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[Noteworthy] E-stop

Discussion in 'ELECTRICAL ISSUES - POWER YOUR MACHINES & SHOP' started by WDG, Dec 28, 2016.

  1. WDG

    WDG United States H-M Supporter - Premium Content H-M Supporter-Premium

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    There is such a vast amount of information on this site that I have not and cannot consume it all. I may be posting an opinion of mine that has been covered before, if so the moderators are welcome to delete it. My experience has taught me to correctly identify the difference between a stop button and an E-Stop button. A stop button will allow a motor to wind down to stop. The speed it slows down with is determined by how it connected or what it is connected to. If using a PLC or a VFD you can program a speed that you want it to stop at. If the motor is connected to something directly thru an on off switch, it will wind down from the load it is connected to. The stop button that is going to a PLC or VFD is just that a stop button. An E-Stop (this is my opinion) should never under any circumstances be connected to a PLC or a VFD. If the contact in the PLC or VFD should stick, (I've witness this in an AB PLC) you can push all the E-Stops you want but the machine will NEVER stop. That is why you hard wire an E-Stop in the control circuit ( as in directly to the motor starter). Now there is going to be some concerns as to how quickly your equipment will stop, but the point is you want it to stop. Anyone who sees a fault in my logic, I would welcome your experience and advice. As I said, I have personally seen an AB PLC contact card stick closed and every E-Stop on the machine was pushed and the monitors overhead all said the E-Stop was initiated. Only when a non-fused disconnect ( which is not used for stopping but for locking out and isolation when working on the machine) did the machine come to a screeching halt.
     
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  2. JimDawson

    JimDawson Global Moderator Staff Member Director

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    WDG I agree 100%. The E-stop should cut power to all outputs on a PLC, and cut all control power to devices like motor contactors. The overall system should be designed to power down in a safe condition on E-stop or power fail. To really do it correctly, there should be a contactor ahead of a VFD so it is powered down also, although this is not normally done in the home shop, but is a requirement for industrial applications. Connecting an E-stop to a PLC is fine as long as the PLC is only monitoring the E-stop and is not an active device in the E-stop system.
     
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  3. mksj

    mksj Active User H-M Supporter-Premium

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    It is in the form/design that it is implemented and one must look at the whole system and the risk/alternative to minimizing injury. I can only conjecture for the sake of discussion on this issue, it is far more complicated from my read of what is available online. It is a valuable discussion point that one should never assume that any device or system will work 100% all the time. I always assume that a switch may fail closed/on/fail to work properly and that as a minimum you should have at least another safety redundant system. I agree that an E-Stop is not a safety stop, but how it is implement varies.

    There is also the definition of what an E-Stop is suppose to do: "Emergency stop systems act primarily by removing power from the prime movers in a machine, ensuring that power is removed and the equipment brought to a standstill as quickly as possible, regardless of the stage of the operating cycle that the machine is in. After an emergency stop, the machine is inoperable until the emergency stop system is reset. In some cases, emergency stopping the machine may damage the equipment due to the forces involved in halting the process quickly. Cycle stop is a control system command function that is used to bring the machine cycle to a graceful stop at the end of the current cycle. The machine is still fully operable and may still be in automatic mode at the completion of this stop."

    There are also specific requirements for different settings, industrial vs. home/shop. There would appear there are different many ways to satisfy the requirements, not all include disconnecting system power but rendering the system inoperable upon activation of the E-Stop. On VFDs and newer PLCs there are redundant safety interlocks and dual contacts (redundancy) that essentially kill all controls and/or VFD output. Newer VFDs have a safety input that disconnects the VFD output, in some cases this can be timed to allow quick braking, then both the drive output and control inputs are disconnected. Alternatively I have seen a contactor used either before or after the VFD which is de-energized when the E-Stop is pressed. If a contactor is used on the output it may be timed to activate after a specific stopping interval. If on the input, the VFD can be set to use its regenerative power to brake to a halt then shuts down. An E-Stop may use an alternate means of stopping the machine quickly, in some cases this may damage the equipment. Most manual machines we are likely to encounter have their power to the control contactors disconnected with an E-Stop, so a form of power disconnect to the motor. On my VFD knee mill the E-Stop trips the power ON relay to off, this disconnects the VFD signal inputs and breaks the Safe Disable VFD inputs which shuts down the output section. This requires a reset button to be pressed once the E-Stop is released. All E-Stops systems require some form of additional power reset once the E-Stop is reset.

    This is a worthwhile discussion of E-Stop Myths. http://www.automationmag.com/opinion/machine-safety/792-myth-busting-three-emergency-stop-myths
     

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  4. 4gsr

    4gsr Global Moderator Staff Member H-M Supporter-Premium

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    So I guess, a foot stop or brake as you see on newer lathes is not a E-Stop device?
     
  5. TomS

    TomS Active User H-M Supporter-Premium

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    This discussion is timely. I'm in the throws of converting my PM-932 CNC to belt drive using a Hitachi WJ200 VFD. I follow the logic about cutting the power to the motor contactor thus stopping the spindle. What I don't understand is how is the E-stop wired to kill both the VFD output and the motion controller? Two separate systems with different operating voltages. Safety should always be the primary consideration. I want to make sure I do it right.

    Tom S.
     
  6. mksj

    mksj Active User H-M Supporter-Premium

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    It does stop the lathe immediately and it does trigger (deactivate) the power relay which may require a separate reset (button or placing the spindle lever to the stop position) of some form. It does not kill power to the machine, but the electrical part is the same as hitting an E-Stop (i.e. the switch is serially connected with the E-Stop, usually before it, along with the cover and shield safety switches). But an E-Stop switch usually requires some form of positive latching mechanism when activated, so it either needs to be twisted or pulled to reset. In higher level E-Stop systems there is a sentry systems which monitors the switch and there are two separate deactivation switches. This does not negate the issue of the E-Stop switch breaking when hit, one reason why I do not use Automation Direct E-Stops, as the lens breaks very easily. In modern PLC systems there is both redundancy and specific timing protocols, any deviation or activation kills specific output "run" protocols and activates an emergency shut down on its outputs, and in some cases it activates alternate braking systems.

    Tom, on the PM932 the E-Stop is the first switch that connects the 24VAC to the main power contactor (KM1) if you take the power to the VFD from the output of the contactor (220V from the contactor to the spindle motor control), then hitting the E-Stop will immediately power down the VFD. See the attached file for some suggestion on the connections. I would probably add high speed fuses or a breaker on the input 220VAC power and you may want a hard power switch disconnect (or pull the plug if you are working on the machine). You can also program the VFD to use the reserve capacitance and regenerative energy to apply braking when the power is disconnected. It is not as fast as when powered, but on my lathe it takes about 3 seconds to stop instead of 1. When VFDs are powered down, they may remain functional for something like 30-60 seconds until the power supply capacitors are drained.
     

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  7. Groundhog

    Groundhog United States H-M Supporter - Premium Content H-M Supporter-Premium

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    Ski lifts have some additional Emergency Stop requirements (fail-safe setting of brakes, speed of stop, reset procedure, etc.) and all requirements are spelled out in detail (down to the minimum size and color of the button labeling). The definition of an Emergency Stop and it's related functions is several pages long. They have to cover many types of speed control and prime movers (electric, gas, diesel).

    In relation to the equipment being used here I think mksj has nailed it. His definition and discussion seems pretty inclusive to me.
     
  8. JimDawson

    JimDawson Global Moderator Staff Member Director

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    The way I have mill set up is with 2 relays. One kills 120VAC power to the servo power supply, the other kills 24 VDC control power to the PLC (motion controller) outputs. The PLC outputs are wired to small relays with very low current coils designed for PLC isolation, those relays then energize the contactors that handle the high current loads. This is done so the PLC is not switching highly inductive loads from the main contactors coils to minimize the chance of damage to the output transistors.

    The E-stop is also monitored by the PLC, and on E-stop, the PLC is programmed to disable all outputs and revert to a safe startup condition, this happens in 10ms or less. This kills all of the servo enable signals, stops motion output, and notifies the CNC program that the E-stop has been pressed, thus resetting the command buffer and a few other things. In order to power back up the E-stop button has to be pulled out, and then the control power button pressed to restart. After that is done then the cut can be started again from the CNC program.

    My spindle VFD is completely separate from the servo control and is not under program control. I thought long and hard about this one as well as having some experience with different failure modes. I'm still thinking about the best way to handle it. In general you don't want to kill the spindle before the table is under control. It is rare that an emergency condition arises that requires the spindle be shut down instantly along with the table motion. The half second or so it takes me to hit both switches is not really significant. Normally things happen so quickly that the tool bit is already gone or other disaster has already occurred before you can react. I am just using the VFD run/stop inputs to control the VFD, it is always powered up as long as the main power is turned on.

    On my router, the E-stop shuts down everything including the spindle, again killing control power thus killing power to all of the contactors. I have never used it to stop the machine. Suction, table vacuum, servo power, and spindle each have their own control switches and may be operated manually or under program control.
     
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  9. TomS

    TomS Active User H-M Supporter-Premium

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    Thanks Jim and Mark. You've given me a lot to think about. As I move forward with my conversion I'm sure I will have a question or two, or three, or four.

    Tom S.
     
  10. WDG

    WDG United States H-M Supporter - Premium Content H-M Supporter-Premium

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    mksj was very accurate in that in some instances there may be considerable damage to a machine. I cannot divulge the machine that I was referring to as it is proprietary information. But I will give a general recount. The machine had a very large HP motor running a long drive shaft the length of the machine. (40') All the timing on the machine at different points was done with different size pulleys running off of the main drive shaft. In order to stop the run away machine, the non-fused (lock out) disconnect had to be pulled. It stopped instantly. Big drive belts broke all over the place, as well as product. No metal parts were damaged and no personnel were hurt, but a lesson was learned. The buttons were all labeled E-Stop and the message on the monitors came up as such. But they were really stop buttons and not E-Stops. Of course this was quickly corrected. I must confess that I don't have an E-Stop on my mill or lathe. The lathe is not a problem as it plugs in at the back of the bed and is easily reachable, but I do have to reach across the bed to do it and that is dangerous. I will change that. Also the Mill has a plug but a lot of damage can be done before I can reach it so I will be putting in a switch to break the power to the motor irregardless of what the VFD does. Knowing this before hand is my lack of respect for my own knowledge it that I was in a hurry to get everything running instead of listening to what I knew and take the time to do it correctly. TomS is approaching it correctly and should be alright.
     
  11. markba633csi

    markba633csi United States Active Member Active Member

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    Good points and it brings to mind one of the problems with the commonly seen "drum switches" which, when grabbed for in a panic, could easily be swung past the center off position and the motor would still be running, usually in the same direction.
    Mark S.
     
  12. TomS

    TomS Active User H-M Supporter-Premium

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    I haven't given up on seeking out further information on E-stops and how to apply them to my CNC mill. I've got a thread going on the Precision-Mathews forum and in the interest of keeping my build thread consolidated in one place I'll continue asking questions over there. Here's the link if you are interested. http://www.hobby-machinist.com/threads/taking-the-cnc-plunge.24858/

    Tom S.
     

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