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Measuring extruder temperature using Adrian's circuitWednesday, June 25. 2008
I prototyped and tested Adrian's proposed circuit for using the resistance of the Nichrome 60 heater on Tommelise's extruder barrel to measure its temperature...
Quite some time ago, Vik Olivier suggested that it should be possible to use the resistance of a Nichrome heater wire to also measure the temperature of what it was heating if one were clever enough. A few days ago, Adrian Bowyer published a circuit design that could make that possible. I intended last weekend to do a lashup of the circuit to see if it would be of use in my next generation extruder design for Tommelise 2.0. Because of some other firmware issues that took longer than I expected I didn't get around to cobbling the circuit together till yesterday. It seems to work quite nicely. A little background, first. While, I've been quite addicted to the notion of a single control board design for literally years now, I recently decided for convenience to have a main controller board run the positioning system and another, smaller board run the extruder(s). The reason for that change was more practical than philosophical. I knew very well what was needed to run the positioning system. The extruder, thanks to the growing number of people making and using Darwins with their Mk II extruder, had entered a period of rapid evolution. What with dribble controls, encoding the pump shaft rotation and now this nice new proposal for measuring extruder temperature, it seemed to me that it would be good to just freeze the main control board design and put another microprocessor to work just looking after the exturder(s). In my design the two boards would talk to each other via a I2C link, a technology that I've recently had a lot of experience with in setting up my EEPROM buffer. To that end I put an add-on board to handle Adrian's circuit onto my old 18F4550 prototype board. You can see the lashup here. 
The yellow circle at the top of the pic encloses the actual barrel extruder barrel. The red circle in the middle marks the add-on board and the brown circle at the bottom encloses the 18F4550 prototype board. Looking more closely at the add-on board (yellow circle) I've marked the two transistors that are TIP 122's in Adrian's design and BD681's in mine. The green circle right beside them encloses a relatively high wattage 1.6 ohm resistor a handfull of which I bought to build up Zach's stepper controlled board and then never did. I couldn't understand why Adrian put the 220 ohm resistors between the transistors and the microcontroller, so I left them out. Mind, I expect that there is some very good reason and I'll find myself putting them back in at some point. Mostly, I didn't have any 200 ohm resistors in my supply drawers, so, since I hadn't used any such thing heretofore I omitted that part of his design. The Tommelise extruder heater is rated at 6 ohms. Figuring a nominal voltage to the heater of 12v that means that I should read about 3.3v at the sense point when the system is cold. Resistance for Nichrome 60 at 250C is about 1.15 times its resistance at ambient. What that means is that the voltage measured should drop about 0.34v over a range of about 230C. Given that the 18F4550 has a 0-5V range, 10 bit A/D circuit on-board what you are looking at is about 3.3C resolution for this kind of temperature measurement. While I've not had time to design the firmware I did a cursory test of the circuit manually by tapping the extruder heater with a 5v supply source and then quickly measuring the voltage on the other side of the circuit with a digital meter after I'd disconnected the power input. The voltages are moving in the directions that Adrian suggested, so I suspect that my circuit is, more or less, right. I fiddled with a range of resistor values for the 2.6 ohm one that I was using and discovered that while I could move the cold voltage measurement up and down fairly easily, there was relatively little change in the range of voltage that temperature differences yielded. Adrian suggested that one might want to put an op amp chip into the circuit to get better resolution. Another way to approach this would be to use a higher resolution A/D chip instead. Microchip offers both a 12 bit, I2C compatable A/D converter and a 16 bit, I2C compatable A/D converter for about US$1. That would get your resolution down to 0.8 - 0.05C. I don't know why you'd want more resolution than that. Errors in other part of the circuit would be much higher than that. Given that I know how to make I2C comms work and next to nothing about op amps, that's the direction I'm likely to take. Trackbacks
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That's very interesting. You could also use an external Vref for the ADC of the PIC and get better resolution with just 2 resistors.
Hi Forrest,
Nice work. I love it when people actually do things, not just talk about it. Something that I lack myself after building the Darwin and then finding no time to actually develop my ideas. I'm forced just to write and discuss short bits when I find some free hour.
To me it seemed relatively easy to measure analog with the 18F4550 at 8 bit resolution. Getting the other 2 bits to make for 10 bits is a bit harder, but there are code examples out there. I'm not sure though, whether this range would be enough to measure more or less precisely at the 50-300'C range... 16 bits should do it, no doubt. I've never gotten I2C to work well. I had trouble getting the device to function as a master or something. If I try it again I might ask you for some advice at some point in time, if you don't mind, since you're doing really well with that :)
Hi
I am working on a project where we are using impulse heater wires (only on for 1 second) to seal plastic film together.
Is the circuit and more details available?
I had considered using a PLC and measuring voltage and amps plus some maths to work the temperature out. Anyone tried that?
Regards
Mike Bowthorpe
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