I would like to create a hot wire foam cutter device. The wire material is nichrome, diameter 0.9mm, resistance is about 1.7 Ohm/meter. For best cutting results, I would like to control the temperature electronically. This will be a 4 axis CNC system where the wire length between the mounting points changes while the axes move. Planned length of the wire is abount 1.5m. Another extra (non-heating) wire is added, that is able to change its length to ensure the tension of the wire while the axes move. So the length of the heated part of the wire would not change, but it would be hard to measure the contraction caused by the heat.
The main question is this: how can I measure the temperature of the wire? I was first thinking about a thermistor, but then I realized that the hot wire itself could be acting as a thermistor. When temperature goes up, then probably the resistance will also go up. However, I have no idea how to create a circuit that measures the current flowing through (it can be 10A), and how to calculate the temperature from that. Another problem is that the hot wire will be heated with a 24V 200W transformator, unregulated AC power. My idea for power control was to use a diode bridge, a high power fet for switching (e.g. IRF44N or similar) and PWM signal to control the temperature. But this would make temperature measurement impossible - PWM controlled AC cannot be used together with the other idea.
The workaround I have in mind is this: use regulated DC and a simple A/D converter to measure the resistance of the wire in every (say) 1 second. Heating would be turned off completely during resistance measurement. This would take just a few msec. Use the PWM controlled driver to heat the wire in the remaining time. Would that work? How can I separate the heating circuit from the measurement circuit? (E.g. to protect the A/D converter safely.)
The secondary question is this: do any of you have an idea about how much power and voltage I need to heat the wire? I thought that 200W would be enough. That is about 8 amps at 24V. But don't have any experience. Also, even if I can measure the resistance of the wire, I have no idea how to convert it to a temperature value. (It would be nice to display the absolute temperature value, but it is not important - all I care is to adjust and maintain a constant temperature that has the best cutting results.) I have a 24V 210W transformator for heating, but the power may not be enough.
Or maybe you have an easier solution in mind - please don't hesistate to share.
Heating goes through the green lines and the red wire. Tension is provided by the gray cable (which is not heated).