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  #1  
Old Mon 06 November 2006, 00:24
fabrica
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Limit / Home switches - Mechanical or Proxy?

I inquired from My Engineer about the prox switch matter. This is what he had to say.

"Two Prox switches to be mounted on each side of the X and Y rails. All together 4 prox switches. (Two switches to be mounted on each rail is to ensure that If one fails the other would do the job). All these prox switches to be connected in series with each other. The switching position of the prox switches to be in a Normally closed position (NC). The Primary side winding of the Torroidal transformer to be energised through these switches".

When selecting prox switches consideration should be given to the current and flowing through the switches to feed the primary side.

Once the motors stop how are you going to reverse the motors?.
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  #2  
Old Mon 06 November 2006, 01:03
Gerald_D
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Fabrica asked in another thread yesterday where to get the proximity switches that are marked as optional on the drawings.

I replied:
"For the proxies, I must admit that I am not using any at all. And I am having difficulty to see how they can be connected in series in a practical failsafe way. We rely on the "soft-limits" in the Mach software to know when the cars are near the end of the rails. Although I have target holes in the rails, I really don't know yet what the best proxy will be.

What are you doing for a control box? Buying ready-made or building your own?"


He replied that he will forget the proxies for a while and that he has somebody building a control box for him.

I replied:
"With the proxy switches, it is easy to connect them to switch off the power if the cars move too far. My problem was to find a way to connect proxy's to tell the PC that it must do a gentle stop and not lose its position in memory. (the cheap click/click type switches can easily be connected, but they are not so well sealed against sawdust)

Ask the guy who is doing your control box if he knows how to use these proxy's (the round one with 2 nuts) to switch off the "coil"."

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  #3  
Old Mon 06 November 2006, 01:15
Gerald_D
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Okay, we are coming to a common understanding now, and we can fill in the details later. The basic question for you today is where to get what proximity switch. Does your engineer recognise those switches as being available in Sri Lanka? (I am very sure they will be, but the prices may be crazy) Google
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  #4  
Old Mon 06 November 2006, 01:32
Gerald_D
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On the technical side, the proximity switches can ride on the cars just above the rails (1mm above the rail) While the switch "sees" the rail it could close a relay which make the toroidal transformer switch on. When the car goes too far, the proxy switch now sees the hole and the relay switches off.

Also, if the car gets jammed and it lifts off the rail then the relay will also switch of. The proxy will switch as soon as iron comes inside 2mm of its nose - take the iron further away and it switches off. The beauty of proxies are that they are water and dustproof and have no moving or wearing parts. But, a big snag is that they are difficult to connect in series.....

Remember that a lot of people don't use any switches. The computer knows where the car is, and if the car does sometimes hit the end-stop it is not serious.

Up to here we have spoken of "Limit" switches. Now, we need to speak of another concept..."Home" switches. (You will see both mentioned in the Mach3 software)

A home switch is used for referencing the cars to an exact position - for example the corner of the table where x,y is zero,zero (0,0). We have also never used a home switch.

The Mach3 software allows the limit and home switches to be the same switch.

If you use mechanical switches (microswitches), they are very easily connected in series, but they rely on moving parts which have to stay clean.

A possible solution is to give a sealed relay to each proximity switch then it is easy to put the relays in series. Just a bit more wiring...and a few more things which can come loose....

I would seriously suggest to start using the MechMate without any switches. You learn to ride a bicycle quicker if you don't have those extra little training wheels.

Reversing the motors is taken care of by Mach3. Realise that Mach3 will "trip" if a limit is hit, and it will require a manual reset.
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  #5  
Old Fri 10 November 2006, 19:26
vadeem
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Another link to prox switches at $19.99 USD

http://www.automation4less.com/store...s.asp?cat=1091
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  #6  
Old Sun 12 November 2006, 01:39
Dick van Randen
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Any reason why magnetic reed switchs wouldn't be suitable as a choice? Dust proof , relatively cheap and reliable , millions of homes and companies use them as part of their security systems.

They are available NO or NC
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  #7  
Old Sun 12 November 2006, 12:04
Gerald_D
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Dick, they don't work so well when mounted on steel parts (the magnetic fields get distorted) and switching distance is not consistent. Another snag is that the magnet collects scrap and this jams up the system.
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  #8  
Old Sun 12 November 2006, 13:12
Dick van Randen
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As a limit switch they just need to switch, a homing switch would be something entirely different. Locating the limit switch would be all important. Did a bit of looking, they make units designed for steel doors and mounting on floors. The inconsistent distance is true, but seems to between approaching and retreating from the magnet. 25mm approach 35mm retreat

I would be more inclined to using a base coordinate and working coordinate system with one set of good homing switches (mechanical) and use the softwares limit checking and file checking to prevent crashes.

I know SB3 has the ability ,ZT and ST. Does MACH3 ?
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  #9  
Old Sun 12 November 2006, 17:49
Mike Richards
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Reed relays have the bad habit of latching (welding) when they need to carry more than a few microamps. Since most logic circuits require 4-10 milliamps (1,000 times more current), I stopped using them in my designs many years ago.
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  #10  
Old Sun 12 November 2006, 21:57
Gerald_D
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I don't know the ZT and ST functions of SB3, but G-Code (and thus Mach3) has good systems of base and working coordinate systems. You can tell Mach3, for example, to move no closer than 10mm to the table limits, except when homing. Typically, the home and limit switch in Mach3 is the same switch - because they are used in different ways, Mach3 knows how to treat them separately.
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  #11  
Old Sun 26 November 2006, 19:17
fabrica
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Have decided anything on the prox switches. Wether it should be NPN or PNP, Sensing distance, NC or No.
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  #12  
Old Sun 26 November 2006, 21:53
Gerald_D
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Mike, Dick, you guys know more about these things - what would be the better choice?

The drawings make provision for an optional 12mm barrel proximity switch looking down onto the rails. When the rail is present, the situation is healthy and this is the "Normal" condition. If the switch passes a hole at the end of the rail, it must change to "Fault" condition. (also if the car lifts off the rail).

My personal thought is that each proximity drives a small ChangeOver relay - so that a set of "potential-free" contacts are available for a safety series circuit. Therefore I would pick a Normally Open sensor that would go Closed when on the rail and close the relay - any loose connection will break the circuit. Switching distance is not important because the switch will have a precise ride on the rail. The last question is NPN or PNP....?
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  #13  
Old Sun 26 November 2006, 23:19
Gerald_D
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Also, proximity switches typically need a power supply of 10 to 30 Volt DC. The control box I've used so far doesn't have this voltage available.

I have used a 9V transformer to supply AC to the PMDX card. However, the PMDX can be supplied with 7 to 12 V DC. Therefore, if one is going to use typical barrel proximities, it makes sense to get a 12VDC power supply (500mA or more) and supply 3 proximity switches and 1 PMDX-122 card from it. (samples)

Regarding a relay for each proximity, the relay coil must then be rated for 12VDC and must use less than 100mA to switch. The relay contacts only need to handle a tiny current...

All of this is guesswork on my part....I havn't tried it!
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  #14  
Old Mon 27 November 2006, 09:20
Mike Richards
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The NPN, PNP question is not easily answered. I spent about an hour with a local supplier a few months ago who sold 12mm proximity sensors. He couldn't get the information that I needed, so I put off buying the sensors.

Basically, an NPN transitor is turned on when you put some voltage to the BASE of the transistor (the amount of voltage depends on how you've built the circuit, but 2.5 volts minimum usually works fine in simple switching circuits). On the other hand, a PNP transistor is turned on when the BASE of the transistor is at ground potential and then turns off when voltage is applied to the BASE.

In a proximity sensor, normally you have no connection to the BASE, so it depends on how the circuit works whether you need an NPN or PNP type sensor. Campbell products specify NPN devices. The sample circuit in the PMDX-122 manual (figure 3, section 2.4) also shows an NPN transistor as part of the Opto-coupler circuit.

Like you, Gerald, I would want the proximity sensor to be active until it sensed a 'hole' or absense of metal in your design, or a bolt head on my Alpha. Maybe that is the question that you have to ask the supplier, whether the output of the proximity sensor is on or off when the sensor senses metal.

I'll try to pick up a sensor today and put it into a test circuit.
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  #15  
Old Mon 27 November 2006, 10:16
Gerald_D
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Mike, I have a choice of 4 variants:
PNP NO, PNP NC, NPN NO, NPN NC - all the same price and availability. (all 3 wire sensors)

The "normal" in the NC or NO is apparently when no metal is nearby, ie. nothing to sense, no target.

I would really appreciate your little test.
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  #16  
Old Mon 27 November 2006, 10:33
Gerald_D
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Mike, just to make it clear, I don't think I want to connect the 3 (or more) proxies direct to the PMDX. They need to form a series chain and you can't do that with proxies unless you have lots of voltage and matching proxies. So I forsee a small relay for each proxy. Suppose then that the PNP/NPN issue becomes moot?
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  #17  
Old Mon 27 November 2006, 10:38
ralph hampton
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Hi,
I am playing with these things too. I have a 5-60v NO type with 2 wires. If I put 12v accross the 2 wires and bring it close to steel it lights up. Havn't a clue what I'm doing mind...
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  #18  
Old Mon 27 November 2006, 12:12
Mike Richards
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Gerald,
Here's what I've found. I've tried two 12mm sensors with a gap distance of 4mm. The Balluff part number for the NPN part is BES M12MI-NSC40B-S04G. The Balluff part number for the PNP part is BES M12MI-PSC40B-S04G. Both parts require a quick disconnect cable. I used the MURR 7000-12221-0140300. Total cost for two sensors and two cables is about $150 - but they are rugged, industrial quality parts, and they have a gap distance of 4mm instead of the much more common 2mm.

On the NPN device with the cable attached, Pin 1 (Brown) is connected directly to +12VDC (the device requires 12 to 30 VDC to operate). Pin 4 (Black) is connected to one end of a 20K 1/4-watt resistor. The other end of the resistor is connected to +12 VDC. Pin 3 (Blue) is connected to Ground. With power on, and no metal near the proximity sensor, the voltage between Pin 4 and Pin 3 is 12 VDC. When metal comes within 4mm of the sensor, the voltage between Pin 4 and Pin 3 drops to 0.7 VDC (and the built-in Yellow LED in the sensor turns on).

On the PNP device with the cable attached, Pin 1 (Brown) is connected directly to +12 VDC. Pin 4 (Black) is connected to one end of a 20K 1/4-watt resistor. The other end of the resistor is connected to Ground. Pin 3 (Blue) is connected to Ground. With power on, and no metal near the proximity sensor, the voltage between Pin 4 and Pin 3 is 0 VDC. When metal comes within 4mm of the sensor, the voltage between Pin 4 and Pin 3 drops to 10 VDC (and the built-in Yellow LED in the sensor turns on).

The 20K resistor was selected because it was the first resistor that I found that I knew would work. Other values (depending on the voltage used) could be substituted. A 4.7K resistor would work better. (Edited: I just tried resistor values between 1K and 20K, all 1/4-watt. On paper, they should have all worked perfectly, and, indeed, they all worked perfectly. The math to determine which resistor or coil rating to use is very basic: You would divide the PS voltage by the resistance of the resistor or coil to find the current flowing through the circuit and then multiply the current by the PS voltage to find the wattage requirement of the resistor or coil. For example, with a 12 VDC power supply, a 1K resistor passes about 12ma of current and requires at least a 0.14 watt resistor. If I used a 1K resistor, and if the circuit was built so that current was normally flowing through the resistor, I would use a 1/2-watt rated resistor so that heat would never become an issue.)

That's the raw data. Instead of using a 20K resistor, the sensors could turn on the coil of a relay or drive some other electronic circuit. The choice really comes down to whether you want voltage when the sensor senses metal (PNP) or you want voltage when the sensor does NOT sense metal (NPN).

Personally, if I were designing an interface with these proximity sensors, I could add some TIL-111 type opto-couplers to allow an easy interface to a 5V circuit. Then, I would use some AND TTL gates or some OR TTL gates depending on whether I wanted the circuit to detect one signal line going 'off' (AND gates), or whether I wanted to detect one signal line going 'on' (OR gates).
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  #19  
Old Mon 27 November 2006, 15:22
Mike Richards
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Gerald,
Now that I've had a chance to run the test, posted above, and a few hours to think about limit switches in general, my preference would be either an NPN-NC or a PNP-NC circuit (again the preference between NPN and PNP is determined whether you need voltage absent when the sensor is active (NPN - sinking circuit) or voltage present when the sensor is active (PNP - sourcing circuit). The reason that I would want a Normally Closed circuit is simply to verify that the proximity sensor is actually working when power is applied. If the proximity sensor was not working, the controller would indicate that at least one of the limit sensors had been tripped.

On the other hand, with a Normally Open circuit, you wouldn't know if the circuit worked until you ran a 'homing' test. If the sensor was NOT working, your only non-visual indication that something was wrong would be when the stop blocks at the end of the axis was hit - not a comforting thought. In fact, the reason that I first looked at 12mm proximity sensors was to build a fail-safe sensor for my Alpha.

Also, I would only use a relay if no other option were possible. The problem with a relay in a 'homing' circuit is the variable delay that is inherent in a mechanical relay's design. For pactical purposes, a 50ms variation probably wouldn't make any real difference when you make a slow, final approach to the sensor, but the possibility for error is literally built into a relay. Of course, building a solid state sensor circuit requires some wiring, at the minimum, and mostly likely a circuit board. That's certainly not difficult for a simple circuit, but probably beyond the comfort zone of most CNC operators.

(Edited: Your design, where the sensor is placed close to the rail, would require the Normally Open configuration, because the sensor would be active automatically when it sensed the rail. It would become inactive when it sensed the 'hole' in the rail. Your circuit would already be fail-safe with a N/O circuit. The same basic theory would work on my Alpha if I laid a strip of metal along the rail under the sensor with the ends of the metal strip placed so that the axis end points were reached when the sensor turned OFF.)
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  #20  
Old Mon 27 November 2006, 22:00
Gerald_D
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Thanks Mike, you had me rattled until I read your last edit, where you agree on N/O being the best option for the MechMate style rail. My priority for the proxy is fault detection and not calibration, therefore "homing" is a secondary consideration. There are 2 types of faults I want to catch with the proxies:
- end of travel, before a car bangs against the stops and the stepper is forced to jump steps, and
- a car lifting off a rail, like when the z is told to dive too deep.
Relay logic appeals to me for the above.

(You could drill only 10mm diameter holes in your Alpha rails to serve as targets - that is big enough to trigger a 12mm proxy. The huge holes in the MechMate rail are for another reason......)
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  #21  
Old Wed 29 November 2006, 01:32
Dick van Randen
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Fabrica's engineer wrote

"Two Prox switches to be mounted on each side of the X and Y rails. All together 4 prox switches. (Two switches to be mounted on each rail is to ensure that If one fails the other would do the job). All these prox switches to be connected in series with each other. The switching position of the prox switches to be in a Normally closed position (NC).
The Primary side winding of the Torroidal transformer to be energised through these switches".

I don't think that is a good idea, it can be done but you would need to use armoured cable or conduit and proper switch boxes to protect the cables with line voltage going to each end of the table. It all adds to the complexity and expense. Stick with low voltage sensing and use the software driving the steppers to do an emergency stop.

Using proximity switches is fine. However, NC mechanical switches may work just as well as long as they stay dry. I have been using NO switches for more than a year and dust builds up on them regularly, so far no problems and they cost all of $5 to buy. A cheap place to start. Added bonus is they are surprisingly accurate, better than .005 inch error returning to where they started from.
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  #22  
Old Wed 29 November 2006, 06:14
Mike Richards
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Yesterday I was going to post something about having limit switches that interrupt power to the machine as well as homing switches that indicate a known location on the machine. When I read the post from Fabrica's engineer, I realized that I'm not the only one thinking along those lines. Like Fabrica's engineer, I'm not talking about little micro switches, but heavy-duty switches similar to Honeywell's EN50041/47 Global Limit Switch that can carry a lot of current.

It seems that putting limit switches,that are normally closed, at the ends of each axis, would be the more proper way to protect the machine. If the power being supplied to the stepper/servo drivers was routed through those switches and the switches were configured in an AND configuration, so that all switches had to be closed for the circuit to work, then, any time a limit was reached, the power supplying the steppers/servos would be cut, and the machine would stop before crashing into the stop blocks. (Now that I've installed 3:1 gearing on my Alpha, crashing into the stop blocks is a much more serious situation.)

Since only the line carrying power to the motor drivers needs to be interrupted, we're probably talking fairly modest current, certainly less than 20-amps and more likely less than 5-amps. At my test bench where I really punish a variety of steppers and servo motors, my 120VAC line is a standard 16-guage extension cord. Running a single 12 or 14 guage stranded cable through a series of switches would certainly not be any more difficult than running power to the spindle or router.

With heavy duty limit switches in place, proximity switches could then be used to 'home' the machine. Not to get too complex, but while I was traveling the 100 miles to a computer customer's site, I started thinking about using a little $2.00 Atmel 8051 type micro-controller as an interface to the 12mm proximity sensors that I tested earlier. With a trivial amount of programming and a total of $10 worth of parts (crystal, auto-reset chip, opto-couplers, resistors, LEDs, etc.) a first-class interface could be built. Even the cheapest of those little micro-controllers can run 1,000,000 instructions per second, so running a tight control loop would be instantaneous, for all practical purposes. An excellent assembler (ASEM-51) is available as freeware. I downloaded the Linux version yesterday and found that it is just as capable as the Intel-51 assembler that I used back 'in the day'.

Maybe what I'm really trying to say is that if we step back away from our favorite machine for just a minute and then ask ourselves how we would build a new machine if we didn't already have a model to start from, we might come up with some innovative ideas that are both practical and useful. I know, that for me at least, I hardly ever re-visit fundament concepts once I've found something that works.
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  #23  
Old Wed 29 November 2006, 06:21
ralph hampton
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Dick,
Glad to read your post. I assume you have them connected to a shopbot control box. Can you confirm my proposed starting point of connecting brown to 5v, blue to an input(3) and as mine are 2 wire NO 5-60v, I assume the earth will run back through the body and my earthed shopbot. I am not an electronics geek, and don't want to blow anything up through misadventure. The software bit seems pretty easy.

Thanks,

R.
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  #24  
Old Wed 29 November 2006, 06:47
Gerald_D
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It is good to step back a bit and look at the wide perspective. Probably the biggest reasons why we don't have any switches on our two routers are because:
1. All the wiring of those switches will make perfect antennae for receiving interference.
2. No single style of switch (mechanical or proxy) is the obvious perfect choice. Proxies great for dirt, mechanicals great for positive click/click.
3. And probably most importantly....we manage pretty well without any switches at all.

I made provision in the design for fitting 12mm proxies, because proxies are what the ShopBot mindset wanted. I am also looking for a mechanical switch that will fit on the same bracket, but havn't found anything as internationally standardised as a 12mm proxy. Personally I am a lot more comfortable to install and fault-find mechanical click-click switches.

If one puts the (toroid) power supply current through mechanical switches, you can't drive the system back after "homing". In this case there will have to be two circuits - homing on the control inputs, and limit switches on the mains power. More wires, more antennae, more loops...........
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  #25  
Old Wed 29 November 2006, 06:59
ralph hampton
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I actually just want to use mine for repeatable XY homing, so can put them anywhere in the table (fitting convenience), run them to a non-stopping input switch and play with the software till it works right.

R.
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  #26  
Old Wed 29 November 2006, 08:02
Mike Richards
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Gerald,
You're right. The mechanical switches that I wrote about are not homing switches, but limit switches. When any one of them opens, the motors stop. Their only purpose is to keep the motors from driving the gantries into the stops. If, in fact, a switch became active (open), you would have to manually push the gantry far enough to close the switch. That brings up another problem. whatever mechanical device is used to push against the switch's actuator cannot let the actuator 'coast' past the mechanical limit. In other words, once power is cut, it has to stay cut until the gantry is manually moved to a safe place.

On the other hand, the proximity switches would work just as they do now on my Alpha. That is, they would have a role as a 'homing' switch and also as a limit switch. The Shopbot software allows you to jog off of the proximity switch after a short delay.

Of course, what I preach and what I practice are two different things. On my Alpha, I only have one target mounted on the X-axis and one target mounted on the Y-axis. Both targets are near the 0,0 point on the table. Because I have a 120x60 model and mostly use it for 96x48 material, I have a rather large buffer area around my working area. Now that I have some good, heavy duty, robust 12mm proximity switches, I'll probably at least mount targets on both ends of each axis. The heavy duty mechanical switches are 'for future consideration'.
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  #27  
Old Wed 29 November 2006, 08:16
Gerald_D
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Ralph, remember this thread on that old forum?
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  #28  
Old Wed 29 November 2006, 09:21
ralph hampton
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I do.
I have to say I miss the old "alternative" shopbot forum - in fact it is still called hotbot on my bookmarks - for maverick shopbotters who want to have highly radical thoughts and schemes away from the crowd. The sort of folk who in their next lifetime will undoubtedly be building orbital space stations off Mars...

R.
(as much as I admire the Mechmate, I am definitely with shopbot for the forseeable present)
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  #29  
Old Thu 30 November 2006, 01:29
Dick van Randen
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Hello Ralph

To connect your NO limit switches wire them in parallel and connect to Port 3 and GND (NOT 5VOLTS) . The limit switches are wired in a similar fashion to the Zero plate. I recommend you use Shield cable, the foil kind with a bare gnd trace wire works very well. Ground the cable at one point only, either in the box or on the table , if you are chaining switches together (like outdoor christmas lights) splice the gnd trace wires back together to continue shielding to the next switch.

Check that Input Switch #3 Mode is " 1 - as Limit Switch ". Thats found in Values near the Bottom , iNput Switch Modes.

Once you get that working there is a document that might be of interest. On your SB computer in Program Files\ShopBot\ShopBot 3\Help\Table Base Coordinates.pdf
Bill Young has written an explanation on how to implement "soft" limit checking using just two switches to protect the table limits. I haven't tried this myself, because I haven't had too yet.
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  #30  
Old Thu 30 November 2006, 16:27
Mike Richards
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This discussion about proximity sensors and limit switches gave me a chance to go back a few years when I first started playing with micro-controllers and process control. Back then, the chips were expensive. Compilers and software tools cost as much as a new car. In other words, the project had to have major funding before I would use a micro-controller. However, after scrounging through my parts cabinet, I found a cache of AT89C2051 chips, which are 20-pin 8051 type microcontrollers with flash memory. In the same box I found a bunch of 11.059mhz crystals, some TI reset chips and various other goodies. After finding the chips, I scoured the Internet until I found some good compilers, debuggers, simulators, etc. What I found amazed me. Anyone can take a few chips, costing less than $10 and build an I/O interface to connect any kind of proximity switch/limit switch to any kind of controller. The 80C2051 has 15-I/O lines which can be split between input or output with the software. That means that I can have three proximity switches (X-axis, Y-axis, Z-axis), two Emergency Stop switches, five LEDs for status on the switches, a fail-safe timer/LED/reset circuit, two lines to my Shopbot controller box and still have two unused I/O lines. The code to make everything work took about an hour to write and debug (sorry, but I had to relearn 8051 assembly code). The end result, is that after a little more than an hour, I had a software simulator showing that $10 worth of over-the-counter chips could interface the outside world to the controller card. Perhaps the best thing is, that if anything changed, a simple software fix could handle things.

I guess what I'm trying to say is that good hardware/mechanics can sometimes be greatly improved with a little electronic wizardry. The hardware/mechanics make everything work, but the electronics can make hooking things together simple.

If anyone would like to try some of this, send me an email (miker@xmission.com) and I'll be happy to send you a list of websites and/or component recommendations that might help you along. Be advised, however, that I'm not offering to design or build you a custom-made circuit board or to write custom code for you. I've done that enough that it's no longer fun for me. If you're willing to do the work, I'll be happy to help you along.
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