optical slot limit sensors

[travis_farmer]

Still doing research on a few things.
rather than mechanical limit switches, i was wondering about optical slot sensors being used for the limits and homes. do they fail or read false in dusty environments? they look pretty inexpensive, and don't really wear out, to speak of. they do require a little more supporting electronics, and pre-made options can be found on eBay with the supporting circuit attached, ready to output a TTL level signal.

~Travis

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[KH0UJ]

A typical sealed mechanical switch will last you 10-15 years. you can use optical switches too like they use on the elevators or on a mouse wheel, to be honest I hate limit switches on our CNC routers sir, I took them all out, all 4 sets of them, there`s a lot of reasons why I need to take it out.

[Al_The_Man]

The slot opto's are easy to integrate, minimum integration, they are used in treadmills etc for speed feedback etc.
Al.

[wizard]

Still doing research on a few things.
rather than mechanical limit switches, i was wondering about optical slot sensors being used for the limits and homes. do they fail or read false in dusty environments?

The answer to the dust question is yes they do fail and frankly it doesn't take much dust at all. In automation, at the plant I work, we use a lot of optical sensors of various sorts, in some cases the sensors are very sensitive to the environment. I've probably used more Q-Tips to clean optical sensors than most people use in a lifetime.

they look pretty inexpensive, and don't really wear out, to speak of. they do require a little more supporting electronics, and pre-made options can be found on eBay with the supporting circuit attached, ready to output a TTL level signal.

There are so many optical sensor product out there that you are likely to find something that will work without any supporting electronics. Most of the industrial sensors we use have an operating voltage range of about 10 to 28 volts (it varies a lot with manufacture and specific product). Price wise I suspect most of the industrial solutions are out of range. There are however a whole bunch of stuff available for board mounting and less robust field mounting.

~Travis

For absolute simplicity I might suggest reed switches which in their bare form can be had under 50 cents and under $5 in plastic housings ready to mount. You do have to consider max operating current and magnetic fields when mounting. With today electronics though current isn't a big deal and the sealed nature of reed switches means they are very reliable in low voltage circuits (assuming contacts designed for low voltage operation). On the other hand they are a bad idea on machines that machine magnetic materials.

As for optical sensors you can always house them in a metal or plastic block, ad a plunger to actuate the sensors. The problem here is that you just made a limit switch and thus might be better off just buying a limit switch.

Speaking of limit switches; industrial mechanical versions are pretty pricey, but do look around as they sometimes show up in surplus shops and such. Almost none of these are suitable for high reliability / safety related use. If you put guarding in place you might want to consider listed safety switches but then you are really talking expensive.

In any event if you expect dust find something different.

[travis_farmer]

hmm, thanks for the responses, all.

i was trying to find a non-mechanical (more reliable) option. what about hall-effect sensors with a strong magnet? i just remembered i happen to have a whole bunch of digital out hall sensors, and some neodymium magnets. personally, i have never tested them for repeatable triggering (for homing as well), but i suspect they should be at least as good as a reed switch. I think i got them on eBay from China, for a few bucks for a bag of 100.

~Travis

[ger21]

Cheap mechanical limit switches are about $1 each. 99.9% of the time, they never even get used. And if they fail, your limit circuit will activate, and you can replace it. But as someone mentioned, they should last at least 10 years.
And, for a small to medium sized stepper powered machine, you don't really need them, as the machine will just stop when it runs into it's stops. Most smaller stepper machines don't have enough power to do any damage.

what about hall-effect sensors with a strong magnet?

Actually, the weaker te magnet, the better the accuracy. Many of us are using hall effect home switches.

http://www.cnczone.com/forums/open-s...cnc-forum.html

[travis_farmer]

ok, so my sensors will work, but not my magnets. good to know.
Also, thanks for the link to forum post Gerry, i'm currently reading it and finding a lot of useful information.

what i take most out of it so far, is when my machine is built, and able to motivate, i will have to do my own tests with my sensors to see what the performance is. I read somewhere about the use of a steel nail to "focus" the magnet, so i will have to test that as well.

I hope this weekend to work on stripping my old camper out in prep of renovating it into a CNC trailer. with the frame jacks under it while the CNC is operating, it should prove to be a stable, yet movable CNC "office/shop". when my father retires, i hope to be able to make a business out of building custom cabinets, with the designs in the door/drawer faces as an option.

~Travis

[Al_The_Man]

Generally the type of hall sensors used on CNC machines for many functions including tool changers etc are the type that sense using a ferrous metal 'flag', these do not require a magnet, as magnets tend to attract metallic dust etc.
Al.

[travis_farmer]

i think, Al, if i recall correctly, placing a correct strength magnet behind the hall sensor turns it into a ferrous metal sensor. the change in the magnetic field from the introduction of ferrous metal causes the sensor trigger. i am not sure if my sensors can be used in this way, but i may experiment with it later.

~Travis

[travis_farmer]

found this PDF to be informative:
https://sensing.honeywell.com/honeyw...05715-2-en.pdf

page 78 describes a "piston sensor" using a bias magnet, and a ferrous piston inside a nonferrous cylinder. the basic idea can be applied though.

i will have to experiment with a sensor and a magnet to see if my sensors will work.

~Travis

[travis_farmer]

my results of my hall effect sensors and my neodymium magnets are as follows:

trigger distance is about 1/2 to 5/8 inches (by eye), and within about 20 degrees of the face or back of the sensor.
Ferrous metal triggering was best achieved by placing the magnet just within the ON trigger distance, and the metal was passed in between the magnet and the sensor.

I realize these aren't very absolute values, but it was a rough test of the idea itself. once my CNC is built, i will have a better test platform to use, with the capability to test repeatability.

I don't currently have plans to mill metals, so i doubt the collection of ferrous material on the magnet will be an issue.

what i would like to test also, when i have a CNC ready, is if it is more accurate with just the magnet, or with the ferrous metal breaking the path.

~Travis

[wizard]

hmm, thanks for the responses, all.

i was trying to find a non-mechanical (more reliable) option.

Reliable as compared to what? The reality in my world is that switches, that is the contact mechanism, don't fail that often. This even on machine that cycle several times a minute. What fails is almost everything around that switch. The cable to the switch will get snagged of wear through in a drag chain. The device they are attached to often wears out first, for example an air cylinder; then the manufacture of that air cylinder changes the mounting method forcing a new switch installation anyways. Sometimes the magnet gets knocked off

Mind you I'm talking industrial machines that cycle several times a minute on a router you would almost never hit an limit switch. Well you shouldn't in normal operation. Here I'm thinking end of travel axis limits. Sometimes it might pay to have positionable limit switches to use when verifying out new code to to prevent axis travel in an area that might cause damage. In this case we aren't talking axis limits per say.

what about hall-effect sensors with a strong magnet? i just remembered i happen to have a whole bunch of digital out hall sensors, and some neodymium magnets.

Why didn't you say so. In my mind it is always better to use what you have before spending hard earned dollars on stuff. The only problem I see with such chips is that you still end up building "Limit Switches" which is hard to justify when they can be had rather cheap.

personally, i have never tested them for repeatable triggering (for homing as well), but i suspect they should be at least as good as a reed switch.

Homing is a different story and in some ways servos can beat steppers because a servo system can home to a high degree of precision with relatively cheap sensors. There are a lot of factors that go into a sensors type to be able to repeat accurately. Generally the ability to repeat precisely goes up with that precision.

One machine implementation I saw years ago simply bottomed the axis out, stalled the servo, for a home reference. I was never extremely happy with that solution but it did work fairly well. Generally I don't like bottoming out.

I think i got them on eBay from China, for a few bucks for a bag of 100.

~Travis

The type of output they have will be a factor. On most low end CNC systems you need to run all your axis limits into one input which means chaining the sensors together. Some outputs will not support that well.

To be perfectly honest you don't need the axis limit switches. You can skip them for good on a small machine or add them later if needed. Homing switches are a slightly different story, personally I'd prefer them but you can manually home a machine out against fixed targets. You will also often zero off the vise or the piece to be machined. That being said I still prefer to see a machine run a homing sequence when powered up positioning all axis in a clear zone.

[louieatienza]

Embarrassingly, exactly the way I "homed" my routers was to slowly run each axis against a "hard stop." As accurate as I needed (though never measured.) Of course this is done slowly. You won't damage your steppers by stalling them as they're current-limited. And you won't damage your machine unless you used cardboard and Elmer's School Glue.

One reason was my machine had an overall travel of 32" x 38" and I routinely had to machine parts that were 30" x 36", using 1/2" and larger bits. That left me only 1/2" on each side clearance. Which is another consideration you need to be aware of, and design in when determining the amount of travel you need for your machine. Limit and home switches will eat machine travel. Again, if you're using steppers, and you size them correctly, you likely will not damage your machine if you run into a hard stop. With DIY linear bearings, it could be an issue. Also you don't want to rapid into a hard stop, especially with ballscrews.

One VERY IMPORTANT reason to have some kind of homing sequence is there will eventually come a time when you're halfway through a multi-hour job, and you'll trip a breaker, or otherwise lose power. Or you change bits, forget to zero it out, and you run into a fixture and stall out. If you have no way to home, you'll have to find a way to datum off a feature of your part so you can find X0 Y0 Z0 again. Not bad if you're cutting a panel. Easy if you were drilling holes (just pick a hole, drop your tool in and get the coordinates in CAD). Very hard if you were doing 3D bas relief carvings and there's no edge or material top left to reference off of.

Teknic's servo drives have featured hard stop detection for homing for a long time.

[travis_farmer]

Alright, here is what i learned from the tons of information here. forgive me if i missed something.

Smaller machines don't necessarily need limits. but if i have any, homing sensors are more important, and end limits are really only needed by preference. I may have them, simply in case i miscalculate something.

limit and home switches eat travel. so i will have to factor this in when figuring my machine size.

I double checked the datasheet for my sensors, and they are open-collector. so i need only a pullup resistor to hold the output high. I think i remember that the output is pulled low when a magnet activates it. so if i tie the end limits together, they will function fine together. any sensor pulled low will activate the limit input.

I hope to get started on renovating my camper into a CNC "shed" today. i didn't get much done on it yesterday as i was helping my father get rid of some yard clutter he had. when done though, i will have a space that is about 8' by 29' to work with.

~Travis

[louieatienza]

If you use Mach3 you can just set up home switches and use "soft limits" badically defining the work area of the machine in Mach3...

[travis_farmer]

I will be using LinuxCNC. I have only played with it a little bit, so i am unsure if it has that feature.

~Travis

[Al_The_Man]

I am not sure if the motion cards you can use in LinuxCNC have the same ability as the Galil Motion cards which use the method used in commercial CNC systems, where the homing procedure is to first rapid to the home switch, then reverse in slow until it comes off the limit, at this point it looks for the 1/rev Z pulse of the encoder and then registers that as zero.
Extremely accurate.
Al.

[ger21]

But requires using servos, which I believe are well out of the OP's budget.

[travis_farmer]

Al_The_Man

I am not sure if the motion cards you can use in LinuxCNC have the same ability as the Galil Motion cards which use the method used in commercial CNC systems, where the homing procedure is to first rapid to the home switch, then reverse in slow until it comes off the limit, at this point it looks for the 1/rev Z pulse of the encoder and then registers that as zero.
Extremely accurate.
Al.

ger21

But requires using servos, which I believe are well out of the OP's budget.

LinuxCNC can use one or two parallel ports for I/O, so it uses similar, if not some of the same hardware for Mach3.
LinuxCNC Documentation Wiki: LinuxCNC Supported Hardware

and yes ger21, i have priced out servos, and they are quite well out of my budget.

~Travis

[wizard]

Alright, here is what i learned from the tons of information here. forgive me if i missed something.

Smaller machines don't necessarily need limits. but if i have any, homing sensors are more important, and end limits are really only needed by preference. I may have them, simply in case i miscalculate something.

That sounds like my opinion. As someone else pointed out it is very important to have a homing routine. I prefer to home against a switch of some sort but acceptable results can be had by bottoming out an axis on a hard stop.

As for hard stops, for end of travel you can get by without them. I do not consider crashing into a axis hard stop to be a good thing. Eventually somthing will lossen upor break. The good thing here is that steppers dont suffer from runaway the way some servos do so the energy in crashes are far less.

From my perspective hard travel limits can be more useful in keeping the machine out of areas you want to avoid especially when testing new code.

limit and home switches eat travel. so i will have to factor this in when figuring my machine size.

Well a home switch shouldn't but axus limits will to dome extent. The problem is axis limits have to happen early enough to stop movement befor inertia slams the axis into the mechanical stop.

I double checked the datasheet for my sensors, and they are open-collector. so i need only a pullup resistor to hold the output high. I think i remember that the output is pulled low when a magnet activates it. so if i tie the end limits together, they will function fine together. any sensor pulled low will activate the limit input.

Sounds good.

I hope to get started on renovating my camper into a CNC "shed" today. i didn't get much done on it yesterday as i was helping my father get rid of some yard clutter he had. when done though, i will have a space that is about 8' by 29' to work with.

~Travis

Interesting point here, one local cabinet maker around here converted one of those smaller touring busses into a traveling shop. The bus being one of those with the massive windows all around. I've never actually meant the guy running the business but have seen the "bus" many times. You could easily see all of his tools obviously set up for cabinet makng. The big windows must have been great for working in the daylight.

The big problem you will have with a router in such a trailer is access. Handling full size sheet goods will be a chore if not close to impossible. You may need to make considerable midifications to the trailer beyond gutting it.