Hey Don, hey all,
I think that’s what you can adjust with Pulloff distance, on the Masso G3. On the buildbotics-derived Original Series Onefinity Controller, after retrofitting inductive proximity sensors, the adjustment of can be achieved this way (search velocity, latch velocity, latch-backoff, zero-backoff).
While my Google skills are letting me down as far as finding supportive evidence either way, my industrial experience leads me to believe that this style of photoelectric sensor is actually inherently more accurate than an inductive proximity one. The caveat being, as mentioned by someone else already, the quality of each individual sensor.
The style of sensor being used here has a separate emitter and receiver for the beam. Because of this layout, the finer the beam can be, the more accurate the sensor will be, as the width of the beam is essentially the only possible margin of error. I’m not sure the exact logic Masso/Onefinity uses here, but typically the position would be recorded the instant that the beam is broken, and then double-checked again when the beam is unbroken when the axis moves back from the switch. In theory at least, this means it can be quite accurate even with a faster homing speed as it can retract slowly to get the most accurate position.
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.
In a nutshell, though I’m sure that there are some very expensive inductive proximity sensors that would be more precise than the broken beam style on the Elite series, at a same dollar value I would expect the broken beam style to be much more repeatable than an inductive proximity sensor.
He timplett,
I like it when one starts to think and to question things 
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:
The sensor triggering point can be fine-tuned with its circular body with external thread, available in M8, M12, M18 sizes.
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.
The exact functioning of electronic components definitely goes at least a little over my head, but that is basically what I was getting at, is that the electronic circuitry becomes the main component of determining the accuracy of the sensor for an inductive proximity sensor, because the “physical” detection mode is a bit more vague. It is a comparatively large area, and the detection value ramps up over a comparatively large range. Even the diagrams you attached show this ramp up in detection strength as the object size increases, and this is essentially what happens as the object approaches the sensor as well, the size of the portion of the metal object in the detection range increases. So getting an accurate and repeatable measurement is down to the electronics maintaining a very consistent measurement field, and triggering at a very specific signal strength.
And that is really all I meant in saying that a sensor based on breaking a light beam at least CAN have an inherent advantage, because you can reduce the detection area to the “physical” width of the beam of light being used. From your diagrams above all the models have a variance of at close to 1mm from minimum to maximum detection using an iron object, so accuracy and repeatability of less than 1mm is then down to the electronics. Below is the engineering data from a similar style Omron through-beam sensor, where there is no ramp up or down, just a virtually instant flip from off to on (or vice versa) as the beam of light is either allowed through or not.
Again, I was really commenting on the theoretical accuracy and repeatability advantages of the design, as obviously people are getting excellent repeatability from the inductive proximity sensors.
As far as being affected by dust, the essentially on-off nature of the optical homing sensors should mean they will not be affected in a way that would cause an accuracy issue. Enough build-up of dust could block the beam and trigger the switch (which obviously isn’t ideal when trying to run, but that would need a lot of dust) but a dusty lens should have a negligible effect on trigger point. When using these in industry that was what we found, even as they got dirty they worked perfectly reliably until they didn’t work at all.
Inductive proximity sensors however could have their accuracy affected by ferrous dust, though obviously that won’t be a problem in this use case, but was in the pipe factory where I worked. Because they are essentially triggered by a certain amount of ferrous material being in their detection range, adding any ferrous material other than your actual detection flag can mean they trigger sooner than you want them to. But again, unless you are machining steel on your Onefinity (and good luck to you if you are), that really doesn’t apply here.
Hey timplett,
yes, but this is the case with every electronic sensor. This is also the case with a photoelectric sensor. On the receiving side of the light, you have a phototransistor and it has a circuitry that in the end will produce a state change if a threshold is met.
You don’t know what a Schmitt-Trigger is? That’s the part that makes the threshold inside the sensor.
yes, but this is only the illusion that the threshold circuit provides.
Like with the inductive sensor, with your photoelectric sensor, there still and always will be light detected on the receiving phototransistor, even if the light is cut by reaching homing position, because it is an analog electronic part. It will never be zero light. Therefore your circuitry (usually inside the sensor) will define a threshold.
The diagrams above just say what you need to know when installing the sensor: E.g. on the first model, it says, If you have an iron part of 10 mm side length, it will trigger the sensor at 1.5 mm distance from the sensor. It will exactly then make a state change on the electrical output, high to low or vice-versa, depending on normally-open or normally-closed.
I think I was convoluting it in my head and not doing a very good job of explaining what I was thinking. Yes, even the through-beam raw sensor input will be analog and will require some sort of circuitry to determine if a threshold has been crossed, I definitely just saw two charts from Omron and said “See?” while actually not applying any actual thinking to that part.
Both styles of sensor will have a certain physical range in which they will generate an analog response which the internal circuitry then converts to a digital on/off. For simplicity’s sake, let’s just say that this response changes in a linear fashion from 0-100 as the detection object moves into range.
If the induction sensor has a range of 1mm where it moves from 0% detection to 100% detection, and it can accurately detect a 1% change in the raw signal to then trigger the actual threshold, it will be able to accurately detect down to 0.01mm.
Now let’s say you have a through-beam optical sensor with a beam width of 0.5mm. As an object moves into that beam, the detected light will drop from 100% to 0% (again, simplifying) as the object moves across the 0.5mm of the beam width. If the electronics have the same accuracy as the ones in the induction sensor, this would result in a detection accuracy of 0.005mm.
The caveat being, none of the sensors I looked at gave such a simple number as beam width. The physical lens on the Elite’s homing sensors are 1mm in width, but that does not mean that the beam is a full 1mm wide. Typically the lens of the receiver is a fair bit wider than the beam to account for any inaccuracies in aiming of the emitter, though that is obviously less of a concern in these one piece sensors, so it is possible they are using a full 1mm wide beam.
Following that train of logic, then an induction sensor with a smaller detection zone should also theoretically be more accurate/repeatable and a quick search for a high precision induction sensor seems to bear that out, as looking at the options for this specific model from Omron does show a correlation between sensing distance and repeatability.
Actually getting it through my head that the through-beam is still initially an analog raw signal does however also dispel my initial thought of the sensor being mostly unaffected by dust, as if the trigger threshold is say 50% light received and dust is already blocking 20% of the light, then the flag only needs to move 30% of the way into the beam rather than 50% to trigger it. It would actually be less affected as some of the dust would be in the same area that the flag is also blocking, but just using a very simplified analogy to show it would in fact be affected. The detection area does again come into play in mitigating this too, as a smaller detection area means any percentage of error is a smaller actual position difference.
I guess to sum up these ramblings, I do still feel that the detection mode of a through-beam optical sensor at least has the potential for inherent accuracy advantages over an induction sensor, assuming all else is equal. I suspect that at Masso’s target precision level, the through-beam sensor was the less expensive option.
Hey timplett,
nice, but all this has no relevance. That is because you have a circuitry attached behind the analog sensor, that produces a threshold, and as the user of any of these sensors, you have a simple state change.
And a diagram provided, to enables you to read off where the state change is.
Hey timplett,
I think that’s the wrong assumption you make. The diagrams do not show such a ramp up. You don’t understand them correctly. The diagrams just show the ratio of object size to sensing distance for the triggering event.
By finding and reading a value at an intersection of X and Y axis of the diagram, you can read off where the trigger point is for a specific distance and object size. That’s all. There is no ramp shown.
What you have in mind is that when you approach a metal to a coil, which with its oscillating current is able to detect a metal part approaching because it will interact with the magnetic field, the hereby induced current changes gradually as it is an analog sensor. But since this is of no big use for the end user, a sensor usually has a circuitry that defines a threshold, so that at the end you just “see” a sudden state change.
And the diagram just gives you the information where that change will exactly (and repeatably!) will occur.
So what I have tried to explain to you, is: What is said applies to a phototransistor too the same way. It will always produce a voltage curve depending on the number of photons that hit it, even in “total” darkness. But in order to make this usable, a sensor usually has a circuitry that defines a threshold, so that at the end you just “see” a sudden state change.
So I would not see myself able to judge on which sensor is more accurate and would provide more repeatability, since both in practice show no measurable deviation AT ALL.
PS: Adam @adamfenn28, I can’t imagine where this deviation comes from. It would be necessary to investigate the possible causes of what could be interfering with repeatability here that in my opinion should be excellent with photoelectric sensors.
What I found useful was:
Example homing sequence
- The axis moves towards its axis homing sensor until MASSO sees the axis Homing sensor changes to High
- If the homing sensor is already High MASSO will assume that the sensor is already found and will move to next step in the homing process.
- When the Sensor changes to High the axis decelerates and stops.
- The distance the axis travels after it starts to decelerate is determined by the axis acceleration setting. ← !!!
- During the deceleration and stop the sensor must remain High at all times.
- The axis backs slowly off the sensor until the sensor changes to Low.
- The axis stops and the Back off process is complete with the primary home position located.
- If a Pull off distance is specified the axis will move at rapid speed the specified distance from the primary home location and stop.
- The current position is now assigned the coordinate shown in the Home position.
– Source: The Homing process – Masso Documentation
Disclaimer: I don’t own and don’t plan to buy an Elite/Masso machine/controller
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