How can I stop an ongoing motion by pressing an Emergency Stop

[Karlheinzs]

How can I stop an ongoing motion by pressing an Emergency Stop botton using 7344 motion controller with break out box.  The emergency botton will make a close contact when pressed.  This is a saefty question.  The motion should not resume when the emergency stop is released.  Only restarting the application should allow the motion to resume.

[tochuck123]

I'm taking a guess and I would think you have a digital in that you could poll in your program.  When the button is pressed it changes the true/false value and then you can use the StopMotion.flx to stop the motion.  It won't restart when the button is released.

[Karlheinzs]

This is correct.  The option exist and can be used if there is not a better one awailable.   However, I was hoping there is a hardware solution in the motion control card that would act like a break.  the limit switches only activate in one direction.  I hoped there is something that stops in both directions..  Any other suggestions?

[Buechsenschuetz]

You can build some external circuitry and activate both limit switches simultaneously with your push button.

When implementing emergency switches you have to respect legal regulations for emergency switches. Since activating the limit switches still is done more or less by software (it is possible to disable the limit switches, and this might happen accidentially) your local authorities or safety supervising boards might not accept this as a safe emergency switch.

[Brian_Beal]

You could use the shutdown input on the card.  If this is a true safety issue, then you really need to use a normally closed contact on the emergency stop.  The E-stop circuit should kill power to the drives, not just inhibit motion at the controller.

[AnalogKid2DigitalMan]

Brian's suggestions hit the mark. One item to be careful of is if any of your mechanisms will 'run away' with loss of drive power. This is typically the case for ballscrew-driven Z axis stages. Under loss of power, gravity may pull them downwards. If this is the case, the stage should have a brake mechanism that is active when unpowered.

Normally closed contact is typically the norm since it is failsafe operation if the wires get cut or connections open up.

[Jochen]

Please refer also to this document in the NI knowledgebase for more information about E-Stop and the shutdown feature.

Best regards,

Jochen Klier
National Instruments Germany

[Karlheinzs]

Than you all for your contributions.

The shut down option is close to what I need.  However, i need to drive four independent and otherwise dupplicate systems with the four axis of the 7344 motion controller.  At any given time only one of the axis might have an emergency stop activated.  If that is the case than only the respective axis shuld be shut down not all four axis since thie will cause interference with four production line and therefore becomes expensive.

The double use of limit switches of a given axis might be another option, however on direction of the motion uses the limit switch as a home switch and it still needs to be active indepentend on the emergency stop function.

Thus my revised question is, how can the 7344  be used for four independent dupplicate systems, each with their respective E-Stop funtion?

[Jochen]

From a functional point of view you could run an onboard program that checks digital lines for the status of an E-Stop switch and turn off the appropriate axes. Onboard programs run independently from the host-PC on the µController of the motion board. Additionally the E-Stop should deenergize the drive.
Disabling the E-Stop shouldn't energize the drive again so the button should be connected to the Reset input of an RS-Flipflop. The Set input should be connected to a digital output of the motion board. The output of the RS-Flipflop turns a relay on or off that energizes/deenergizes the drive.

In this scenario the E-Stop sequence would look like this:

1. User presses E-Stop button:
• Axis gets deenergized
• Onboard program detects E-Stop button and stops motion
• Host program gets information from onboard program about the E-Stop.
  
2. User releases E-Stop button:
• Onboard program returns information about released E-Stop button to host program
• Host program asks user if it's ok to energize the axis again
• If user acknowledges the host program energizes the axis again by switching a digital line.

This is just an idea and I don't know if from a legal point of view this approach is feasible. But on the other hand this method should be fail safe as the axis is safely disabled by the E-Stop button without software involvement. Even if the onboard program failed while the motion control is still running (which is extremely improbable) the axis wouldn't be energized after the E-Stop button is released so I feel quite confident about it.

Jochen

[Buechsenschuetz]

It is not always desirable (let alone legal) to deenergize an axis when the E-stop is activated. Think of elevators, lifts, and any lifting and fixing equipment.

I think devices like this rather should stay in a safe position after pressing the E-stop than getting de-energize since gravity will cause motion then. Always think which forces other than the driving motor may cause (dangerous) motion in your system.