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Best Fieldbus For Conveyors

In the Beginning: Cleanroom Conveyors

In the past few years, I’ve only worked on small systems.  But in the early days of my automation career, over 15 years ago, I was involved in a clean room conveyor system that read, labelled, and sorted disk cassettes.  Since I was a young pup in those days, I had absolutely no input on electrical or mechanical component selection.

The system featured a conveyor with various gates, pushers, barcode readers, labelers, photo sensors, an Omron PLC, and a PC to control everything.  All the I/O was hardwired to the PLC.  Since it was in a cleanroom, I got used to bunny suits – and quickly learned that if you want to type in a cleanroom, you need to put on the tightest gloves that won’t break.

We did get the system working reliably after various adventures such as blowing a PLC I/O module (that wasn’t me!) and reconfiguring the conveyor.

Reconfiguring the conveyor meant moving gates and sensors around. I still remember how much work it took: since all the sensors were hardwired, we had basically had to re-do the wiring when everything was changed.

So how could I wire the conveyor electronics so change isn’t painful?

Two Example Situations

Let’s make up a couple of likely examples, and look at s0me possible solutions:

Scenario 1

  • We have a conveyor gate and bypass that has four photo-electric sensors and four outputs (say, to 3 pneumatic solenoids and 1 barcode reader trigger) that have to be moved.
  • It is currently 20 feet from the PLC, and needs to be moved 10 feet down the conveyor (so it will be 30 feet away).

In all cases, the sensors and pneumatics will have to be moved; normally these are mounted on rails or slots on the conveyor and should be easy to move.  But the cables can’t be moved as quickly.

Scenario 2

  • Oops, we forget we needed to add 2 extra photo-electric (PE) sensors, 15 feet from the PLC.

Use Hardwired Cables

If we stay with hard wired cables, there are a couple ways we could solve Scenario 1:

  1. Remove all the 8 20 foot cables and replace them with 8 30 foot cables (that’s a lot of waste and wasted work, ugh!)
  2. Splice 10 extra feet onto all 8 cables to make them longer (yuck!)
  3. Add connectors and add another 10 foot extension cable to all 8 cables, or if the cables already have connectors, add another 10 foot extension to all 8 cables.  This solution isn’t so horrible, except we have to do it 8 times, and connectors, while often necessary, aren’t all goodness:  they add another potential failure point (corrosion, operator error, etc).

My memory is hazy at this point, but I’m sure we didn’t use standard cables (such as M12 or M8) and thus used solution 1 or 2…

For Scenario 2, we simply run two more cables from the PLC to the input sensors.

Use Multi-Port Junction Boxes

One possibility would be to use multi-port junction boxes, available from  Automation Direct, ifm, Balluff, Lumberg, Turk, and many others.  These boxes have standard M8 or M12 circular connectors for sensors and actuators, and then run all the signals back, over one cable, to the controller.  The big advantage is fewer cables: a 8-input box will need only one cable, instead of eight.

I’ll assume I can get an eight port junction box that will work with 4 input and 4 outputs.  So for Scenario 1, moving the wiring requires moving the junction box, and adding one 10 foot extension cable – a lot easier than dealing with 8 cables!

For Scenario 2, if there’s a junction box nearby with 2 spare inputs, then we can connect the PE sensors to it; otherwise we have to add a junction box at 15 feet, connect both sensors to it, and run the box’s cable back to the PLC.

Overall, using junction boxes is a big win: although it has added a some extra cost, it’s already saved us a lot in labor.  Another plus: because the junction box just collects wires together, our PLC’s setup doesn’t have to change at all.

Use A Fieldbus (CANOpen)

Another approach is to use a fieldbus or industrial Ethernet.  I’ll use CANOpen here, because that’s what I know best, something like these IP6x products available from Schneider, Phoenix, Beckhoff, etc.  I will assume that the CAN network uses M12 daisy-chained cables, covering the whole length of the conveyor, while each fieldbus box will have a separate M12 power cable.

So for Scenario 1, the move will require moving the CANOpen box, adding an adapter to connect the existing cables together (replace the box’s connection between cables), and adding an extra cable at the box’s new location to connect to the next CANOpen box.  The power cable to the CANOpen box will have to be extended by 10 feet (e.g. add an extension cable).

For Scenario 2, if there is a CANOpen box nearby with 2 spare inputs, then we can connect the PE sensors to it; otherwise, we have to add a CANopen box at 15 feet, add another CANOpen cable to get to the next CANOpen box, and run a power cable to the new box.

We will also have to make sure the PLC can talk to CANOpen, by using a PLC with CANOpen built-in or adding a gateway.

Compared to hard wiring, the cost is significantly more (due to the cost of the CANOpen gateway and I/O boxes), our PLC has to change (to talk to CANOpen) but changes are much easier.

Compared to using Junction Boxes, the cost is more, the PLC has to change more, but there is more flexibility (easier to add I/O, and support for more types of I/O).

Use The AS-i Fieldbus

AS-i gets its own section because of its unique cabling.  AS-i components are available from a wide variety of companies including Siemens, ifm, IDEC, and Festo.

I will assume an 4-in/4-out AS-i fieldbus box, with two flat AS-i cables (one for the network + power, the other for output power) running the length of the conveyor.

For Scenario 1, the move will require disconnecting the AS-I box from the cables, moving it to the new location, and reconnecting it.  That’s it.  Nothing needs to be done at the old location, because the AS-i cables are self healing, and nothing needs to be done to the AS-i cables at the new location, because the AS-i connectors are insulation piercing and can tap into the flat AS-i signal and power cable at any location.

For Scenario 2, if there is a AS-i box nearby with 2 spare inputs, then we can connect the PE sensors to it; otherwise, we simply add a new AS-i input box at the new location, attach it to the AS-i cable, and connect the PE sensors to it.

Of course, the control system will change a bit: our PLC will either need an AS-I interface module or gateway.

Overall, the cost should be roughly comparable to other fieldbuses, but significantly more than hardwiring or using junction boxes.  However, initial installation time should be the shortest, and it’s definitely the quickest to re-configure.  AS-i doesn’t have the flexibility to add exotic equipment such as encoders and servo motors, but it’s hard to beat for this conveyor system.

I Learned About AS-i Early On, But Too Late

I first learned about AS-i a year or two after we installed that conveyor system, but I have always thought that the initial added expense and complications (e.g. finding a PLC that would work with AS-i) of AS-i would have been worth it in time saved, especially when making changes after installation.  But I didn’t know about it before we started, and I didn’t get involved in specifying control systems until much later.

For our current systems, AS-i does not make sense: they are compact, do not use sensors with M8/M12 connectors, and the I/O requirements are well defined but varied.  So I have still never used AS-i, but if I ever do a conveyor or similar system again AS-i will definitely be one of the leading options.

Disclaimer: as noted, I haven’t used AS-I, junction boxes or such, but I believe my descriptions give a reasonable idea of how the different approaches would work out in practice.

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