Grounding PT100 to eliminate heater faults

sonderzug · 28 Mar 2019, 06:49

Hi,
I've had a great deal of problems with spiking temperature readings when I first installed PT100 sensors in my machine. they would only occur when a print had started and/or the heater was at above a certain temperature level. In my thread you can look at the temperature graphs. I solved this by using a temperature sensor with shielded leads (the shield provides mechanical shielding as well) and grounding the shield at one side.

dc42 · 28 Mar 2019, 08:56

@vacalos said in Grounding PT100 to eliminate heater faults:

I have also noticed that I sometimes have an issue when running bed calibrations in which the Z Probe is triggered before it probes the bed.

The most common causes of false triggering of the Smart Effector are:

Magnetic fields generated by fans;
Some bed heaters generate large magnetic fields, and the switch on/switch off of the bed heater may induce a voltage in the strain gauge amplifier. A workaround for that is to use the B1 parameter in the M558 command.

vacalos · 5 Apr 2019, 23:26

Thank You,

I tried using the B1 parameter and it didn't have any effect.

I plan on rewiring my PT100/Z-Probe and Heater/Fan cables using (4) 4 conductor twisted pairs to see if this helps.
But I don't know if any of the cables should be shielded and grounded. And if they should be grounded I don't know how.

Can anyone advise on the grounding issue?

vacalos · 28 Mar 2019, 16:29

Also,

I am having a difficult time finding (4) conductor twisted pairs that are not shielded. Would it cause any problems to use shielded cables without grounding them?

genghisnico13 · 28 Mar 2019, 21:41

@vacalos regular UTP, ethernet cable, it would probably be best to cut one of the short ones made to connect to the devices because as far as I know they are made with stranded wire, so it's more flexible. I don't know how durable they are in a constantly moving printer.

SupraGuy · 28 Mar 2019, 22:12

Cat5, Cat5e, and Cat6 ethernet cable are typically solid core wire, and are not intended for constantly moving applications. That said, they would probably be fine for a number of years without failure, but it's likely that some place in the cable it would eventually fail.

A Former User · 29 Mar 2019, 03:01

@supraguy said in Grounding PT100 to eliminate heater faults:

Cat5, Cat5e, and Cat6 ethernet cable are typically solid core wire, and are not intended for constantly moving applications. That said, they would probably be fine for a number of years without failure, but it's likely that some place in the cable it would eventually fail.

Both Cat5 and Cat6 are made as both solid core and stranded. Cable used for installation is typically solid core, and cable used for patching is stranded.

dc42 · 4 Jun 2019, 09:11

@vacalos said in Grounding PT100 to eliminate heater faults:

Also,

I am having a difficult time finding (4) conductor twisted pairs that are not shielded. Would it cause any problems to use shielded cables without grounding them?

You could solder a wire to the shield and ground it to the ground pin of a spare endstop connector.

We'll try to include a ground pin on the next version of the PT100 daughter board.

A Former User · 4 Jun 2019, 12:19

@vacalos

Hi, as dc42 pointed already out, I can - at least for me of course - confirm that using GND(dc) instaed of PE(from AC) was in my case better. To avoid a catastrophal event if that ground should ever touch something it should not, I put a resistor in the cable that runs from GND to the PT100-shield

If you are looking for TP-cables besides ethernet-cables for moving applications have a look at (sorry as somebody living in germany these are the things google threw up when googling in german language...):

unitronic FD CP (TP) plus, EDIT: this has 1 overall shield
metrofunk kabelunion Li2Y(C)H-C11Y 2x2 AWG24, EDIT: this has the benefit of shields around the twisted-pairs + an overall shield
"single pair ethernet" for moving applications with two of them in one cable -> 2x 2x ... TP

If you do not need those kind of cables, alarm-wires are often twisted-pair and seem to be cheaper then the often twisted microphone-cables I read in this forum at some other place... some industry-cable companies also sell ethernet cables for moving applications if searching the web and or electronic-reseller-websites you will find lots of those...

Other things that might help:

Did you check out Fnn on the M305 command: https://duet3d.dozuki.com/Wiki/Gcode#Section_M305_Set_temperature_sensor_parameters
Did you disable all steppers that are not populated on the board via "R-1" within M569? https://duet3d.dozuki.com/Wiki/Gcode#Section_M569_Set_motor_driver_direction_enable_polarity_and_step_pulse_timing

Question to the whole forum: Could anybody give me a hint how to come up with a macro that optimizes the fan(s)- & heater-frequency to something that has the least possibility to interfere with the PT100-frequency?

dc42 · 6 Apr 2019, 09:11

@lb said in Grounding PT100 to eliminate heater faults:

Question to the whole forum: Could anybody give me a hint how to come up with a macro that optimizes the fan(s)- & heater-frequency to something that has the least possibility to interfere with the PT100-frequency?

See my reply in the other thread.