Closed loop servo control with custom driver board

Hi,

sorry for the long text in advance, but i want to explain in detail what my goal is...

I want to build a CNC mill/router/whatever with homemade dc servo motors and homemade encoders. I am currently using ordinary dc motors, controlled by an Arduino, which controls them with a PID controller according to the encoder input. That's all working well, but my arduino is a replacement for GRBL, which means i am doing all G-Code parsing and trajectory planning myself, which i clearly shouldn't be doing.

It's time to take it to the next level. I started writing a program, which acts like a PoKeys Ethernet device and talks to Mach3, but that is also not a permanent solution. In the best case i would want LinuxCNC to do all the work, realtime motion control and servo tuning and my homemade board would only forward signals to the dc motor drivers and count the encoder pulses.

Specifically, i want to make an STM32 based board, which LinuxCNC talks to in the following way:

"Hey, set motor X to 50% power and tell me where axis Y is right now. BTW, is the emergency button pressed?"...

If that's not possible because ethernet is too slow or something similar, i'm fine with PID controlling the servos myself, but i still want LinuxCNC to tell me the current positions and ask for the actual positions.

I do NOT want LinuxCNC or Mach3 or any other software to generate Step/Dir pulses, as the STM32 would need to count those, which is ridiculously unnecessary.

Is there a way i can let LinuxCNC talk like this to my homemade board, maybe over ethernet or Beckhoff EtherCAT or the parallel port?
Do i need to write a custom HAL driver for that or is there something available?
Is there something like an ethernet mesa card driver i can imitate, so LinuxCNC thinks it's talking to a mesa card, but is really talking to my own board?
And last, but not least, can all of this be done without buying extra hardware, so it only uses ethernet or the parallel port?

Sorry for the long text, any help or ideas appreciated...

Regards,
Florian

The Pico Systems universal PWM controller works pretty much like this. It reads encoder position and generates a PWM pulse train as commanded by LinuxCNC.
So, the trajectory planner sends out a bunch of positions, and the PID component adjusts the PWM output to get the most accurate following of that commanded position.
Pico Systems makes some servo amps (brush type and brushless) that take in the PWM and output power to the motor.

Pico Systems also makes the PPMC which does this with an analog velocity command to an analog verlocity servo amp.

Jon

I have already looked at the PPMC board and the others, but couldn't find anything helpful. Is there any information on how to interface with them or is there a driver for LinuxCNC that needs to be installed?

Also, is it connected to the parallel port or ethernet port? I only found PCi versions, which i can't manufacture easily...

Florian

The driver for all 3 of our products is included in the LinuxCNC distribution. In the source, it is under hal/drivers/by_interface/pico/hal_ppmc.c
The driver is "installed" with the hal command loadrt, which loads a pre-compiled kernel-level real time driver and makes it available to hal via the addf command.
The driver scans the parallel port for devices and exports pins and functions to hal based on what boards it can enumerate.

It is interfaced via a parallel port. Using the EPP protocol, there are registers in the device that are accessed by selecting the address and then reading or writing a contiguous block of registers.

Our boards are not PCI, but could be interfaced through a PCI or PCIe parallel port card, or an on-motherboard parallel port.

I think Tommy is right. Linuxcnc is the motion controller so it knows if the estop is set
and it also knows the axis position at any time.

You summed up your scope nicely.

my homemade board would only forward signals to the dc motor drivers and count the encoder pulses.

Take the time to understand how the mesa cards work with the hm2_eth component

Thank you for your answers!

The thing with the emergency stop was just an example for an input, i was thinking of a physical button like a limit switch. Forget about that.

I have now installed LinuxCNC and i am trying around with different mesa card configurations and decided i want to emulate the Mesa 7i80HD ethernet card. I would use a laptop with software to emulate it and when it's finished, i would port the code to the microcontroller, just like i did with the PoKeys device.

I can set the ip address of the card and when LinuxCNC is started, it stops right away and says it can't find the card.

Does anyone know some kind of specification for the ethernet interface, which exact packets need to be sent? Is it even open source? I can't really find anything helpful...

Did you look at the 7I80 manual?
It describes the interface protocol

I am reading through it and it does give some good hints, but sadly it doesn't give me the information i need...

Like it for example specifies all eeprom addresses and what specific bits it the message mean and the UDP port, but it mostly doesn't specify what should come back from the board: If it's the same socket, what packet length, ...

It returns data to the requesting socket
And the returned packet size depends on how much
read data has been requested in the request packet

(this can also be determined by looking at the hm2_eth driver source)