measuring and controlling the temperature of a wire - Page 1
Forum section: electronics / projects

author: phil from seattle
author_rank: Super Contributor
timestamp: March 12, 2021, 12:03:32 am
content: I'm thinking of making a hot-wire foam cutter temperature controller. Since the wire will be buried in EPS (aka Styrofoam), there is no way to directly measure the temperature. So, I was thinking of using the resistance of the wire which changes (increases) with temperature. I've read a number of sites that talk about how to calculate the temperature for continuous current but I want to control the temp probably via PWM using a PDI algorithm. I think the relationship between temperature and PWM duty cycle is going to be linear.I have not seen anything like this out there so maybe there are problems with this approach? Most foam cutters use manual control of the power. I'd like to be able to get more precise control, especially when the length of the wire can vary as well as the amount of wire that is embedded in the EPS block.

author: bob91343
author_rank: Super Contributor
timestamp: March 12, 2021, 12:19:11 am
content: This is one case where true rms measurement is called for. The energy is current squared times resistance times time. The temperature can be calculated from the voltage, but that is the average temperature. You need to define what temperature you want to measure. Since the temperature is varying, so will the voltage (assuming constant current). If the current is pulsed, then its rms needs to be used in the calculation.

author: langwadt
author_rank: Super Contributor
timestamp: March 12, 2021, 12:40:34 am
content: eons ago I build a soldering iron that worked like that, the heating element was a temperature dependent resistance. The power to the heating element was rectified but not smoothed AC and the resistance was measured during the zero crossings

author: penfold
author_rank: Frequent Contributor
timestamp: March 12, 2021, 03:40:09 am
content: If you can sense the voltage and the current, then you can calculate a power and use that as a basis for feedback. There's no need to worry too much about RMS or anything, just low pass filter both voltage and current and multiply in the MCU... it won't be "true" power but it will be a value proportional to the power, I think this method needs a wire with a positive tempco of resistance to work well

author: phil from seattle
author_rank: Super Contributor
timestamp: March 12, 2021, 06:56:24 pm
content: Quote from: penfold on March 12, 2021, 03:40:09 amThat is pretty much what I thought. I have some nichrome 80 wire on order to play with. It does have a positive temperature correlation. Probably the biggest unknown is the interaction of the thermal mass of the wire and EPS ambient. There is lots of info about nichrome wire in open air, not so much about other ambient materials. ThePDIPID (sigh) algorithm will take a fair bit of tuning, I think. This is likely to use a DC supply (for safety) so simple calculations all around.

author: langwadt
author_rank: Super Contributor
timestamp: March 12, 2021, 07:12:00 pm
content: Quote from: phil from seattle on March 12, 2021, 06:56:24 pmnichrome is probably not a great choice, it is made to have a small temperature coefficient, iron is ~15 time higher, copper is ~10x higher

author: phil from seattle
author_rank: Super Contributor
timestamp: March 12, 2021, 07:49:37 pm
content: Quote from: langwadt on March 12, 2021, 07:12:00 pmTrue enough. Though Nichrome is kind of the standard. Aggressive filtering is called for here. Copper - a durable enough gauge will take way too much current. I am hoping to stay below 10A at 48VDC, 20A at 24VDC. Low voltage for safety. Iron - I'll take a look but would be more inclined to use Stainless Steel. It is an alternative that I have been considering.

author: penfold
author_rank: Frequent Contributor
timestamp: March 12, 2021, 08:47:37 pm
content: IQuote from: phil from seattle on March 12, 2021, 06:56:24 pmRather than a full PID (PDI, IDP... depending on preference) I would consider perhaps running the wire current through some current mode control (just bang-bang control or hysteretic, nothing complicated) designed around some series inductance and use a power calculation at a low frequency (10Hz or whatever) to do a compensation closer to the thermal time constant of the wire.Just as a side thought, if you were to take voltage feedback from a "tap" on the wire in like the top 2cm of unused span, I'm pretty sure with a little maths it could be used to temperature compensate the current control for neg tempco metals.Piano-wire can be a reasonable option if the target temperature isn't high enough to upset its temper too badly, but even when annealed its pretty strong.

author: tszaboo
author_rank: Super Contributor
timestamp: March 12, 2021, 10:57:20 pm
content: Way to complicated.Connect setpoint to input, replace Rg with PTC wire.Boost opamp output with transistor if needed.Probably possible with 1 transistor, but I cannot be bothered to figure it out.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 13, 2021, 02:20:54 am
content: Such things indeed exist! Check out hot wire anemometers.In that case, the goal is to measure the power dissipation and thus mass flow rate, which you'll also get more-or-less, not that the foam flow rate will be all that useful; just that it's a byproduct of the temperature regulation.To measure the resistance, you need the ratio of voltage to current. This can be done with an analog multiplier, PWM (suitably arranged, can also perform multiplication), or digital control. You may also find it's helpful to measure two points alternately, and calculate the resistance slope between them, much as a MPPT does. This especially helps in the general nonlinear case (like from a solar panel), but the trick is, for a fixed difference ΔI, the voltage has a proportional difference ΔV = ΔI R, and so average current or voltage can be adjusted, while applying a fixed AC dither. Just detect ΔV (which is again a mixing process, but a synchronous detector or sample-and-hold can be easier to build than a full multiplier) and servo against the setpoint.Tim

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 13, 2021, 06:39:02 pm
content: I endorse your plan to actively control temperature. I have used open loop hot wire cutters and it is tricky and tedious. The heat required varies with the foam, length of cut and cutting speed all of which change often. While the range of heat that works is not tiny, it is small enough that goofs occur frequently, either from too much or too little heat. The easiest manual control is to vary cutting speed, but if you don't cut foam regularly you lose the feel and skill.

author: phil from seattle
author_rank: Super Contributor
timestamp: March 14, 2021, 12:11:19 am
content: Quote from: CatalinaWOW on March 13, 2021, 06:39:02 pmExactly! And this will be for a CNC foam cutter with 2 independent X/Y platforms at each end of the wire. One will be fixed position and the other can be moved to accommodate different sizes of foam. So the wire length can vary in 2 ways. The system should not require any input on wire length - just target temperature. The goal is to be able to put the temperature setting in GCode.

author: beanflying
author_rank: Super Contributor
timestamp: March 14, 2021, 01:27:01 am
content: With the several cutters I have made over the decades (old)Stainless Mig wire 0.8mm worked really well. As already mentioned Nichrome was always the go to partly as it is widely available in the past. Another alternate with the reverse tempco is Kanthal which I have a stash of for other purposes which I got of evilbay from a surplus seller in Europe haven't tried it yet.I have in the past used anything from a 12V battery with a sliding alligator clip on the bow to adjust the temp to more recently a 0-15V 60A supply I own. a 3-4' bow typically with the Stainless wire runs 12V and 8-12A depending on length.With Temperature setting we always did a suck it and see test and as we were hand cutting and you could feel the drag on the wire and adjust the wire on the fly as you cut.Sometime project below as it might spark a few ideas lots of other things to do ahead of thisTabletop is two torsion boxes 1200x600 joined together and will go on top of some trestles and serve double duty for woodworking. The plan is for the cutter and frame to be removable for easier storage. so 1200x1200 base and likely a 600mm X axis.

author: SilverSolder
author_rank: Super Contributor
timestamp: March 14, 2021, 01:56:29 am
content: One issue is that as the foam "sucks heat out of the wire" - you have no way of knowing if it is a 5cm thick foam piece sucking heat out of 5cm of wire, or a more compact 2cm foam sucking a lot of heat out of 2cm of wire. So keeping the temperature constant at the point of cutting will probably only work well if the length of the wire is not too different from the thickness of the material?Imagine the situation where 3cm of the wire is glowing hot, yet the wire is still too cold inside the material where the heat is being carried away.How do you deal with that?

author: beanflying
author_rank: Super Contributor
timestamp: March 14, 2021, 02:04:14 am
content: Quote from: SilverSolder on March 14, 2021, 01:56:29 amBy controlling speed to allow the wire to do it's job without mechanical pressure. Providing the wire is above the melt point of the foam it will cut and like soldering temperature above that point allows for more speed. So looking purely at temperature is not taking into account the whole system.It is actually a misnomer that it is 'cutting' it is an ablation or melting process.

author: SilverSolder
author_rank: Super Contributor
timestamp: March 14, 2021, 02:55:31 am
content: Quote from: beanflying on March 14, 2021, 02:04:14 amInteresting, so like a soldering iron, you have to pump more energy into it when soldering large blobs quickly (more energy to maintain a given set temperature)?

author: beanflying
author_rank: Super Contributor
timestamp: March 14, 2021, 03:13:15 am
content: Sort of that analogy. More energy into the wire will get you more speed potential and ability for the wire to continue melting the foam as you progress. The downside to simply pushing more energy into the cut is the kerf (width) of the cut goes up so the accuracy and surface finish quality goes down. In the case of CNC cutting (misnomer but it is what it is) as you have no 'feel' over the process you will need to figure out a speed value that works in particular for strongly tapered parts where the speed ratios can vary a lot. So with an energy to maintain a certain speed at one end will produce a more heavily cut (over melted) at the other narrower end.The set point also depends a lot on material being cut. EPS or simple Styrofoam being on the low end and DOW Blue or Spyderfoam being higher so you finish up with rules of thumb for a given wire/bow length for different foams.Minor thing to consider is also the mechanical properties of the wire when hot our typical bows held the wire under a fair bit of tension when cold to allow for softening/stretching when hot. The rough design above won't run a bow and the wire will be on runners with a weight at one end to maintain tension.

author: pqass
author_rank: Super Contributor
timestamp: March 14, 2021, 03:46:31 am
content: What if you you were to measure the ease of travel of the wire through the foam directly by attaching a load cell at one end?Use the load cell signal to modulate the current through the wire; >deflection => >current.You would need a knob to set the minimum current and another knob for the deflection signal gain;ie. delivered_current = min_current_setting + deflection_signal * gain_settingThe fixed end of the wire should to be very stiff.

author: SilverSolder
author_rank: Super Contributor
timestamp: March 14, 2021, 04:35:04 am
content: Quote from: pqass on March 14, 2021, 03:46:31 amThat sounds like a pretty cool idea - it may be easier to measure than the temperature of the wire. - We still have the issue of a thin dense sheet vs a thick fluffy one (i.e. wire is unevenly heated). Ideally, the length of the wire should be modulated to suit the thickness of the material? (Unless it doesn't matter that the portion of the wire outside the cut gets hotter than the part inside.)

author: beanflying
author_rank: Super Contributor
timestamp: March 14, 2021, 04:55:54 am
content: If your foam is of constant width then that could work but generally this cant be relied on to be constant in the real world. An eyeball gauge might still be the best option to generate a table of Power and SpeedsWith unknown foam densities it was fairly normal practice to run a manual test cut and look at the wire drag/sag and bump up the current to reduce it.A Few potted thoughts on powering the wire now the Coffee level has increasedConsider it an ideal resistor and the few percent change at the operation point is a tweak more than a major issue. If you want to try and run a compensation curve then sure but it is creating more bs than is likely fixes.The Supply for the wire should be a Constant Current Source and say allow for 24-36V maximum to allow for fine cutting wires to be used at up to a 2m length. Maximum likely current based on sub 1mm wires of 20A with finer wire being sub 10A. This can be trimmed a lot but this then limits wire options.

author: SilverSolder
author_rank: Super Contributor
timestamp: March 14, 2021, 05:01:17 am
content: lol 2m? Those are some serious foam slabs!

author: beanflying
author_rank: Super Contributor
timestamp: March 14, 2021, 05:26:28 am
content: Diagonal or tapered cuts across a 1.2m wide block + a 'fudge factor'In cutting R/C model wings for example 900-1200mm wide blanks are common and if you look at some of the current industrial foam cutters and the stock materials available then 1000-1500 wide beds and blocks are common.Below just for info this would be fairly 'typical' wire drag across a 1000mm block as shown with the wire to cool so about a 20mm drag. While this might seem a gross effect the difference between this and a straight cut is not much in terms of power or speed. So just for a concept wire shown above the block but a simple rear to front cut 'typical' wire shown, increased wire tension will not overcome this issue only a reduced speed or increased power will.Interested to know if a strain gauge will be effected by heat or if it would need to be isolated from the wire?

author: pqass
author_rank: Super Contributor
timestamp: March 14, 2021, 05:45:07 am
content: Quote from: SilverSolder on March 14, 2021, 04:35:04 amI don't have experience with doing this but it sounds reasonable.I was thinking of a lightweight load cell like this:https://www.sparkfun.com/products/14727Surely it should be able to detect low digit gram movement in the wire. I don't know if the cold-to-hot wire temperature swing (increasing its length) would exceed the load cells range. This will be exacerbated by wire length. You may need to move up to the 500g load cell but then you'll lose resolution (low digit grams detection). Or you could bring the wire up to minimum open air temperature (min current) and then use a thumbscrew to tighten the wire up to put a minimum load on the load cell before starting the cut.If the foam thickness varies during the cut, the deflection amount should just pump more current to compensate. Although, it may be a slow feedback loop. At least you won't be fiddling with a current knob; just maintain an even pressure during the cut.Foam density+initial thinkness should govern the minimum current knob's position; lighter material starts at a lower setting. Although, denser material will need a higher deflection gain knob position. This is going to be trial and error; enough current to cut but not too much to make a ragged edge. Does that sound right?The only wire temperature that matters is that in the material; cooling off as you make the cut. The air parts will naturally be hotter but it should't matter.I'm pretty sure it can be implemented old-school; all-analog with op amps.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 14, 2021, 05:57:17 am
content: A position sensor would be better than a load cell: you need a spring to hold tension in the first place, and then tension only varies due to displacement of the spring. (When I made a hot wire, so many years ago, I used a bit of coiled up coathanger for the spring. Good enough.)Which... I suppose a slider (pot or whatever) would work. There are many kinds of position sensor. One with low hysteresis/stiction would be preferred, or at least with a stiff enough mount that the spring tension easily overcomes it.Neat thing is it could be simply a rheostat. Go super old school.Maybe even just a switch is enough. Hysteretic temperature control. Insensitive to resistance tempco, uses wire length as the standard. Probably not fast enough to do that way, but you can see how simple it could potentially be.Also, more CNC ideas, if this is at all relevant -- could simply put the power supply on another axis and wrap it into the G code. Then you can tune feedrate and heat for whatever depth of cut. Good luck calibrating it, but you could get some very nicely controlled cuts (in terms of feed rate, kerf, evenness, edge burn, etc.) once it's all done.Tim

author: pqass
author_rank: Super Contributor
timestamp: March 14, 2021, 07:07:40 am
content: Quote from: T3sl4co1l on March 14, 2021, 05:57:17 amWhen I suggested a load cell I was thinking about the typical foam cutter; vertical wire cutting centimeters thick foam not cutting two meter long slabs horizontally. So choose the position sensor based on the maximum deflection expected. Still, I wouldn't think you'd want it too slack otherwise it won't be a straight cut when moving in both x and y at the same time. Besides, the heat melting the material does the work; it's not a wire cheese slicer.So, 1mm deflection (over 2 meters) tops? Maybe an analog hall-effect sensor then?If you've got a lot of slack in the line (with a light spring on the end), then your mode of operation is more like:push on the material, wait for the spring to tighten up the wire, push some more, etc.I think using a tight wire with gram-resolution feedback modulating the wire current will at least allow the feed rate to be less precise, maybe even constant. ie. relieves the operator from calculating feed rate and current at any given part of the cut.EDIT: Maybe something like the arrangement shown in the attached diagram.

author: beanflying
author_rank: Super Contributor
timestamp: March 14, 2021, 07:43:44 am
content: Slabbing type cutters are easy as you tend to be feeding the same density and foam types into it multiple times so a set and forget more or less. The shape and profile cutting where you are moving four or even five axis is where is gets harder as the wire is not a static length. For the setup shown above I would be looking at a 5-600mm length change available to allow tapers and up to 45 degree cuts across the bed.Interesting kicking around some ideas

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 14, 2021, 09:00:26 am
content: Quote from: pqass on March 14, 2021, 07:07:40 amThat too, you want to keep tension to avoid cutting much of a hyperboloid!The deflection, axially, due to thermal expansion, will easily be much more than 1mm -- the thing I played with used stainless wire over about a 30cm span, and needed a couple mm of travel I think. (The frame was stiffer than needed, hence the extra spring. No shame in using the frame itself as a spring by the way, but do mind the stiffness of the mounts, don't want it tipping relative to the base you're pushing against, of course.)The lateral or longitudinal deflection inside the cut, can easily be much more, unless the tension is very high indeed; in which case, be very careful about the temp-strength curve of whatever material the wire is, too!(I wonder what shape that would be, anyway? It ought to be a somewhat lower radius of curvature, at depth, but would it be a hyperboloid exactly? A hyperboloid is a quadratic surface formed by the rotation of an offset straight line, not the deformation of a more-or-less-equally tensioned cable. Maybe it works out the same anyway? Hmm, or revolution of a catenary? No, not necessarily anyway; the cable is probably not even making a plane curve inside, it's also being twisted around as it goes. Hmm...)(Looking, that would be a catenoid. Which is also a minimal surface, hence the shape of otherwise-cylindrical bubbles, for example. It has a transcendental form, not quadratic, so definitely not equivalent to a hyperboloid. As for this case, who knows what exactly it is.)Anyway, yeah, it's notquitea cheese slicer, but I think you don't want to sit and wait in the cut, you want it to drag just enough that the melting foam sets the feedrate for you. Taking it slow, will inevitably end with a jagged cut. Just like, heh, compare machine to hand guided torch cuts, that's a good example. If you could just move your hands smoothly, it'd be fine...I think, for the most precise work, it's always going to be a compromise. A fast cut requires high power, which will excessively overheat the in-air part, widening the kerf at the edges. A precise cut might be taken slowly, but also maybe not too slowly as you don't want the molten-foam-gunk welding back together behind the wire. It helps to have it just hot enough that the foam melts away a little more than the wire width; or, leaves the cut entirely as assorted toxic vapors...So, I think in terms of a hand operated machine, it's simply a matter of adjusting the tradeoffs for best usability and repeatability, given the limitations of our hands.In an analogy that I realize is probably nearly useless, because of how few people likely have experienced it -- a similar situation is induction heater control. So, let me explain that a bit.Induction heating, is the heating of metal, using an AC magnetic field. The power supply is only capable of so much output voltage and current, and the work coil and associated components (usually a resonant capacitor and transformer) form a matching network between the inverter and work. The impedance and resistance vary with material type, size and proximity. So, if we set up a coil to receive power, we can run it at fixed voltage for example, or current, or just sit on full power at resonance all the time.The first case (CV) is about what those cheap ZVS drivers do -- output voltage is close to supply voltage (times a conversion factor), so output power is inversely proportional to Q factor. If we start with an unloaded work coil, Q is high and power is relatively low (but still significant: we're typically dumping a lot of amperes through a hunky copper coil). As we introduce some work, Q drops, and power goes up -- it's as if it adjusts up to meet our load. This is efficient for intermittent applications like stovetops, forging, and as a handheld torch or sorts. However, if we introduce too much work, it may be that the Q drops too far (e.g. lossy magnetic materials like steel) and either draws excessive supply current, or quenches oscillation (for the ZVS oscillator kinds). Or for large nonmagnetic materials, the inductance drops, raising frequency and lowering Zo, drawing more current but not necessarily delivering more power into the work (a solid bar of copper might be a better magnetic reflector (diamagnet) than the coil, and end up raising Q instead; I'm not sure that a practical situation would ever see actual reduced power from a heavy copper load, but more just that the power might not be as high as one expects, and certainly the efficiency into the workpiece is very low.)So that's one situation, insert work and it gets hot. Sounds nice, eh?Other control methods, and matching networks, give different results. We might have a series resonant system, with current or phase control. Control can be implemented by adjusting supply voltage, or inverter PWM, or frequency. In the former, phase would be tracked only to stay on resonance (or somewhat nearby, whatever's convenient), making the system a simple PLL'd oscillator. With frequency shift control, phase control is a possibility.Anyway, for this case, say we have a CC/CV supply driving an inverter, switching at resonance. The inverter effectively downconverts the resonant tank's response to DC, so its ESR looks like a resistive DC load. When unloaded, Q is high so ESR is very low, and the inverter probably sits at low voltage, in a current limit. When heavily loaded, Q is low so ESR is high, and the inverter probably sits at low current, in a voltage limit. Here, we have a situation that, as work material type and size/distance are varied, there will be a maximum power point. So as you insert a workpiece, power goes up, and up, but gradually slows down; it may hit a peak and actually reverse, going back down.So, having explained that:Depending on control method, you may have a situation where more and more power is available; or there's a sweet spot, and much like using a gas torch, the hottest part of the flame is just at the tip of the inner cone, although in this case there's no cone to compare to.And so, applied to the hot wire:It may be that a constant-resistance control is more convenient, as it tends to keep the temperature up. But it fails for low-tempco wires, and it allows sausage effect (in-air section overheats).Mind, nothing can fix the sausage effect, there's simply no way to avoid that -- only compromise between cutting rate, kerf and overheat.It may be that a (lateral) force response is more convenient, giving more heat on demand, at expense to accuracy (including tension somewhat, as the overheated wire takes up more compliance range of the spring). It's automatic, you don't need to touch a knob, the work is the knob itself effectively.Or an axial force or displacement response, which tracks with lateral force as well as temperature.Or maybe a combination of all three is best -- that's the thing with human interfaces, they're messy. It's nice to conceive of some ideal control method, that perfectly separates voltage and current, or whatever; but that method might turn out not all that practical. A drill motor for example needs a resistive response: it should have more or less constant speed for the most part, but when it bogs down, it certainly should not simply stall out, as a CV/CC supply would. The CC region should have a resistive slope. Maybe the CV-CC corner is very wide, so that it's not even very CV at light loads, but once loaded past a point, it very obviously drops off, giving tactile feedback that you're overloading it.Or maybe CC is valuable anyway, as an electronic implementation of the adjustable torque clutch most drills have! (Do they do that, these days? I haven't used a nice new drill in years, come to think of it..)Or consider the automobile: we expect throttle to correlate with torque, but it's a combination with RPM as well. If it were purely torque, we'd see pretty wide variation in speed as the vehicle's mass is an effective integrator. Control adjustments would be too frequent, it would feel slippery and inconsistent, overly sensitive to change in road grade, or wind. So it should have a resistive characteristic. (Modern control systems, with electric throttle, and CVTs when applicable, can do all manner of weird things, anyway, in terms of dynamic response. I mean, even with basic mech throttle, ECU and automatic transmission, dropping into low gear for acceleration might take a second or two. It might take as long for the electric throttled engine, meanwhile the CVT adjusts as it goes, or with some delay too; they even do bizarre things, at least in the US I think, like emulating a shifting transmission with the CVT. Humans are,weird...)Tim

author: SilverSolder
author_rank: Super Contributor
timestamp: March 14, 2021, 03:56:13 pm
content: I really liked the idea of modulating the power along with the other parameters straight from the CNC - that almost has to be the best way for a mass production setup (max speed / quality, but a lot of work to set up?).For general cutting, perhaps the tension on the wire could be used to modulate the feed rate? So, as tension rises, the feed slows down... That would compensate for the material having variable thickness etc.? Seems that might be better than trying to modulate the current in the wire (or, in addition to modulating the current)?

author: pqass
author_rank: Super Contributor
timestamp: March 14, 2021, 05:49:17 pm
content: I was curious of exactly how much a heated wire would expand (fixed on one end, spring on the other). See attached.It easily cuts low density EPS insulation at 10VDC@2.1A but not at 7.4VDC@1.5AThe expansion stops after about 10 seconds; when at equilibrium I assume.15 seconds after power-off, the wire is cool again and back to its original length.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 14, 2021, 06:12:52 pm
content: Quote from: pqass on March 14, 2021, 05:49:17 pmData rules. It is too easy to get buried in alternative theories when a few minutes of data collection can narrow the speculation greatly. Also opens up another alternative for feedback source. Instead of a load cell a LVDT or other distance measuring option. Even turn a pot through a gear train or other mechanical motion gain.

author: twospoons
author_rank: Frequent Contributor
timestamp: March 15, 2021, 05:34:51 am
content: Set the wire up in a bridge, with feedback driving the bridge excitation so that resistance remains ~constant. Like they do with Pirani gauges.You can make one of the resistors in the reference leg variable to adjust the temperature.

author: phil from seattle
author_rank: Super Contributor
timestamp: March 15, 2021, 06:43:14 am
content: Quote from: pqass on March 14, 2021, 05:49:17 pmThe time to equilibrium is dependent on the thermal mass of the wire, the total wattage dissipated and ambient temperature. I'm not sure of what the actual ambient will be when the wire is in the EPS given it is a very good insulator. Higher voltage will be faster than 10V. But no matter what, the PID algorithm will need to factor in a slow response. I'll play with the coefficients a lot once I get an experimental setup going.Thermal expansion is a small part of the problem. Cutting on angles could require as much as 50% more wire, maybe even more. I am thinking of using another stepper motor channel to control a spool to maintain proper tension. Haven't put a lot of thought into that yet.

author: phil from seattle
author_rank: Super Contributor
timestamp: March 15, 2021, 06:54:35 am
content: Quote from: SilverSolder on March 14, 2021, 03:56:13 pmInteresting idea. One area I want to investigate, in a controlled way, is the relationship between speed of cut and quality. Anecdotally, quality is related to speed when manually controlling a hot wire. Faster is better. However I suspect this is due to hand shake that is spread out over a longer cut at faster speeds as well as the increased resistance/drag at faster rates. You can hold heavier objects steadier than light ones.Generally, EPS and other foams have a uniform density.

author: phil from seattle
author_rank: Super Contributor
timestamp: March 15, 2021, 07:03:58 am
content: Widening of the kerf at the material's outside edge generally isn't a problem with manually controlled temperature. I don't expect it to be different with a CNC cutter though worth keeping an eye on.

author: beanflying
author_rank: Super Contributor
timestamp: March 15, 2021, 07:15:23 am
content: We used to make our old Foam cutting templates from Laminex/Formica and they could cause a problem with not being easily able to see the kerf properly. The only issue could be a slight deviation on the kerf due to heat with this method. As I mentioned earlier though test cut was normal and tweak on the fly as required.Even with a CNC set temp I would likely still want the ability for a small range manual override to it.

author: phil from seattle
author_rank: Super Contributor
timestamp: March 15, 2021, 07:52:59 pm
content: Quote from: beanflying on March 15, 2021, 07:15:23 amAbsolutely! The system I will be using is grblHAL which has the ability to override a number of GCode settings. In this case speed and temperature.