PrntrBoard – bringing it to live May 13th.

In my previous tests my PrntrBoard prototype was having some issues talking to the TMC2130 motor driver chip. It took a while, but I figured out the issue – the SPI library was not initializing correctly, so the chip was using the SPI0, hardware block, while on my board the drivers are hooked to SPI1 pins.

A few configuration settings later and Marlin was booting up, without complaining. In the process of debugging I also created a small test program to control the motor driver. You can see it here.

From that work I found that the motor driver chip was getting quite hot if I set the motor current at 900mA. The default was 600mA – it was getting warm but not as bad. Anyhow I decided I want to further stress the thermal design of the board and ordered this 4 channel thermometer.

The thermometer arrived today and I set to use it for a few tests. Keep in mind the accuracy of the thermometer is questionable at this point. I did a very simple test:

  • at ambient room temperature all 4 channels showed values within 0.3 degree Celsius.
  • I held all 4 probes in my hand and all 4 showed 35.1C – again within 0.2C of each other.

First I tried to test the extruder heater control logic. From previous attempts I know it was working, but I wanted to see how accurate the temperature is. I set the printer to heat the E1 to 180C:

M104 T1 S180

This is a video from that test. My thermometer shower 163.5C or thereabouts – so there is a significant offset between the firmware and the thermometer. I also verified with an infrared thermometer and it was showing ~160C. So it seems the thermistor setting is not quite correct. The bad thing is that this heater came from China and I have no clue what thermistor they used, so I’ll have to measure it a few times to determine the correct parameters that should go in the firmware. That would be a post on its own.

Disappointed from my temperature control test, I set to test the thermal design of the board. I added a thermocouple to the bottom of the board – where the motor driver ship is mounted.

as well as a second thermocouple on top of the chip

This was to test a theory, which suggested that the chip dissipates more heat on the bottom side.

Attempt one was not very conclusive. This was using Marlin, I enabled the extruder motor and with the default settings it had 600mA of drive current and 300mA standby current, but this was not enough. The driver chip was barely at 30C and both top and bottom were at similar temperature.

Attempt two almost ended in a disaster. I used the above test program to make the motor running with 900mA drive current, but the program had an unfortunate side effect of leaving the extruder heater on. I smelled smoke and turned the power off. The smoke smell was from the extruder heater melting the temperature probe cable.

Crisis averted and on to attempt number three. My test program was spinning the motor, the power consumption was about 0.5A at 12V. The chip was heating with steady rate, even with my big fan blowing over the board. It was clear the board would not sustain such current. However the top and bottom of the chip were within 1-2C of each other. I decided that maybe the fan was interfering with the measurements and thus ensued attempt number five.

The chip was heating rapidly, but finally you can see the top was 5C hotter than the bottom. Another observation was how quickly things cooled down after the power was turned off.

After all this what is the conclusion:

  • the motor driver does get hot on the top, so putting a radiator on the top has some merit
  • my thermal design sucks, and I decided to re-route the bottom of the board, so there is a large copper area with no traces which can dissipate the heat from the chip.
  • the thermistor coefficient needs to be calibrated in the firmware
  • on the bright side almost all controls are working and Marlin is operational to a degree.

Well, ’till next time

~V

Prntr Board V1

Prntr Board V1

PrntrBoard is a 3D printer controller board designed to work with STM32 NUCLEO dev kits. The current version (V1) supports NUCLEO-64 series kits. The design is being developed on F446-RE kit, but other models could work as well. IMO the STM32F446 NUCLEO-64 kit offers very good performance (180MHz CPU) for the price ($15).

Some features of the board:

  • 5x Trinamic super quiet drivers (TMC2130 or TMC2660)
  • Marlin firmware
  • 4x controllable fans and 2x “always on” fan connectors
  • selectable fan voltage (5V or Vin)

Because the NUCLEO-64 has limited number of IO pins, some compromises had to be made:

  • No sd-card
  • No display support

Generic shortcuts I don’t plan to improve:

  • 5V power supply is external. These are available from various resellers and fairly cheap – less than $1. No need to waste board layout space and component count.
  • Heated bed MOSFET – large heated beds consume a lot of power, and it is challenging to provision the design for > 10A current. External heated bed MOSFETs are very affordable < $10 and claim to support 20A minimum. If you have large bet use one.

Details

I use the daily build of KiCad (soon to be released V5) – let me know if you have trouble opening the files.

Once the design is validated I would add support for NUCLEO-144 kits, these have many more available I/O pins, which would enable more extruders, SD-card and LCD screen utilities to be added. The downside is that they are quite big and I’m trying to limit the design to 10x10cm to reduce the cost of the PCB fabrication.

The current design is using 4 layer board, 6/6 mil clearance, 12 mil via hole size and 20 mil via diameter.

There are two active branches:

  • tmc2130 – the board design for TMC2130 series drivers
  • master – the board design for TMC2660 drivers

Both PCB designs use the QFP version of the driver ICs because they can handle a bit more power.

Status

I have made prototypes of the TMC2130 board design (rev0) – the board has a few bugs, that are corrected in the rev1 version. I have validated the heaters, and fan controls are operational. Working the kinks out of the motor driver wiring.

The TMC2660 branch status is: the rev0 board is fully routed and passes DRC checks. I have not made any prototypes of the board, because 2660 drivers are hard to find due to low stock levels at suppliers.

Software

The software for the board is a clone of Marlin at bugfix-2.0.x branch of my repository. I keep it relatively in sync with the Marlin branch. Note: there are other branches of Marlin as well only the bugfix-2.0.x supports the STM32 microcontroller.

Marlin is using Arduino IDE or Platform IO. I personally had issues with Platform IO not supporting the STM32F4 board, so I use Arduino version 1.8.5. To compile the firmware code, you would need to install the STM32 Arduino port.

Leave a comment if you have any questions.