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I installed a capacative proximity sensor on mt 3D printer to auto level the bed. I've noticed some inconsistencies in how it reads and was wondering if it is/can be affected by heat?

The print bed is heated, and I noticed that it will sometimes fail to read the bed before the safety rocker limit switch activates. I've also noticed that if I leave the heat bed on, with the sensor close to it for an extended period (thus, the tip of the sensor heats up sitting a few MM from the bed).

Is it the heat that is causing these flucuations in readings, or is it more likely the sensor itself not being high quality and just flucuating in it's readings?

I don't want to bother working out a heat-related solution if it's likely not the heat.

The sensor I'm using is this one: http://smile.amazon.com/uxcell-1-10mm-Capacitance-Proximity-Sensor/dp/B00542U3M4?ie=UTF8&keywords=capacitive%20proximity&qid=1462033236&ref_=sr_1_1&sr=8-1

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There may be other concerns. For instance humidity.

I would recommend not using a capacitive touch switch if measuring distance is a concern.

Capacitor touch switches are charged and discharged continuously. Parasitic capacitance changes this and it is the changes that trigger the switch. Anything that changes the parasitic capacitance will change when and if the switch will trigger.

Consider the micro soda dispenser. It has a capacitive touch screen. But when it comes to dispensing ice or liquids the machine relies on electrical contact switches.

Assuming you want to integrate a switch into your print head assembly but want it out of the way (i.e. non contact). I would suggest an optical parallax type detector. Sharp makes several for various ranges. Select one for close range and higher resolution. Note, even though the Sharp detectors have an analog output, the output is "stair stepped". This granularity may get in the way of your requirements. Take it into consideration when designing with this part.

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  • \$\begingroup\$ Ah, after looking at the switch I realized you were likely using it as a substitute for a end-stop switch. After reading the question again I am guessing you want to sense the head to heat plate distance. I doubt these types of switches will get you the accuracy you need (literally sub mm measurements). \$\endgroup\$
    – st2000
    Commented Apr 30, 2016 at 17:23
  • \$\begingroup\$ Except they seem to be the industry standard (for auto-leveling systems, instead of a standard endstop rocker switch for base models) and even the cheap $9 one I got on amazon gives me .1mm accuracy. The only time it's "off" is if I calibrate it against a cold plate, then I heat the plate up, and it seemed inconsistent once when I left it 2mm from a 95C plate for about 10 mins, then tried to level - hence my question about if it's the heat, or possibly something else. \$\endgroup\$ Commented Apr 30, 2016 at 23:30
  • \$\begingroup\$ I'm afraid all I can do is make guesses. My guess is heat drives away humidity which I think will change the parasitic capacitance more than the heat. My other guess is that the current in the heater influences the proximity sensor. Lastly, there is likely a processor in the sensor which is trying to adapt to changing conditions. I'm guessing you may get better results if you program in an idle state between measurement where the sensor is raised for, perhaps, several seconds to a known off position above the heater plate between critical measurements. \$\endgroup\$
    – st2000
    Commented May 1, 2016 at 1:45

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