M42 using variable value is slow/not working well

usinjin

Hello all. I have a custom built pen plotter (operates very similarly to a laser cutter/engraver in practice). The mechanism to raise/lower the pen is a voice coil driven by an external controller that takes a single-wire PWM signal (51% - 100% duty cycle moves the actuator up, and 49%-0% moves the pen down). This gives me a precise way to control the pressure of the pen on paper.

At any rate, the VCA controller is connected to OUT6 of my Duet Mini 5+. I'm using M42 commands to drive it. So after building my gcode files using Lightburn, I simply replace M3 commands with something like

M42 P0 S0.35

for example.

This works perfectly for static values of S. But if I try and use a variable for S, like the following:

var d = global.pressure32
(...)
M42 P0 S{var.d}

The plotter no longer reliably moves the pen. Adding a 100msec dwell after moving the pen works better, but results in some slow/jerky motion and doesn't seem like a real fix.

I don't know if the issue lies with the motion planner somehow or if reading the variable repeatedly is just slow.

My config.g:

; Configuration file for Duet 3 Mini 5+ (firmware version 3.3)
; executed by the firmware on start-up
;
; generated by RepRapFirmware Configuration Tool v3.3.16 on Sat Jul 22 2023 22:18:39 GMT-0500 (Central Daylight Time)

; General preferences
M453                                    ; CNC mode
G90                                     ; send absolute coordinates
M550 P"nPLT"        	                ; set printer name

; Network
M551 P"duet"                            ; set password
M552 P192.168.1.14 S1                   ; enable network and set IP address
M553 P255.255.255.0                     ; set netmask
M554 P192.168.1.1                       ; set gateway
M586 P0 S1                              ; enable HTTP
M586 P1 S0                              ; disable FTP
M586 P2 S0                              ; disable Telnet

; Drives
M569 P0.0 S0                            ; physical drive 0.0 goes forwards
M569 P0.1 S1                            ; physical drive 0.1 goes backwards
M569 P0.2 S1                            ; physical drive 0.2 goes forwards
;M569 P0.3 S1                           ; physical drive 0.3 goes forwards
M584 Y0.0:0.1 X0.2 P2                   ; set drive mapping
M350 X16 Y16 I1      	                ; configure microstepping with interpolation
M92 X100 Y59.26 				   	    ; set steps per mm, shorter pulleys on X
;M92 X80 Y59.26 				   	    ; set steps per mm, longer pulleys on X
M566 X500.00 Y500.00                    ; set maximum instantaneous speed changes (mm/min)
M203 X18000.00 Y18000.00                ; set maximum speeds (mm/min)
M201 X500.00 Y500.00                    ; set accelerations (mm/s^2)
M906 X1800 Y1800 I30            		; set motor currents (mA) and motor idle factor in per cent
M84 S1                                  ; Set idle timeout

; Axis Limits
M208 X0 Y0 S1                           ; set axis minima
M208 X290 Y290 S0                       ; set axis maxima

; Endstops
M574 X1 S1 P"^io1.in"                  ; configure switch-type (e.g. microswitch) endstop for low end on X via pin !^io0.in
M574 Y1 S1 P"^io0.in"                  ; configure switch-type (e.g. microswitch) endstop for low end on Y via pin !^io1.in

; Pen
M950 P1 C"io4.out"      ; allocate GPIO port 1 for the enable/disable of the voice coil amp
M950 P0 C"out6" Q30000	; allocate GPIO port 0 to out6 on expansion connector, 30kHz

M42 P1 S0               ; disable motion of pen 
M42 P0 S0.6				; pen up command
M42 P1 S1               ; enable motion of pen
G4 P500

M42 P0 S0.5				; set to 50% (which is 0% output)
M42 P1 S0               ; diable motion of pen

; Sensors
M308 S0 P"io3.in" Y"linear-analog" A"Pen pressure" F0 B-8.5 C1160

; Variables
global pressure_on_pause=0.5
global pen_up_on_pause=0
global pen_is_up=1
global pen_is_on=0
global pressure32=0.38
global pressure85=0.36
global pressure105=0.32

And a snippet of a gcode file:

; Layer C02
M42 P0 S{global.pressure85}
G1 Y250.831
M42 P0 S0.6
G0 X65.497Y250.831
M42 P0 S{global.pressure85}
G1 Y56.831
M42 P0 S0.6
G0 X173.447Y56.831
M42 P0 S{global.pressure85}
G1 Y250.831

The immediate fix I can think of simply hardcodes in all values into M42 commands before running the plot, but I wanted to see if there was a better/easier way of doing things. Thanks for any assistance.

T3P3Tony

@usinjin this is one for @dc42 to confirm but adding the delay (or M400) will almost certainly not be what you want as it means the following movement commands wont be handled until the delay/M400 completes. even without the delay using M42 in between the movement commands like that is probably sub optimal. The way this is handled for Laser cutters is with a S parameter of the G1 command. If that can be made to work with your VCA controller then that may be the better solution:

https://docs.duet3d.com/en/User_manual/Reference/Gcodes#m452-select-laser-device-mode

you can set the same pin you are using now for M42, but instead as a laser pin. I expect you will want S1 mode of M452 so there is a continuous output and then use commands of the form:
G1 Xxxx Yxxx Sx.xx

What I don't know is if commands of this sort:
G1 Xxxx Yxxx S{<variable>}

Will work better but its worth a shot.

dc42

@usinjin this is a known issue, see https://github.com/Duet3D/RepRapFirmware/issues/767. Meanwhile I suggest you either use a M400 command prior to each M42 command, or use laser mode and the G1 S parameter as @tony suggested.

T3P3 created this issue in Duet3D/RepRapFirmware

open M280 M42 etc. commands are executed too soon if they use any expressions e.g. involving variables #767

usinjin

Thanks to both for the help. I did forget to mention that I had tried M400 already, and seeing as it's the same as G4 P0 that's when I tried to increase the dwell time, neither of those work very well.

I avoided laser mode before because as I'm controlling an actuator and not a laser, the "pen down" time is dependent on the force being applied. I want to be able to add padding with G4 in the event that I use a very low pressure so that I can make sure the pen is touching the page before moving occurs. Using M42 allowed me to separate "pen moves" and "XY" moves. Is there still a way to do this with M452?

usinjin

Actually, could I do this with M670?

o_lampe

@usinjin Never tried it, but it seems to hit the nail for your use case. From what I gathered, it will apply the advanced timing on both moves (up+down).
Is that a problem?

usinjin

@o_lampe I think it would work okay, there could be some wasted time if I set the timing to the worst-case for all pen movements. Not 100% sure.

I'm still not convinced that my hardware will support laser mode. I suspect the G1 commands will be fine, but the problem arises with G0 traversals when the pen needs to be "up"/"off". Won't G0 set the laser duty cycle to 0%? If so, that is actually pen "down" at full force on my hardware. Maybe I can turn the driver off first (also with M42, but hard coded value of 0 for off and 1 for on), so it won't matter what the duty cycle is?

I also figured out how I could turn this in my gcode files:

...
M42 P0 S{var.pressure32}
...

Into this:

...
M42 P0 S0.355
...

Based off another file that has had pressure variables written to it:

M28 pressure_values.g
set global.pressure32=0.355 ;calculated from a load cell input value
...

I did this with a Python program running on the host that transfers the files via FTP. It's a pretty ugly solution to basically just do a text substitution, but it does work...if anyone is interested in how I did that I can elaborate. I will also keep trying the laser mode option (which would be far nicer I think).

dc42

@usinjin said in M42 using variable value is slow/not working well:

Won't G0 set the laser duty cycle to 0%? If so, that is actually pen "down" at full force on my hardware

You could reverse the sense so that 0 means pen up and 1 means down, by inverting the control pin name in the M452 command.