Coming Up with the
Right Control and
Data Acquisition Software

A supplier of refrigeration control systems made a strategic choice when it selected
the software that would implement its products.

Gary Marchuk, Think & Do Software

Screen 1. Think & Do Software provides continuous monitoring of the refrigeration system's compressor parameters, as well as sequencing on/off cycles to improve efficiency.

Techni-Systems, a leading supplier of atmosphere and industrial refrigeration control systems, has carved out a niche by providing state-of-the-art equipment to food industry clients throughout the northwest. The company's PC-based systems monitor and control refrigeration system parameters to increase efficiency and energy savings for their customers.

Techni-Systems recently installed three industrial ammonia refrigeration systems using Think & Do Software to manage control and data acquisition. All three systems were delivered to Washington state customers, each with its own food processing requirements. All three customers saved processing time, energy, and maintenance costs through automated control implemented by Think & Do Software.

Control Systems Overview

For Techni-Systems, industrial ammonia refrigeration systems require integrated monitoring and control of all components, including the compressors, condensers, and evaporators. The systems help maintain a precise temperature and optimize energy use through variable speed fan control, wet bulb temperature control, auto-suction pressure compressor control, intelligent compressor sequencing, and demand defrost scheduling.

The industrial refrigeration control system performs many functions to achieve maximum energy savings and precise temperature control. Typical features found in a Techni-System solution include:

• Temperature monitoring and control
• Defrost cycles
• Fan cycling
• Float counting
• Vessel level monitoring
• Condenser control
• Condenser VSD fan control
• Ammonia leak monitoring
• Compressor control
• Compressor sequencing
• Compressor variable speed control
• Auto-suction set point control
• Kilowatt monitoring and control
• King valve control
• Underslab temperature monitoring
• Glycol heat exchanger monitoring and control

For each installation, control panels provide the analog and digital I/O interface with pressure transmitters, compressor control circuits, temperature probes, valves, pumps, fans, and other related devices. Auto/manual switches allow manual operation of equipment for testing or backup control.

To ensure maximum system integrity, both local and remote alarms are initiated. Critical alarms include high/low zone temperature, high suction pressure, high/low refrigerant vessel level, compressor failure, ammonia leak, and system failure.

A Strategic Choice

Techni-Systems chose Think & Do Software, a fully integrated, PC-based control software package to run its control system. Think & Do runs on standard PC hardware and communicates with remote I/O bases for monitoring and control. The I/O bases have as many as 100 analog measurement points for monitoring system variables, and as many as 100 digital I/O points for sequencing pumps, fans, and compressors.

Techni-Systems has evaluated a number of control software products over the last ten years. But according to Dan Black, a Techni-Systems controls systems engineer, "Most so-called solutions were very inadequate in terms of direct PC control functionality. I wasn't interested in just an HMI or data acquisition package--I wanted to do control with the PC because it is so much easier and more cost effective." Black added, "I knew that PC reliability was not a concern because my company has been using direct PC control for over a decade with a very low failure rate."

To increase efficiency, you continuously monitor all process parameters (see Screen 1, page 58). The parameters are tightly coupled because the status of any single value or parameter could dictate a different control response. The appropriate control actions are programmed using the Think & Do flowchart programming environment (see the sidebar). Black said, "I was attracted to the flowchart programming language running under the Windows NT/real-time kernel architecture. We are not proficient with computer languages and have no desire to become so, and therefore depend on a versatile and powerful language that is easy to use. Think & Do has really achieved this combination. With no training, I was able to begin development on a project that had to be operational in less than a month--a very risky decision, but one that has proved to be right. I was particularly impressed with the way Think & Do organizes system variables into data tables by type (e.g., input, output, float, number, flag, and timer). It is easy to lay out a project by initially defining the variables to be incorporated and then starting the flowchart development. This approach keeps the project organized and makes it easy to replicate repetitive control functionality."

A relatively new paradigm in control programming, this patented technology allows the programmer to use standard flowchart blocks for sequential control and logic. The graphical language uses diamonds for decisions and boxes for actions. The icons are dragged and dropped into the control program and then configured for various tasks, such as turning on and off outputs, math functions, motion control, serial communications, and PID loop control. Flowchart programming is taught and used in a wide variety of situations, and the basic concepts are understood by many people. It is an intuitive method for defining process flow and a standard practice for many computer programmers. Because the basic concepts are well understood, customers can bring process and machine experts into the control design area, which accelerates the development process and improves the quality of controls. For some customers, the process and machine experts have even been able to write the control programs. For ease of use, flowcharts provide graphical representation that enables visualization of control logic and quick debugging. Ladder diagrams have been the primary language of PLCs for years. One of the great strengths of ladder diagram programming for the factory floor is its ability to represent the programming graphically to maintenance personnel. Other programming languages have tried to displace ladder logic as a preferred method for control programming, and they failed because of the absence of a graphical display. Flowcharts provide the graphical representation, both for programming and debugging, and have gained rapid acceptance in the control and automation market. Flowcharts are also extensible. They do well for simple logic and sequential control but also can provide for more sophisticated action blocks in the realm of the logic and sequential program. Think & Do Software has used this extensibility to add functions (e.g., PID loop control, serial communications, motion control, and ASCII string handling) to its standard programming environment. This allows a programmer to interact with all the control devices in a common programming environment, with a common database for the entire control system. This facilitates the development process and lets the control system accommodate changes and additions. Two additional benefits of flowchart programming are the inherent diagnostics and the state orientation of its programming method. Because flowcharts are state oriented, they allow you to program functional tasks or machine states and enable and disable these functions depending on whether they must be active. For example, many machines or processes have modes (e.g., automatic, manual, jog, setup, maintenance, and fast). A flowchart can be developed for each state, or mode, and rendered inactive when not needed. This prevents the typical logic conflicts found with other methods of programming. Additionally, flowchart programming provides for easy diagnostics. At any decision or compare block, if the result is not what is expected, an action block can be inserted to create an alarm or diagnostic action.

Another requirement of the system was remote monitoring and troubleshooting. Techni-Systems uses standard PC software tools to remotely access the Think & Do control systems via a modem link. This allows operators to view color graphic displays, make set point adjustments, and perform technical support and/or training from anywhere in the world. They can even monitor the control logic remotely and, if necessary, make online changes while the system is running.

Techni-Systems has to customize each of its systems to accommodate the requirements of each customer and the associated product being refrigerated. Think & Do's modular flowcharts and full support for subroutine flowcharts allowed Techni-Systems to develop libraries of reusable logic. As a result, the company can create semi-custom control system solutions in a fraction of the time required to write them from scratch. The company just pulls existing control flowcharts from a central repository and designs the high-level logic to link them together.

Conclusion

The comprehensive control system that Techni-Systems designed has reduced energy consumption by 25% or more on an annual basis. Additionally, product quality is improved because process parameters are tightly monitored and controlled. These benefits were realized by Techni-Systems through the use of the PC-based control system from Think & Do Software.

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