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Draw-Wire Calibration/Positioning System

accuracy
calibration

#121

Yeah, that’s a good point. But maybe one could make the eyelet with a large radius and then account for how the cable comes off it it like we do with chainAroundSprocket.


#122

I wonder how the ‘Goliath’ unit solves this - their sensor seems to use a cable exiting through a hole in the top of the unit. That would simplify a few things.


#123

I don’t know for certain, but they probably aren’t using 1/16 inch steel cable for it since they aren’t supporting any weight. Something of smaller gauge or maybe synthetic is all that’s needed for the application.


#124

We wouldn’t need to support weight either - just measure a distance. I’ve wondered about kevlar thread, but never used it.


#125

Actually, in this sub-thread we have drifted into talking about replacing the
chains with steel lines (motors moving to the sled)

I agree that the difference between just measuring position vs supporting the
sled at different tensions makes a giant difference in the size of line needed,
and the resulting problem of ‘set’


#126

A couple of additional options!

  1. Draw-TAPE encoder. Uses a stainless steel tape (similar to a regular tape measure) in place of a cable. Stronger, less stretch.

  2. Optical, magnetic, or capacitive LINEAR encoder. Usually the “scale” or “tape” (a strip of material that is marked with visual, magnetic, or conductive markings) is attached to the machine and the “reader” or “head” moves along the tape, taking readings. Because many of these scales are flexible (especially the optical ones) one could be wrapped around a spool and the end of the tape would be attached to the router. As the router moves, the tape is pulled past a reader which is fixed in place. The advantage? Accuracy at the micro (to Nano!) meter level. No worries about spool diameter (because the tape itself is being read)!

Thoughts?


#127

Either or both sound interesting, do you have links to examples?


#128

Draw-Tape Encoder


#129

Or: https://www.amazon.com/gp/product/B0792T5WD7/ref=as_li_ss_tl?ie=UTF8&psc=1&linkCode=sl1&tag=kipkvide-20&linkId=eee36192178dcee432fd1a9615ddebf0&language=en_US

Uses a string instead of tape


#130

can you findout the cost? we would want the digital output version.

David Lang


#131

note that even if this reaches it’s claimed accuracy of 0.01" (as opposed to
just displaying numbers to that many digits, which is what I expect to be the
case), this is not quite good enough for what we need as there are places where
the chain length error translates into 2x error in position.

But I’m picking one up to experiment with.

David Lang


#132

This looks promising! I don’t know the cost, however.

http://www.p-coretech.com/en/sokucho/

http://www.p-coretech.com/en/sokucho/MSZ.pdf


#133

Here is the reply from CoreTech:
—————————
Thank you for your inquiry.

We will let you know our price as follows.

MSZ-300Mini @ 350,000 yen.
Magnet tape type rotary encoder 3m measure @380,000 yen.
FDI @15,000yen.
Delivery time 45 days.

Best regard.
—————————
350,000 yen is about $3000 !!


#134

lots of nice toys out there is you have the budget for them :slight_smile:

David Lang


#135

Here is a thought:

We purchase the tape separately from the head. We glue the tape onto the plywood, and put a sensor on the head of the machine (the sled). This could be used to measure the position of the sled as it sits on the plywood.


#136

please diagram how you would do this. I’m not visualizing how it would work.

David Lang


#137

I’ve spent a lot of “thought-experimenting” with this and I don’t think it will improve corner performance, but rather just side performance and vertical movement is likely greatly harmed by it. Imagine a situation where all the left side slack is being taken up by the motor… that would help the sled pull to the side, but the improvement will decrease as it approaches the side… just like the current setup can pull to the top center, but it takes lots of force to do so… The same would be the case here… and because those two pulleys can rotate freely the sled can be freely moved up and down, with the only constraint being the cable associated with the right side of the machine. So to drop downward, the right side of the machine would have to let out more cable and and allow gravity to do its thing. That’s why I think there would be little corner improvement… But worse, to go to the top left corner, the right side cable system would have to pull in its cable, probably rather tightly to hope to get the sled to move upward… and I don’t even know if it would really raise it above center-line. I think the overall tendency of this design is to move to the middle/center of the work area.


#138

Optical encoder strips are pretty cheap, I’ve seen them up to 360lpi.
https://www.adsignmark.com/solvent-printer-encoder-strip-epson-seiko-konica-printhead-150-180-200-300-360-lpi-raster-film

The sensors for these strips are pretty cheap too.
https://www.ebay.com/itm/300-LPI-Encoder-Strip-Sensor-HEDS-9740-250-Optical-Sensor-for-Inkjet-Printer-1pc-/282571117098

Dust might be an issue but if the sensor/spool are put inside an enclosure maybe some felt pads could wipe the dust off as the strip is retracted.

Connecting them to the sled is the largest issue I see.

I’m thinking we need three encoder strips measuring from the top center and lower corners. Attached to the sled in a corresponding triangular pattern with each attach point equidistant from the center of the router bit. That should allow us to calculate the location of the bit, even if the sled rotates.


#139

Hello Jonathan

Pull wire sensors are a great idea.

They are used a lot a in industrial automation and often in harsh environments.

There is no need to worry about the wires sagging as they have internal springs to hold tension as well as spooling devices to ensure that the wrap is consistent.

We have used them over lengths of up to 6m and the accuracy very high < 1mm over this length, most of which comes from the mechanism not the encoder itself.

Would provide closed loop control which some of the other members have mentioned.

Have not got a Maslow myself but intend to.

Best Regards
Paul


#140

Wow, long thread and lots of ideas here…
I’m liking the idea of using the current motors and chain for support and movement, and Kevlar cord running from the sled to the bottom corners for position encoding. The tension on the measuring cords would also serve to reduce sag in the chain, and thus improve accuracy in the bottom corners.

I’m visioning it something like this: one end of cord on another ring or linkage at the bottom of the sled, going to a very small pin or pulley beyond bottom corner (mirroring motor placement?), then up to encoder shaft/spool mounted on top bar (axis parallel to top bar), two turns around encoder shaft, then down to a weighted pulley, then up to anchor point on top bar.

We can save money by using relative encoders rather than absolute, We can define a ‘zero position’ by disconnecting both cords from the sled and attaching them to a storage peg mounted somewhere below the working area, somewhat near the center. Once calibrated, we can disconnect the cords, attach them to the ‘zero storage peg’, and direct the firmware to store the current position in non-volatile memory. We just have to remember to have the measuring cords on the peg when we power up the Arduino controller.
This would also get the measuring cords out of the way while loading and unloading sheet stock…

One problem I see with the thought of driving the sled with a triangle above, and locating it with a triangle below, is that the opposite corner vectors will seldom be collinear. I can imagine a scenario when the sled is somewhat off-center, and the direction of motion desired is collinear with one of the sensor vectors, but not collinear with either of the chains, so the intuitive use of the bottom right sensor encoder to control the top left motor by simply flipping the sign in the current code would fail.