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How do you think of the design of Mightyboard by Makerbot?

Posted by thuwrx10 
How do you think of the design of Mightyboard by Makerbot?
April 18, 2017 08:56AM
Mightyboard my Makerbot, a once open-source hardware, seems have been abandoned by the open-source community. There are a few differences in design between Mightyboard and popular RepRap electronics. I found these design considerations interesting and want to discuss here.

1. Mightyboard uses thermocouples with dedicated ADCs to get the temperature reading from hot ends while the majority of RepRaps use thermistors. What's the difference? Thermocouples do have a flaw that it is a good conductor. It took out electronics on board with +24V when hot end heat resistor accidentally shorted with the heat sink in my printer.
2. Mightyboard has digital potentiometers so that MCU can adjust the current limits of stepper motors automatically. What's the benefit?
3. Sailfish firmware use .x3g file claiming it will burden less on the MCU. Is RepRap firmware more likely to use up processor time?
Re: How do you think of the design of Mightyboard by Makerbot?
April 18, 2017 09:59AM
If you are interested in alternatives to Arduino/RAMPS, take a look at 32-bit controller electronics. They have much faster processors (e.g. 120MHz ARM Cortex M4 in the Duet WiFi and Duet Ethernet, see duet3d.com), so processor load isn't an issue. They nearly all have firmware-settable motor currents. This is well worth having IMO: no need to twiddle ports while looking at a voltmeter, or setting the current by trial and error, you just set the current to what you want e.g. 75% of your motor's rated current. The motor currents can be reduced by the firmware at appropriate times, for example when homing or when the motors are idle.

Thermocouples are typically more accurate than thermistors, but PT100 sensors are better unless you want to measure very high temperatures. With a thermocouple, you need to run thermocouple wire all the way back from the thermocouple to the interface board, to avoid temperature errors. This makes it hard to have a connector on the print head for easy hort end removal.Thermocouples only generate a few millivolts or even less, so the wiring picks up interference by induction easily.

The Duet WiFi and Duet Ethernet have daughter boards available providing digital interface for thermocouples and PT100 sensors. The chip on the PT100 board is specified for +/-45V over-voltage at the inputs, so a short to 24V should not cause damage.

32-bit boards offer have other advantages soo, such as firmware configuration from a test file (no need to rebuild and re-upload the firmware to make configuration changes), and the better ones provide a web interface via Ethernet or WiFi.

The reality is that 8-bit controller boards such as Mightyboard are long obsolete by current engineering standards. But Arduino/RAMPS is still widely used because the Chinese-built versions are so cheap.

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
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