Temperature Control

Ive opted to go with the Marlin firmware on my machine, It seems to be one of the main ones out there and offers several advantages. Firstly it has this feature called lookahead whereby it buffers incoming movement commands and if there are no drastic changes in direction it keeps the movement of the axis high along with the extrusion rate. If there are changes in direction above the set limit then it slows down the movement enough to keep quality high.

Secondly it supports PID control of the extruder heater. Basically this looks at the current temperature value and the desired temperature value and calculates an error value. It then adjusts the control input in an attempt to minimise this value. This offers greater control than then old-style Bang-Bang control method where the heater would switch on full below a set temperature and off above another. You can actually look at the temperature graph on the computer as the machine is heating up and see the temperature stabilise around the desired value with the control inputs becoming smaller and smaller with each swing.

One of the issues with this control method however is that you need to input three separate constant values that determine how the heater is controlled to minimise the error, the proportional, integral and derivative values, hence PID. Calculating these can be a pain in the ass when the thermal characteristics of a extruder head are unknown as how the heater heats under power and cools in the surrounding air affect the control input required to minimise the error. In response to this Marlin includes a feature that will auto calculate these values for you. To start this process you send a M303 command to the printer with the extruder at room temperature. The extruder will then heat up to 150 degrees and attempt to stabilise around this value using sets of heating commands of a decreasing magnitude. After working down through about 6 sets out will pop the P, I and D values. These then get written into the firmware uploaded to the printer.

Most places suggest that if you run through the process 3-4 times you should be able to end up with temperature swings of around 0.3 to 0.5 of a degree around the desired value. I’ve run the process twice and seem to have got a pretty good result so far so itll be interesting to see if the resulting PID values begin to show greater consistency each time the process is run.

I’ll leave you with some pictures of the finished machine and the issue that I face with the cables on the extruder stepper. The section of heatshrinked cable is to help fix this problem, but doesn’t quite do it.

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First Extrusion

I’m amazed at how well this has all gone. Most of the time when setting something like this up there are always a couple of things that have been cocked up, crossed wires, some issue in the programming that makes it do weird and seemingly unexplainable things etc. Kudos to the guys that wrote the firmware and set the default values (which have to work with a massive range of machines) and to the guys that manufactured the RAMPS board.

I just haven’t had this problem yet however. All the motors have been running fine without any of the current limiting circuits being adjusted, the thermistors seem to have been reading the right temperatures and the heated bed and extruder both heat up and cool down really quickly.

I’m not 100% happy with the way that the wires are routed off the extruder carriage yet however. The positioning and the path that they take is fine but I’d be happier if they had more support at the carriage to prevent the smaller wires on the motor connector from suffering fatigue. I also think that I need to find a couple of stronger springs for the idler on the extruder body so that the filament is held with more pressure against the hobbed bolt.

Here’s a first squiggle of extrusion

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Not a Time Machine – Part 6

I believe that after this last weekend you could call the tub definitely workable and for camping almost practical. Six hours of sitting next to the campfire throwing on wood got it up to an almost scalding temperature. When you compare it to the last time it was used there was no doubt that it is getting up to temperature quicker and staying hotter for longer so the insulation really is a must for efficient operation.

The only issue now is that we used a good cubic M of wood over the a weekend…

I don’t think there is anything that we can do to the actual tub to solve this problem and that the solution has to be found in the design of the boiler. Several options have come to mind when thinking this over.

1) The pipe. Would increasing the diameter from 25mm to 50mm allow a significantly greater amount of water through? The issue here is that we’d need to coil the thing and that can be a dog, plus we’d need to find 50mm fittings for the tub.

2) The boiler shell.

a) The last two times we have used the tub we have placed the boiler in the fireplace and lit a fire around the outside as well as inside the boiler.

b) This last time we actually got the fire around the outside hot enough that the bottom of the gas bottle became red-hot and started deforming.

c) When we have used the boiler by itself we ended up with a layer of charcoal in the bottom which (along with the deformed base) seems to indicate that whilst the fire is hot enough there is not enough oxygen.

I wonder whether it would be worth cutting some bigger vents in the bottom of the boiler and just setting a fire inside it in the hope that the increased efficiency would provide the same heat output and transfer to the coil but with a decreased fuel consumption.

The other idea that has been tossed around is whether it would be better to stick with the large fire concept and just remove the coil from the boiler and place it directly into the embers?

Any thoughts, comments or ideas would be greatly appreciated.

 

Photos courtesy of Ash and Matt

First Light

So astronomers have this event called first light. It’s similar to a roofing party but on an astronomical scale…

When building a telescope the pivotal moment is when the mirror is first exposed to the night sky allowing the “first light” of a astronomical object to fall on the mirror. Now I’m not building a telescope but surely the first use of a reprap has to be of similar significance?

Yep, fired it up for the first time tonight, nothing more than running the motors back and forward, testing endstops, thermistors and heaters but none the less kinda exciting to see several months work come together and actually do something for the first time.

The next couple of weeks will be spent getting used to the firmware and host software and fine tuning it while waiting for the last two parts but then once it’s all up and running properly I’ll get back with some pictures of the completed printer and some test prints.

Not a Time Machine – Part 5

The Time Machine Hot Tub is having its second proper outing this weekend with a holiday to the Coromandel. Last time it was used in the wild we had an issue in that we weren’t able to get it quite hot enough. Kauaeranga Valley is notorious for being several hundred degrees below zero at night so it was decided that something had to be done.

The only difference between the first time it was used (when it worked really well) and the second was that we were able to use a 12v computer fan as a blower for the burner. The second time we didn’t have the luxury of a 240v outlet for the transformer for the fan to plug into and as a result weren’t able to get the fire as hot as the first time. As this will often be the case going forwards out only option was to increase the efficiency of the burner and insulate the tub to decrease heat loss.

We really wanted some sort of closed cell foam to prevent it getting waterlogged, something cheap and something flexible. Foam bed rolls provided to be an ideal solution and 3 double bed rolls were acquired from The Warehouse on sale.

These were then cut into strips

And inserted between the tub frame and outer cladding

Until it looked like this

We managed to attach the remaining bed roll to the roof of the tub by “Man Sewing” it on with wire.

Hopefully that should make enough difference to the performance or it looks like someone is going to have to spend most of saturday blowing air into a fireplace.

FedEx = best shipping company ever!

The FedEx gremlins have been kind to me this morning, I challenge you to find someone else who can pick up a package on Friday afternoon in Dongguan China and deliver it Monday morning to Avondale, NZ!

Anyway below are a few pics of the items that they have brought. Behold, the 2nd to last piece of the puzzle. Only waiting on the couplings for the Z-Axis now. I fear however that they may take a while longer as they are coming by economy international mail…

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The bottom of the Mega 2560 is stamped GENUINE but looking at some of the soldering im not quite sure, That and it did come from China. Anyway we shall see how it goes.

Whilst I could have got the Mega in New Zealand it would have been twice the price. The RAMPS board however is near on impossible to get if you want the 1.4 version in NZ and so your only option is really to order it from overseas. It’s also a lot cheaper to order them fully assembled as opposed to just ordering the PCB and sourcing the parts in NZ. I’m not sure yet but am hoping that it wont come down to “you get what you pay for” and that the whole thing isn’t going to fail in a couple of months. You can get some long-lasting / decent electronics from China aye?

Current Progress

Progress has been made in dribs and drabs this last month. The end is in sight however. The rest of the printed parts have been installed and a lot of the cabling has been completed. There is still a little bit that needs to be done on the extruder but its at the point where its the small finishing off stuff that takes time but doesn’t look like a lot of progress.

Still to arrive is the motor controller board (RAMPS 1.4), Arduino MEGA and the some misc electronics. I’m also still waiting on the couplings to connect the Z motors to the threaded rod.

The aim this next week is to get all the small annoying bits out of the way so that when the new parts finally arrive installing them is all that needs to be done.
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