Reprap extruder heater

My cheap polymide tape arrived from Hong-Kong yesterday so I was able to get the heater built on the extruder for the reprap. No problems with the construction of the heater itself, although after running some tests I discovered the thermistor I had chosen is only rated to 155 degreees Celcius. I obviously wasn’t looking very hard when I ordered it or perhaps it was just the fact that it was one tenth the cost of a more suitable device that convinced me to buy it. I should have a new thermistor in this week and replace this one.

Before going too far with my heater I wanted to test the system and check that the temperature measured was accurate. I ran three sets of tests. Using the Butterfly Logger with some DS18B20’s and a SHT71 I logged the temperature of the barrel at the edge of the extruder (see close up above). The SHT-71 was used to monitor the extruder temperature with the DS18B20’s monitoring ambient. The first test was logged at 10 second intervals with the later two logged each second.

The first test was a 0.2 deg C/s ramp from near ambient up to 75 deg C and then a step change to 100 deg C and then passive cooling. This is shown in the plot below. The period of 10 seconds seemed too slow to give me a good idea of the stability so n the following tests it was decreased to 1 second. This did show rough correlation between the set temperatures and the measured temperatures although not really as accurate as I had hoped.

This test was a controlled ramp of 0.2 deg C/s from near ambient up to 100 deg C. After holding at 100 deg C the system is passively cooled to 50 deg C. The system holds at 50 deg C momentarily before being given a step change to 100 deg C, after which the system is allowed to cool to ambient.
The better time resolution allows the system stability to be better assessed. The system looks reasonably stable at the 100 deg hold mark. Here it is cycling around 5 deg around the set point. The ‘stable’ temperature is slowly rising which I attribute to the thermal mass of the barrel and thermal barrier warming up. It is not 100 deg C as it is not measuring at the same point where the control thermistor is measuring. Looking at this initially lead me to check the characteristics of the thermistor I was using and is what lead me to discover that it was only rated at 155 deg C. In checking the data sheet I also noticed a diference in the Beta value fromt he look up table used in the extruder firmware. I recalculated the look up table accordingly and repeated the tests in test 3.

This was a repeat of the previous tests with the new lookup table ( Beta = 4400). This seemed to give a ramp rate twice of what was programmed (0.45 deg C/s compared to 0.2 deg C/s). The temperatures seemed hotter which is expected given the change in thermistor table for the control firmware.

The next test will of course be to see how the system performs when loaded i.e. extruding some plastic.

Reprap Electronics build

I ran out of time this weekend to get anything done on the MP3 shield or the MLMC projects. I did however manage to find time to solder up the electronics for my reprap I’m building. After a couple of hitches I also got the firmware on and up and running.I had to download the latest from SVN else I got a clash between the stepper-motor drivers and the servo motor drivers in the firmware for the extruder.

I built the mother board to use a standard PC power connector even though I’m building a reprap. It just seemed silly to power this PCB via USB and then rig the power-supply to power all the other boards. I temporarily used the USB 5V to power the PCB via a pin on the JTAG connector during programming the firmware. Before the firmware on the mother board was programmed the PC PSU wouldn’t fire up so I needed a temporary power supply.

I managed to test the extruder board with some test software I found at….29. But I haven’t managed to get it working through the mendel firmware via the host software yet. The thermistor was working so I know the RS485 link is functioning properly. Probably just a configuration.h option I’ve over looked.

It took a couple of hours to get the three boards all soldered up and tested. I’ll post more when I have more done. I’ll probably be focusing on the mechanical side of building the Cartesian robot for now so not much electronics left to do, although I still have the firmware to sort through…

[EDIT] It turns out that I had overlooked the I2C connection between the motherboard and the extruder board. I should really have read the instruction.