RE: What to use? EtherCAT or Ethernet Powerlink with CANopen protocol

Hi Kees,

You bring up an interesting point.

A traditional PLC implementation, designed to be modular with say one PLC
per section will communicate with simple Ready, Not Ready, Available, Not
Available signals on inputs and outputs.  Say we're dealing with a machine
that provides a tray of product to be worked on by the adjacent module.

Section 1 turns on an output labelled Product Available once it places a
part in the correct position.  Section 2 has that output connected to an
input called Product Available.  As soon as Section 2 has grabbed the part
and moved it out of the way, it an output called Product Ready.  Section 1
reads the Product Ready Input and knows that there is now room for another
product.

And so on.  The point is with a few hardware inputs and outputs it's
possible to set up hardware handshaking to transfer product.  That product
could be data (like a centronics printer interface) or a bottle ready for
a label, or filling or a cap.

That's the traditional way.  Easy to single step and troubleshoot since
the technician can use a meter to see the handshaking.  And there are a
number of wires from each PLC between each system.  Want to add a Product
Waiting signal?  Add two more wires and one input and one output.

The alternative is CAN bus messages on a single bus with information as to
Product Available, Ready, Busy etc.  Adding signals is easy.  Diagnostics
require a monitor that shows the sequence and time stamps of each signal.
Race conditions show up.  For example showing slow solenoids that used to
close a switch in 30mS and now take 55mS.  Need a scope for the hardware
method to figure that problem out.

Where CANFD (possibly) comes in handy is that included with each product
available message is the product serial number or manufacturing code.  The
Available signal could even have bits set that identify which optional
process is done within the manufacturing line.  Verification can happen
along the way with a bar code scanner that identifies the sticker on the
product with the product available message.

So can you do this with Ethernet or CANOpen?  Yes.  Although it might be a
bit harder with CANOpen a customized CAN protocol would have no trouble.

John Dammeyer

> Twenty years ago Honeywell introduced the CAN-based Smart
> Distributed System (SDS). Simple on/off sensors could be
> programmed with a batch counter. Don't count every bottle,
> but after 24 counts the system should place the next box. The
> PLC does not count; the sensor does. This is called "Thinking
> out of the box"?
>
> Kees Zagers

"ELS! The Solution"
Automation Artisans Inc.
http://www.autoartisans.com/ELS/
Ph. 1 250 544 4950

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