RE: CAN bus -- Interview question

Yep,

as a design rule we remove the muscle only and leave the brain alive, even 
in full 24 VDC systems.
With PLCs etc it is fairly simple, shut down the power supply from the 
outputs and leave the cpu on to have diagnostics capability.
Sometimes in hydraulic systems one does need extra shut down valves to 
actually stop the movements.

isto



"John Dammeyer"

From:
"John Dammeyer" <[email protected]>


To:
"'Canlist'" <[email protected]>


Cc:



Date:
11.10.2012 19:07


Subject:
RE: [CANLIST] CAN bus -- Interview question



Not sure I understand your question.  The Link you provided clearly states 
the reason for the pull cord along a conveyor belt.  Since it's an ESTOP 
the CAN bus has nothing to do with it other than perhaps reporting via a 
second switch inside the box that the ESTOP was activated.
 
What to do with the ESTOP comes up often in the home hobby or small 
business CNC groups all the time.  There's a big red button on the PC user 
screen marked stop and it's attached via a port pin to a physical button 
or some other sort of switch.  Once active this big red soft button starts 
flashing the CNC software will no longer issues stepping pulses or provide 
control outputs.
 
But it's not an ESTOP even though it looks like one.  Intrinsically safe 
systems remove power to all life threatening equipment when pressed.  So 
AC power vanishes since the AC is dangerous.  The AC also moves the 
conveyor belt and with it removed the belt stops.  Motors stop.  That 
operator being electrocuted by a frayed wire no longer is frozen by the 
current to the wire and drops away.
 
There are arguments that if the power is removed that then the machine 
cannot be manually operated to help remove someone pinned by the machine. 
This is true.  But that is an after the event activity to be dealt with by 
emergency response people.  They may well restart the system in a manual 
override mode that lets them then reverse motors slowly etc. 
 
But an ESTOP should initially remove all power.  Lately though I believe 
PCs and user interfaces are being exempted by that rule and only power to 
motion gear is physically removed.
 
Let the counter arguments begin.  <GRIN>
 
John Dammeyer
 
 
"ELS! The Solution"
Automation Artisans Inc.
http://www.autoartisans.com/ELS/
Ph. 1 250 544 4950
-----Original Message-----
From: [email protected] [
mailto:[email protected]] On Behalf Of raj 
katoch
Sent: Thursday, October 11, 2012 3:46 AM
To: Canlist
Subject: [CANLIST] CAN bus -- Interview question

Hi, 

There were few question which i was not able to answer in an interview 
today.
One of them i will like to ask on list :---

What is the use of this Pull cord switches on Conveyor belt ?
What role CAN bus plays it may be the field bus to transfer event to stop 
the belt power supply, is it right ?

http://www.martin-eng.com/resources/blog/emergency-stop-switches


thanks & regards,

katoch




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