Re: CAN bus -- Interview question

Thanks for your reply John & Rufus.

regards,
katoch

On Thu, Oct 11, 2012 at 11:04 PM, Rufus V. Smith
<[email protected]>wrote:

>  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:
>
> 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]<[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
>
>
>