Controlling the temperature of a hot wire.

DavidGreg

I have a project where it would be handy to be able to control the temperature of a hot wire under different thermal conditions. This requires varying the power applied to the wire via PWM. The resistance of the wire corresponds to the temperature and so controlling to a resistance setpoint should give a reasonably repeatable wire temperature. But the trick is, how do you measure the resistance of the wire while heater is operating?

I thought of maybe using a series resistor with a known resistance and an ADC, but that seems to require quite a bit of power to be dissipated in the measuring circuit. Is there a more clever way to do this task with a prop?

Mike Green

You can use a small value series resistor so the voltage drop across it is small and therefore the power dissipated is small. You can use a Hall Effect sensor. There are some specifically designed for measuring current and have a (relatively) high current conductor in the package itself so the Hall Effect sensor is at a known distance from the conductor. I don't have a link to one of these, but I found it with a simple websearch.

Can you measure the wire temperature directly using an IR temperature sensor?

Toby Seckshund

If the wire is cutting something then there will be discrete zones of temperature changes. The mountings will be a sink of heat, the free wire will be a zone, the wire as it enters the cutting zone and a short distance into the cutting zone will be another, the cutting zone towards the core another - and then the reverse on the way out. It will be tricky to measure all those zones accuratly. The guage of the wire and the current is probably as good as it would get. Power loss in the measuring would only be a problem if it were battery powered ( or are we talking huge powers here ?? )

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Dr_Acula

I did this years and years ago with op amps and a PWM using a mosfet to control the temperature of a glowplug. It would be much easier with a microprocessor.

As you say, temp is proportional to resistance. The 'sense' resistor does not need to be a very high value, and ideally shouldn't get hot. I used a 30cm length of thicker hookup wire - the same thickness as is used in automotive wiring. All connections must be soldered.

Turn on the power with the PWM. Wait a little. Then sample the volts on either side of the sense resistor. V=IR and even though R is not known exactly, it is fixed and you can work it out if you know the wire thickness. So you can work out 'I' from the voltage drop. This current will be the same through the heating wire. Now measure the volts across the heating wire and R=V/I and V and I are known so you can work out R and that is proportional to the temperature. Use this to adjust the width of the next pulse.

This technique works regardless of the supply volts (which will droop a bit whether it is a mains supply or battery).

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ElectricAye

My comment here is a question instead of an answer: is it possible to power the cutting wire via PWM, then momentarily shut off the PWM and switch to a purely sensing mode wherein the cutting wire resistance is measured, the PWM duty cycle variable is changed, then the system switches back to the PWM power mode, and back again, etc, etc. really, really fast???

Dr_Acula

Yes, you could do that. Lots of options when you have a micro and you can do maths like V=IR. You could even display the temperature if you can calibrate it somehow.

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Graham Stabler

Lem make very nice current sensing modules based on hall effect: www.lem.com or you could use a sensing resistor (small value say 0.1ohm) this would not dissipate much power compared to your cutting wire P = I^2 R. You would want to take the average value so a simple low pass filter (RC) might be an idea.

To get resistance

R = V/I

where I is measured and V = Supply voltage * duty

Cheers,

Graham

Graham Stabler

Or just a thought, could you make the wire part of an RC circuit and measure time it takes to discharge in the off time of the PWM or at a regular but lower frequency than the PWM is driving at as ElectricAye suggests (lets face it the wire will not respond super fast).

Graham

DavidGreg

Thanks for all of the good ideas!

First, the wire is actually part of a glow plug. Not different than the cutting wire in any real way and the idea is the same (I used to cut foam wings with a hot wire cutter).

I did some playing on the bench tonight and the wire seems to respond amazingly fast (to me at least). The same duty cycle at 500 Hz and 100 Hz produce visibly different temperatures. 1000 Hz is different than 500 at the same duty cycle, but not as much. 10 Hz you can see the wire basically flashing on and off.

The ICs look interesting, but I'd like to avoid the extra complication. The RC circuit idea intrigues me, as low parts count is a big plus. However, I really have no idea how to implement it.

The plug consumes about 1 amp at 1.25 V when its at the right temperature (1.25 Ohms). A 0.1 Ohm shunt would consume 10% of the load's power, meaning I could control the wire temp up to 50 Watts with a 5W resistor, or 10W with a 1W resistor.

Could I use a prop pin as a 1-bit ADC with the serial resistor and simply turn on the heater when the pin goes high and turn it off when it goes low? Its sort of hard to do with the 1.65V threshold with out using an amp, and then I'm back to the parts count thing.

EDIT: Forgot to mention that this is a battery powered app, so consuming less power is better. I'm thinking that doubling the power with a 1 ohm shunt might not be such a bad thing...

Post Edited (DavidGreg) : 12/14/2009 1:47:46 AM GMT

ElectricAye

DavidGreg said...
... A 0.1 Ohm shunt would consume 10% of the load's power, meaning I could control the wire temp up to 50 Watts with a 5W resistor, or 10W with a 1W resistor...

With PWM at 500 Hz, I wonder if you have to worry about inductance in the type of resistor you select? There are "non-inductance" power resistors, which I think cost a little more. I suppose inductance might be a consideration, too, if you are coiling this "glow plug" wire into some fancy shape. These are questions, just thinking out loud.

CannibalRobotics

With the RC circuit model there is no 'constant' extra resistance. With a small capacitor there is only some front end load on the HIGH cycle. The wire could be the resistor. You just wait some number of uSec in the PWM = LOW state.
Right?
Jim-

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Rayman

You can find very precise milli-ohm resistors made for this application... They're called current-sensing resistors...

I have some 5 milliOhm ones from Newark: Part# 20C1972

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Graham Stabler

I think you could add a little capacitance across the wire, once the power is removed the cap will be discharged through the wire. You could connect the negative end of the capacitor to a propeller input via a high value resistor, when the power is switched off this point will rise in voltage and eventually count as a logic one. The time that takes will be proportional to the resistance. This could probably be incorporated into the PWM code.

Graham

mctrivia

since resistance changes with temperature then would it not be fair to say that the current draw also changes with temperature. why not just use a constant current source to provide the power? you then set the current at the temperature you want. No micro needed.

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Graham Stabler

That is the usual way of driving glow plugs, for model aircraft at least.