Re: CAN bus -- Interview question

Just one case where complete power removal isn't the best:

If you have a conveyor controlled by servomotors, you can decelerate it 
quicker with the
servo controller active, rather than by removing power (and letting it 
coast to a stop).

If something has to *STOP!* , you probably need external emergency 
braking in the system, that
functions when ESTOP removes power (as it should).

(P.S. The fact that my car brake pedals aren't directly tied to brake 
shoes still bothers me....)

Rufus




Rufus V. Smith Faria Watchdog [email protected] On 10/11/2012 
12:07 PM, John Dammeyer wrote:
> Message
> 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
>