The emergency stop

[Youri]

Hello,

I'm using a PCI-7354 motion control board and a UMI-7774  motion interface. I'm trying to implement an emergency stop button. I think i'm not the first one trying to do this, but i'm a little bit confused about how this is implemented in the NI hardware.

First let me explain how i think an emergency stop should work. It should be a normally closed switch, so if the button is pressed the contact is released (no more electrical connection). If the button is pressed the drive should be disbaled and there should be no more movement. Using a normally closed contact means that the system doesn't work or stops working when the emergency stop button is disconnected or if there is a break in the cable leading to the button. In this way you  make sure that the emergency button is functional before you start moving. I should think this is the standard and most safe way to implement an emergency stop and i know most motor drives have it implemented in this way.

But now back to the NI motion hardware. I know about the shutdown input and this document :

http://digital.ni.com/public.nsf/websearch/622529A109BAD05E862569EA0060304F?OpenDocument

Here it says that the shutdown circuit requires a rising edge to be activated. A rising edge looks like a normally open switch which is closed when pressed. This system doesn't work when the button is not connected or the cable is broken. What am I missing here?

Thanks in advance,

Youri

[Jochen]

Youri,

E-stop is sometimes a complex topic, as you have to decide from case to case about the conditions of a safe state (e. g. a vertically mounted axis is probably not in a safe state, when it gets deenergized).
There is already a good thread about the whole issue here in the forums. Here you can find some more information about the shutdown input.

I hope that helps,

Jochen Klier
National Instruments

Message Edited by Jochen on 09-26-2007 09:12 AM

[Giridhar_Rajan]

Hi,

I got a solution for your problem which I have successfully implemented in a couple of projects. Wire up your E-Stop to the Fault I/P of the control connector (15-pin connector) for each Axis in UMI. The fault input can be configured as NO or NC using MAX and the DIP switches provided in the UMI for End Limits and Home limits. An indication of this is shown through an LED in UMI called Enable. The Enable output of the control connector should enable disable your drive. Now, also wire your E-Stop to one of the digital inputs in the motion controller so that you can read this input in your software and call the stop motion VI.

Rgds,

Giridhar Rajan

NextFirst Engineering Technologies,

Bangalore

[Youri]

Thank you both for your replies.

Jochen: I agree with you that there is discussion about what action is taken after the emergency stop button is pressed. This depends largely on the application. But I still think that the way the button is connected to and handled by the motion controller should be in the way i descriped. I've read the threads you mentioned, but they both seem like a lot of work-arounds to me.  In my opinion (and by the knowledge i have on this moment) the shutdown input doesn't respond the way i would like it to do. So i'm wondering how this shutdown input is intended to be used and why there isn't a proper e-stop input on the NI motion controller. These motion controllers are widely used and easy to set up by inexperienced people.  There should be a easy, simple and most of all safe input for an e-stop button. But again, i might be missing something.

 Giridhar: Thanks for the tip. I'm going to look into it.

Kind regards,

Youri

[Jochen]

Youri,

I agree with you that we could do better with an E-Stop feature, but on the other hand there are so many aspects to consider when implementing an E-Stop, that regardless which implementation we chose, it would only cover a small percentage of all applications.
As already discussed, depending on the application there are also legal regulations for this feature and in many applications E-Stop will have to be implemented by turning off the complete power supply for the motors, maybe in combination with an active failsafe break system. In these scenarios no software involvement is allowed.
We definitely have an open ear for good suggestions, but in most cases the E-Stop feature will have to be implemented completely independently from the controller and the drive, so our options are very limited.

Jochen

[Youri]

Hi Jochen,

You must have the speed record for quick replies or something :). I do agree that its difficult (not to say impossible) to make an e-stop input which is suitable for all applications. But in my opinion it would be a good start to change the working of the shutdown input. I would like it to react to a level instead of an edge. As long as there is a high level on it, everything works, motor can turn and all is fine, but when there is a low level on it (e-stop button press or a cable snaps) nothing moves. Just clear and simple like that. But again this is just my opinion and i assume that NI had a reason to design the shutdown input in this way.

Kind regards,

Youri

[Giridhar_Rajan]

Jochen,

Regarding your suggestion to completely cut the power supply, I have seen that when we resume from E-Stop, the motor starts to hunt as the controller is still in the loop and trying to control the motor. This can sometimes be very dangerous for the operator. Anyway, an easier solution to the problem can be to rename the Fault i/p as the E-Stop input, as it gives similar functionality as described by youri.

Rgds,

Giridhar Rajan,

NextFirst Engineering Technologies

Bangalore, India

[Jochen]

Giridhar,

shutting down the power completely could also involve shutting down the power for the PC. I know this is not a very nice solution but I wanted to point out that this might be a legal requirement which has to be considered.

Jochen

[Simon Tew]

From my view,The most effective way of "E-STOP" is to turn off SERVO ON signal at Driver side and also active the Shut Down input of motion controller Also "E-STOP" signal must use Normally Close type contact.