Doubl-Kold’s control system:
Over the last 25 years Doubl-Kold developed a very extensive control system for the refrigeration and controlled atmosphere storage industry. Having engineering and refrigeration departments in the same company gives us a large base of experience to draw from, and with our substantial customer base we have developed some software that initially meets or exceeds most plant operations.
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Also data integrity is key for users to be able to ship product out of the country, so we encrypt all the data that’s stored in each daily file.
Some of the things history has taught us is that no matter what control you have whether PC or PLC based, all equipment will fail eventually. So this has lead us to develop a split system consisting of one HMI computer connected to one or more control computers (referred to as controllers).
The split system has multiple benefits:
Along with the HMI and controller setup we design our systems with another kind of backup, a mechanical kind, that can be engaged in a worse case scenario. Like if all the computers have been stolen or a power failure in the room that houses the computers.
Here is our normal manual backup setup:
The User Interface aka HMI
What we did is give them a system they can be comfortable with by allowing customization of the user interface. Colors, fonts, screen layouts, alarm setup, security and other types of customizable tweaks.
When in editing mode all the items on the screen become movable
re-sizable, and can have any of their individual properties changed.
This not only helps legibility, but also lets users design ideal layouts to fit their individual needs.
The Plant Layout
The picture on the right you can see the basic structure of how we will setup a facility tree. With this we can quickly adapt new models of equipment to our existing code.
As an example, we currently cover 18 different types of zone control. If a new type of zone was needed we would get the specks, logic, equipment, and various options needed and develop that into a new chunk of code.
We use a configuration file that is read into the system at startup and constructs a simulated version of the facility based on what was in the configuration file. This helps to standardize on equipment for the computers and the hardware it talks to.
What makes our control system different from others is that every facility uses the same software to run their system. This was developed so we would not have a custom program per customer.
How much impact does this control system put on these computers? Generally, with our control system maxed out it uses about 2%-5% of the CPU time running the software. This is a big difference between PLCs or the popular canned software systems that can grab up to 95% of all system resources leaving no room for anything else to take place including a simple screen saver.
What makes a PC superior? When a PLC is developed, specialized tools just for that PLC brand is also developed to program it. These specialized tools run on PCs, and you need a PC to upload your project to the PLC. As computers and their operating systems change older PLC development tools may no longer be supported for a PLC that's 10 years old.
PLCs tend to only run ladder logic, but that ladder logic is not transportable to another brand/model of PLC, causing the developer to adapt all that old logic to a different brand of PLC, costing the company time and money in redeveloping and or retraining of their personnel. Another downfall is that all the I/O is built right in to the PLC. If a single I/O point burns out, the logic will need to be rerouted to a different point or the whole PLC will need to be replaced if it is too old for upgrade.
We developed our system with Microsoft's .Net runtime and we stay current with programming software changes. We maintain older versions of our software, and make sure that all our customers will have a direct upgrade path to the latest software version, with little or no down time. The other benefit is if one I/O point is burned out, one I/O point is replaced, because the I/O is separate from the brains and easily swapped out.
I/O Hardware and Communications
The Opto 22 hardware is solid, and has been known to take a lightning strike now and again with minimal damage.
With the Opto hardware we have two ways of communicating with it. Serial 422 (multichannel), or TCP/IP. Both communication packages have advantages over the other, so it usually comes down to what will work the best at that particular facility.
Modbus - We currently support several packages that talk over Modbus (serial 485 2/4 wire and TCP/IP) these range from VFDs to compressor micro packages, scrubbers, Nitrogen generators, and other pieces of equipment.
We have built in hardware trouble shooting that allows you to view the communication, I/O points and any errors.
When any changes are done on the system, or when someone logs in or out, the system keeps track of it all by recording the event into a daily log. These can be back tracked up to a year or more and can help someone see when a problem had occurred.
Other events such as computer reboots, errors, and when an alarm clears are all saved in the daily logs.
Graphing and data logs
Why not simply use a database? Well databases tend to get too large, are not vary portable, are easily corrupted, and some databases are easily opened and manipulated. This is not an option for us as data integrity is top priority.
A one day file for a 12 zone facility is about 700 kilobytes on our system, so it's easier to copy to a remote computer for viewing. Database files could easily average several hundred megabytes making it difficult to transport or download.
To make the data easily and quick to navigate, we developed shortcut links in the graphing program. This allows the user to select only the information they are interested in each day. Once the shortcut is saved just click on that shortcut, the dates to be viewed and the system will generate a graph and a log sheet of all the data selected.
After the data has been generated, the logs can be searched and highlighted. The program can also flag any data that's missing and give a quick report.
Another tool is also available to record custom data that an operator may need, like trying to catch an event that might require someone to sit all day watching. Unlike our standard data this one gives more options for recording interval (1 second to once every 24 hours) and gives you some points not normally found in the regular data list.
Plus the all the data can be saved or viewed in an Excel format (no encryption).
Proactive vs. reactive control for energy savings
What we have done is proactive control. Meaning the plant supervisor can schedule parts of
the plant to scale back during known peak times, but the control will still monitor any zones currently off during cycling for high temperature limits and zone to run for awhile to keep the product
in tact. This will have the effect of shedding the load needed to run the plant, so less compressor and condenser will be needed without a complete system shutdown.
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