[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]

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]

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.

[KenoSlim]

Okay, thanks for the tips. I had to order in some transistors so while I was at it I ordered a few relays with a lower amp rating. It's just a signal change that's needed after all.

[RomanLini]

Dozer56- That Hall Sensor (see image) looks nice, ready to use. Your link (in post #124) shows Digikey sells them for $9.52 which seems pretty good.



I don't know if they are temperature compensated though, or how sensitive they are. It would be good to see one used on a machine and get some data!


Crevice Reamer- thanks for the links!

I too use soft limits, it hasn't caused a problem. Generally when my machine has a crash it's loud, nasty and immediate, so it gets turned off ("E-stop") and then reset etc to re-home it.


KenoSlim- Thanks for the nice words. Sorry to hear you get noise problems, and I read down the thread you fixed it with a relay.

However I'm not sure a relay is the way to go as it has a few mS pull-in time, so the home position will be in a different place depending on how fast your machine is moving.

Maybe you could use an opto-isolator (optocoupler) instead of the relay? That will be much faster as it is fully electronic.

You may even be able to use JUST the transistor as a buffer (with no relay) and if that is set up properly might fix your noise issues.


Update on the hall switches I installed on my own CNC machine;
My Build Thread- DIY hobby small plastics mill/router

I have had some minor problems in the cold weather now (it is winter here in Australia) and although the hall sensors are temperature compensated my aluminium frame CNC machine gets a couple thou of thermal expansion from cold to warm which is interfereing with the X home measurement.

My build thread above tells about the symptoms I'm getting and the work-around I'm currently using.

[ger21]

Looking at Mouser, it looks like there are new part numbers. Mouser has an SS441A and SS441R. Which is the one I'd want?

And would a small magnet like this work? It would be easy to drill a small hole and glue it in place.
http://cgi.ebay.com/5-PIECES-1-8-x-1...item3a5f734f1e

[TarHeelTom]

>>And would a small magnet like this work?

No personal experience, but I've read that this type of magnet is very fragile, and is normally glued to a mounting bracket or whatever.

Tom

[RomanLini]

Ger21- I just checked the SS441 datasheet (it's posted earlier in this thread) and it appears the suffix -A -S and -R refer to different lead lengths and types;

-A = standard long straight leads
-S = surface mount, ie shorter bent leads
-R = reduced length straight leads

I think the -A is the best, you can always cut the leads shorter if you like.

Re the neodymium magnets on ebay, these super magnets have very high field strength. The result is that the sensor will detect the magnet from further away, that might be good.

The bad side is that the distance is also increased for the change in magnetic field. That means the field will go from (say) 90% to 91% over a LARGER distance, the result is that the accuracy and repeatability is not as good. Stronger magnets will also tend to pick up more crud. My suggestion is to use older style black (ferrite type) magnets, these are still available and can even be pulled out of cheap fridge magnets etc that you can buy in any supermarket.

TarHeelTom- The supermagnets are usually plated with a shiny nickel outer layer that makes them a bit more durable.

[brtech]

I've got my home switches installed. I'm using ferrite magnets from cabinet latch mechanisms. I'm seeing what appears to be over 1/2" on to off hysteresis. The gantry moves till the home switch is detected, then it backs off 1/2" or so to get the switch off. That seems excessive. What are you getting?

One more problem for those with Gecko G540s. On mine, I can measure 11 volts on the inputs! I was running the SS441s on a 6v supply, but it's really bad for most electronics devices to have an input at a higher voltage than the power, so I changed the power supply to 12V. It works, but it sure seems weird. It's as if the Gecko has some kind of an internal pullup to some internal 12V supply or something. Not mentioned in the Geckodrive manual. I sent their tech support an email about it; no reply. My Gecko died in an unrelated problem and I needed to send it back. I mentioned the problem in the return form. When they returned it, no mention of the issue, and no change in the input voltage. I have tried with and without a 470 ohm pullup resistor on my side; no significant change.

[RomanLini]

brtech- The high level of hysteresis is both normal and good! The hall switches have an internal brain that operates like a "schmidt trigger" which gives a very reliable ON transition as the south magnetic field rises above a set value.

Then the magnetic field needs to drop by an overlap (hysteresis) that is deliberately designed into the sensor, before it switches off again. The SS441 turns ON at about 115 gauss (magnetic field strength) and doesn't turn OFF until the field drops to about 20 gauss.

This hysteresis effect means the sensor will trigger very accurately and reliably even if the magnet is moving slowly in relation to the sensor. Without the hysteresis effect there would be a point at the trigger threshold where the sensor might "chatter" on and off as it hovers around the trigger threshold.

Don't worry about the hysteresis, it is there for good reason! In practice what you need to do is always sense the home position by moving toward the sensor in the same direction, until the sensor turns on. This is pretty normal for homing routines anyway.

Also, if starting from home, you need to move the machine way past the sensor (by 3/4" or so?) then bring it back towards the home sensor until the sensor turns on.

To answer your question my sensors have about 8 or 9mm hysteresis, my magnets must be a fraction weaker than yours (or a bit further away).

So don't sweat the hysteresis! When homing in the right direction toward the sensor they should still be accurate to maybe 0.01mm or so.

I can't speak for Gecko but I think the G540 inputs have an internal pullup resistor. This will not matter, the output of the SS441 sensor is an "open-drain" type, this is designed to be perfectly compatible with 5v equipment or equipment that has a pullup resistor to 5v, 12v even up to 30v or so.

So it will not hurt the SS441 sensor to be powered from 6v DC on its power pin and have an 11v pullup on it's output pin. That would be all ok. However if you want to power the sensor from 12v that is also ok, but will not make any functional difference.

[revwarguy]

Just in case this helps anyone from reinventing them, attached are three files for machining the switch case from Roman's drawings:

switchcase.jpg image of final machined part from these files. Includes holding tabs.

switchcase.dxf - dxf files for the above parts, holding true to Roman's dimensions with two exceptions: screw holes were made sightly wider and the pocket over the screw holes was made slightly wider so that the whole thing can be carved in a single operation using a 1/8 bit.

switchcase.crv - aspire (or Vcarve Pro) file for the project assuming 10 mm thick material

Roman, you really ought to write this up and send it to Digital Machinist. George Bulliss is always looking for articles. If you do, you can include these files if it would help. :stickpoke


Post edit - It appears that CNCzone doesn't like aspire files. I get the message "invalid file" when I try to upload it. If anyone wants the .crv file for this, I guess you'll just have to PM me.