Binary Construct

Temperature Tuning Part 1

Thermistors work very well when measuring temperatures near the middle of the specified range. They operate nearly linear and have a high resolution (large resistance change per degree). These properties make it easy to determine the actual temperature using the RepRap electronics.

There are two parts to this problem. The first part is when you get close to the upper end of the range of the thermistor the resistance changes per degree are very small. Depending on the resolution of the DAC being used, this can allow for more error in measurements and loss of individual degrees (e.g. incrementing the digital value may be 2 - 3 degrees).

The other part is the resolution of the lookup table in the firmware. Since thermistors are nonlinear and ATmegas are relatively slow at float-point calculations, a lookup table is required to convert the output of the DAC measuring the thermistor to an actual temperature. This table approximates the curve of the thermistor’s change in resistance to actual temperature.

With careful thermistor choice you can reduce the effect of this problem. I will attempt to explain below.

I finally got my Mendel working, as it turns out almost all my problems were being caused by bad thermistor calibration. You can see from the screen shot, 250C indicated by the software is actualy around 225C in the real world. On short notice I found the correct look-up table here: RepRap 1mm Thermistor Lookup Table. Brian helped me fix a leaking thermal barrier. My spool is mounted below my machine, so I put together a Bowden cable to feed the extruder to keep it from jaming.

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I have assembled most of the Mendel now, just waiting on a new Z-belt from McMaster. The BoM for the Mendel has the wrong one listed, try this instead: 6484K512

The NEMA17 I have for the extruder is slipping. It is a KL17H247-168-4B rated at 63oz-in but I don't think I am getting that kind of torque using the Extruder Controler board. I have a spare stepper board I might try and drive it from.  I am also going to attempt to build a Bowden cable with teflon lined tubing (McMaster Part: 4mm I.D. 5557K33 or 3mm I.D. 5557K38). This will allow me to use a NEMA 23 motor rated much higher (282 oz-in).

I moved the stepper controller mounting plate outside the frame to allow for the 4th stepper controller. I will probably move it as it might restrict the Y axis movement.

The next step is to attach the Z belt and finish the extruder.

Photos below: