Repetitive layer defects

deckingman

@edgars-batna said in Repetitive layer defects:

@deckingman Wouldn't the high inrush current trigger the breakers? What about switching this 10 times a second?

What I've got now is this: https://docs-emea.rs-online.com/webdocs/16bc/0900766b816bcc73.pdf (the 77.11.8.230.8251 variant)

It's recommended to be switched around 2000 times an hour in the spec sheet. I can't find this sort of information in the other spec sheet anywhere. Am I just blind?

Mine has been running for about 3 years with no problems. It's what they are designed to do. This isn't really my field of expertise but they usually work as zero crossing switches. That is, it only switches when the AC sign wave crosses between positive and negative so not when a high current is flowing (no doubt someone with more knowledge than I will correct me if I'm wrong or give a better answer).

An SSR is a Sold State Relay and inside it is triac or some such solid state device, so in operation, it's very much like a Mosfet that we use on hot end heaters - just capable of switching higher currents. IIRC, the PWM frequency that the firmware uses for bed heaters is 10Hz so very low.

Trust me, hundreds if not thousands of people use them to control their mains powered heaters.

Edgars Batna

@deckingman Well, I guess that's encouraging. I would almost go for it, but last time I tried the lights in the room would flicker, and since there's 8mm metal in between, I don't think the switching mode affects much. Did I chose the wrong type of SSR?

deckingman

@edgars-batna This is not my field of expertise by any means. This might help https://en.wikipedia.org/wiki/Solid-state_relay

Extract from text in that link

"In AC circuits, SCR or TRIAC relays inherently switch off at the points of zero load current. The circuit will never be interrupted in the middle of a sine wave peak, preventing the large transient voltages that would otherwise occur due to the sudden collapse of the magnetic field around the inductance. With the addition of a zero-point detector (and no adverse circuit inductance and resultant back-e.m.f.), the individual SCR's can be switched back on at the start of a new wave. This feature is called zero-crossover switching. "

Perhaps the one that you tried didn't use zero crossover switching which is what made your lights flicker? Dunno - just a guess.

Edgars Batna

@deckingman There it is - I bought a "Random switch-on" relay. Learned something new today.

Phaedrux

@edgars-batna said in Repetitive layer defects:

last time I tried the lights in the room would flicker

I had this happen too with some pot lights. You can reduce the PWM frequency on the bed until it stops.

dc42

@denis, from your config.g I see that your machine is a delta. What layer height are you using? It's normally best to use a layer height that is an exact number of full steps for the motors. Your steps/mm is 200 in config.g, so I presume that's using 0.9deg motors and 16-tooth pulleys. So your layer height should be a multiple of 0.08mm. Using 1.8deg motors and 16-tooth pulleys, you would have 100 steps/mm, so layers should be a multiple of 0.16mm.

If you use other layer heights, you may or may not get good results, depending on your motors and the amount of friction in your machine. Increasing the tower motor current may help. I am able to print with 0.2mm layer height on my delta, which also has 200 steps/mm. The fact that this is an exact number of half steps may be helping.

dc42

@edgars-batna said in Repetitive layer defects:

@deckingman There it is - I bought a "Random switch-on" relay. Learned something new today.

Zero-crossing turn on is recommended for resistive loads such as heaters. However, heaters have no inrush current, so using a random switch-on SSR (which is the right sort for driving inductive loads) will make very little difference.

Denis

@dc42 Yes, David. You are absolutely right. This is a delta, motors 0.9 degrees, 16 teeth pulley. And I used the height of the layer 0.1, 0.15, 0.2. This is very valuable advice, but since I am not familiar with the delta kinematics, can you explain where the figure 0.8 came from? Some use 0.1618, but I didn't get a good result with such a layer.

dc42

@denis said in Repetitive layer defects:

@dc42 Yes, David. You are absolutely right. This is a delta, motors 0.9 degrees, 16 teeth pulley. And I used the height of the layer 0.1, 0.15, 0.2. This is very valuable advice, but since I am not familiar with the delta kinematics, can you explain where the figure 0.8 came from? Some use 0.1618, but I didn't get a good result with such a layer.

You have 200 steps/mm @ x16 microstepping, so 100/16 = 12.5 full steps/mm. 1mm/12.5 = 0.08mm.

Edgars Batna

I followed some advice from here and got some results to report for reference:

Switching to a zero-crossing from random-crossing SSR did not visibly affect the light flicker issue.
Switching from bang-bang mode to PID at the bed heater did not affect the horizontal banding issue.
I was able to further tune the PID and keep light flicker to a minimum.
The Z axes construction was worse than I expected. I could measure +/-50% variation every Z motor revolution with just a basic electronic caliper. Apparently the Z screw rods were misaligned so just a little rod bend was being exaggerated.
The weight of the bed was high enough to press the Z stepper rotor downwards on high acceleration moves (this obviously depends on its construction), so I added some flange bearings at the bottom.
The Z smooth rods were not really smooth in the Z direction and needed some 800 or more grit sanding.
I remodeled and fitted bigger, bulkier variants of all brackets, couplings, etc on the Z axis.

Now after improving 2 out of 4 Z axes the banding is nearly gone!