Remora - Rpi Software Stepping Using External Microcontroller via SPI

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16 Dec 2021 21:17 - 16 Dec 2021 21:24 #229205 by Bari

 Also, we could double the step rate by completing the step in one thread cycle vs the two that are currently used.

I thought that was already the case.

I forgot to mention that many servos offer Step Dir inputs now and have torque curves that reach in to the many thousands of RPM vs ~1K RPM for steppers.
Last edit: 16 Dec 2021 21:24 by Bari.

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16 Dec 2021 21:42 #229207 by drewnabobber
In our quest for higher step rates, it's important to realize that most step/dir servos max out at 500 KHz step rate. DMM servos for example.

The JMC integrated servos which are fantastic for hobby or even prosumer machines max out at 250 KHz, which is the same as most external stepper drivers. MHz step rates are really not required for the vast majory of machines. So there is definitely a point of diminishing returns after 200 KHz or so.

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17 Dec 2021 00:27 #229221 by Cncninja
I think even a 100Khz would suit 95% of uses cases. I personally needed 45KHz using 4mm pith and 16x micro-stepping to get the speeds I needed. So the LPC17xx SKR 1.4 turbo didn't work out for me it was maxed around 30KHz which is close to the step rate a Pi can do with its own I/O. So I'm watching closely at the ARM 32-bit progress here. I know there are different ways to generate the step pulses but I still struggle with why the same hardware fluctuates so much. the LPC17XX for example in Grbl is 150K X5 axis, in marlin it's 200k, in Mach 3 its 100k x4 axis but in Remora it 30k.. or the stock Rpi I/O on Linux CNC is around 35KHz but on pandapi is making step rates in the 10+ MHz using the rpi4 gpio. The pi does not have a PRU like the beaglebone or orangepi so what voodo is going on?

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17 Dec 2021 00:36 - 17 Dec 2021 00:37 #229222 by Bari
Yes 100KHz stepping for 3-5 axis would cover the majority of applications.

Pandapi is a new one to me. Looks like it uses a stm32f103
www.pandapi3d.com/forum/pandapi-fdm-3d-p...ns-about-the-project

is there a schematic somewhere? Found it seconds later   gitee.com/markyue/pandapi_wiki/raw/maste...atic_PandaPiV2.8.pdf
Last edit: 17 Dec 2021 00:37 by Bari.

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17 Dec 2021 00:42 #229223 by Cncninja
The stm32 it just to run the heat bed the Step Dir and Normal I/O is being done on the RPI.

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17 Dec 2021 00:47 #229224 by Cncninja
PandaPi: use RPi to control 3D printer directly,except the temperature control which is just to maintain the temperature.

Klipper: uses a RPi to parse G-code,map out curves,set accelerations,and then send the motor command to the MCU via uart.

the obvious difference is that PandaPi control the motor directly.the gpio on the RPi speed being able to signal at 10+ Mhz as compared to 8/32bit MCU limit of about 10Khz/200khz for steps.that is one of my reason to explore this project. although the <100Khz speed is enough for our FDM printer recently but not the future.

Why is there a mcu?

RPi has not enough GPIO pin for handle all the motors,hotend,bed,endstop,LCD,auto bed level,run out sensor.

how do this assure the real time control?

about the real-time, here is the result that is almost perfect for real time control drivers. the output signal of the one raspberryPI's GPIO,and displayed by the oscilloscope.

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17 Dec 2021 00:50 #229225 by Bari
I also see Step and Dir from the STM32 but who knows?. The docs are terrible and you'd have to dig into their source.

There are just many different ways to write firmware. Getting the most out of a micro requires you to know how the microcontroller works and designing accordingly.

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17 Dec 2021 00:50 #229226 by Cncninja
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17 Dec 2021 00:52 #229227 by Cncninja
Yep Agree, that's why I'm a bit lost why the speed difference on the same hardware.

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17 Dec 2021 00:55 #229228 by Cncninja

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