Electronic home switches made easy!

[RomanLini]

I just don't believe that about mechanical micro switches being the most accurate. Here's my take, coming from an electronics background;

1. Optical interuptor sensors. Lousy accuracy, affected by ambient light, IR, even radiated IR (heat). The optical receiver is not temperature compensated and worst of all they age over time so the receiver properties change over time. Also a big problem with dust of any type.

2. Mechanical microswitches. Even if this was to have the initial repeatability of a "no moving parts" hall system, there are parts that will wear over time and change the home point. They usually incorporate a plastic lever and pivot, and plastic wears. It also has a high thermal expansion (much greater than metal) so if the inbuilt lever is 5mm in size and expands 2% over normal operating temps (quite typical) that would be an error of 0.1mm even before the wear starts to occur. Contact wear is also a given, and any mechanical interface may get some dust or swarf between the lever and actuating surface at any time.

Most of these error problems also apply to hall sensors that have a mechanical lever interface (like the type mentioned by CreviceReamer above) which is why I would avoid these type of hall sensor.

3. No moving parts hall switch. Using a "slide by" operation gives very high repeatability, probably as good or better than 0.01mm. There are no parts to snag or trap swarf. Obviously these are not suitable for cutting steel or ferrous metals as the magnet will attract steel swarf but wood, plastics, aluminium, brass etc are ok.

The hall switch has 4 sensors in a bridge so it self-cancels ageing effects of the silicon. Also some brands are temperature compensated for both the hall switch AND the magnet itself (see the datasheet) so even though temperature changes may affect the silicon and the magnet strength that is all compensated out.

When you crunch the numbers, the hall switches look miles ahead for accuracy and repeatability! I'm really looking forward to seeing Vladimir's (vsengineering) test results as his machine allows much finer testing than my machine results shown on page 1 of this thread (which are only in resolution of 0.01mm).

[Glennluo]

I am Glenn from Shang Hai China.I am a design engineer in electronics field

[jke]

Hi, (just new here)

To make a hall switch fail-save or "normally closed" you can add a transistor in the housing.

Jurgen

[Dozer56]

Just going through and re-reading this thread again, Great info here!! Even though I don't understand alot of all the technical discussion I think I could make these sensors even with my limited (non existent) knowledge of electronics. I am about half way through my first CNC router. From reading I think I understand that mach3 with a g540 is going to have to have limit switches wired up for the system to run. I might be wrong about that. I thought I read a few threads where some only run on soft limits, but I think since I really have no idea what I am doing and think I should have protection against crashing though it's probably inevitable and probably part of the learning experience, at least it usually is for me....

Is the Hall sensor that CR posted a link to back in reply #107 & 108 http://parts.digikey.com/1/parts/948...0-3h-02-a.html basically the same as the home made one in this thread? I'm just thinking I like these better than the mechanical ones and if I could buy them it would save me time and probably much aggravation getting my machine started.

Thanks

[Crevice Reamer]

Originally Posted by Dozer56 Just going through and re-reading this thread again, Great info here!! Even though I don't understand alot of all the technical discussion I think I could make these sensors even with my limited (non existent) knowledge of electronics. I am about half way through my first CNC router. From reading I think I understand that mach3 with a g540 is going to have to have limit switches wired up for the system to run. I might be wrong about that. I thought I read a few threads where some only run on soft limits, but I think since I really have no idea what I am doing and think I should have protection against crashing though it's probably inevitable and probably part of the learning experience, at least it usually is for me.... Is the Hall sensor that CR posted a link to back in reply #107 & 108 http://parts.digikey.com/1/parts/948...0-3h-02-a.html basically the same as the home made one in this thread? I'm just thinking I like these better than the mechanical ones and if I could buy them it would save me time and probably much aggravation getting my machine started. Thanks

Your link doesn't work. If you mean THIS switch:
http://search.digikey.com/scripts/Dk...me=480-1993-ND

Then yes. The output is digital, which is what the G540 expects. Just connect the Red wire to +30V or less, the black wire to negative and the Green wire to any G540 input.

In my G0602 lathe project, I'm using these with G540 as a spindle speed threading sensor and as the Z home switch:
http://cnczone.com/forums/showthread...=106312&page=6

And a Voltage divider that converts the 48V from PSU into 24V For sensor power:
http://cnczone.com/forums/showthread...312&page=7.#78

I don't use limit switches with steppers, just soft limits:

http://syilamerica.com/docs/Homing_V...%20Limits.html

CR.

[Dozer56]

My link wasn't correct, fixed it now. I was thinking about that type too, but I was thinking these might work better for me with the magnet .

[Crevice Reamer]

Ok. I see you fixed the link. THAT switch will also work with G540, but you will need a magnet south pole to operate it. The switch I linked to above has the magnet already mounted internally and operates when a ferrous metal vane passes through its slot.

CR.

[KenoSlim]

Nice job with the hall switch idea RomanLini. Sure has a lot of folks working on it.

Here's my problem... Any time I stay with 5V for limits or home switches I get line noise. I have shielded wire, capacitors, grounds everywhere, just can't get rid of the noise with 5V. I use 12V through the switches with a relay to signal Mach 3.

I hooked everything up according to the diagram and everything works fine till I add the relay. When the relay is added, the output from the hall switch sinks to about 2V and the LED turns on. I also lose the switching effect of the hall switch. The magnet makes no difference.

Would anyone have a suggestion as what I should try? I don't want to give it up and I have already cut the mounts for the hall and LED.

[jke]

Hey,

You can not drive a relais directly from the hall sensor, you need at least a transistor.

Jurgen

[kineticstep]

Originally Posted by KenoSlim Would anyone have a suggestion as what I should try?

How big is your relay? Remember it has to be driven by less than 20mA if you aren't using an extra transistor to drive it.

If you use this approach with a relay you might consider including a diode to protect against the back EMF when the relay shuts off.

[KenoSlim]

Thanks for the responses guys!

I'm using Omron G5V-1-DC12 relays. I also have some G5LE-1A4-DC12 available. I'm using a diode with some other relays at the moment, using contact switches.

I found on the Honeywell site a schematic using a transistor with the hall sensor. It calls for a PNP type. Shows a basic part number of 2N3638 but it is no longer available as far as I can find at Mouser. There is a truck load of PNP transistors available so I'm not sure which to order.

[kineticstep]

NTE129 is supposedly very similar, but almost any general purpose transistor is going to work. If you have a bunch of junk circuit boards you could pluck a few off of them.