We still have the market leading TMC2209 stepper motor drivers with good heat spread allowing high current to be drawn. The same with the input stage which is able to handle up to 30 A. Clever use of solid state power handling gives the board programmable over current protection which is both fast acting and incorporates over temperature.

The Allwinner A64 is still a great choice as the CPU (or SoC to be exact) since it is plenty powerful for a 3D printer, it has great upstream kernel support for all functionality and it has that integrated AR100 CPU giving us a fully fledged 300 MHz real time capabilities built into the silicon.

Recore A8 comes with Rebuild v1.0.1 pre-installed (the Fluidd version), but the user can choose to switch to MainSail or OctoPrint if they want.

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Better Wi-Fi support

For Rebuild, the Wi-Fi has now gotten an overhaul and so the board should come up as an Access Point from the start. This has been well tested with Rexfer dual band Wi-Fi dongle which is the “official”/works out of the box Wi-Fi dongle for Recore.

SSH integration

We have also worked on better integration of SSH between Moonraker, Fluidd, Reflash and OctoPrint. If SSH is not enabled in Reflash, it can later be enabled via either of the top level user interfaces. It is now possible to start SSH as a service in MainSail/Fluidd, but it’s not good to leave the SSH interface exposed with the default password. Therefore there is a timeout on the SSH service, so it is shut off again after an hour is the password is not changed. We consider the top level UIs well enough protected to be able to start the SSH service.

For Reflash the most important updates are support for Wi-Fi access point, serial number update functionality, touch panel rotation and adding the IP-address to the screen.

If there is a Wi-Fi dongle attached when Reflash is booted or if one is inserted after a boot, Recore will come up as an access point. So now it’s possible to connect to the board and do a complete flash without requiring an ethernet cable and finding the IP address. Since the board is in AP-mode, it will not have access to the internet and thus Rebuild can not be downloaded, but instead it must be uploaded from the host computer.

Serial number tool

For all Recore A6, A7 and A8 boards, the serial number and a calibration file is present in a small 4 MB hardware partition in the eMMC. This is nice because it will survive a complete wiping of the eMMC which is what happens when installing a new version of Rebuild or Refactor. For Recore A5, this was not present. But those calibration files exist in the Recore repository, so it’s possible to add them simply by downloading them from github. If the serial number is missing, a pop-up with an option to add this comes up in Reflash.

It’s important to know the hardware revision when using Reflash so that the right device tree binary (DTB) is set. The DTB describes the hardware and it is now more or less the only thing that separates the different hardware versions. There are some minor differences in the Klipper config file as well, but Reflash will automatically install the right config file based on the UI and hardware revision.

IP and host name on the screen

A small change that is nice for those with multiple Recores in their office is to quickly find out what the IP-address is. The Reflash UI now shows the resulting hostname/mDNS name even if there is a conflict.

Magic mode on file uploads

A small and fun thing is the addition of “magic mode” for file uploads. Magic here refers to the direct transfer of a complete distro (Rebuild or Refactor). The distro then goes directly from the internet and to the eMMC without the intermediate step of first storing it to the USB drive. This has now been extended to also work with file uploads. For us developers creating images, uploading and testing, this is hopefully a nice addition.

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As explained in the video, there are 4 different sensors that can be used with Recore: Thermistor, Thermocouple, PT100 (using an amplifier) and PT1000. Both Revision A5 and A6 can get very accurate measurements, but A6 can use the upstream Klipper implementation for all sensors without any custom calculations. A5 requires the Intelligent-Agent branch of Klipper.

The dry well calibrator (sometimes called a dry block calibrator) is a chunk of highly temperature conducting material, for instance aluminum, that has holes that fit the temperature sensors to be calibrated well. It’s also important to keep the plane in which the sensors reach equal among the sensors, to get the same readings. One possible source of error in this implementation is the depth of the holes. The reference should have a hole of 20 x D, which for the SDL S385 is 6 x 20 = 120 mm, but achieving that with a 2 mm hole (for the thermocouple) was unfeasible, so a depth of 40 mm was chosen instead.

The dry well in the video was constructed from a 55 mm diameter x 300 mm cylinder of 6061 aluminum stock that was purchased on eBay. The heater is a 220 V, 500 W, 10.5 mm heater from Ali Express. The heater is controlled with a solid state relay ED24D3 from Sensata-Crydom (Digi-key Part number CC1825-ND). The PT1000 is a 31500989 from Heraeus Nexensos (Digi-key part number 1759-1044-ND) and the reference temperature sensor is an SDL model S385 from Sensing Devices Ltd. A model for the dry-well was created in FreeCad.

In the video, the reference temperature sensor is calibrated using de-ionized water with with ice cubes made of de-ionized water and boiling de-ionized water. The boiling point of water is determined using a barometric pressure senor from Seeed Studio (Digi-key part number 1597-101020812-ND). The uncertainty of the pressure sensor was given at +/- 8 m. The 1 mA of current is supplied by an HP 3245A and the voltage is measured by an HP 3458A.

The results achieved was within about 1 degree of each other for the 4 different sensors tested. It should be noted that even though temperature measurements seem simple to carry through, it was much work calibrating the reference, measuring the sensors several times, tuning the input values to the sensors and even creating models in SPICE to reverse engineer the effects on the inputs and model them in code for Klipper.

For those with a Recore A5, I will be updating the Intelligent Agent Klipper branch with the new input stage model, so a software update is needed. The new way of doing it is to disable the offset which makes the offset become a pull-down which is easy to model in code (in contrast with a pull-up to a voltage which is lower than the maximum input voltage).

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Functional testing

The testing procedure for Recore is done by the board itself. This is convenient because it makes it easy to access the data available on the board, such as values that must be collected from various sensors. These are:

• Voltage on the input
• Current used by the board
• Board temperature
• 4 thermistor/thermocouple inputs
• Cold junction temperature

All the analog values are gathered by the microcontroller on the board and sent to the SoC via UART. In addition to this, all inputs and outputs must be tested. These are:

• 4 High power outputs
• 4 Fan outputs
• 6 Endstop inputs
• 4 USB
• 6 Stepper motors
• 1 HDMI output
• 1 Ethernet
• 23 Voltages

The stepper motors and endstops are tested together, simply letting the motor run until the endstops are hit. The HDMI is tested by sending a known image to a Manga Screen with a USB webcam pointed at it and using ImageMagick to check for similarity. Ethernet speed is tested using Iperf. Mosfet outputs are tested by checking that the on and off values are correct using a separate microcontroller on the testing jig.

Calibration

It’s great to have good equipment for running the tests, but where the equipment is really needed is during calibration of the gain and offset for the programmable instrumentation amplifier. The thermistor/thermocouple inputs can be programmed to have either a 1 time gain or a 101 times gain. This is done by either pulling a resistor low using a GPIO pin or by setting it to high Z which will eliminate it from the equation. There is also a programmable pull-up that can be used to create a voltage divider for a thermistor.

With this setup, the amplification at various voltage levels can be measured. From this, using linear regression, the gain and offset can be calculated as G = ΔVo/ΔVi. The offset is simply the output value measured at 0 volt input. The offset value can be measured by the end user by shorting the input to ground, disabling the pull-up and setting the gain to 101.

Details on accuracy of thermocouple

In the video, I mention that the tolerance of the resistors used are ±1 %, but a good thermocouple can have a 1 degree accuracy across the temperature range. These are values that do not easily compare, so allow me to clarify. A type S thermocouple is actually not that much in use any more, but regardless, the tolerance is very good. ± 1 ºC from 0 to 1100, which is < ± 0.1 %. Below is the IEC 584 chart used as reference.

Thermal experiment

The video ends with an experiment comparing three thermocouples. Unfortunately, I don’t yet have a proper thermometer that I trust, so the results must be taken with a grain of salt. Also, the test was conducted with a fast temperature rise followed by a gradual cool down. This is not ideal since the temperature in the aluminum block needs time to reach equilibrium. A better test would be to use a dry-well and allow the well to settle at different levels. Still, during the cool-down, the temperatures actually matched pretty well for a large range of values.

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