Replacing Linear Encoder Read Head Help With LPI to Micron - Page 1
Forum section: electronics / general

author: Pork
author_rank: Contributor
timestamp: July 04, 2020, 03:29:10 pm
content: I need to replace the read head for a linear encoder and the only data is stated in LPI.The data sheet states 1250 lines per inch (.0008 inch pitch).I believe this is equivalent to 20 microns but I'm not an encoder expert.Can anyone verify whether or not that is correct?Thanks

author: TimFox
author_rank: Super Contributor
timestamp: July 04, 2020, 04:34:07 pm
content: Usually, the read head works in phase quadrature, so you get a higher spatial frequency using two read sensors 1/4 period apart in the head.The 1250 LPI encoder itself is a square wave with 8 x 10-4inch period. If you follow the "x4" case in this link, you can get an even value of 1x10-4inch per increment, or 100 microinches resolution. Since 1 inch is exactly (by statute definition) 25.4 mm, 100 microinches ("one tenth" in American machinist jargon) is exactly 2.54 micrometers (microns).See:http://www.ni.com/tutorial/7109/en/

author: Pork
author_rank: Contributor
timestamp: July 04, 2020, 04:50:33 pm
content: Tim, I'm not sure I'm following you exactly.Are you saying the 1250 LPI is actually 100 microns or are you saying with a x4 multiplier it's capable of 100 microns?The read head is an x1 design, if that helps.Thanks

author: TimFox
author_rank: Super Contributor
timestamp: July 04, 2020, 05:08:07 pm
content: No.One micron = 1 micrometer = 1 millionth of a meter = 1 thousandth of a mm.The resolution of this encoder should be 1 x 10-4inch = "one tenth" = 1/10 of a "thou" = 1/10 of a "mil" = 1/10 of a thousandth of an inch.The micron is a metric unit, your encoder is inches (conventional units). Seehttps://en.wikipedia.org/wiki/MicrometreFor American machining, 0.0001 inch is a useful resolution.The NI app note discusses different read heads and how they are counted in the digital controller.

author: Pork
author_rank: Contributor
timestamp: July 04, 2020, 05:14:03 pm
content: Tim, I appreciate your response but I just need a direct confirmation as to whether or not 1250 LPI is equivalent to 20 microns.I know that 1/1250 = .0008 inches and that .0008 X 25.4 = .02 mm and that .02 x 1000 = 20 microns.What I don't know is whether or not there is something I'm missing in the interpretation of the encoder datasheet.

author: TimFox
author_rank: Super Contributor
timestamp: July 04, 2020, 05:18:56 pm
content: 0.0008 inch x 25.4 mm = 0.02032 mm = 20.32 micrometers (exact), not 20 micrometers.Metric encoders are available with lines/mm calibrations.Production accuracy on encoders is much better than that 1.6% discrepancy.I am not nit-picking on the difference between 1250 lines/inch and 1270 lines/inch = 50 lines/mm: A decent machinist's 6" rule has an overall tolerance of 1/64", which is 0.26%. I used linear encoders in my work, when we required good accuracy.The discussion in the NI app note on read heads seems clear to me, so I won't repeat it.By the way, the difference in sampling in optical read heads is not done with multiple microscopic photosensors, it is done with precision-made masks that cover a reasonable number of cycles over a reasonable-size photosensor.

author: Pork
author_rank: Contributor
timestamp: July 04, 2020, 05:39:38 pm
content: The data sheet for the read head I need to replace states that the scale was originally a metric scale and that all English (inch measurements are rounded off).It provides an example of 120mm being interpreted as 4.72 in. (not 4.7244).This is why I question whether or not a 20 micron read head will work as a replacement.I've not seen a scale designated 20.32 µmPerhaps you could look at the data sheet and give my your thoughts.My encoder is the "LB" type. You will see the note #2 addresses LB scales.Thanks

author: TimFox
author_rank: Super Contributor
timestamp: July 04, 2020, 05:43:31 pm
content: Well, that's not what you asked about in the original post. A true 1250 line/inch scale is normal for inch usage (as I described above), and that corresponds to 20.32 microns.In the manufacturer's data sheet, he sells scales with either inch or metric increments as separate models, which is typical for this kind of product.Of course, one can calculate inches from mm, and vice-versa in post-processing.

author: Pork
author_rank: Contributor
timestamp: July 04, 2020, 05:48:50 pm
content: Yes, I understand that the direct conversion is not exactly 20µm. That's why I need assistance to determine an acceptable replacement.I'll start another post with a more concise title, I suppose.Thanks for contributing.

author: TimFox
author_rank: Super Contributor
timestamp: July 04, 2020, 06:15:34 pm
content: From that manufacturer, the LB2 series uses a 1250 lines/inch scale, and the LB4 series uses a 50 lines/mm = 1270 lines/inch (exactly).I think your confusion might have started with the manufacturer's comment in Note 2 abouttotal lengthdimensions being derived from a metric prototype: this has to do with overall travel and mounting dimensions, not the resolution.Note that the optical read head must correspond to the scale, since it contains masks that must match the line pattern on the scale. Of course, the complete unit from the manufacturer has corresponding components.Where things get really interesting is if you use the analog interpolation hardware, that starts with the two sine waves generated by the optical reader and interpolates them trigonometrically after analog-to-digital conversion (relying on the sin2+ cos2= 1 trig identity to cope with light intensity variation] to get extremely high resolution.

author: Pork
author_rank: Contributor
timestamp: July 04, 2020, 06:22:16 pm
content: Thank you, that is where the confusion comes from. I realize the mask for the optical portion must match the scale but, I was confused regarding the LB note #2.The manufacturer for this scale has been out of business for some time now so I'm looking for another solution to replace the bad read head.Thanks for the clarification. It looks as though I'll need to find a LPI scale after all.

author: Pork
author_rank: Contributor
timestamp: July 04, 2020, 06:35:21 pm
content: Alright.Well I guess the question becomes, what would the output square wave look like if I used a 50 l/mm read head on a 1250 LPI scale?

author: TimFox
author_rank: Super Contributor
timestamp: July 04, 2020, 06:35:27 pm
content: Yeah, it sucks when a necessary supplier goes under. It is easy to find other manufacturer's replacements for the DRC shaft (rotary) encoders, but nobody seems to make a direct replacement for the linear encoders, where the dimensions and mounting are not so easy as with a shaft.For what it's worth, I always had good results with Heidenhain linear encoders, but they may be too expensive for your application. We used the analog interpolation boxes to get 1 micron resolution from a normal scale. With any of these encoders, be very careful about the mounting since they cannot tolerate misalignment between head and scale, and can be damaged easily.

author: TimFox
author_rank: Super Contributor
timestamp: July 04, 2020, 06:38:58 pm
content: I never tried doing anything so mis-matched, since the head always came with the scale. Can you actually buy a replacement head without the scale that fits your scale? The fit is critical.You need coherence between the scale and mask. I don't know how many cycles are in the mask, but it would decrease the amplitude and mess up the phase. With 64 cycles, I think the square wave would vanish, with 1.6% difference in the periods. The signal in the photodiode is the average of all the openings.

author: Pork
author_rank: Contributor
timestamp: July 04, 2020, 06:51:44 pm
content: I can't buy just a replacement head.The problem is the scale on my machine has two read heads one L and one R handed.The L hand one is bad but but is very simple circuit wise. The R hand one is good but the PCB is vastly more complicated.At this point I'm looking for another scale with the same resolution to remove the optical portion from and then I will reverse engineer the bad PCB and build a new board from scratch.I was hoping to avoid doing that though and just swap in a 20µm read head.It's not looking promising though.

author: philipz
author_rank: Contributor
timestamp: August 19, 2020, 09:15:22 am
content: Did you try to open and repair L hand head? is there something broken? May be it's need to clean photocell from dirt. I know it is difficult task, but it is not working and you not loose so much (except time). If you take your time and carefully open a head you can have success. If I was in your place I will tryI saw your other tread where you explain that you try to repair it. You have progressbut not complete. Sorry. But I think that you can solve the issue with missing Z pulse, just keep your enthusiasm and continue. One suggestion - can you find with magnifying glass where exactly is Z mark? After you change optical alignment of head is there opportunity to move head so Z mark is not in position to read by head?