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  1. #41
    My breakout board has a charge pump output that I'm currently not using. Mach3 is using it but I think I will look at incorporating it into the hardware stop circuit as your drawing shows.
    Also, and this is very important but open to interpretation, I am not removing power from the spindle inverter during emergency stop, I mentioned this is another post. The reason is that the inverter can stop the spindle faster than it will stop on it's own, it does this by applying dynamic braking. Other options for bringing the spindle to a timely halt would be a spring loaded mechanical brake or as in the case of my table saw, a DC injection braking system. Removing power to the inverter via a contactor is the obvious emergency stop strategy and it could be said that removing power from the inverter under load will damage it, that might be true but better to have a damaged inverter than a damaged limb. According to the machinery Regs. which won't apply in a private houshold situation, the spindle should stop within 10 seconds, now I've not had my spindle up to speed yet so time will tell how long it takes to stop on it's own.
    Last edited by EddyCurrent; 14-11-2013 at 03:20 PM.

  2. #42
    Quote Originally Posted by EddyCurrent View Post
    Also, and this is very important but open to interpretation, I am not removing power from the spindle inverter during emergency stop, I mentioned this is another post. The reason is that the inverter can stop the spindle faster than it will stop on it's own, it does this by applying dynamic braking. Other options for bringing the spindle to a timely halt would be a spring loaded mechanical brake or as in the case of my table saw, a DC injection braking system. Removing power to the inverter via a contactor is the obvious emergency stop strategy and it could be said that removing power from the inverter under load will damage it, that might be true but better to have a damaged inverter than a damaged limb. According to the machinery Regs. which won't apply in a private houshold situation, the spindle should stop within 10 seconds, now I've not had my spindle up to speed yet so time will tell how long it takes to stop on it's own.
    Good info, thanks.
    If I had your VFD rather than my cheap Chinese one, I might be more inclined to trust it to stop the spindle during an Estop as well.

    Quote Originally Posted by EddyCurrent View Post
    Removing power to the inverter via a contactor is the obvious emergency stop strategy and it could be said that removing power from the inverter under load will damage it, that might be true but better to have a damaged inverter than a damaged limb.
    Why would the inverter be damaged in this situation? Is it because once the power is cut to the inverter the spindle acts as a generator and this generated current can damage the inverter?

    I would like to have a safety system that does not compromise at all on safety but has the best chance on leaving the equipment undamaged (also how do you test an estop circuit if testing it may damage the equipment?!?).

    I'm not that keen on the mechanical breaking solution, so that leaves the following:-

    EStop
    • Dynamic/Rheostatic braking
      Cut cables from inverter to spindle and attach spindle to a bank of breaking resisters to apply dynamic breaking (is this even how it works!?!). At same time cut the run cable to the inverter.
    • DC injection braking system
      Same as above but DC injection brake cuts the link from inverter to spindle and applies DC to the spindle to stop it.


    I like the idea of the DC injection braking system but they seem pretty expensive (RS DC Injection Brake @ ~160 with VAT!). I wonder how easy/cheap it would be to do dynamic braking (without the vfd)?

    edit - Just noticed my Huanyang vfd has terminals to add a braking resistor. If power is removed from the inverter would this still be used/effective? would it cause damage to the inverter?
    Last edited by cncJim; 14-11-2013 at 05:04 PM.

  3. #43
    Some reading.

    LPC Siemens Micromaster440
    HMK Direct High Performance Drives and Positioning Systems
    Motor drives & safety interlock circuits: AC Drives: FAQ (part 3)
    Variable Speed Drives - E-stop

    My conclusion is that when someone hits the emergency stop button the drive control signals must instantly tell the drive to stop and after a short delay the power is removed from the drive input via a suitable contactor. A braking resistor would be used to slow down the load as quickly as possible but as far as I'm aware it would only be operational if the drive was powered up.
    As it happens the ABB drive I'm using has the STO function built in. I'll add a new drawing ASAP.
    Last edited by EddyCurrent; 14-11-2013 at 06:26 PM.

  4. #44
    Ok lets have a reality check.!!

    The spindle is a low powered affair, even at 2.2Kw and if the shit hit's the fan then reality is the situation will be one of just few things gone wrong.
    E-stop is hit due to Limit or some other none spindle related condition then doesn't really matter how long spindle takes to slow down and coasting to a stop isn't long or problem.
    E-stop is hit due to tool sticking etc then bloody thing will have stopped anyway or will grind to a halt dam quick.!!
    E-stop is hit because you have been stupid enough to stick a hand or some other bodily part in the spindle then Yes you'll want the Spindle to stop quick has possibly but again your flesh or clothing will do a good job of acting has a braking and the spindle isn't powerful and doesn't have massive torque so will stall.!! . . So what I'm saying is the VFD's built in DC braking will be enough no resistor required.

    Now regards the Kill power or Kill signal to VFD then I used to be Kill Signal guy but I'm now I Kill all power. Reason being what happens if the signal breaks or Even the VFD goes faulty.? Kill power and all doubt is removed. In practice the reality is the spindle will be jammed, either in material or flesh or in a situation where doesn't affect safety and the DC braking allowed without need for a resistor is more than enough.

    For DIY use then it's more than good enough to just kill power and Let the spindle coast.
    Last edited by JAZZCNC; 14-11-2013 at 11:59 PM.

  5. #45
    Dean, you are correct in that every situation has to be risk assessed on it's own.
    I also think the braking resistor is not required because of the fairly low inertia of the load.
    I agree that for DIY either strategy is fine but maybe not everyone reading this is in a DIY situation, in which case they should be referring to the relevant regulations.
    The main aim of this is to get a good emergency stop system using the gear we've got.

    It would be useful if we could define some scenarios and strategies for stopping a cnc machine, your input is valuable here.

    I've listed some ways to stop the machine and the strategy to adopt, I'm not saying it's comprehensive so please add or amend as you think.

    *emergency stop button pressed - strategy 1 or 2
    *limit switch activated (not home limits) - strategy 2
    *spindle over temperature switch activated - strategy 3
    *Stepper driver alarm relay activated (e.g. AM882) - strategy 3
    *'Stop' button pressed - strategy 4
    *Charge Pump error - strategy 1 or 2

    Strategy 1
    ----------
    * remove power to all devices immediately

    Strategy 2
    ----------
    * remove power to stepper drives immediately
    * remove power to ancillary devices immediately e.g. air, water, dust extraction
    * issue stop command to VFD then after delay remove power to VFD

    Strategy 3
    ----------
    * issue audio visual alarm before taking action (with time delay)
    * controlled stopping of all machine actuators and ancillary devices
    * leave power supply on to VFD

    Strategy 4
    ----------
    * controlled stopping of all machine actuators and ancillary devices
    * leave power supply on to VFD
    Last edited by EddyCurrent; 14-11-2013 at 09:06 PM.

  6. #46
    Quote Originally Posted by EddyCurrent
    Removing power to the inverter via a contactor is the obvious emergency stop strategy and it could be said that removing power from the inverter under load will damage it, that might be true but better to have a damaged inverter than a damaged limb.
    Unlikely to cause significant damage to your limbs due to spindle's low inertia, so personally I'd rather it damaged the limb as that'll get fixed for free on the NHS, unlike the VFD.

    Quote Originally Posted by cncJim View Post
    Why would the inverter be damaged in this situation? Is it because once the power is cut to the inverter the spindle acts as a generator and this generated current can damage the inverter?
    Vaguely.

    Quote Originally Posted by cncJim View Post
    Just noticed my Huanyang vfd has terminals to add a braking resistor. If power is removed from the inverter would this still be used/effective? would it cause damage to the inverter?
    If you remove the power, the inverter can't do anything as it will only remain switched on for less than 1/10th of a second before the capacitors discharge. So no, it wont apply the brake.

    Quote Originally Posted by cncJim View Post
    I would like to have a safety system that does not compromise at all on safety but has the best chance on leaving the equipment undamaged
    In that case you need to separate 'fault conditions' from safety concerns - e.g hitting a limit switch or stepper driver fault signals do not pose safety hazards, so treat them as such and wire it so the machine just pauses. The only one that warrants fast and complete shutdown is pressing the e-stop, as that's what you can supposedly do if you're about to get hurt.

    Quote Originally Posted by JAZZCNC View Post
    Now regards the Kill power or Kill signal to VFD then I used to be Kill Signal guy but I'm now I Kill all power. Reason being what happens if the signal breaks or Even the VFD goes faulty.?
    The signal should be wired such that if the wire's broken or otherwise interfered with the signal changes state and the system stops. The VFD going faulty sounds exceptionally unlikely, but if you're worried about that then just have a timed relay to switch the VFD power off a couple of seconds after the signal (as mentioned earlier), since by that point the spindle will have stopped so you wont damage the VFD by cutting the power, and if it's not stopped then the VFD has gone faulty so you probably don't care about risking damage to it by cutting the power.

    Quote Originally Posted by JAZZCNC View Post
    E-stop is hit due to Limit or some other none spindle related condition then doesn't really matter how long spindle takes to slow down and coasting to a stop isn't long or problem.
    My new spindle takes 107 seconds to stop from full speed if left to coast. I tend to agree that it's not really a problem though.

    Quote Originally Posted by EddyCurrent
    *emergency stop button pressed - strategy 1 or 2
    Two, see above. You could take advantage of the enable signal on the stepper drivers to disable them, then link cutting the power to the delay associated with the VFD.

    Quote Originally Posted by EddyCurrent
    I also think the braking resistor is not required because of the fairly low inertia of the load.
    Yes - the internal resistor is sufficient to stop the 2.2kW spindles in less than a second, so you can easily implement the above method.

    (3000th post... )
    Last edited by Jonathan; 15-11-2013 at 01:00 AM.

  7. #47
    Quote Originally Posted by EddyCurrent View Post
    I agree that for DIY either strategy is fine but maybe not everyone reading this is in a DIY situation, in which case they should be referring to the relevant regulations.
    The main aim of this is to get a good emergency stop system using the gear we've got.

    It would be useful if we could define some scenarios and strategies for stopping a cnc machine, your input is valuable here.
    Yes Agree in industrial situation then machine needs to be safe and Regs followed to keep the HSE police happy but mostly folks here are DIY and there's world of difference between Reg compliant and being used safely in a shed.!! . . . Practical common sense is all that's needed IMO.

    Regards Strategy's then here's my take.!! (which has changed over the years.! I used to kill everything regardless)

    E-stop means Foooooooking Hell EVERYTHING STOP NOW.!! . . . So Strategy 1 only.!

    Limit/s trip means machine is outside it's working parameters so do something about it.!! . . So disable the drives using the drives enable signal, Not killing power to drives so maintaining holding torque. At same time inform the Control software so it's halts program execution.
    Send Stop signal to Spindle/VFD again leave power on has it's not an emergency, it's a machine position error so we just need it to come to a controlled stop.
    Along side this you could if wanted turn off other things like vacuum etc either by informing control software or by locally controlling on/off thru Relays but all this should just be on the Limit circuit NOT The E-stop.
    Same regards the Fault signals from drives, It's a machine Error not an emergency so same applies just stop the machine in a controlled manner.

    If any of the above happens then it should be wired in such away that some form of Latch is dropped and not allowed to re-latch until fault is cleared and then only with Push of a Momentary button not Reset switch.

    The E-stop should have ultimate control and Kill power to everything in the event of an emergency so all Latches are dropped and won't reset or start again without some input. Ie Pushing Reset momentary button.

    Charge pump and over Temp etc are again machine errors not emergency conditions so just bring machine to stop in controlled manner.

    Personally I wouldn't have Temperature control anything over than alert the user thru audible or Visual means. Problem comes from the manner in how the machine is stopped.? To safely stop the machine and not wreck the work piece or lose position requires a Feed hold not a Stop command.! If you stop the machine while it's moving, either thru E-stop, limit trip/machine error or just pushing Stop button on control screen then you MUST presume you have lost position thru inertia pushing, THE ONLY SAFE WAY to bring the machine to stop and not lose position is to use Feed hold and allow the control software to do a controlled stop.
    Problem with this is it's doesn't do it instantly so there's always some delay until look ahead buffer is cleared or it's finished the current move. If you have an overtemp problem you won't want to wreck the job so a controlled stop is required so feed hold must be used.
    It is possible to use an input and have Mach watch it then use an OEM trigger to activate the Feedhold if the state changes but personally I wouldn't and just have a Spindle siren scream at me.!!

    This also answers your "Stop Button" option.! . .Pressing stop will only stop the machine and other attached devices in a semi controlled manner but it won't be controlled and can't reliably be resumed from without first homing the machine to get back into position.

    Also noted you said Home limit's.? Home switches are not limits of any kind neither are they part of any system, e-stop or limit etc. They solely define the Machine coordinate Zero position and are purely input's to the Control system and nothing else..!! . . . .(Eddy I know you probably know this but others may not)

  8. #48
    Looks like we're all largely saying the same things now.

    Quote Originally Posted by JAZZCNC View Post
    This also answers your "Stop Button" option.! . .Pressing stop will only stop the machine and other attached devices in a semi controlled manner but it won't be controlled and can't reliably be resumed from without first homing the machine to get back into position.
    For the non-emergency stop button, you could just wire it to an input set to activate the feed hold, so the machine does in a controlled manner. It's still likely to leave a mark on the work, but prevent something worse.

  9. #49
    Quote Originally Posted by Jonathan View Post
    The VFD going faulty sounds exceptionally unlikely
    Not that unlikely really has I've got one here that did just that in that it wouldn't turn off thru a signal or the control panel. Turned out to be faulty panel and was replaced by ABB but the fact was it stayed bloody on so no signal would have turned it off.!!

  10. #50
    Quote Originally Posted by JAZZCNC View Post
    Not that unlikely really has I've got one here that did just that in that it wouldn't turn off thru a signal or the control panel. Turned out to be faulty panel and was replaced by ABB but the fact was it stayed bloody on so no signal would have turned it off.!!
    What's your point? The timed switch off mentioned twice (now three times) in this thread would have turned it off. Also you can't judge how likely something is with such a small sample size.

    A couple of months ago I got a 15kW ABB VFD. Wired it up correctly and 5 minutes later the input rectifier blew up leaving my ears ringing for about half an hour!

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