Limit Switches - optical / mechanical / proximity

[Gerald D]

I really don't understand why problems arose when the first guys used the common 12mm diameter proxy on alu rails. These proxies typically work at 2 to 4mm sense range for steel and 1 to 2mm for alu, and this should have been okay for either material. As you point out, those going for much extended ranges (18mm dia. and/or 8mm sense on steel) are going to make a new set of problems for themselves.

[smreish]

Gerald,
The 2-4mm sense - 12mm targets worked just fine, but I found that the clearance of 1mm was little too close. (little things liked to get jammed in the clearance area)

Electrically, they worked fine. Mechanically, they were a cow pusher for dust.

Thus, went with the extended distance units for the aluminum for peace of mind.

Sean

[David Bryant]

Hi Gerald
The problems might be as the switch off distance is what being used, it is obviously longer than the switch on distance.
European standards suggest a plus or minus ten percent of sense distance in manufacturing tollerance is ok to meet the standard.

It is suggested that the off state is clear for three times the sense distance.

So if we start with a 4mm sense distance the "Ideal" target is either a square the size of the diameter of the sensor or three times the sense distance, whichever is the largest.

Also the recommended off state is clearance for three times the sense distance or 12mm. This is the guideline that the installation of sensing the hole in the Aluminium supported by steel breaks. Hence my earlier suggestion of a hole in the steel under the aluminium as well.

Regards
David

[PEU]

Here are my 2 cents to this thread. After reading the options available to wire the sensors and buttons I liked Mike (Richards) idea. Then Gerald added suggestions to this schematic and based on the design an ideas I did this in expresspcb:



changed 2x4 TTL AND gates by 4x2 CMOS AND gates
Added LEDs to all proximity sensors
Added an 8pin connector to the BoB
Added 3 connectors to pushbutton stations

Then I continued reading the PMDX125 manual and I found the inputs section:



They suggest using the sensors in parallel without extra interfasing as shown in fig.4. PMDX122 users will need to provide 12V to the sensors

The similar section for PMDX 122 is not this detailed:



But I guess it can be used the same way, I mean without relays and the design by Mike.

Am I missing something?

[Richards]

Pablo,

I like your design. It's clean and efficient. So far, the only issue I have is using a CMOS chip to drive (I assume) a TTL chip. Traditional CMOS might have a problem with TTL. To get around that limitation, the HCT and VHCT chips were designed, i.e. 74HCTxxxx or 74VHCTxxxx, where 'xxxx' is the part number. Some of my older design books show the use of a 1k resistor connected between the CMOS output and ground. Other designs used a transistor per output gate as a buffer.

I haven't found data on the chips used on the PMDX-125 board, but the manual states:

Inputs (including E-Stop and Fault):
Low must be able to sink 6 mA at less than 0.8V
High must be open circuit or greater than 3.0 volts (and no greater than 25VDC)
Response time is 20 µs

That's a huge load for most CMOS.

[PEU]

Its not my design, its yours!!

Changing back again to TTL is no problem at all, just need to change to 74LS21 or 74HCT21 as you suggest. Im drawing the part since its not available.
I will try to make the PCB single layer, so it can be machined with any router

I still wonder if the pmdx125 wiring suggestion have any problem or drawback. [edit] I mean, Im no expert in the differences between TTL and CMOS, switching the design back to TTL solves what you posted in blue color?
Attached is the new expresspcb drawing

[Richards]

Pablo,

I'm comfortable with the 74HCT logic family as the replacement for the CMOS part. The HCT series has the best of both the CMOS and the TTL worlds. It is used to bridge logic families so that different types of logic can be used without unforeseen problems.

Fairchild Semiconductor has an application note, AN-368, that explains when and why to use HCT. It's the kind of document that I like to read instead of reading the latest fiction best-seller.

If you Google "the differences between LS, ACT, HCT logic families", you'll find a lot of other documentation that helps to explain why there are so many logic families and when to use each type of logic. In a perfect world, all of the logic families would have chips with the same functions, but we live in an electronically imperfect world, so knowing what happens when you need to mix logic families helps to keep the glitches out of the design.

When I'm designing circuits for use with other products and when I don't have a schematic and parts list for that other product, I usually try to use 74ACTxxxx logic. The ACT family sinks OR sources 24mA per pin (usually with a 100mA total current draw per device). The problem is that the 'ACT' devices are more expensive and the logic family contains fewer parts, so it's not always possible to find a perfect match.

Things were easier when I started out. If an output had to drive more than one input, I used standard TTL. If the output only had to drive one input, or if the input used 'LS logic, I used 74LSxxxx chips. It was a given that all logic was Active Low, meaning that the output was a current SINK. Now, designers tend to mix and match logic families. Some outputs SOURCE current and some outputs SINK current. The Geckodrive G201 and G202 have inputs that are driven by current sinking outputs. The Geckodrive G203v has inputs that are driven by current sourcing outputs. The Geckodrive G201X can be driven by either sourcing or sinking outputs.

Because I don't like to repair circuit boards, I normally use simple, off-the-shelf parts. I always use chip sockets. (One of my pet peeves is the manufacturer that solders chips directly onto the board. When chips are soldered in, a 30-second chip replacement requires careful unsoldering with a high likelihood that a trace will be damaged by the heat.) I leave the surface mounted chips, the programmable chips and the microprocessors to the more exotic designs for customers who are willing to buy spare boards, which means that the customer's down-time is only long enough to swap out a circuit board.

Although I use the PMDX-122 break-out-board (and I really like that board), the PMDX-122 is much more than is required to do the job. The purpose of the board is to protect the Parallel Port and to amplify the signals. A 3-chip board could do that. One chip for the Step/Direction signals. One chip for the auxiliary input signals. One chip for the auxiliary output signals. Mach3 can easily handle the polarity of the signals. By adding opto-isolators to the auxiliary inputs and outputs, noise would be kept to a minimum. Every time I get ambitious and decide that it's time to design a new break-out-board, I take a reality check and ask myself why I would want to spend a week designing a board and $500 to get two prototypes made when I can buy a very nice, very reliable PMDX-122 for less than $100. So far, logic has won out. As senility progresses, I might someday abandon logic and design a board that is not needed in the market place just so that I can say that I designed and manufactured my own break-out-board.

[Red_boards]

Hi Pablo,
I was reading a document from PMDX earlier and it had suggestions on wiring e switches for J10 and J13 to use. This is a bit more detailed than in the manual. The document is AN125-1_MachinePower_02.pdf

Doesn't really help with J12, but I hope it's useful.

I'm looking forward to hearing how you go.

[PEU]

Originally Posted by KenC

I don't know how I left this question unanswered... Better late then never ..
Yes, it came with the cable attached directly to the sensor, no connector in between.

Received mine today, I was happy to see they have a LED where the cable meets the sensor, did not checked them yet.

[silverdog]

I assume that from the box situated on the gantry to the pdmx the connection is with a 7 core cable this means that proximity and pushbutton has a common ground. This is possible only if the power of the pdmx is +12 vdc from the same supply of the +12vdc of the proximity isn't it ?
if the pdmx is powered with 9 vac I can't use the same ground, right ? in that case I need 8 core cable right ?
maybe just a question by a dummy .... but ...
thanks

[smreish]

Sergio.
7 core is for the pause, estop and resume buttons. The 7 core does used a shared ground for electrical connections to pins 11, 12, 13 and 15....with a common ground.

The proximity circuits are outside this requirement. Pending your choice on limits/prox sensors, you will need additional

[zigmart]

Robert correct me if I am wrong, but the optos are not in the circuit to provide isolation, but just as a quick 12v to 5v conversion. Therefore sharing a ground is not a problem.

[silverdog]

Well, my supposition came frome post n°122 in this thread:
http://www.mechmate.com/forums/showt...&postcount=122
ciao

[smreish]

Sergio, if you wire it that way...your good.

I was concerned you were trying to put the M12 connectors and their power on the 7 core cable...which would leave you short a number of strands.
Personally, I prefer to run 12 core....those extra cores have saved my rear lately when a core breaks or I want to add that little "laser or duty purpose button" out on the gantry.

[Red_boards]

I'm contemplating making proximity target M1 18 027
It's listed as fitting to the car stopper M1 18 000, but I can't see where.
I'd appreciate some guidance on whether to make this part and where to install it.

Bright flash: These fit into the 20mm diam holes in the rails and can be turned to tune the proximity switches? In which case the correct higher level drawing for M1 18 027 are 10 10 246 and 10 20 246? If this is the case, do they get glued in place somehow? Won't vibration let them shift?

[Robert M]

Your « bright flash » will serve you right !
As for my opinion & method I’ve practice on this, I’ve made a smaller hole ( 18mm) hole on the underneath beams to serve as a “through hole stopper” for these eccentric discs . This way, this M1 18 027 will not fall through !
Then, in the rail, they can get fixed in place by means of “gluing then” with slow set silicone, after making sure they are in correct position.

Not something I can recall being exchange / discuss much here …. Or maybe I too getting affected by age

Amicalement, Robert

[smreish]

I real world use, I have found no real need for the disk if your not using the "auto squaring" feature in Mach. The open hole is for two things:
- homing machine
- end of travel "soft" limit

The homing can be resolved to an accurate position with home offsets in the settings in Mach3.

If you want to use the autosquare feature, then the holes in your rails will have to be "dialed" in as the disk/hole method suggests. It's a nice feature in the MM design, I just never really had a use for it.

Sean

[PEU]

Originally Posted by PEU

Today I tested this config with a PMDX125 BOB and it work as PMDX says, without extra interfacing, I used it for a spindle sensor and it worked like a charm at 1300RPM. Even the color cables were the same!

[JamesJ]

John (Zigmart) had some boards made based on the scematic in post #38 of this thread of which I purchased one. For whatever reason my proxies (AECO NPN NO purchased at factorymation.com) would not work correctly with the 74LS08 AND logic chip. I had to use a 74LS32 which uses OR logic in order for the proxies to trigger the correct output.

Any idea why that would be? Thanks.

BTW: Proxies rock! At least for me.

[zigmart]

Actually this is doing exactly what Mike designed it to do. He wanted all the inputs to be in an "open state" so when one sensed a target at the limits, it triggered the circuit ( 3 off, 1 on).

Unfortunately we want them to all be sensing metal for the whole run "closed circuit", switching to an open state at the limits. ( 3 on, 1 off)

Glad you found a work around, and I will be doing the same to get mine working.

Thanks

[JamesJ]

Tks, John. I had not considered that he would design for seeing targets instead of always seeing metal. By wanting to be able to sense the gantry lifting in addition to the home/limits I had to use "OR" logic. Luckily there was a "drop in" replacement IC with OR logic.

I plan to post some pictures tomorrow of my proxy board installation.

[Richards]

Sorry, guys. My Shopbot had targets. Your MechMates have holes. The designs are opposites. The 74LS32 chip uses "OR" logic which is opposite to the 74LS08's "AND" logic.

I replaced that design a long time ago with another design that uses a microcontroller. That little 20 pin computer allows me to use a jumper to select whether a proxy is normally On or whether it is normally Off.

[zigmart]

I would probably give you my first born to get ahold of that circuit (he has a strong back, and follows instructions most of the time, but he eats ALOT)

[Red_boards]

Here is more input on the wiring of proxies from Steve Stallings at PMDX. PMDX have a guide sheet pending, but it's been a while in arriving. My question reflects my lack of knowledge. Apologies to Sean for using his circuit as a comparison, but I was struggling to get my head around the wiring. I hope the info helps others.

> > Hi Steve,
> > I'm having trouble finding a
> > specific answer to wiring proximity sensors that I can use for homing
> > and limit switches (I've been anxiously awaiting the app-note for a
> > while now, but I realize that you have other things that need priority).
> >
> > I see in the PMDX 125 Users Manual that the NPN switches are set up in
> > parallel (Figure 4 in Section 6) and wired to a single input on J12 or
> > J13. However the best info I can find off the user forums is for a
> > series connection as in the attached pdf. (Note: the circuit diagram posted by Sean earlier in this thread)
> >
> > I just wanted to
> > confirm that the circuit shown in the Manual is correct.
> > (or are the two diagrams the same and I'm misreading the load?)

REPLY:
If you have NPN type sensors with N/O (normally open) outputs,
then the diagram in the PMDX-125 manual will work fine.

The diagram shown in your attachment also used NPN sensors with
N/O outputs, but added relays. This does provide isolation between
sensor power supply and the breakout board, but I do not know
why the builder decided this was necessary. His design will
work properly, but it is more complicated. Please note that
he must be using relays with normally closed contacts, even
though he does not say this on his diagram. He may have been
trying to follow the failsafe series wiring approach, but this
really does not work in his case since cutting a wire between
one of his sensors and the relay would go undetected as would
a failure of the 12 volt power supply. If he had used normally
closed sensors and normally open relays, still in series, then
the circuit would be failsafe.

You should also be aware that the circuit in the PMDX-125
manual produces an active low signal and the circuit in your
attachment produces an active high signal, so your CNC software
must be configured accordingly.

If you actually have a PMDX-126 instead of a PMDX-125, then
the above still applies, but you could also use PNP style
normally open sensors with the PMDX-126.

Normally closed electronic sensors such as proximity sensors
can also be used with any breakout board, but their outputs
cannot be directly wired in parallel or (for most sensors)
in series. If one had a supply of normally closed sensors,
then using relays would allow you to use more than one to
drive a single breakout board input.

Regards,
Steve Stallings
PMDX

[smreish]

Steve's notes are correct.
I choose to wire mine specifically this way so the interaction with the MM rail (so it detects a derail situation within my logic profile designed into my system) The machine will not operate in my scheme without all the sensors active and working.

Each works - just remember that the sensors are always ACTIVE during operation and fault out...like an estop

Good luck

Sean

[Red_boards]

Thanks Sean. Since I really don't follow these circuits I needed further clarification. I posted Steve's reply because I hoped it would help other people. Thank you for your clarification.

[Red_boards]

I have NPN N/O proximity sensors J12M-D4NK (it's possible that the label is incorrect and they are J12M-D2NK or J12-D2NK).They are rated 10-36V, 200mA and 4mm. My search on the code only turns up Chinese sites.

I have them wired to ports 10 and 11 and the 12V output on the PMDX board.
The "Autodetect" feature in Mach 3 /Config/Pins and Ports can to detect a single sensor and the home and limits work fine. When the sensor fails to see metal it triggers an error state.

But when I connect a second sensor in parallel as per the circuit in PEU's post of 18th Feb 2011, then the sensors fail to switch and Mach 3 error states are never triggered.

Is this situation familiar to anyone?

[JamesJ]

Red, I had, incorrectly, assumed that it would work with the PMDX-122 board. As soon as I paralleled two sensors it quit working correctly. What board are you using?

[Red_boards]

Jim,
PMDX-125

[bradm]

We're using proximity sensors to sense the absence of metal (the hole), so that's "not proximity". So the normal state of a "Normally/Open" proxy on a MechMate is "not normally open", known as closed, meaning that it's really a "Normally/Closed" sensor as we use it.

N/O switches are wired in parallel, but N/C switches are wired in series. So the reason that paralleled sensors aren't working is that one of them is always conducting, so you can't see the changes on the other one.

Proxies are a little more electrically complex that a simple switch, but they can be daisy chained with a little careful attention ( one example )