[Project] 3+Pi (3PlusPi) A 3+ Axis Raspberry Pi 'hat'

[tkdaichi]

G'day everyone.

I'm a graduate EE from Brisbane with a passion project. I humbly welcome your constructive feedback and opinions. I --- as many others might agree --- am always learning to improve and I am willing to always endeavour to learn even more.

Allow me to introduce to you my project: 3+Pi. https://hackaday.io/project/163498-project-3pi

A 3+ axis Raspberry Pi Hat. The concept of this board was originally to be used for 3D printers/CNC machining/Laser cutters or just about any application that requires the need for 3+ axis control. Acquiring inspiration from the RepRAP model ATMEGA2560 based boards, I thought it would be nice to have a rather all in one package whereby the end user wouldn't need to have a computer within proximity or have a gcoded SD card that will plug and play. I thought it'd be nice to have an all in-one package allowing end users (especially to those who might be new to this domain) to have the ability to see what is being built or what they wish to 'slice' and 3D print. This all can be done in one system similar to a deploy and go but with visual feedback and learning experience.

However, it felt as though I was limiting the project to only accommodate a closed target so I thought that, maybe this can be used for robotics application or perhaps even more. I mean, there are 4 (currently although thinking of adding even more later hence the 3+) stepper drivers on board right now so that can be used to do kinematic robotic applications right?

Anyways, it's currently a work in progress as of now (23rd Jan 2019) I am waiting for the boards to be manufactured and trying to cut down BOM prototype components costs as this is currently being funded entirely on my own savings. (It's a passion project after all)

Key parts that might interest some of you:

- MCU is the STM32F401RE. A cortex M4 (arguably overkill) processor for such purposes. My reasoning behind this was to pragmatically apply FreeRTOS implementation onto it and learn something. I'm also thinking of adding more features to the board which will fully utilize the M4 core properly.
- Trinamic's TMC2100. Originally wanted to use the SPI variants thinking I'd drive the motors through SPI... After some research...yeah that didn't quite work out well. Regardless, these drivers have teh ability to drive the steppers quietly compared to the common variants like the DRV8825 or the A4988s... They are however, pricey 
- MAX17574 Buck controllers (pair... 3v3 and 5V)  3A rated.. Should be sufficient enough to have it powering the Pi and 3v3 logic on the 'hat'. Again, prototype stage so might have to increase my power budget. Speaking of which... Power supply is a barrel jacked 24V 4A supply. 96W should be enough to power everything.

Firmware is using FreeRTOS with the help of STMCubeMX HAL drivers which saves time on dev time. I intend to optimise this further on with time.

Software will be running Raspbian and the GUI will run Python for the time being because of time constraints (I work practically every day so I only get minimal time to work on this)

Do I plan on selling these? Perhaps... I'm not sure who'd like to use them buy this but that would be nice. I do however, take great pride of my creations so I might not even sell it if I am not happy with the end result because of how buggy or malfunction-al it might end up being... Don't want to end up on Mail bag having Dave tearing me apart now do I  haha

It's obvious that the parts I've chosen are rather expensive in comparison an 8 bit 2560 running 4x A4988s at a fraction of the price which can be cloned rapidly from China to tank the price even more. I acknowledge that indeed.. This is really just a passion project that I just wanted to do for myself and hope to share it with people if they are interested in it. But with that being said, I am not going to be 100% sure if I will even want to sell it soooo..... there is that.

Anyways, thanks for reading up to this point. I welcome your feedback, please have a look on the Hackaday page I just linked above if you want to see progress of how it's going... I might take decades working on it so do expect that.

Cheers   

[awallin]

how good or bad does the Pi run low-latency linux kernels (rt-preempt, xenomai, etc)?
the whole software stack from G-code to stepper pulses is done by linuxcnc, and it's not worth repeating that effort: http://linuxcnc.org/

The M4 runnig RTOS could still be useful, for generating high-speed pulses, or perhaps encoder-counting and PWM-drive if you plan to be able to drive servos also.

[tkdaichi]

I haven't benchmarked the Kernel latency of the pi thoroughly so I am not well informed about that right now. I can have a look around and get some information. However, Pi will communicate to the F401RE through UART at 11200baud or more to provide high speed comms and also human readable debugging. Plus there will be twin GPIO pins both ways acting as secondary ACKs and BUSY flags. If Latency becomes a serious issue, I might even create a NV buffer using a uSD card if needed... But that's going back to square one... ha-ha.

Thanks for pointing that out, I will have a look into it more

I've heard of Linuxcnc, but haven't toyed around with it a lot. Thanks for pointing it, I've downloaded the ISO and might run it on a VM and see if I can get something out of it.