choosing proximity sensors?

[the great waldo]

Hello all.
I'm in the process of choosing proximity and homing switches. I have noticed that there are different sensing distance switches from 1 to 4 mm are there any benefits in choosing switches with a 1mm sensing distance (are they going to be more accurate for homing?) Any other information gratefuly accepted ie npn or pnp.
Cheers Andrew

[driftspin]

Hello all.
I'm in the process of choosing proximity and homing switches. I have noticed that there are different sensing distance switches from 1 to 4 mm are there any benefits in choosing switches with a 1mm sensing distance (are they going to be more accurate for homing?) Any other information gratefuly accepted ie npn or pnp.
Cheers Andrew

Hi waldo,

I spend only €13,- on 5 units... m8 6-36 vdc 2mm sensing distance.

Aliexpress

lj8a3-2-z/bx npn

Just a single meter of wire on the sensor.

I use a sloped metal surface of about 20 deg for sensing.

I am very impressed by the accuracy.

homing is spot on on a cheap 0.01 mm tdi every time.

They switch to negative / ground on contact.Almost no resistance compared to grond.

In the off position they have about 10k series resistor in the switching wire

Had to do some tinkerring with resistors to interface with the cheap 5 axis Bob i have.

Not sure what your budget is for them or what controller electronics you have.

Grtz Bert.


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[magicniner]

With proximity sensors you could use two switches in series, one sensing axis position directly and the second sensing a cam on the rotating screw - instant multiplication of homing resolution ;-)

[driftspin]

With proximity sensors you could use two switches in series, one sensing axis position directly and the second sensing a cam on the rotating screw - instant multiplication of homing resolution ;-)

That is actually a very good idea.

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[magicniner]

I'll be using it on my Chester mill conversion, I've decided to get my new VMC up and running first and make the rest of the parts for the conversion with the big CNC.

[the great waldo]

With proximity sensors you could use two switches in series, one sensing axis position directly and the second sensing a cam on the rotating screw - instant multiplication of homing resolution ;-)

Hi Magicniner

Any chance of explaining the cam on the rotating screw (picture/sketch for a dummy might be helpful) If it's too much hassle don't worry, but I am curious.

Cheers
Andrew

[magicniner]

Anything with a high point that a proximity switch can sense at a repeatable point in the homing direction will do, you could use a single bladed chopper too, anything which sticks out significantly more at one point in it's rotation will do.
Mount the cam near one end of the screw and mount the proximity switch so that it is triggered by the cam.
Simples! ;-)

Here's a pic I found, imagine a gear with just one tooth.

[driftspin]

Anything with a high point that a proximity switch can sense at a repeatable point in the homing direction will do, you could use a single bladed chopper too, anything which sticks out significantly more at one point in it's rotation will do.
Mount the cam near one end of the screw and mount the proximity switch so that it is triggered by the cam.
Simples! ;-)

Here's a pic I found, imagine a gear with just one tooth.

HI magiciner.

Only small disadvantage i can think of is that the sensor must be on the moving part.

Good idea !

Grtz Bert.





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[the great waldo]

Anything with a high point that a proximity switch can sense at a repeatable point in the homing direction will do, you could use a single bladed chopper too, anything which sticks out significantly more at one point in it's rotation will do.
Mount the cam near one end of the screw and mount the proximity switch so that it is triggered by the cam.
Simples! ;-)

Here's a pic I found, imagine a gear with just one tooth.

Hi Magicniner.
I hope this isn't a silly question but apart from the obvious what is a single bladed chopper in this case?
Cheers.
Andrew

[Neale]

Bert - I don't think that the sensor needs to be on the moving part - I can imagine a rotating blade on the end of the ballscrew with a fixed sensor next to it.

Andrew - the term probably comes from the kind of operating disk used with optical sensors - imagine a disk with part cut out that blocks or unblocks the light path as it rotates, hence chopper. In this case, you could use something very similar but with the rotating disk triggering a proximity switch.

Nick - I think that you need to wire the switches in parallel. As long as they are both the same type, you could use NC or NO with appropriate change to the detection logic. The only downside apart from a very small increase in wiring complexity is that you lose the "fail-safe" capability of a single NC switch. There are some failure modes that could let the machine overrun the end position.

It's an interesting technique, though - I did consider something similar when i was building my machine (although I didn't spot the simple way of doing it!) that involved switching to the "cam" sensor when the usual sensor triggered, However, on testing, I found that a single sensor in the conventional "pass-by" position was more than adequate, so I didn't bother exploring. But that's on a router rather than mill, so accuracy requirements are not quite so high.