Most Tektronix scope probes of relatively recent vintage have a groove about half-way back from the tip to the entrance point of the cable. A clip, sort of like a small version of a hairpin clips into this to connect the ground clip to the scope probe's ground. For low-frequency use, sometimes the ground at the probe is best omitted. On rare occasions the little ground clip has been known to detach from the ground point (in high sproing factor installations) And what, may I ask, is a LOW sproing factor installation? I've never seen one except when a ground plane hit the edge of a single-sided circuit board. How often do you see those nowadays, hmmm?  and flip deftly through the air to land on the nearest high voltage terminal point. Across separate HV and/or ground points together, of course. With predictably exciting results. The Fry Factor is high. Not that *I've* ever done this. Ahem. We've only heard rumors of it being done, right? I've heard of screwdrivers being gently laid upon circuit boards with interesting results as well. And probe tips hitting more than the intended single IC pin during probing, and... Sure glad that wasn't me. -- "Most Folks Are As Happy As They Make Up Their Minds To Be" -Abraham Lincoln ----------------------------------------------------------- www.diversify.com - Happy Website Development |
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http://www.google.ca/search?hl=en&q=...ment+cnc&meta=
instead of re-inventing the wheel... "Eric R Snow" wrote in message ... This may seem off topic but it's not really. Measuring is important to machining. And I need to make a measurement without putting pressure on the item being measured. Now, for the purists, I know that light will put a little pressure on what I'm measuring, but this will have way less effect than any kind thing I can measure. I have an encoder that outputs 10,000 pulses per revolution. With a shaft measuring .19515" (radius) each pulse equals .0001 on the circumference. So, if the radius keeps multiplying by 10 then the amount traveled by the circumference will increase by 10. Here's the plan: Put a mirror on the shaft. Shine a laser at the mirror. Adjust the mirror until the laser is visible on a target 1591.5" (132.625 feet) away. This will increase the amount traveled to .1" for every pulse. So, measuring the distance the spot travels will show how many pulses should be generated. The reason for this measurement is to rule out backlash in the encoder. It appears that there is a 9 pulse error. In other words, if the encoder shaft is turned one way and the count is noted, and then turned the other way until the count changes, it appears that the shaft turns an amount equal to 9 pulses. I need to rule this out because this is the error I'm getting is a mechanical system and it appears that all the mechanical lash has been reduced to less than .0001". The last thing seems to be the encoder itself. Thank You, Eric R Snow, E T Precision Machine |
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His 1/10000 revolution is going to give a spot motion of 0.2 inch at a
distance of 13 feet. He won't have any trouble seeing this with a halfway-decent laser. If his laser has a divergence of 2 mrad full angle, at 13 feet the spot is only a little more than 0.3 inches diameter, and I'll bet he can see a spot motion of 0.05 inches with a 0.3 inch spot. You're right that people don't generally realize how big and fuzzy the laser spot is going to be, but in this case it should be no problem. I would use an autocollimator or jig transit or theodolite if I needed to resolve 1 arcsecond or 10 arcseconds, but he has to resolve only 130 arcsec. When I set up an autocollimator measurement I use a diode laser attached to the autocollimator to get it roughly aligned; with the target 13 feet from the mirror, it's easy to measure angular changes quite a bit smaller than 130 arcsec using just the laser and mirror. Don Holly Richard J Kinch wrote: Eric R Snow writes: So, measuring the distance the spot travels will show how many pulses should be generated. This won't work with a simple beam. The laser spot will inevitably be many times the size of the motion of the spot you are trying to measure. We have hashed this over and over with people who speculate they can align their round-column mill drills using a laser. I'm always surprised by people who think laser pointers have near-zero angular diameter. Not the case. |
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On Mon, 03 Oct 2005 22:43:11 -0500, Don Foreman
wrote: On Mon, 03 Oct 2005 18:19:35 -0500, Richard J Kinch wrote: This won't work with a simple beam. The laser spot will inevitably be many times the size of the motion of the spot you are trying to measure. (snip) True, barring exotic lasers and optics. Now having dismissed what won't work ..... let's devise what might work. Same idea, Eric ....optical leverage .... but without the laser. Tape a scale on the distant wall. View the reflection of the scale in the mirror using a powerful (spotting) scope, preferably one with a reticle. The scope can be close to the mirror because it's focussed on the distant scale via the mirror. Your line of sight via the mirror will move 0.1" at range of 159 inches, as you noted. The distant scale will appear to move that much when the mirror rotates. With a 20X scope, it will be as if you are viewing the scale from a distance of about 8 inches. You can easily discern 0.1" from that distance. At least I hope you can! Your scale may need to be marked with lines .025" thick for clear visibility in your scope -- depends on the scope. I can very easily discern .025" lines at 100 feet thru my inexpensive spotting scope, though it doesn't have a reticle. A good riflescope of 20X or more magnification, or a transit or theodolite, would work well. Oops. Range is 159 inches, not 159 feet. You should be able to see an ordinary machinist's scale thru a 20X scope. Not all scopes will focus that close (about 13 feet) but some do. Mine does. I tried this. From 200 inches I can easily discern the 1/64" gradations on the ruler of a machinist's square when looking at it directly thru the scope at 20X. Resolution with the mirror I had handy was not nearly as good. The mirror would have to be a good quality first-surface mirror -- mine was a crummy inspection mirror. Good first-surface mirrors are easily found at surplus stores for a buck or two. The "spot" from my $9.99 HF laser level is about .25" dia at about 17 feet -- but there is a central "bright spot" that is probably only about .050" dia. So, with a good first-surface mirror, I think your experiment would work OK with a laser. A small prism may also work well. |
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On Tue, 04 Oct 2005 17:27:46 -0500, Don Foreman
wrote: On Mon, 03 Oct 2005 22:43:11 -0500, Don Foreman wrote: On Mon, 03 Oct 2005 18:19:35 -0500, Richard J Kinch wrote: This won't work with a simple beam. The laser spot will inevitably be many times the size of the motion of the spot you are trying to measure. (snip) True, barring exotic lasers and optics. Now having dismissed what won't work ..... let's devise what might work. Same idea, Eric ....optical leverage .... but without the laser. Tape a scale on the distant wall. View the reflection of the scale in the mirror using a powerful (spotting) scope, preferably one with a reticle. The scope can be close to the mirror because it's focussed on the distant scale via the mirror. Your line of sight via the mirror will move 0.1" at range of 159 inches, as you noted. The distant scale will appear to move that much when the mirror rotates. With a 20X scope, it will be as if you are viewing the scale from a distance of about 8 inches. You can easily discern 0.1" from that distance. At least I hope you can! Your scale may need to be marked with lines .025" thick for clear visibility in your scope -- depends on the scope. I can very easily discern .025" lines at 100 feet thru my inexpensive spotting scope, though it doesn't have a reticle. A good riflescope of 20X or more magnification, or a transit or theodolite, would work well. Oops. Range is 159 inches, not 159 feet. You should be able to see an ordinary machinist's scale thru a 20X scope. Not all scopes will focus that close (about 13 feet) but some do. Mine does. I tried this. From 200 inches I can easily discern the 1/64" gradations on the ruler of a machinist's square when looking at it directly thru the scope at 20X. Resolution with the mirror I had handy was not nearly as good. The mirror would have to be a good quality first-surface mirror -- mine was a crummy inspection mirror. Good first-surface mirrors are easily found at surplus stores for a buck or two. The "spot" from my $9.99 HF laser level is about .25" dia at about 17 feet -- but there is a central "bright spot" that is probably only about .050" dia. So, with a good first-surface mirror, I think your experiment would work OK with a laser. A small prism may also work well. All I've had time for today is to make a quick measurement to a closer target. At 75 feet the spot is 1.5". And I calculated that the spot will move .568" at 75 feet. So, the spot is about 3 times as big as the movement of .0001". But, the edge is pretty well defined, so I can see the edges move. If I draw lines at the edges I'll be able to see how much things have moved. I have a couple lenses that I used on a laser pointer to get a smaller spot at 30 feet. They worked pretty well. But laser pointer spots are not round and so any improvement really helps. I was surprised how much the beam expanded. As for the other ideas posted here, they have merit and if I can borrow a 20 or so times scope these will be explored too. It sure is fun to learn new stuff. Thanks, Eric |
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Eric,
I think you forgot a factor of two again -- the spot should move 1.13 inches if the mirror rotates 1/10,000 of a revolution. No problem seeing that with a 1.5 inch spot. I like the spotting scope idea too -- tape a scale to the side of a spotting scope which has a reticle and view the scale through the rotating mirror, and you've got a cheap autoreflection alignment scope. But to get a clear bright image with the scope, you need a good mirror that is about the size of the scope objective. The laser method has lower resolution but can use a much smaller mirror. Don Holly All I've had time for today is to make a quick measurement to a closer target. At 75 feet the spot is 1.5". And I calculated that the spot will move .568" at 75 feet. So, the spot is about 3 times as big as the movement of .0001". But, the edge is pretty well defined, so I can see the edges move. If I draw lines at the edges I'll be able to see how much things have moved. I have a couple lenses that I used on a laser pointer to get a smaller spot at 30 feet. They worked pretty well. But laser pointer spots are not round and so any improvement really helps. I was surprised how much the beam expanded. As for the other ideas posted here, they have merit and if I can borrow a 20 or so times scope these will be explored too. It sure is fun to learn new stuff. Thanks, Eric |
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On Wed, 05 Oct 2005 08:01:13 -0500, don holly
wrote: Eric, I think you forgot a factor of two again -- the spot should move 1.13 inches if the mirror rotates 1/10,000 of a revolution. No problem seeing that with a 1.5 inch spot. I like the spotting scope idea too -- tape a scale to the side of a spotting scope which has a reticle and view the scale through the rotating mirror, and you've got a cheap autoreflection alignment scope. But to get a clear bright image with the scope, you need a good mirror that is about the size of the scope objective. The laser method has lower resolution but can use a much smaller mirror. Don Holly OOPS! You're right of course. Shows what happens when you can't keep all your attention on one thing. Eric All I've had time for today is to make a quick measurement to a closer target. At 75 feet the spot is 1.5". And I calculated that the spot will move .568" at 75 feet. So, the spot is about 3 times as big as the movement of .0001". But, the edge is pretty well defined, so I can see the edges move. If I draw lines at the edges I'll be able to see how much things have moved. I have a couple lenses that I used on a laser pointer to get a smaller spot at 30 feet. They worked pretty well. But laser pointer spots are not round and so any improvement really helps. I was surprised how much the beam expanded. As for the other ideas posted here, they have merit and if I can borrow a 20 or so times scope these will be explored too. It sure is fun to learn new stuff. Thanks, Eric |
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On Wed, 05 Oct 2005 08:18:11 -0700, Eric R Snow
wrote: On Wed, 05 Oct 2005 08:01:13 -0500, don holly wrote: Eric, I think you forgot a factor of two again -- the spot should move 1.13 inches if the mirror rotates 1/10,000 of a revolution. No problem seeing that with a 1.5 inch spot. I like the spotting scope idea too -- tape a scale to the side of a spotting scope which has a reticle and view the scale through the rotating mirror, and you've got a cheap autoreflection alignment scope. But to get a clear bright image with the scope, you need a good mirror that is about the size of the scope objective. The laser method has lower resolution but can use a much smaller mirror. Don Holly OOPS! You're right of course. Shows what happens when you can't keep all your attention on one thing. Eric Hey again Eric, Ummmmmm.... well, I'm no physics major here, but if I'm not mistaken, it is the "angle of incidence" that "doubles". So every time the mirror rotates at all, one "step" or more, unless the laser position goes "with it" (so that angle DOES NOT change), then the angle the laser beam strikes the mirrored surface the will change, and it is THAT angle that "doubles" the laser-beam deflection. That would mean some relatively complicated method of figuring the amount the angular change of the mirror, and how much the "spot" should move at the 132 feet distant surface. Wow!! Try it quickly, just for fun....mount the mirror fixed to something, and fasten the laser to a moveable/swingable "radius bar" with the "centre" of that radius at the mirror, and with the laser aligned down the radius bar to strike the mirror. Create a surface at approx the same distance as the radius of the laser, and starting at a point roughly perpendicular to the mirror/ turn on the laser and note the point it strikes the "wall". Move/swing the radius bar a certain distance, and note the position it intersects the wall. Move the radius bar again the exact same distance, and I believe you will find that the point of intersection distance "doubled" the distance of the first move, and the next move would double the second. Even just thinking about this.... imagine that you shone a light at a mirrored surface directly in front of you and perpendicular to you. The reflected light would come right back at you. Rotate the mirror only 45 degrees, and the light will reflect a right angle. Rotate further to almost 90 degrees, and the light will be reflected to almost dead away from you, or near 180 degrees. Pool ball off the cushion idea. Take care. Brian Lawson. All I've had time for today is to make a quick measurement to a closer target. At 75 feet the spot is 1.5". And I calculated that the spot will move .568" at 75 feet. So, the spot is about 3 times as big as the movement of .0001". But, the edge is pretty well defined, so I can see the edges move. If I draw lines at the edges I'll be able to see how much things have moved. I have a couple lenses that I used on a laser pointer to get a smaller spot at 30 feet. They worked pretty well. But laser pointer spots are not round and so any improvement really helps. I was surprised how much the beam expanded. As for the other ideas posted here, they have merit and if I can borrow a 20 or so times scope these will be explored too. It sure is fun to learn new stuff. Thanks, Eric |
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On Wed, 05 Oct 2005 12:26:16 -0400, Brian Lawson
wrote: On Wed, 05 Oct 2005 08:18:11 -0700, Eric R Snow wrote: On Wed, 05 Oct 2005 08:01:13 -0500, don holly wrote: Eric, I think you forgot a factor of two again -- the spot should move 1.13 inches if the mirror rotates 1/10,000 of a revolution. No problem seeing that with a 1.5 inch spot. I like the spotting scope idea too -- tape a scale to the side of a spotting scope which has a reticle and view the scale through the rotating mirror, and you've got a cheap autoreflection alignment scope. But to get a clear bright image with the scope, you need a good mirror that is about the size of the scope objective. The laser method has lower resolution but can use a much smaller mirror. Don Holly OOPS! You're right of course. Shows what happens when you can't keep all your attention on one thing. Eric Hey again Eric, Ummmmmm.... well, I'm no physics major here, but if I'm not mistaken, it is the "angle of incidence" that "doubles". So every time the mirror rotates at all, one "step" or more, unless the laser position goes "with it" (so that angle DOES NOT change), then the angle the laser beam strikes the mirrored surface the will change, and it is THAT angle that "doubles" the laser-beam deflection. That would mean some relatively complicated method of figuring the amount the angular change of the mirror, and how much the "spot" should move at the 132 feet distant surface. Wow!! Brian, I just drew the setup. With the laser hitting the center of the mirror. I drew it with the laser origin the same and just rotated the mirror. So the laser is stationary but the mirror was drawn at two angles. The difference beteween the laser included angles appears to be twice the angular rotation of the mirror. Try it and see if you get the same thing. Since the laser is shining on a flat target there will be some error because the radius will be changing with rotation. But since the measurement is only going to be, at the most, 1/1000 of a full rotation, or .36 degrees, it won't matter much. Eric Try it quickly, just for fun....mount the mirror fixed to something, and fasten the laser to a moveable/swingable "radius bar" with the "centre" of that radius at the mirror, and with the laser aligned down the radius bar to strike the mirror. Create a surface at approx the same distance as the radius of the laser, and starting at a point roughly perpendicular to the mirror/ turn on the laser and note the point it strikes the "wall". Move/swing the radius bar a certain distance, and note the position it intersects the wall. Move the radius bar again the exact same distance, and I believe you will find that the point of intersection distance "doubled" the distance of the first move, and the next move would double the second. Even just thinking about this.... imagine that you shone a light at a mirrored surface directly in front of you and perpendicular to you. The reflected light would come right back at you. Rotate the mirror only 45 degrees, and the light will reflect a right angle. Rotate further to almost 90 degrees, and the light will be reflected to almost dead away from you, or near 180 degrees. Pool ball off the cushion idea. Take care. Brian Lawson. All I've had time for today is to make a quick measurement to a closer target. At 75 feet the spot is 1.5". And I calculated that the spot will move .568" at 75 feet. So, the spot is about 3 times as big as the movement of .0001". But, the edge is pretty well defined, so I can see the edges move. If I draw lines at the edges I'll be able to see how much things have moved. I have a couple lenses that I used on a laser pointer to get a smaller spot at 30 feet. They worked pretty well. But laser pointer spots are not round and so any improvement really helps. I was surprised how much the beam expanded. As for the other ideas posted here, they have merit and if I can borrow a 20 or so times scope these will be explored too. It sure is fun to learn new stuff. Thanks, Eric |
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On Mon, 03 Oct 2005 09:39:54 -0700, Eric R Snow
wrote: SNIP The driving method so far is a belt. I can detect no slipping at all. But I keep getting this .0009 error. I don't know if this is a display error because it's reading the encoder disc in quadrature. I thought about looking at the disc itself. It's 2.000 inches in diameter with 2500 marks on it. which makes the spacing about .0025". This is readable with a magnifier. But I thought that amplifying the error would be easier because I can see the movement at the same time as the display changes. I can very easily see 9 inches movement at 132 feet. And it turns out I have a target just about that far away. A nice clean cement wall. Thanks for reading and responding. Eric It is almost impossible to get a belt or friction drive to work with ZERO backlash. Reversal of rotation generates an an error of twice the compressibility of the drive system and this may well be enough to account for your error. Try bolting a rigid arm to your encoder a few inches long and spring loading this against a solidly anchored stop. Insertion and removal of feeler gauges between the arm and the stop will soon show whether your .0009 error is inherent in the encoder or your friction drive. Jim |
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