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Emergency Stop Wiring Question

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Cl(VII)

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OK, so only loosely rocketry related, but as I'm moving through designing my home brew CNC router I want to incorporate an Emergency Stop. A full on the power is off NOW kind of switch. Because I have seen that the capacitors in the 24V power supply are capable of driving the Arduino and steppers for a few seconds after power is disconnected I can't just put the panic button on the line in. Also, I want the router to lose power simultaneously. This pretty much necessitates using a relay. This is unfamiliar territory for me, so I drew up how I envision this below. Advice from someone who knows what they are talking about is extremely welcome. Below is also a link to the relay I envision using. This I am even less sure about than the wiring, so advice welcomed here as well.

Relay: http://www.mouser.com/ProductDetail/TE-Connectivity-PB/K10P-11D55-24/?qs=sGAEpiMZZMtSzCF3XBhmW0G7%2fNjY%2fkWBPkUMKb%252b4K50%3d

 
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markkoelsch

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Even with this, the motors will spin for a few seconds. I am not sure this really buys you anything that a normal power switch will not.
 

Cl(VII)

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Even with this, the motors will spin for a few seconds. I am not sure this really buys you anything that a normal power switch will not.
Why will the steppers continue to spin, Honestly, what have I missed? There is virtually no capacitance on the Arduino and gShield. Where will they get the power? I know the router will take a moment to spin down, but it is the steppers I am most worried about.
 

ttabbal

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Why will the steppers continue to spin, Honestly, what have I missed? There is virtually no capacitance on the Arduino and gShield. Where will they get the power? I know the router will take a moment to spin down, but it is the steppers I am most worried about.
I don't think the steppers will keep going. They require an active control signal to move. The power cutting off the controller will keep that from happening. Even with some capacitance on the control circuit, the steppers triggering once will drain that pretty fast. The smoothing capacitors in the main power supply are larger and could keep things running a little longer, so your design looks pretty good to me. As you noted, the router will have some inertia and take a moment to spin down. Less if the bit is in contact with material.
 

OverTheTop

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Is there an eStop function (possibly also shown as a limit function maybe?) input on the driver card that supplies the stepper? They quite often command the drives to drive to a halt and are possibly quicker than just cutting the 24V power.

I am guessing the rotational inertia of the drives is not very large anyway and even if you just de-power the drivers they would come to a halt relatively quickly.

The router bit is another matter entirely :)

A guy at work ran his router with a 1.5kW watercooled head into a holddown clamp. Big bang. Destroyed the spindle motor and VFD driving it :facepalm:
 

les

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Why will the steppers continue to spin, Honestly, what have I missed? There is virtually no capacitance on the Arduino and gShield. Where will they get the power? I know the router will take a moment to spin down, but it is the steppers I am most worried about.
It will depend on how fast the steppers were moving when you hit the Emergency Stop. They could have some rotational inertia.

I assume your emergency switch will be closed for the system to work, and when you hit the switch it will open the line?

Since the relay has 2 form C contacts you may want to open both AC lines to the router.

Otherwise everything, including the relay selection looks good. I see you are getting the style to mount to a printed circuit card. Are you designing this card or where are you getting it from?

Also, remember safety. You will be wiring high voltage. Make sure all connections are covered to prevent accidental contact.
 

Lowpuller

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The diagram as shown will not shut your router down.

You DO NOT WANT A RELAY IN YOUR EMERGENCY STOP CIRCUIT, it adds a failure point.

You need a latching button with one or more contacts that are normally open. Hitting the estop should remove power to the transformer and to the router itself.

However the estop will NOT REMOVE OR STOP any mechanical energy, and it will also not immediately eliminate any stored electricity such as in a capacitor.
 

SpaceManMat

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The diagram as shown will not shut your router down.

You DO NOT WANT A RELAY IN YOUR EMERGENCY STOP CIRCUIT, it adds a failure point.

You need a latching button with one or more contacts that are normally open. Hitting the estop should remove power to the transformer and to the router itself.

However the estop will NOT REMOVE OR STOP any mechanical energy, and it will also not immediately eliminate any stored electricity such as in a capacitor.
I know that shorting a DC motor will act as a break on the motor so will remove mechanical energy, does anyone know if this holds true for AC or Steeper motors? If so you could use a relay to both isolate and also short the motors when the estop cuts the power.
 

OverTheTop

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I know that shorting a DC motor will act as a break on the motor so will remove mechanical energy, does anyone know if this holds true for AC or Stepper motors?
Yes it does. Shorting across the windings of a stepper or a DC motor will greatly increase the braking torque on the shaft. The same applies to universal motors. Most AC hand-held power tool motors are "universal motors", which are basically a DC motor (with commutator, brushes etc) and a field winding in place of magnets. That field winding makes them universal. You can run them on AC, or DC (either flavour) at the required voltage and the motor will always run in the same direction :) .

If you are going to put a relay contact across the router make sure there is a resistor in series with it to limit the current. It might otherwise chew out the relay. Make sure the sequencing (power off, then apply short!) is correct also.

What do commercial CNC routers do for their eStop functionality? Could be a good indication if the feature is really necessary. A commercial unit is likely to have to comply with the Machinery Directive which has provisions for the safety of equipment operators enshrined in very dry reading... Worthy of consideration though.
 

Cl(VII)

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First off, thanks for the well thought out advice. I knew there would be several folks around here with the knowledge to advise on this.

Let me clarify a few things about what I am trying to achieve.

1) I want the steppers to loose power NOW.
2) There are pins for limit switches for all three axis on the gShield, and I will use them for that purpose. I could wire in a stop button to these pins as well, and it would stop the steppers. In doing it this way I was trying to get the power shutoff to the router as a bonus.
3) I could pause steppers from the laptop if I liked, but if something gets wonky I want a big red button to mash instead of needing to think.
4) Loosing power to the router is a bonus, and yes shorting the motors (steppers or router) would slow it down much quicker, but my confidence in designing or executing that is not high.
5) Yes, I will keep all of my connections clean, and covered to prevent accidental contacts.
6) Yes, switch normally open then off when pushed.

I am looking at this feature as a relatively cheap backup plan, and try to abide by the advice "don't let the perfect be the enemy of the good enough." This is only going to be a hobby size and power tool after all...my table saw doesn't have that detects flesh and breaks feature either, but that would be best practice no doubt.

Thanks again for the input.
 

markkoelsch

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Why will the steppers continue to spin, Honestly, what have I missed? There is virtually no capacitance on the Arduino and gShield. Where will they get the power? I know the router will take a moment to spin down, but it is the steppers I am most worried about.
Sorry, I meant the router itself. They usually will spin for a few seconds do momentum.
 

Cl(VII)

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Thanks for advice, answers and more questions in red.

It will depend on how fast the steppers were moving when you hit the Emergency Stop. They could have some rotational inertia. noted

I assume your emergency switch will be closed for the system to work, and when you hit the switch it will open the line? Yes

Since the relay has 2 form C contacts you may want to open both AC lines to the router. No reason not to, if I make the wires from the router the part that switches couldn't I ground the router motor this way when it flips from the 120V and ground?

Otherwise everything, including the relay selection looks good. I see you are getting the style to mount to a printed circuit card. Are you designing this card or where are you getting it from? Recomendations welcome, I would prefer a flange mount, or better yet a fully contained one with wire leads.

Also, remember safety. You will be wiring high voltage. Make sure all connections are covered to prevent accidental contact.Absolutely, not worth getting fried over.
 

ChrisAttebery

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On most CNC builds there is an e-stop input on the break out board. Once it's tripped the BOB will cut the inputs to the stepper drivers. You need to read the manuals for the stepper drivers and see if they can handle having their power cut while in use. Some can, some can't. You can use the same E-stop switch to trip a relay for the power to the spindle. If it's driven by a VFD make sure that if can handle having the power cut. It may have its own E-stop input.
 

rharshberger

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While you are at it I would recommend putting a magnetic latching power switch on the main power supply line, that way if you loose power for some reason the switch will automatically open and the machine will not restart without the ON being pressed again. I retrofit my tools with these type switches if they do not come with them they are cheap insurance. Something similar to this http://www.grizzly.com/products/120V-Magnetic-Switch/D4155
 
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Cl(VII)

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While you are at it I would recommend putting a magnetic latching power switch on the main power supply line, that way if you loose power for some reason the switch will automatically open and the machine will not restart without the ON being pressed again. I retrofit my tools with these type switches if they do not come with them they are cheap insurance. Something similar to this http://www.grizzly.com/products/120V-Magnetic-Switch/D4155
Would be a good addition to a set it and forget it type tool. Thanks.
 

Reinhard

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OK, so only loosely rocketry related, but as I'm moving through designing my home brew CNC router I want to incorporate an Emergency Stop. A full on the power is off NOW kind of switch. Because I have seen that the capacitors in the 24V power supply are capable of driving the Arduino and steppers for a few seconds after power is disconnected I can't just put the panic button on the line in. Also, I want the router to lose power simultaneously. This pretty much necessitates using a relay. This is unfamiliar territory for me, so I drew up how I envision this below. Advice from someone who knows what they are talking about is extremely welcome. Below is also a link to the relay I envision using. This I am even less sure about than the wiring, so advice welcomed here as well.
The schematics in the first post will work as intended, but I'd suggest doing it "backwards". Use a double-pole switch to fully disconnect the 120V line and use a relay with a 120V coil to disconnect the 24V line. The major advantage is, that it maximizes the scope of your emergency cut off. If your power supply starts smoking or, even worse, some error led to an electrocution hazard, a full disconnect of the system is the thing that you want to do.
As an added minor bonus, a power loss will lead to the same power down sequencing (but your power cord shouldn't be a tripping hazard in the first place ;-) ).


You DO NOT WANT A RELAY IN YOUR EMERGENCY STOP CIRCUIT, it adds a failure point.
This is standard industrial practice, although there are special "safety relays" used. Multiple emergency switches are often used for quick access from all directions. There are also safety interlocks, for example on things like access hatches. All these switches are usually connected in series and no matter which one is triggered, any potentially harmful source of energy (not only electricity but also pneumatic pressure etc.) is cut via the safety relay(s).


First off, thanks for the well thought out advice. I knew there would be several folks around here with the knowledge to advise on this.
1) I want the steppers to loose power NOW.
2) There are pins for limit switches for all three axis on the gShield, and I will use them for that purpose. I could wire in a stop button to these pins as well, and it would stop the steppers. In doing it this way I was trying to get the power shutoff to the router as a bonus.
If you really want to stop your steppers even harder and faster, you can short your steppers coils, as has been already mentioned. For a bipolar stepper, you need a double-pole double-throw (DPTP) relay. On end every coil, one end, lets call it 'A' gets directly connected to the stepper driver. The other end, 'B', gets connected to the "common" pin of the relay. The "normally open" (NO) pin gets connected to the stepper driver instead of 'B'. The "normally closed" (NC) pin of the relay gets connected to 'A'. Without power the relay, the motor is shorted and exhibits a much higher braking torque. You don't need additional resistors for this. The coils of the stepper motors will limit the current. You can play around with your steppers before you integrate them into your system. With open wires, you can usually turn the shaft with your fingers, with shortened wires you can't (depends a bit on the size of your motors). This active braking mechanism is, however, likely overkill and I would only consider them if you think your emergency switch does not stop it fast enough after some testing.


Sorry, I meant the router itself. They usually will spin for a few seconds do momentum.
In the most common error scenarios on a CNC mill ("Ooops, I didn't expect my program to do this"), it is advantageous if the axis drives stop before the spindle does. This will usually stop milling in a way that won't break your cutting tool. Scenarios were you want to stop the spindle as fast as possible independent of what the axis drives are doing are conceivable, but personally I haven't experienced them so far.

Reinhard
 

Cl(VII)

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NateLowrie

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At Fine Line Automation, we have traditionally configured our machines in one of 2 ways:

  • Often, the breakout board will have a E-Stop connection. We tend to always wire it here. The E-Stop connection is normally hardwired on the board to stop all motor outputs, VFD outputs and relay outputs.
  • If the board doesn't have an E-Stop (and it sounds like yours doesn't), my recommendation is to put your E-Stop inline on the AC side of the box. You need to size it bigger in most cases but this is the best route. Your industrial PLCs actually program the ladder logic this way and use a relay and start button in conjunction with the E-Stop. I wouldn't worry about leftover power as the effect should be pretty instanteous.

If you want me to provide a wiring diagram I can. Also, NEVER rely on just the software E-Stop.

Make sure you wire the E-Stop as a Normally Closed switch. The reason for this is safety. If a wire breaks or becomes loose, the E-Stop will trigger if wired as a NC switch. If you wire it Normally Open then that broken wire never registers and the E-Stop fails to function. Same goes for the limit switches.

If you need help with your machine, PM or email me or give the shop a call.

Regards,

Nate
Owner, Fine Line Automation
 

Cl(VII)

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Thanks very much for all the input and offers of more help. I think I have boiled this down to a workable solution that is similar, but not exactly like the one above. I have some parts on order, and will rig the entire thing up for testing, first with a multi-meter standing in for the router, then with the router in place before I finalize anything.
 
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