6 axes robotic arm - rc servos - raspberry pi 4

Hi !

I would like to make a 6 axes robotic arm with rc servos (mg995 and sg90). I would like to use LinuxCNC to be able to move the robot end-effector in the "world mode". (as in moving the end-effector in the x, y, z, directions and a, b, c, angles).

This is will be just for demonstration, since the robot won't really have a purpose, just a toy. I don't really care about precision or tolerances, I know RC servos won't be really precise.

I saw this video on youtube : 
His robot has 5 dof, but this is exactly what I want to reach, except that my robot has 6 dof. Unfortunately I don't understand german and there isn't much info about how he obtained this result.

I have read that LinuxCNC is using a realtime operating system, so I should be able to plug the signal pin of the rc servos directly on the Rpi, right ? Or will I have some jitter problems as shown in this video (at 4:35)  :  ?

How do I assign a pin on the rapberry pi to control the signal pin of a rc servo ?
I found this : wiki.linuxcnc.org/cgi-bin/wiki.pl?RC_Servo_Test but this isn't for a Rpi.

Also I learned about rcpwmgen. But it looks like it's only made for mesa boards.

I am looking for documentation about using rc servos with LinuxCNC on a Raspberry Pi but can't seem to find much. 

I hope I am not asking questions that are too simple, but I am having trouble finding information about what i'd like to do.

Thank you

Hi Andy,
sorry for noc documentation. I tried to write a man page but dont know how! Is a man page wanted? Where do i read about how to write it? Then, what files are submitted and how is 'it' submitted? ( i dont get notifications -- long story -- ask JT -- so i will try to monitor this thread )
tomp

Thx Andy will read now
tomp

update: the attached text file produced the attached manpage .
I used a text editor, the manpage source from streamer, thenthen gzip hal_pi_gpio.9 to get the format i see in /usr/share/man/man9
hth
tomp
 

I have never used the pwgen with servo directly into LinuxCNC.

However I tried to use HAL with two Hitec Hs-422 servos to manage the pan-tilt movement with hal_pi_gpio and the Raspberry pi 4.
The two motors are powered by 5V with an external power unit.
The control pin instead is connected directly on pins 33 and 35 of the Raspberry

Attached you will find a .hal file with commands and a file .xml for the graphical interface in PyVCP.

By copying these files to the Raspberry where the LinuxCNC is already installed, the test starts with the command:

halrun -I -f servopwm.hal

By moving the two sliders you can move the motors.

In this link there is an explanatory video:
drive.google.com/file/d/1-3U6u_qYzBzSIDH...hyN/view?usp=sharing

A little calculations.
The standard RC servo uses pulse length control. 1.5ms is the middle position, about 0.5ms and 2.5ms are the extreme positions of the 180 degree deflection.
So 90 degrees corresponds to 1ms.
The base-thread jitter on RPi4 is around 30us. This corresponds to a deviation of about +-2.7 degrees. So the position of the servo will shake in a range of about 5 degrees.