Closed-Loop Vs Open-Loop Control Systems Part 2 of 3: Understanding Your Process

In a previous article, Temperature Control of Air Heaters, we provided a general overview of control for process heat systems and explained the difference between closed- and open-loop controls. Briefly, a closed-loop system is output driven; the output of the system is continually measured and fed back to control components which adjust the operation of the tool to bring its output into alignment with a pre-set target. An open-loop system has no feedback loop and, as a result, the output of the tool does not impact its continued operation. For example, in a closed-loop process heat system if the inlet air increases in temperature the tool output temperature will briefly increase before the control system brings it back to setpoint, however in an open-loop system the tool output temperature will increase and no corrective action will occur.
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​In this article series we will expand on this topic, exploring when it is advantageous to use a closed-loop system and when it is acceptable to use an open-loop system. The first installment looked at the benefits of automating your system. This second installment will look at an important secondary benefit that emerges from using a closed-loop control system: developing a better understanding of your process.

Know Your Operating Parameters

​Open-loop systems are often controlled using a percentage power setting (i.e., setting the dial on the tool to 6 out of 10). This is because without a feedback system, there is no way to verify if it has reached a target temperature setpoint. If you are installing a closed-loop system for the first time, it is possible that you won’t know your exact target temperature for certain. This is normal and determining that temperature setpoint will be an important step during the commissioning of the system. Once established, there are considerable advantages to knowing your exact operating temperature.

Troubleshooting Process Issues

​Whether your product drifts out of specification, a defect emerges, or some other production/quality issue develops, knowing your exact operating temperature can allow you to quickly confirm or eliminate it as a possible source. Is your operating temperature still on target? If the answer is yes, then we can quickly move on in our investigation.

Reset to Optimal Temperature Easily

​Whether your operating temperature was changed as result of a one-off production run, a maintenance program, tampering, or any other reason, when you know your correct operating temperature you can easily reset your closed-loop system to the correct setting.

Catch Maintenance Warning Signs Early

In addition to the operating temperature, a closed-loop temperature control system should allow you to easily monitor your percentage power output at any given moment. Access to this type of information can be very beneficial, you may even want to incorporate regular checks into your preventative maintenance schedule. Below are a couple of hypothetical examples:

A heat system is installed and operating with full air flow from the blower and a temperature setpoint of 200°C. At first installation, the controller indicates that the heater is using 50% of its power capacity to maintain the operating temperature. However, when the system is checked 3 month later the controller indicated that heater is now only using 40% of capacity! This could be an indication of a potential maintenance issue that should be addressed proactively. Perhaps air flow to the heater has been reduced, possibly a clogged blower inlet filter or an object placed in close proximity to the inlet is choking the flow.
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A heat system is installed and operating with full air flow from the blower and a temperature setpoint of 350°C. During the last system check the controller indicated that the heater was using 60% of its power capacity to maintain the operating temperature. However, only a week later the system indicated that the heater is using 90% of its capacity! This could be an indication that one phase in your 3 phase heating element has blown out and the other two phases have been amped up by the closed-loop system to compensate and maintain the setpoint.

System Updating and Long-term Reliability

​When you know your operating parameters, replacing old and out-of-date equipment becomes a lot easier. You can have confidence when changing the make or model of a heater, because you know the operating parameters that really matter. You can also make changes to the process based on efficiency and what you know about your process. For example, if your previous system never operated above 60% capacity, perhaps you can save some money and switch to a smaller heater.
 
STANMECH is a big advocate of knowing your operating parameters. This is a critical aspect in having complete control over your process. 

Read part 3 of 3: When Open-Loop is the Correct Choice