proximity sensors

[FordGalaxy]

It's time to put some limit/home sensors on my X2 conversion.

I would rather use proximity sensors than micro switches.

Anyone have some suggestions as to which ones work well, which ones to avoid, any help would appreciated thanks.

Bill

[Zaggy]

Hi,
I've tested Honeywell Hall SS443A associated with DealExtreme rare eath magnets (Super-Strong Rare-Earth Square RE Magnets (30-Pack) - Free Shipping - DealExtreme) with good success.
The working distance between the sensor and the magnet is around 5mm. Precision and repetability are satisfying : 1/100 mm.
Zaggy

[wizard]

I ask because for limits anyways a mechanical switch might be more reliable. The last thing you want is some swarf triggering an axis limit.

It's time to put some limit/home sensors on my X2 conversion.

I would rather use proximity sensors than micro switches.

Are you sure about that's?

Anyone have some suggestions as to which ones work well, which ones to avoid, any help would appreciated thanks.

Bill

I'm sure there are different opinions here but for limits I'd suggest mechanical switches.

[Flexo!]

Another vote for proximity switches.

ALL limit switches have to be covered... chips can get into mechanical switches, or optical switches so its not a valid argument.

Inductive switches are easier to mount - espcially the 2 hole ones like these: SNO-4 PNP Proximity Switch, they run off breakout boards 5v line, and give very good accuracy (better than a 10 microstep stepper can resolve ~0.003mm)

[RTTIM]

I ask because for limits anyways a mechanical switch might be more reliable. The last thing you want is some swarf triggering an axis limit.

Depends on what type of proximity switch you use - there are several options, including inductive, capacitance and magnetic/hall effect. The ones Zaggy mentioned, for example, are hall effect units that are triggered only by a specific pole of a magnet (usually S). They are very reliable and will not be triggered by swarf.

OTOH, mechanical limit switches can be less expensive (depending on type), and the wiring is certainly simpler.

Tim

[FordGalaxy]

The two hole inductance sensors seem the easiest to use. I'm wondering about how repeatable they are. Do they return to zero, or is there a plus or minus a few thou?

[FordGalaxy]

109jb
I thought someone might answer how repeatable the inductive proximity switches are, but RPC jacked the thread and took it to the basics.

[RTTIM]

109jb
I thought someone might answer how repeatable the inductive proximity switches are, but RPC jacked the thread and took it to the basics.

Too general a question - again it depends on type and packaging. Some are very precise, other less so.

Repeatability in application is more a function of the control software. Most CNC programs stop the axis drive motor when the home/limit switch actuates, then slowly reverse it until the switch just clears. If your system is calibrated correctly, repeatability should be just fine. For really precise applications you'll still want to zero to the part prior to machining.

[HimyKabibble]

109jb
I thought someone might answer how repeatable the inductive proximity switches are, but RPC jacked the thread and took it to the basics.

You're kind of asking "What gas mileage does a car get?", or "How bright is a light bulb?". Is the car a Pinto or a Ferrari? Is the light bulb 15W or 100W? Different prox switches have different specs. You have to look at the specs for the specific switch you're interested in using.

Regards,
Ray L.

[Flexo!]

I thought someone might answer how repeatable the inductive proximity switches are

The cheap $5-$10 chinese proximity switches will give you better than 0.01mm. I've tested them pretty thoroughly on my 2 CNC mills and you get a bit of variation in multiple home attempts (likely from the CNC electronics/software etc).

[FordGalaxy]

Thanks flexo
For homing .01mm should be fine.
When I was a machinist we would home the machine and zero the registers when we powered up, and then set X,Y,Z zero on the part.

[wizard]

Ah! I didn't even think of doing it that way. You have to forgive me, I come from an electronics background where I am always thinking of tying every to an individual input (I guess it comes from my microprocessor background).

It really depends upon how fancy you want to get with your controls. On smaller machines though there is little value in wiring up axis limits to discreet inputs. So you can either electrically AND them or OR them together.

That being said if you want to directly disable your motor drives with the axis limits then you should pay attention to how the ENABLE works on your axis amps.

So, I guess I can just tie them all into my E-Stop circuit?

Thanks!

No! Repeat No. An E-Stop circuit is an entirely different animal from an Axis over travel limit. This is an area that is often not given the right level of importance on home built machines, but I might suggest that it is a critical area for any automatic machine. Thus I'd reccomend becoming familiar with proper E-Stop implementations. Oh and buy good quality E-Stop buttons.

On a side note people often use the axis over travel limits as an homing limits. I generally don't like this approach, better to implement entirely separate circuit(s) for that.

As to the discussion about prox switches I will just mention that I don't see many of them used on metal working equipment. For machines destined for other uses, yes you will see prox switches being used for limits. Maybe machine tool manufactures are old fashion, but I also see it as a sign of experience.