add servo motor support using cprezzi and DWiskow approach

[lavolpecheprogramma]

I merge the changes done by @DWiskow in this repository with grbl 1.1h version.

DWiskow explains his work in this post and I replicate it in 1.1h release.

Now you can enable the servo on z axis directly in config.h like the @cprezzi approach.

Thanks for your work.

[BenjaminPoilve]

Hello,
I don't know if this is the right place for this discussion, so feel free to tell me If that's not the case.
This PR worked quite well for me, and I think servos are a common enough usage to be integrated. Though the current method (relying on the pre-scaler to have a PWM "close enough" to 50hz and use the PWM rate to set the signal length) does have a significant shortfall: the precision is limited to the available steps. For me that's 31 steps over 270°, so almost a 10° step. This makes it impossible to program smooth tool movement using G4 commands.

Do you have any idea how to solve such a problem? My knowledge of C is quite limited, but one thing is for sure, it could not rely on the standard PWM! I think solving this issue would make this inclusion to the grbl controller much more versatile.

Thanks, and thanks to the grbl maintainers !

[cprezzi]

The hardware PWM approach was the only option to fit in the extremly limited space of the atmega328 chip. Grbl already uses almost all available space.

[BenjaminPoilve]

Thanks for the answer! That's what I feared. For those who face the same issue as me, the alternative is to use a separate board to convert the PWM to a servo signal. You can then use the standard spindle code to precisely control the servo.

You can make such a board with a simple 555, or find some sellers that make them (for exemple, see here)

[cprezzi]

By the way. The servo option was not made to give you a fine controll of the servo position, it's made for pen plotters where you just need pen up or down. The exact positioning is made in hardware.

[Mazenalimam]

Hi I used this version 1.1h for plotter but I noticed that when homing the servo/spindle is disabled giving me unwanted pehavier.
I want the servo/spindle to stay up while homing can you help me what to change in the code to do that? Thx

edit:
i solved this by using the 3 files here and change the code in limits.c after line 163
#ifdef SPINDLE_IS_SERVO
pl_data->spindle_speed = gc_state.spindle_speed;
pl_data->condition = (PL_COND_FLAG_SYSTEM_MOTION|PL_COND_FLAG_NO_FEED_OVERRIDE|PL_COND_SPINDLE_MASK);
#else
memset(pl_data,0,sizeof(plan_line_data_t));
pl_data->condition = (PL_COND_FLAG_SYSTEM_MOTION|PL_COND_FLAG_NO_FEED_OVERRIDE);
#endif

[Kenabi31]

Inverting the Servo makes it move up and down whenever i want to home or move the axis.
#define SERVO_SHORT 15
#define SERVO_LONG 31

And the dual homing function does not work sadly.