Predicting Temperature Controller Maintenance
The electronic controllers of the 1970s and early 1980s had an analog design. While these instruments improved upon the performance of their predecessors, they were limited to providing a single function - controlling the process's temperature. With advancements in microelectronics and integrated circuit development, it was only a matter of time before electronic temperature controllers would become microprocessor based. In the late 1980s and 1990s, digital technology provided control engineers with the ability to implement more complex control strategies. This technology also provided a wealth of new capabilities such as autotuning, soft wiring, mathematical function blocks, digital communications, improved displays and smaller size. Yet, with all of these enhancements, the microprocessor's true power had not been harnessed.
Diagnosing Problems Before a BreakdownIn today's competitive environment, it is important to extract the maximum value from a process. It is no longer sufficient to simply control the process to the desired temperature and produce a quality product. The product must be manufactured in the most efficient manner while maintaining a high standard of quality. To derive maximum value from a process, factors such as labor content, maintenance costs and downtime must be controlled. You cannot wait until the process breaks down and then fix it. The cost of such a breakdown can be extremely high in terms of lost production and substandard quality. The ability to monitor the process, analyze data and predict failures is key to improving operations and minimizing costs.
Because it is coupled directly to what is likely the most important variable being controlled, the temperature controller is a critical process component. Internally, the controller stores a vast amount of valuable data relating to the process. The controller tracks:
- Temperature value (setpoint).
- Actual process temperature (process variable).
- Percentage of power required to obtain a desired temperature (output).
- Error signal or setpoint deviation.
The controller also knows if the process is in automatic or manual mode, any alarm conditions and the tuning constants required to keep the process on track (i.e., the process dynamics). How can this information best be used to provide insight into the process's performance and the equipment's overall health?
Begin by considering what areas you want to target for improvement. Do you want to improve the performance of the process equipment (freezer, refrigerator or chamber), or do you want to optimize the process itself? Are there things that can be done that will allow you to anticipate required maintenance in the controller itself?
Determining When Maintenance is NeededTemperature controllers are available with diagnostic functions such as digital counters and digital timers. Using these features, process engineers can track important variables related to the process. For example, a process engineer could use the counter function to track the number of times:
- An alarm was tripped.
>li>The control and output relay cycled on and off.
- The temperature was out of the acceptable range.
- The controller was put in manual mode.
- The controller automatically retuned in response to the process dynamics.
- A digital input closure occurred.
- The controller went into fail-safe mode.
- Power to the controller was interrupted.
Likewise, an engineer could use the timer function to track:
- Total process operating time.
- Time in manual or automatic control mode.
- Time in alarm.
- Time since last input activation.
With this information, it becomes only a matter of matching important process parameters with available information. For example, if you want to determine if downtime is being caused by a failed contactor, use a digital counter to track contactor cycles. If you know the average life expectancy of the particular contactor used in the process, you can count the total number of cycles and set a diagnostic alarm to trip when you reach approximately 90% of the contactor's life. Then, the contactor can be replaced during a normal maintenance cycle. Likewise, counters also could be used to monitor wear in the alarm controllers and control relays to predict failures, reduce equipment downtime and increase product throughput.
Counters and timers can be used in combination to monitor the process's health. By tracking the number of times that the temperature is out of spec and the length of time for each occurrence, a trend might be noticed. Then, a routine maintenance schedule could be established to reduce downtime.
These are only a few examples of how diagnostic data provided by a controller can contribute to improving process and equipment integrity. Consider your process's unique attributes to uncover additional opportunities to leverage the data captured by the controller.
Closing the LoopIn addition to diagnostic functions, consider the value that communications can add. Communication between the controller and other devices has been possible for many years, and manufacturers routinely develop networks to tie processes together. It is easy to visualize how the diagnostic data within the controller can be communicated to a personal computer (PC), analyzed and reviewed by a process engineer.
It is no longer necessary to locate the PC near the process. Instead, process information can be provided to any interested party such as the production manager at a remote location. Further, if the controller's communication card provides a connection to the Internet, anyone with the proper passwords and a standard Internet browser can access this data.
As control technology continues to evolve, you can expect additional functions and benefits that today might only be imagined. As with other technologies, temperature controllers are evolving from fixed function devices to valuable tools that can help the user improve a process and remain competitive.