Control solution withstands temperature extremes

24 March 2015

Unitronics distributor, MSI Tec, was tasked with designing a panel to control natural gas engines at the City of Tucson’s well pumps.  The control panels would need to be installed at 43 sites around the city. 

The cost per engine and shaft can be greater than $400,000.  Rapid changes in RPM can cause a large column of water to shift up and down quickly, so a robust and reliable controller for these control panels was vital. 

The previous controllers had screens that became increasingly difficult to read after a couple of years, which is why they needed to be replaced with a controller that had improved readability. Other necessary requirements were the ability to remotely connect to the customer's SCADA system, historical data availability to diagnose possible causes of any issues that may arise- and an easy to use interface. "The Vision570 was selected because of its size, colour touch screen, and terrific feature set for the price," explained Matthew Cronin of MSI Tec. 

The Vision570s are mounted in un-insulated panels outdoors.  

Temperatures in Tuscon can climb above 110°F and the low often drops below freezing. Meanwhile, humidity ranges from an average low of 5% in June to an average high of 75% in July. 

The high quality of Unitronic's touch screens allowed them to be resilient to the temperature and sun exposure. "The temperature was so hot that we couldn’t keep our computers running for more than 10 minutes in the shade.  In order to update the stations, we used cars with air-conditioned seats.  We had to leave the cars running the entire day to keep the air and the seats cool enough for the laptops to not lock up.  Even if there were multiple PLCs at a single station, we still had to go put the laptops in the car for a few minutes between each code download to keep the laptops running.  Despite this excessive heat, the controllers kept running without a problem.  And they continue to do so to this day,” said Cronin.

Using Unitronics’ Modbus TCP functionality and a cell modem, Tucson Water’s SCADA system is able to communicate with each well site individually. They are also able to use the V570's SD card functions to write data out every 1-120 minutes depending on user settings – which aids in problem diagnosis. 

After the systems were all installed, an upgrade was requested.  The systems are not networked, so this required an onsite code change to approximately 40 stations… in July.  Interesting hurdles were experienced during this upgrade, but none of them were because of the Unitronics." Said Cronin. "The Vision570 was the most cost effective solution for this application. By combining low cost I/O, communications, PLC functionality, and an HMI in a single package, Unitronics delivered more capability for less money. Because Unitronics is an all-in-one solution, programming is fast because no time is spent trying to get the HMI to talk to the PLC. This system required a diverse combination of analogue inputs and outputs, digital inputs and outputs, and high speed pulse outputs.  Unitronics had a flexible and affordable set of I/O cards to accomplish this. The display screens are also still as easy to read now, as when they were new which is great considering that this was a major problem with our old panels." 

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