Looking for opinions

Prayder

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Jun 2014
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If you were building a wireless set up for PLC control, would you prefer MODBUS TCP or something like the Allen Bradley Stratix 5100 wireless AP to read tags remotely and possibly control things remotely?
 
The first thing you would do is stop and ask yourself why you're planning on using wireless.

I've lost track of the number of times I've clenched my fist and mentally calmed myself down while meeting with people who say "but it works fine in the office". This includes a meeting with an IEEE Senior Technical Fellow, who told me "your protocol simply needs to handled dropped packets better".

What exact functions do you have in mind, and why do you think they need to be wireless ? What other transmitters, protocols, and equipment are using the spectrum you have in mind ?

Stratix 5100 is just a hardened Cisco wireless access point.

"I want to control things remotely" is also something that should trigger a safety analysis of the system.

I literally have a car I can drive short distances with my smartphone. I am appropriately skeptical of the technology and supervise it closely.
 
You’re talking about two different things. Modbus TCP is a protocol that is transmitted via a network regardless if it is wired or wireless. The wireless access point bridges a wired network to a wireless one.

I would ask you to share what problem you’re encountering and what you’re trying to accomplish. There are a bunch of ways to solve a problem and an infinite number of types of problems. The solution for one isn’t necessarily suitable for another. Without more information I’m not sure we can help
 
To be honest, it is just a thought my boss ran by me. We currently use MODBUS TCP wirelessly right now, just as a notification to maintenance personnel. So when one of our machine center operators hits a button on a console HMI the plc turns on an output to a wireless node that goes back to a red lion cadet through modbus. So we were thinking what else could we do wirelessly.
 
We use modbus tcp through a wireless bridge all the time. We specialize in monitoring/controlling oilfield storage tank, pumping units, Injection pumps controlled by Vfd's running PID loops that needs to be tweaked from time to time.

We communicate thru this bridge sometimes miles apart. And as Ken mentioned anytime you allow control make sure safety comes in to play. We allow people to shut pumping units down from their smartphone but we don't allow start's remotely. Once shut down a HUMAN has to be on site to restart!!
 
You have to ask about the difference between want to and need to. We have a portable support rig (about the size of a small semi-trailer) that is PLC controlled. Inside the unit there is a touchscreen workstation running the HMI that controls the machine, which operates as it is being towed down the road. There is an alarm beacon on the top, so if it went into alarm, the guys in the van following would see the beacon, call for a stop and investigate. Then someone came up with the bright idea "Why don't we install a wireless communication? That way we can see what it is doing continuously." So they bought a Wireless Access Point from Best Buy for about $50, and added a laptop running the same HMI. Now that wireless communication capability was advertised to the customer, and it turned from want to into have to. That is when we learned about all the pitfalls of wireless and an industrial control system. We had to design a system that would be robust and immune to noise and the potential for a random hack attack. We are using a pair of Cisco Aironet wireless bridges, set up with direction patch antennas, multiple authentications and a 100 MB Ethernet network. Not cheap, or but it works flawlessly. So you have to ask yourself, do I want to or do I have to, and how critical is the system?
 
I admit I was only alluding to one of the issues I've encountered: the difference between I/O and ordinary messaging.

Modbus/TCP is nice and robust because it relies on good old TCP. Dropped packets just get retried, stuff gets reassembled if out of order, and it's not very time critical.

I/O protocols like EtherNet/IP often use multicast or use protocols that can't tolerate a lot of lost packets, retransmissions, or errors. Their default update times load up the network with a lot more traffic than the default times of other typical protocols.

The wireless router I use at home drops out for about 2 minutes at least a half dozen times a day. I don't know why. That wouldn't fly if it was an I/O bridge for a material transport shuttle.
 
Not all wireless is the same. Different technologies will provide different capabilities and depending on what you want/need to do you may or may not be able to deploy wireless technology that will work as needed. The simplistic rule of thumb is the faster it goes the less distance it will cover and the more fragile it is. However with the advancement of time and demand this is changing but only because no one is advancing slower technology (there simply isn’t any demand for it). For example, Frequency Hopping Spread Spectrum is as about as robust as it gets depending on the quality of design and build and what I mean by that is if you pay $80.00 for a pair of FHSS radios they aren’t going to perform very well compared to a $2000.00 pair of radios. FreeWave and XetaWave build (in my view) the best FHSS radios on the planet (why two different companies? Same guy designed both radios). I will stand them up against anything else when it comes to distance, reliability over distance and ability to deal with noise. So why not just use them for all your wireless needs? Because FHSS has a significant speed limit because of some pesky laws of physics. It takes time to turn transmitters on and off and you have to leave time in between and it takes time to go from one frequency to another etc, etc, etc… WiFi and all the derivatives that have spun off of it uses a continuous carrier (in simplistic terms) so it will run much faster however it is far more susceptible to noise, fade loss and a whole host of environmental factors that limit its range and reliability.
In my past life I supported (among other tasks) some modems that used FreeWave radios and 90% of the time when an application did not work as designed it was because the wireless aspect was viewed as this magical black box that you simply connect the communications cable to one end and the antenna to the other and you now had a virtual serial or cat5 cable with no other differences. You have to educate yourself on all aspects of the application including the different wireless technologies and treat the wireless as a component that has its place in automation but not in all applications.
 
another option if you already have SCADA at that site is open a team viewer account and remote into your SCADA for viewing and control (this can be done by smartphone too).


a remote notifier like WIN911 will allow you reports to smartphone via email / SMS /voice. its read only though.
 
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