Homing repeatability

[Aiph5u (Aiph5u (not affiliated with Onefinity))]

He timplett,

I like it when one starts to think and to question things

@timplett

[…]
my industrial experience leads me to believe that this style of photoelectric sensor is actually inherently more accurate than an inductive proximity one.
[…]
By contrast, inductive proximity sensors have a detection field whose area is much larger than the width of the light beam (though generally a small area as far as sensors go), meaning the potential margin of error is much larger. It’s accuracy comes down to how consistently it can both create and measure that field, as the detection is basically a gradient. Moving metal into it’s detection field will slowly increase the detection strength until a threshold is passed, unlike the break beam style that is used which is just a straight on off signal. This means that the accuracy of an inductive proximity sensor can be affected by the consistency of system voltage and consistency of the mass and position of the metal being measured.

But in practice, an inductive sensor is enclosed with a circuit:

Elements of a simple inductive proximity sensor.

1. Field sensor
2. Oscillator
3. Demodulator
4. Schmitt Trigger
5. Output

– Source: Inductive sensor – Wikipedia

At least in the datasheet of the inductive proximity sensor I use (Omron E2B / Omron E2B), I do not see any diagrams with which the system voltage range could influence the threshold. Also even if you were the sensor manufacturer, once you have designed such a circuit, it is not to expect that it will change during the operation of a CNC machine.

And regarding mass and position of the metal part that you use to trigger the sensor, and also material of the metal(!), you have such a nice diagram:

Source: Omron E2B Inductive Cylindrical Proximity Sensor Datasheet

So since you probably don’t often change the metal part that triggers the sensor, or its mounting position on your machine, you should get an excellent repeatability. As was confirmed here:

@WaywardWoodworker

Over the past 2 weeks I’ve added proximity sensors for homing on the X and Y axis (still working on Z but need some more downtime) and I have been pleasantly surprised on the repeatability and accuracy they have provided to the homing cycle and using that as the basis for work offsets. I have both a metric digital dial indicator and a imperial mechanical dial indicator and through 50+ homing sequence tests with controller reboots between some the error is immeasurable on my indicators .005mm and .0008" resolution respectively although you can estimate the dial indicator to .0002". I was hoping to hold .003" on homing but it has proven to be far more repeatable than expected.

@WaywardWoodworker

I used two different methods using the stock Onefinity controller

First I performed testing to see the general repeatability of the sensor treating it as a probe wired to the controller where I created a g code program to run the axis toward the sensor at a feed rate of 50mm/min, when the sensor was activated it would stop and I had it record the offset from machine 0, then it would back off 10mm and run again with the following results (this is the Onefinity measuring itself in mm):

I then reconfigured the switch as a limit and changed the homing configuration to use the proximity sensor instead of stall homing. Homed it one time, move it 100mm away from the sensor and used that to zero the dial indicator. Ran the homing cycle again, back off 100mm and measure delta, repeated about 20 times with no variation in the measurements. Moved on to rebooting the controller then homing the machine followed by backing the axis off 100mm and again no delta in the measurements. Started thinking the digital indicator was playing tricks so I went to the mechanical dial indicator, same results. I moved the dial indicator base in a couple mm and re-zeroed the indicator just to make sure I wasn’t hitting a odd spot in the equipment but the results remained the same throughout.

Examples of pieces of metal to trigger the sensor.

The sensor triggering point can be fine-tuned with its circular body with external thread, available in M8, M12, M18 sizes.

@TMToronto

@WaywardWoodworker

The basics of the mount looked like this, note the metal screw in the plate for the proximity sensor to detect.

The Masso photoelectric sensor may have an integrated trigger circuit too.

I don’t know if the Masso/Elite photoelectric sensors are susceptible to wood dust in day-to-day practice, I just know that many people neglect dust extraction, and an inductive sensor is completely immune to dust.