That number is the fraction of a step variation you might see from one step to another for an unloaded motor. Yes, it is due to magnetic inhomogeneities, and could be quite irregular around the full circle. Note that since a full circle add up to exactly 360 degrees and (usually) 200 full steps, for every step that is too big, there have to be small steps to make it all add up still. They can't all be 102%, since the number of steps per full circle is exact and constant.

Note that I use this principle to calibrate ultra-high precision x-ray diffractometry systems. With no external standards, and a highly repeatable (bot not accurate) angular scale, one can use 'circle closure', which is this property of cancelling errors, to define angular scales of truly extraordinary accuracy.

See, for example, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4911639/
which is one of the papers I have published on this technique. The technique has been used since the 1800's (!).

@Corexy The advantage of the Pt sensors over thermistors for wide temperature ranges is that the temperature coefficient is very well defined, and fully standardized across all the sensors. Thermistors have varying beta values, and when used over a wide temperature range can give significantly different results between two nominally identical units. In my day job, I work in a ver highly temperature-controlled lab with 0.01C regulation, and logging to 0.001C. This is a US Government standards lab. For this, we use calibrated thermistors because of their very high sensitivity. They are perfect for narrow-range applications. However, to cover a few hundred C or more, they are far from ideal. Although a Pt sensor is less sensitive (0.3%/C at room temp vs. typically 6%/C for a thermistor), they are very linear and interchangeable. A thermistor has too big a coefficient for wide ranges; a 100k thermistor at 25C is < 100 ohms at 280C. That's a huge dynamic range to cover. One the other hand, a Pt1000 is 1k at 0C and 2k at 273C. This is an easy range to digitize, and the resistance is high enough that modest-length wires don't affect the value too much.
The Pt100 sensors are well liked for heavy industrial and high precision applications because they are fairly robust, and the very low impedance makes them relatively noise resistant. On the other hand, the signal levels are low, and you must use a four-wire Kelvin connection unless the leads are very short, since the wiring resistance will be a big contribution to the total resistance. For high-precision work you can't even correct for the wiring resistance in a 2-wire connection, since you don't know the temperature of the wires.

Just in case someone on this thread missed the link from the original thread (linked at the very top of this thread), here is a paper I wrote on the topic of measuring stepper errors and encoder errors.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4911639/

It may be of significant interest. This link is to the non-paywalled pubmedcentral source. The original (paywalled) article is in Metrologia (the official journal of the Bureau Internationale des Poids et Mesures, which oversees the metric system worldwide).

Also, here is a second paper, which discusses periodic errors in interpolated encoders. However, I don't think any of the discussions above refer to encoders that interpolate between their reference marks. The long, ugly token at the end of the link de-paywalls it. (Note that this is legal; it is an official U.S. Gov't publication, and free of copyright, at least in the USA). (Changed link... de-paywalling wasn't really working). It seems that the PDF link on Google Scholar may de-paywall this correctly. Try this:

https://scholar.google.com/scholar?hl=en&as_sdt=0%2C21&q=An+algorithm+for+the+compensation+of+short-period+errors+in+optical+encoders&btnG=

and use the PDF link

My experience with PETG is that it likes the bed quite hot, to get good adhesion. I have an Ultrabase, which has 3mm glass on the aluminum base. The temperature is measured at the aluminum layer. A rough estimate of the heat flux gives me about 10 degrees lower at the surface of the glass than at the thermistor. If I set the thermistor to 90C, so the glass is about 80C, PETG sticks quite nicely. Much lower than that, and its adhesion isn't too good. One characteristic of PETG is that you don't want the nozzle too close to the bed on the first layer. If the nozzle is close, the PETG pulls itself along off the bed. It likes a bit of a drop height, as opposed to most filaments which like a lot of squish.

Also, I print PETG at a fairly high nozzle temperature, 250C. I am using a PT1000 sensor, so I am fairly confident of this number. The viscosity is fairly low when it is this hot, so one can print fairly fast (240um layers at 90mm/sec with a Hemera extruder and 0.4mm nozzle).

@amit-bandarwad I would go HTTP; you already have the network available, avoid extra wires, and (as mentioned elsewhere) ground loops. There is also a lot better handling of non-determinism in HTTP (well, in TCP really) that makes such communications very reliable. Communications hiccups are 'someone else's problem' in that they are handled by the TCP stack. USB/Serial you have to make sure you never get any surprises, such as cable jiggles causing disconnects, or extra characters due to diagnostic messages, etc. The HTTP protocol guarantees exactly what to expect.

A lot of times, thermistors are swapped and no attention is paid to the fact that the beta values are different. various sources have beta between about 2800 and 3100, and this can result in 10s of degrees difference at 200+ degrees. They all look about the same at room temperature, since they are standardized to be 100k at 25C (so they usually look like about 120k at a reasonable room temperature).

I gave up guessing beta and switched to a Pt1000 sensor. It is less sensitive than a thermistor, but has much better wide-range temperature response. There is only 1 (common) flavor of Pt sensor. (The parenthesis is because there is a 'USA standards grade' Pt sensor with higher purity platinum than the common one, and a slightly different coefficient. I don't think any of the industrial sensors will be one of those.)

I would always add an explicit line, so you are not dependent on defaults, and so that you can easily see what to change if there is an issue. 'hidden' configuration can be very hard to diagnose.

LabVIEW does have a quite complete HTTP integration library that comes with it. It should be entirely trivial to make requests and send commands to the Duet.

@gtj0 I'm printing the Prusa rc3 face shield frame, and sending them off to https://www.wethebuilders.com/projects/11 who are assembling them and distributing them at cost to the Maryland area. I will pass 50 printed by Monday, and I think many thousand have been produced by the whole team.

I think your bed is rocking back and forth, so that on the left-hand scans the height has one value, and on the right-hand scans the height has another. This could be because the x belt is pulling off-center on the carriage, or the carriage is just loose.

Have you checked the PT100 with an ohmmeter? At 20C it should be 108.9 ohms. Is it possible it's a PT1000 instead?

I got mine from Filastruder in the U.S.A. but they seem to be out right now. They had Precision Piezo branded one, but I'm sure it is a completely generic unit, except for being mounted in the right size tube tube fit most extruders. Any good industrial electronics place (Digikey, etc.) will have them. I'm sure various Amazon stores will, too. Normally, I would shy away from unknown brands, but I think these are all made to industrial standards, and only come from a limited number of actual manufacturers, so generic is probably fine. Doo look at whether the mounting will work, and if the wires are long enough.

The PT1000 is directly compatible. It is a way better idea than a thermistor for the hot end. Thermistors are very good for super-high resolution thermometry over a narrow temperature range. Nothing is better. On the other hand, a platinum resistance thermometer is really hard to beat for covering the range from -100C to 500C with absolute, assured accuracy. You will actually be able to believe your temperatures. Note that even tiny beta variations between thermistors can mean to units that agree at 25C can be more than 10 degrees off at 250C. Thermistors are stable, not accurate, without calibration at their working temperature.

@arhi There's no particulars limit to the amplitude of a Moire pattern. You can get 100% modulation. Usually, when talking about the surface of a 3d print, it is subtle because it is heavily attenuated by things like the frequency response of the filament flow. Reading an encoder, with an offset between steps and encoder period, has no such limits.

@arhi That's a Moire pattern (beat pattern) between the stepper steps and the encoder steps. When both have fixed integer resolution, and they are incommensurate, you will see something like this.

Just in case someone on this thread missed the link from the original thread (linked at the very top of this thread), here is a paper I wrote on the topic of measuring stepper errors and encoder errors.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4911639/

It may be of significant interest. This link is to the non-paywalled pubmedcentral source. The original (paywalled) article is in Metrologia (the official journal of the Bureau Internationale des Poids et Mesures, which oversees the metric system worldwide).

Also, here is a second paper, which discusses periodic errors in interpolated encoders. However, I don't think any of the discussions above refer to encoders that interpolate between their reference marks. The long, ugly token at the end of the link de-paywalls it. (Note that this is legal; it is an official U.S. Gov't publication, and free of copyright, at least in the USA). (Changed link... de-paywalling wasn't really working). It seems that the PDF link on Google Scholar may de-paywall this correctly. Try this:

https://scholar.google.com/scholar?hl=en&as_sdt=0%2C21&q=An+algorithm+for+the+compensation+of+short-period+errors+in+optical+encoders&btnG=

and use the PDF link

Or, to make it more sensible, they have changed from a signed representation on [-64,63] with 63 being least sensitive to an unsigned representation on [0,127] with zero being least sensitive and 127 being most sensitive, with 7-bit math.

My experience with PETG is that it likes the bed quite hot, to get good adhesion. I have an Ultrabase, which has 3mm glass on the aluminum base. The temperature is measured at the aluminum layer. A rough estimate of the heat flux gives me about 10 degrees lower at the surface of the glass than at the thermistor. If I set the thermistor to 90C, so the glass is about 80C, PETG sticks quite nicely. Much lower than that, and its adhesion isn't too good. One characteristic of PETG is that you don't want the nozzle too close to the bed on the first layer. If the nozzle is close, the PETG pulls itself along off the bed. It likes a bit of a drop height, as opposed to most filaments which like a lot of squish.

Also, I print PETG at a fairly high nozzle temperature, 250C. I am using a PT1000 sensor, so I am fairly confident of this number. The viscosity is fairly low when it is this hot, so one can print fairly fast (240um layers at 90mm/sec with a Hemera extruder and 0.4mm nozzle).

@arhi As to your comment about array in python: the numpy package is the default scientific array package. It is a lot like matlab. It is extraordinarily powerful. The python 'array' class, and python's own lists and tuples, are for entirely different purposes than handling what you want.

I support the 'check for rattles first' approach. I was almost driven bugsy by a resonance in my AM8 printer. I looked all over the body of the printer for it, listening to different parts of the frame, etc. Finally, I noticed an Allen wrench left sitting on the printer base (which is a ceramic bathroom tile). That, not the printer, was the source of all the trouble. The tile is very hard, so anything on it will vibrate and buzz mercilessly. I use the tile since it is a nice, stable base, and fireproof.