Cellular Antenna and your PLC

One of my customers is having fun with one of their own customers. My customer uses Digi gateways running a Python application to collect hourly tank levels, which are fetched by cellular once per day. The tanks hold 250-gallons of a chemical additive which is injected at a variable rate into crude oil pipelines. Small battery-powered ultrasonic level sensors (www.massa.com) push the last 8 hourly samples every few hours using wireless IEEE 802.15.4 (aka Zigbee). The end customer's goal is to forecast when the tanks need to be refilled ... and I don't mean just receive an alarm when it is running low, they literally want to plan truck routes in advance.

Fun stuff.

The problem is that one of the pilot sites has bad cell coverage, meaning many days the central system cannot upload the log. Of course the data eventually it all uploads since it is held for over a month. Now, never mind that this system sits down in a dry wash (a small valley), the end-user says "Hah, that's because carrier A*** stinks; we all love carrier S****." My customer does not want to mix carriers - especially since negotiated pricing between the two in question is so different.

So remember that "coverage" for fixed RTU must be viewed differently from "coverage" for mobile uses.

For a mobile device (or user) "coverage" is defined by the probability that a valid carrier connection will be available as the device moves around. Thus the number of towers (etc) is important, and if there is no signal in one spot, then hopefully will be one a mile or two away.

Unfortunately, our little RTU bolted to a power pole in that gully won't be moving a mile or two ever. So in the end, "coverage" for a fixed device is defined only by the tower(s) which can be seen. So the carrier with the best cell-phone coverage might not be the best carrier to support a particular fixed RTU.

The first step will be to move the RTU up out of the gully, which is easy since the data signals are all wireless and power can be tapped from any of the privately owned power poles. Probably carrier A*** (stinky or not) will work fine once the RTU moves. As plan B and make the end-customer feel listened to, a S****-based RTU will be placed on the same pole as the relocated A*** one.

Could they run out a long external antenna? Sure, they could. But why bother? Put the RTU where it has a nice signal. Even in 2 or 3 store buildings we suggest customers put the cellular router up in the roof-area and run the Ethernet UTP it's 100 meters instead of trying to deal with the signal loss in long antenna cables. We even have customers using 900Mhz Ethernet bridges to link a cellular router placed where it must be placed back to the Ethernet devices which want access.

Worst case, a directional yagi antenna could be used, however you need to understand that cell towers are routinely turned off without warning. The carriers are truly geared towards mobile users who expect bad signals in some places. Towers (or the active elements) also move. Most of the cell towers you see along the highway work like strip-malls; a company owning the tower and supplying power leases tower space to a mix of carriers. This allows everyone to be flexible.

So prematurely locking down a cellular device to a single tower with a directional antenna can cause future problems since it will not see other weaker towers should the targeted tower be turned off or even moved.

Cellular to Wireless Zigbee

An interesting new market we are moving into is cellular access to wireless mesh (Zigbee as example). In some sense it's a supply-chain dream come true. Imagine you supply a product to customers and EVERYONE (except the customer's IT department) want you to be able to see what the stock level is and auto-schedule deliveries.

The customer benefits because they can treat the product as a 'utility' - turn the tap and there it is.

You benefit because you can minimize emergency truck-rolls - no more angry, panicked customers demanding you send a truck over two-thirds empty because someone forgot to schedule a special delivery because the customer needed to use 60% more product for two days. Of course as a supplier the cost of the truck, fuel, and driver are critical parts of your margin/profit. You desire to only send out full trucks which return gracefully empty!

So we are now working with several of the largest chemical suppliers in the world to enable:

1. drop in a powered cellular unit at the customer site
2. drop in powered or battery tank sensors
3. log levels hourly, for the supplier to upload daily (reduces cellular data charges) The supplier uses this as their 'secret-sauce', their own proprietary value-add to predict when trucks need to roll to maximize efficiency
   
4. enable alarm call-out if the levels hit unexpected low-low levels
   

How this works varies by suppliers. The one I'm working with is using Modbus/TCP to pull up the logs daily. Some other suppliers are having SMTP clients push emails back to the supplier with XML formatted reports. The next supplier I might work will wants the binary logs to be compressed (ZIP'd) and then pushed upstream once a day by FTP, where their accounting system will convert from binary to XML to import and issue bills on product usage per MINUTE.

Of course key to all of this is the wireless drop-in-network concept. The supplier doesn't want to invest thousands of dollars pulling wires through SOMEONE ELSE'S PLANT - especially when the supplier's contract might end in a few months.

Wireless sensors aren't new; cellular data access isn't new; supply-chain systems which auto-detect product levels aren't new. What is new here is the merger of many technologies which reduce infra-structure costs, and thus increase ROI.