Best posts made by mendenmh

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 (!).

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

@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.

@elvisprestley
The VIN voltage at the terminal block fluctuates from 24v - 9.5v. The voltage at the bed heater terminal block is constant at 9.5 V.
Your power supply has a big problem. Are you sure it doesn't have a 120/240 volt switch, set to 240 volts, and you are running on 120. This results in a power supply that typically fades in and out. Your Vin should vary at most a couple percent.
If you don't have the input voltage set wrong, the supply probably is defective.

@ignacmc Yes, a PT1000 sensor will have far better absolute temperature accuracy than a thermistor, if you don't have a precise way to calibrate the thermistors.

Thermistors have very high resolution, and can be easily calibrated over a narrow temperature range. I make much of my scientific living working with system controlled to absolute temperatures within 0.01C. Thermistors are great for this, if you only need a 10 degree span and have access to a calibration lab. However, without calibration, tiny errors in the Hart-Steinhart coefficients add up to very large temperature errors when used over a wide span.

Starting with something very close to linear in the first place, and with a temperature coefficient which is set by the properties of a (nearly) pure element, such as platinum, results in a a sensor which can be used over a very wide range with no calibration. A PT100 or PT1000 sensor has lower resolution than a thermistor (it's very hard to read to 0.01C), but a much wider reliable span of readout. It should be within 1C over the entire temperature range from room temp to 300C or higher.

You should get a decent quality calibration resistor (0.1% accuracy) of about 2000 ohms, to check that the duet readback is working right in PT1000 mode. If it works out right, you will be in great shape. The reason I bring this up is that, for example, my duet2 board (1.02 revision) seems to have a significant charge injection problem on the scanning ADC. Putting the PT1000 on channel zero shows a significant (5C) offset at room temp, but putting it on channel 1 is fine. This is likely due to residual charge on the sample/hold capacitor left over from whatever port the ADC scanned before reading channel 0. I haven't looked into the software to see in detail, but it is a classical scanning A/D problem.

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).

One could apply a statistical hack if one really knows the problem is every 20 samples, and either the first or last sample in a batch. One can just replace the (known) bad point with the average of the point before and after. This doesn't change the power spectrum significantly. This is equivalent to adding a random Dirac comb of small amplitude (the difference between the correct, missing point and the average) to the data, which has only a very weak effect on the spectrum. The noise floor at frequencies which are harmonics of (sampling rate / 40) will be slightly raised.

@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.

@mrehorstdmd I think they were magnetized by a capacitor discharge, with the flux going through an armature which could be pushed out at the same time the real rotor was inserted, so the flux path never got a chance to open open. AlNiCo was sensitive to this. High-strength AlNiCo magnets required 'keepers' (flux shorts) to not lose strength when stored.

It's worth realizing just how much the performance of permanent-magnet motors has improved in 30 or 40 years, due to rare-earth magnets. They were really wimpy before, even if not accidentally demagnetized.

@jay_s_uk You and I said the same thing, about the negative temperature coefficient. I said resistance is higher at lower temperatures; you corrected to lower at higher temperatures.

However, your statement about the value for a thermistor being specified at zero degrees is wrong. Most thermistors are standardized at 25C. Pt100/Pt1000 are, indeed, standardized at 0C. But it is still correct that the 100000 is the right number to have put in the file.

@gnydick Is it possible your meter had autoranged to a millivolt range, and you were reading 3987 mV, or about 4 volts, due to leakage currents?

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.

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.

@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.

It would be interesting to put a small load (10k resistor, e.g.) across the fan, and re-measure that voltage. Maybe the board has a leaky FET.

Also, it is possible that the fan draws very little current at low voltage. That may be low enough that nothing is biased on until the voltage is much higher. In that case, you may just be charging some tiny input capacitance, and a small current might create a significant voltage. The load resistor would tell more about this.

I suspect the improvement in behavior with increasing PWM frequency has to do with stored energy in the filter capacitors. You may be very close to the limits of your power supply, ad it cannot provide 100% to the bed. At low PWM, you are fully depleting the filter capacitors, and requiring the power supply to ante up a full-power pulse during the 'on' phase. At higher frequency, the filter capacitors are supplying enough energy, and the power supply only sees the average load. It is likely to work much better with a bigger power supply.

Although an IR sensor may make a useful remote diagnostic for general conditions, I doubt it will work too well for the hot end control. IR sensors are very sensitive to surface conditions. A clean, shiny nozzle will have a low emissivity, and read cold. As soon as it gets a little dirty, the emissivity will rise, and it will read hotter.
I see a lot of people on numerous forums who have tried to 'calibrate' their thermistor (or other readout) temperature against an IR thermometer, and somehow, the assumption is made that the IR thermometer is the correct value. This is almost certainly wrong, in most cases. There are way too many optical factors that go into these to make them really useful. Expensive, commercial, multi-band sensors can be good, because they can make a reasonable assumption about the emissivity, but basic single band ones are only useful for general surveys of temperature differences across a uniform surface.
On the other hand, as a general diagnostic, it might be interesting to watch how the surface of the sample cools at different fan speeds, print speeds, etc., and use this to develop better printing conditions. Let us know what you learn.

@deckingman I won't flame you... What I am looking for is interchangeability of sensors. If one breaks, and I can put in a new one and have everything stay the same, it is good. The variations between thermistors is HUGE under the conditions they are used in 3d printers. They are standardized at 25C. However, tiny variations in beta can result in mismatches between thermistors of tens of degrees at 250C. Besides, I am a thermometry nerd. In real life, I require absolute temperature measurements of a few thousands of a degree.

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.)