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Jan 12, 2016 at 8:01 history tweeted twitter.com/StackElectronix/status/686820377876688896
Jan 11, 2016 at 16:49 vote accept nagylzs
Jan 11, 2016 at 6:37 comment added nagylzs Added sketch for the mechanical part. (But yes, this is not strictly part of the question.)
Jan 11, 2016 at 6:36 history edited nagylzs CC BY-SA 3.0
added 225 characters in body
Jan 10, 2016 at 19:31 comment added Transistor I don't understand 'only the non-heating element changes length'. Can you provide a sketch? (You're providing more details which really belong in the question.)
Jan 10, 2016 at 19:18 answer added WhatRoughBeast timeline score: 2
Jan 10, 2016 at 19:13 answer added Transistor timeline score: 1
Jan 10, 2016 at 19:10 comment added nagylzs I can split the wire into two parts: one heating element, and one that has (almost) zero resistance. I can arrange the tensioning system in a way where only the non-heating element changes length. As a result, the length of the heating element will be almost constant, making it possible to calculate the temperature from the resistance. But calulcating the temperature based on contraction is not possible (or would be very difficult)
Jan 10, 2016 at 18:56 comment added Transistor It suddenly got much more complicated with the addition of variable length. With fixed length cutting wire we could calculate temperature by using current and voltage feedback, calculating temperature rise and getting temperature from lookup table. With variable length the cold resistance value will change. Is the length changing on the fly or could you stop, change length, allow wire to cool, recalibrate and resume? Another, simpler option is to control current. A set current will give constant power per unit length of wire and automatically compensate for change in resistance.
Jan 10, 2016 at 17:53 history edited nagylzs CC BY-SA 3.0
Added more description about the mechanical parts and the length of the heating element.
Jan 10, 2016 at 17:45 comment added nagylzs Yes, the power required depends on the thickness of the foam and the cutting speed too. This is why I want at least try to keep the temperature of the wire constant by measuring its resistance. Heat conduction of the wire is good enough, so if I could measure the total resistance then I could interpolate its temperature with an MCU, and modify the PWM accordingly. This was the idea, just don't know how to do the heating and the resistance measurement at the same time.
Jan 10, 2016 at 16:03 answer added Transistor timeline score: 0
Jan 10, 2016 at 15:51 comment added Transistor It's even trickier than that. At high cutting speed you may remove a lot of heat from the portion of wire in the foam and only air cooling from the rest. Running the cutting wire hot enough may overheat the air-cooled portion.
Jan 10, 2016 at 14:48 comment added Marla To keep temperature constant, the amount of power required will depend upon how fast you are cutting the styro-foam. Notice that the styro-foam is removing heat from your hot wire.
Jan 10, 2016 at 13:23 comment added Jarrod Christman Look at a Wheatstone Bridge, always the general solution I use when needing to measure a varying resistance. Though I am not sure how efficient it'll be with that high of a power, I've only used it in low power applications.
Jan 10, 2016 at 13:12 history asked nagylzs CC BY-SA 3.0