FSR Problems

JRCL

Hello,

I'm trying to use FSRs as a z probe and have them mounted on my bed. However, I get the error "G28 Z Error: Probe already triggered at start of probing move" immediately upon trying to home my Z. I'm using a 10k potentiometer, but tuning the resistance on it seems to have no effect. Here is how I have it wired and my config. g.

; Configuration file for Duet WiFi (firmware version 3.3)
; executed by the firmware on start-up
;
; generated by RepRapFirmware Configuration Tool v3.3.15 on Sun Mar 12 2023 14:20:36 GMT-0400 (Eastern Daylight Time)

; General preferences
G90                                     ; send absolute coordinates...
M83                                    	; ...but relative extruder moves
M550 P"FSR Test Bench"                  ; set printer name

; Network
M552 S1                                 ; enable network
M586 P0 S1                              ; enable HTTP
M586 P1 S0                              ; disable FTP
M586 P2 S0                              ; disable Telnet

; Drives
M569 P0 S1				; physical drive 0 goes forward
M569 P1 S1				; physical drive 1 goes forward
M569 P2 S1        			; physical drive 2 goes forward
M584 Y0 X1 Z2                         	; set drive mapping

M350 X16 Y16 Z16 I1                     ; configure microstepping with interpolation
M92 X80.0 Y80.0 Z400.00           	; set steps per mm
M566 X900 Y900 Z60.00           	; set maximum instantaneous speed changes (mm/min)
M203 X6000 Y6000 Z180.00      		; set maximum speeds (mm/min)
M201 X500 Y500 Z20.00       		; set accelerations (mm/s^2)
M906 X800 Y800 Z800 I30                 ; set motor currents (mA) and motor idle factor in per cent
M84 S30                         	; set idle timeout

; Axis Limits
M208 X0 Y0 Z0 S1                        	; set axis minima
M208 X100 Y100 Z100 S0                      	; set axis maxima

; EndStop
M574 X1 S1 P"xstop"
M574 Y1 S1 P"ystop"
M574 Z1 S2				;define Z probe as low end

; Z-Probe
M558 P1 C"!^zprobe.in" H5 F100		;mode 1 on z-probe in
G31 P500 X0 Y0 Z0.7			;set trigger value, xy offsets, and Z trigger height 
;M557 X0:20 Y0:20 P2

and homez.g in case

; homez.g
G91               ; relative positioning
G1 H1 Z-205 F1800 ; move Z down until the endstop is triggered
G30

Phaedrux

@JRCL said in FSR Problems:

M558 P1 C"!^zprobe.in" H5 F100

Your probe input is inverted by the !. Are you sure that is required for your probe setup?

Also, your homez is using a homing move for an endstop switch instead of a probing move (G30).

JRCL

@Phaedrux That helped me clean up a few things but now it's just not triggering at all. Here are my new files

; Configuration file for Duet WiFi (firmware version 3.3)
; executed by the firmware on start-up
;
; generated by RepRapFirmware Configuration Tool v3.3.15 on Sun Mar 12 2023 14:20:36 GMT-0400 (Eastern Daylight Time)

; General preferences
G90                                     ; send absolute coordinates...
M83                                    	; ...but relative extruder moves
M550 P"FSR Test Bench"                  ; set printer name

; Network
M552 S1                                 ; enable network
M586 P0 S1                              ; enable HTTP
M586 P1 S0                              ; disable FTP
M586 P2 S0                              ; disable Telnet

; Drives
M569 P0 S1				; physical drive 0 goes forward
M569 P1 S1				; physical drive 1 goes forward
M569 P2 S1        			; physical drive 2 goes forward
M584 Y0 X1 Z2                         	; set drive mapping

M350 X16 Y16 Z16 I1                     ; configure microstepping with interpolation
M92 X80.0 Y80.0 Z400.00           	; set steps per mm
M566 X900 Y900 Z60.00           	; set maximum instantaneous speed changes (mm/min)
M203 X6000 Y6000 Z180.00      		; set maximum speeds (mm/min)
M201 X500 Y500 Z20.00       		; set accelerations (mm/s^2)
M906 X800 Y800 Z800 I30                 ; set motor currents (mA) and motor idle factor in per cent
M84 S30                         	; set idle timeout

; Axis Limits
M208 X0 Y0 Z0 S1                        	; set axis minima
M208 X100 Y100 Z100 S0                      	; set axis maxima

; EndStop
M574 X1 S1 P"xstop"
M574 Y1 S1 P"ystop"
M574 Z1 S2				;define Z probe as low end

; Z-Probe
M558 P1 C"^zprobe.in" H5 F100		;mode 1 on z-probe in
G31 P500 X0 Y0 Z0.7			;set trigger value, xy offsets, and Z trigger height 
;M557 X0:20 Y0:20 P2

; homez.g
G90                ; back to absolute mode
G1 X0 Y0 F2000 ; put head over the centre of the bed, or wherever you want to probe
G30                ; lower head, stop when probe triggered and set Z to trigger height

Phaedrux

Can you give some details on the FSR kit you're using?

M558 P1 is unlikely to be the correct probe type. That's for analog probes. P8 would be more likely.

https://docs.duet3d.com/en/User_manual/Reference/Gcodes#notes-88

JRCL

@Phaedrux Yes I'm using these DF9-40 FSR's all wired in parallel with no board. Switching to mode 8 didn't have any effect

dc42

@JRCL your FSR wiring looks correct and P1 is the correct probe type to use for that wiring. However, you may need a potentiometer with a value greater than 10K to make that arrangement work. I can't say what value you need because the page you linked to gives no indication of how low the resistance drops; but it may be as high as several megohms, because the resistance when no force is applied is quoted as being greater than 10Mohms.

You can use the probe reading displayed by DWC to get an idea of the correct series resistor. Look for a low but nonzero reading (e.g. between 10 and 100) before the nozzle contacts the bed, rising to more than 500 when it does push down on the bed.

With the wiring scheme you have, don't use the ! character in the pin name string of the M558 command.

JRCL

@dc42 The first half of that makes sense. I'll play around with some other potentiometers, revert back to P1, and take the ! character out.

However, I'm uncertain about the series resistor you mention. Are you recommending adding another resistor in series with my pot in order to find the resistance that gets it triggering? or how do you mean?

dc42

@JRCL the series resistor I referred to is your potentiometer.