Power loss recovery idea
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Just a strange idea that wondered through my brain sludge. Would it be possible to use a lipo battery possibly plugged into the micro USB as back up. Once mains drops out have a way to monitor it so it can use the lipo to keep the location at the power loss. Possible even raise the z slightly. Could this be possible?
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Nice idea! 6s LiPo for RC modelling are quite inexpensive now.
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To raise the head a little you need to provide VIN power for a little longer, which you can't do via the USB port because of the power requirement.
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To raise the head a little you need to provide VIN power for a little longer, which you can't do via the USB port because of the power requirement.
And what about making reserve battery from 6S LiPo for RC model? It would be 20 - 25.2V and with enough capacity. If make PCB with charger and power switchover logic and LiPo battery it would be nice addon I think.
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Making a reserve power unit for the printer out of Lipos is certainly possible but a small second-hand UPS would do a better job probably cheaper and almost certainly safer. I was thinking more along the lines of a large capacitor like those used in car audio systems. This would give much more time for head movement and recovery file saving, than the capacitors in the PSU. The issue is the voltage drop would take time to be noticed as the capacitor (or lipos) would run the machine for a while, so maybe it wouldn't do a better job, just make the process take longer. Although with the large capacitor in parallel to the PSU, you could set a fairly tight voltage limit, i.e. for 12v system set the voltage for going to recovery to 11.8v, the capacitor should "stiffen" the supply so that sudden loads don't result in voltage drops, only loss of power would result in a measurable voltage drop.
This is where a ups works well on a PC, if its usb linked and monitored, as soon as there is a problem it notifies the PC which takes whatever pre-determined actions you want, i.e. save everything and shutdown or whatever. If you had a unit monitoring your mains supply going into the PSU, and as soon as it detected a voltage drop/problem notified the duet board, then you'd have much more time to move the head away and save the recovery file.
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Maybe something like an amp meter tied in line with the mains wiring? That way it wouldn't have to require waiting for the DC voltages to start dropping before triggering.
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You can also use such module:
http://fr.farnell.com/phoenix-contact/2902879/module-redondance-alimentation/dp/2778143
which manages a 24V lead battery, and switches without power cut. We used some, at work. Expansive, but very usefull.
Or, this one:
only using capacitor. But taking much longer to switch.
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@DjDemonD: I'm agreed that ups is better and safer than lipos, right. But a large capacitor has its own drawback: not every PSU can work with it, stability problem, high startup current surge and so on.But in any case of UPS or a large capacitor it would be nice if we could detect main power problems and signal them to Duet right ahead of it loses its power. So maybe an addon board that takes USB input from UPS or detect main power supply lost (current sensor like Allegro's ACS722 may fit here as it has galvanic isolation) is a way to go.
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Maybe something like an amp meter tied in line with the mains wiring? That way it wouldn't have to require waiting for the DC voltages to start dropping before triggering.
Exactly, and now you don't need all the overhead and expense of a UPS-type solution (not that those are a bad idea). You detect the mains voltage has dropped out, then rely on the charge in your switching power supply's caps to store the gcode location (and whatever other status is helpful) on SD and move the head out of the way as far as you can with whatever juice you have left.
This is apparently what the new Prusa i3 MK3 is doing. There are many excellent ideas in that printer…
http://www.prusaprinters.org/original-prusa-i3-mk3-bloody-smart/
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For larger builds, simply interfacing with a UPS unit to trigger a script which would save whatever necessary would be sufficient. A decent UPS can allow you as much 30 minutes of full power, so you can do whatever you need to preserve a print in order to be able to resume it.
