Is Wireless the Right Solution?

What Are Readers' Experiences With Machine-Mount Wireless Controllers and I/O?

By Control Design Staff

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We build most of our machines by combining modules, so machine-mount controllers and I/O have been particularly effective because they allow us to test each module separately. But the cost and time required to wire the modules together at our customers' sites is a concern, and we're wondering if wireless is the right solution. Can anyone relate their experiences with machine-mount wireless controllers and I/O?

—From February '13 Control Design

SEE ALSO: Machine-Mount Automation

Consider Carefully
In my discussions with our customers, only a few today are seriously investigating wireless I/O. If I move past the initial request for wireless I/O and inquire more about the actual problem they are trying to solve, I find out:

•    For a machine builder, it is typically that they want to reduce the time, hardware and labor costs of installing the I/O products and simplify the connections required.

•    For an end user, it is typically that they want to dramatically reduce the downtime caused by simple cable failures and connection issues.

When I run into the rare customer that has implemented a wireless I/O solution, it is typically fraught with new problems like completely dead communication zones, intermittent connections, limitations of proprietary systems and, the biggest trouble of all, they still have to get power to these devices. So, although the wireless communication solution in theory seems to be cable-free, power connections still have to be run through the machine.

For the past few years, many device manufacturers and consortia have been discussing the possibility for not only a wireless I/O solution, but a wireless sensor actuator network (WSAN) or wireless sensor network (WSN). These types of networks would have a master data collector communicating with the controller, a cohesive communication between all the sensing devices, and the master via an open standard. But today, in my opinion, developments still need to be made in battery and power technologies to make it cost-effective and reliable in mainstream industrial designs.

For machine builders looking to implement wireless I/O technologies today, I would suggest they consider three things:

1.    Think simple. What am I actually trying to solve by implementing wireless? Is there a simpler solution available?

2.    Predict problems. What is the potential for noise, communication interference or dead zones in my design?

3.    Consider power. How will I get power to these devices? Cables, batteries, wireless? What is the total cost of ownership of the solution I selected? Is it energy-efficient?

Will Healy III,
marketing manager, networking,

Not Zero
Wireless is nice because — at least on the surface — it looks like a way to get rid of the cable needed for the I/O. But looking at this a bit more carefully, the sensors (and outputs) have to somehow be powered. Consequently, at least two wires (let's say +24 Vdc and GND) need to be brought to the I/O.

Although this is of course better than having to run dozens, if not hundreds, of cables for power and data, the number of wires is not zero. The other concern is that data over a wireless connection is, in most instances, slower and less reliable.

In a situation where a user wants to drastically reduce the number of wires on a machine, I typically suggest AS-Interface, which is a two-conductor power and data network. Modules are connected to those two wires, receive power over those two wires (just as in the case of the wireless approach), and exchange I/O data. In all fairness, I have to say that outputs are typically (but not always) powered from an additional two-conductor auxiliary power cable, so worst case, four wires will give this customer everything that is needed with the following benefits:

•    High data rate and deterministic update times in the order of milliseconds (248 input update every 10 ms, 248 output update in 20 ms)

•    The possibility to connect digital I/O and analog I/O

•    The ability to connect functional safety devices up to Cat. 4/SIL 3/PLe to the same solution (mix and match standard and safe hardware as needed)

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