Inductive sensor - Z probe returning wrong measurements



  • Hello,

    I have been using duet products for the last 1 year and have built 2 large 3d printers that work with duet electronics. I Have used Duet Ethernet, WiFi and expansion headers for delta and core xy printers which produce amazing prints.

    For a new Cartesian printer with duet wifi board the motor drives, extruders everything work fine.

    But i am facing a strange problem with Z probe and after a lot of unsuccessful search on forums, posting it here.

    I have an inductive Z probe which detects the metal object and also works good during the static and dynamic test. I have done G30 S-1, G29 bed probing, G32 S1 calibration which have all worked as expected.

    But later i found out that the Z measurements returned by the probe are wrong because the analog values being produced seem to be of very short variations. In my previous machines i can see the Z probe values in DWC differ from 0 to 500 as the bed approaches near and the average values returned by G30 S-1 are constant for a set of 5 measurements.

    But with the new wifi board, the sensor measurements differ only between 958 and 1000 on DWC. The Z probe measurements also vary with a constant pattern. they either increase by 0.5 or decrease for a set of 5 to 10 measurements at the same place on bed.
    If my first G30 s-1 returns 1 mm, after 10 measurements it returns 2.5 or 3 mm which is clearly wrong.

    The voltage reading shown on DWC is 24V and probe value range from 960 to 1000.
    I have tested the sensor and the electronics with my previous board and it works fine.

    I feel the problem is either with a firmware update that i am not aware of or somewhere in my settings i am missing something.

    ; Endstops
    M574 X2 Y2 Z2 S1 ; Set active low endstops at high ends

    ; Z-Probe
    M558 P1 H10 F80 T10000 ; Set Z probe type and the dive height + speeds
    G31 P930 X0 Y0 Z0 ; Set Z probe trigger value, offset and trigger height
    M557 X50:1100 Y50:1000 S100 ; Define mesh grid

    Any suggestions or possible solutions would be of great help.

    @dc42 and team - you people have been doing great, i have always found answers to my problems on the forums.


  • administrators

    Inductive Z probes normally have digital outputs, so you should use P5 or P8 in the M558 command instead of P1. The P parameter in the G31 command can be set to 500, or when using M558 P5 you can use a lower value (e.g. 100) for a slightly faster response.



  • Thank you for the response.

    I changed the parameter and tried it but doesnt seem to solve the problem. The Z probe value on DWC changes rapidly from 0 to 1000. G30 S-1 command is returning measurements in the range of 0.79 to 1.4 mm for the same place. its almost double.

    I tried connecting the same setup with my old board and the Z probe measurements are varying from 0 to 400.

    So the setup seems to be correct but the measurements returned by the probe are wrong.

    i measured the actual distance, it is 1 mm but the z probe returns 1.5. I think 0.5 mm is a huge difference.

    Can the problem be with any update ?


  • administrators

    Can you link to the details (preferably a data sheet) for your inductive sensor, and tell us how you have wired it to the Duet?



  • Sensor - https://autosen.com/en/Position-Sensors/Inductive-sensors/Inductive-sensor-High-Resistance-Class-IP69K-AI039

    The sensor is powered externally by 24 v and the output and ground are connected to a relay. Output of relay is connected to z probe in port of duet and ground to ground.



  • I have reverted back the firmware to an olden working version. But it dint make any difference.

    In DWC the older board shows voltage value as 24.5 and the new one shows 24V . Is this difference of any significance ?



  • After struggling a lot.

    I decided to change the board with my old one. With same settings and same equipment, now i get perfect values.

    I think the problem is with duet board, the z probe pin might me floating more than required.


  • administrators

    That's a PNP-output inductive sensor, and it needs a voltage divider on the output to connect it safely to a Duet 1.03 or earlier, or at least a pulldown resistor to ground when connected to a version 1.04 Duet. See https://duet3d.dozuki.com/Wiki/Connecting_a_Z_probe#Section_PNP_output_normally_open_inductive_or_capacitive_sensor.



  • Yes. I have followed that page and made the connections similarly. The signal is driven to ground and enable port. There is a resistor that pulls down the signal. I found the problem to be with the duet board because of two reasons.

    1. The E0 driver was not working and always giving me an current error on DWC and had to change the driver with Y driver.

    2. The same machine and software setup works perfectly fine with another duet board, but with this board it produces wrong measurements.

    Can i claim for a return or exchange for this faulty board ?


  • administrators

    If the E0 driver is not working and the board is within warranty then you can claim a warranty replacement. However, I suspect that the problem with the Z probe may be because the pulldown resistor has too high a value or is not connected properly. If it is a 1.04 revision board, then there is an extra protection diode in the circuit, and the small amount of leakage current from that means that the Z probe input will tend to float high if the pulldown resistor is not present or has a very high value. But please mention the Z probe issue in your warranty claim anyway, to make sure that it gets tested too when we repair the board.



  • Maybe it can be with the resistor because i was not aware of this change of electronics. As the new board is a month old and the other board is around 8 months old.

    I have ordered two new boards. So i shall try and figure out with the new board, if it persists.



  • Panel due is not able to read the sd card. I have done all the connections as per the duet documentation and used the two way wire.

    I ordered duet wifi, panel due i7 and expansion header all together. But now i find that only the expansion header seems to work good. Initially duet wifi extruder driver failed and then later the Z probe pin on duet and now panel due seems to be giving trouble.

    When i try to work with external sd card it shows only one thing.

    "Error: 1 accessing SD card"

    Even in DWC i am not able to access the SD card. - "Cannot initialise SD card1 : card is unusable"
    What could be the possible problem ?

    I was a happy customer of duet with good experience from my last two boards. Being a programmer myself i respect the amount of work being put in to make such a good product. But this particular order seems to be ruining.


  • administrators

    As I said before, the faulty E0 driver is sufficient for you to claim a warranty replacement for your Duet, assuming it is a genuine UK-manufactured one purchased from us or from one of our distributors. See https://www.duet3d.com/warranty.



  • Thanks for the reply @dc42 .

    Maybe a bit of miscommunication, but its more about understanding the fault than about the warranty. 🙂

    I dont understand the problem with panel Due right now. Its a I7 brand new one directly from duet3d UK website.

    But the panel due is not able to read the micro sd card. What could be the possible reason ? Am i missing any connection ? i think there is nothing much from user end because its all straight forward, just connecting the two way wire and power pins.


  • administrators

    In order to use the SD card on PanelDue, you need to use the ribbon cable connector on PanelDue to connect it to CONN_SD on the Duet, instead of using the 4-way connector.



  • yeah. I have used the ribbon cable and connected as per the documentation.
    From the panel due 'connect SD' to the duet 'Connect SD'.

    Is the panel due i7 compatible with duet Ethernet 1.0 electronics and firmware version 1.19 ?
    I tried with new board and 2.04 firmware too but it was not successful.


  • administrators

    All Duet Ethernet boards shipped from the factory are PCB revision 1.02 or later. If yours really is a 1.0 then it must have been converted from a Duet WiFi.

    Duet PCB revisions 1.0 and 1.01 do not provide the UTXD and URXD signals on the CONN_SD connector, so they need the 4-way cable as well as the ribbon cable. Without it they will remain in the "Connecting" state.

    How long is the ribbon cable? The length needs to be limited to about 300mm to avoid signal degredation.



  • Both 4 way cable and ribbon cable are connected. I double checked the connections, they are as per the documentation.

    The length is also less than 250 mm because i read that maximum of 400 mm can only be used.

    I think the panel due is not able to read or detect the memory card. I ran the firmware diagnostics also and it reports only one memory card i.e the one in duet.

    And the panel due throws an error "Error: 1 accessing SD card" . I tried with different memory cards but it still shows the same error.


  • administrators

    If you send M21 P1 from either PanelDue or from DWC, what is the response?



  • "Cannot initialise SD card1 : card is unusable"


 

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