Fieldbus Controls

How will pneumatic circuit Fieldbus technology affect your projects, if at all?

It depends.

If you are building a small air circuit, with only one or two valves or actuators, the benefits of Fieldbus technology can be yours, but it may be price prohibitive.

On the other hand, if the application that you are building the air circuit for has many air valves, many actuators, position sensors and so on, then using fieldbus technology to supply power to and control all those compressed air components, may be just right for you.

Wiring The Machine

For purposes of example, let us mentally create a compressed air circuit containing 10 double solenoid air valves, 10 double acting air cylinders, each cylinder having two position sensing switches to help with tooling positioning and logic control.

That compressed air circuit typically would need to have forty wires to power the solenoids, and forty more wires to the cylinder position sensors, all of which would have to travel from the item they were powering back to a location on the machine frame. That location might be a terminal strip, or it might be the inputs and outputs for a PLC, or maybe a combination of both.

If there were any other devices on the circuit, similarly, all of their wires would have to travel the full length back to the terminal or PLC as well.

That is a lot of wiring. Hours and hours of labor to wire the machine, and maybe hundreds of yards of expensive wire.

The Fieldbus Solution

Now, rather than having at least eighty wires all running from the application back to a single location, let us just take one wire (maybe it's one cable, maybe it's a twisted pair of wires), running from just one location and moving throughout the machine.

That is the concept and the beauty of controlling complex air circuity using a Fieldbus.

How

Each power using device, and each signal generating device would have its own address on the machine.

Just like when a mail person walks down the street delivering mail to each unique address on the street, the address for each air circuit component in the Fieldbus controller would be unique.

The fieldbus system would recognize that device address as being individual, and would be able to power it and, if needed, receive data from it, transmitting power and information both ways on the single twisted wire or cable.

Fieldbus Protocol

What runs a particular brand of Fieldbus equipment is its protocol.

What protocol means is software. The Fieldbus has an integral logic device that runs of software (just like Windows runs so many PCs).

You need to watch out for this if you are selecting Fieldbus technology for your project.

Selecting A Fieldbus

Identify the numbers of, and what type of air devices you would like to have controlled by the Fieldbus controlled circuit.

Some manufacturers offer expansion modules allowing their system to grow as numbers of components increase while others may only allow a fixed number of inputs and outputs.

My advice is to design the machine air circuit to its entirety first, take off a bill of materials, and use that B of M as part of your selection process when sourcing your Fieldbus unit.

Other Fieldbus Considerations

The usual considerations of:

• unit size
• placement location
• power supply required
• interface with other PLC's
• cost
• maintenance/warranty

still need to be reviewed as the Fieldbus selection process proceeds.

If you have a standard for pneumatic valves and cylinders in your plant, talk to that manufacturer too. Major manufacturers of industrial pneumatic equipment have their own line of Fieldbuses and those might be the best to interface with your valves, cylinders and sensors.

Connecting Devices

One of the ways that various manufacturers of Fieldbus technology differentiate their products is the method of connection of those devices to the single fieldbus cable or wire.

The neatest I've seen so far (but not the least expensive) is similar to the connection of automotive wires in wire harnesses. A small plastic piece with teeth is attached to the wire from the device, and clamping that plastic piece over the Fieldbus wire instantly, and correctly, connects the device to the line.

If the need occurs to remove the connection, removal of the clamping piece leaves only very small holes in the wire which are easily caulked or taped over.

It is worthwhile checking out the various methods of attachment as, if you have many devices to connect, a lot of time can be saved using some sort of wire quick connect system.

Fieldbus technology is used in a host of other industries than just industrial pneumatics. The over-arching concept is similar.