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Definition: The adjustment of a loop’s control parameters (proportional band/gain, integral gain/reset, derivative gain/rate) to the optimum values for the desired control response.
When commissioning a control loop, the purpose is to evaluate the building controls system for optimization. There are different types of control loops, and with each comes a different method for commissioning. No matter the situation, what is critical is the comprehension of the control loop, how to recognize when the loop is out of tune, and how to provide stabilization for the control loop in question.
Types of Control Loops
Open Loop – The simplest form of loop – No feedback – Output is not used to adjust the input – Best judgement input is used
An example of an open loop would be a sensor reading outdoor air temperature and a controller actuating a control valve as a function of only the temperature outside. There is no feedback delivered to the controller regarding the temperature within the area being heated / cooled, and therefore no adjustments can be made to further optimize the system.
Closed Loop – A more precise form of loop than open – System automatically adjusts the input to achieve the target output – Adds efficiency and functionality to the loop
An example of a closed loop is a room heated by a hot water radiator. The control elements include a sensor, a controller and a controlled device such as a valve actuator and valve. The set-point is adjusted on the thermostat. The sensor measures the actual value and sends this information to the comparison device, which sends the difference between the two values, or “error”, to the controller, which in turn sends the signal to the controlled device which varies the flow of hot water to increase or cool the room temperature.
Cascading Loops – When one closed loop determines the set-point of another closed loop – Useful in separating a slow control loop from a faster control loop
An example of a cascading loop is pressure-independent controls. These controls use cascading control loops. The first loop is used to control space temperature. The feedback from this loop is sent to the second loop, which controls the airflow required to cool the space by varying the VAV damper.
Adaptive Loops – Learning loops that change their own PID factors over time – They use historical data to further trim their damping (see Damping Loops below) – Can be difficult to commission because they don’t react well to large or artificial changes to inputs
These are loops that are used in the same applications as closed loops, except that they use their own feedback to make changes to their tuning procedures in order to streamline their functionality and increase efficiency. They are designed to work smoothly as long as they are left to their own devices. When tampered with, they can be erratic and unpredictable.
Tuning / Damping Loops
The damping of a control loop refers to the process of fine-tuning a loop for efficiency and effectiveness.
Control loops that are under-damped will hunt forever, never settling down. The loop output will continue to swing above and below the target set-point forever. Over-damped loops approach the target set-point so cautiously that they never actually reach it. The ideal performance of a loop, known as a critically damped loop, is when a loop ramps up quickly when far from set-point, then slows down when nearing set-point and settles perfectly on target.
Trend logs can be used to identify prolonged warm-up periods and sluggish responses, indicating an over-damped loop, or consistent cyclical behavior where the loop never flattens out on set-point, indicating under-damping. Request trend logs of all set-points and feedback of all loops, usually in 1 minute sample rates, prior to commissioning in order to help identify out of tune control loops.
A bump test is also an important method of commissioning control loops. By watching the control valve while changing the set-point, you can see if a loop has been properly tuned. Change the set-point 5 or 10% up or down as is appropriate, and look for the proper, critically damped response, which is a single loop followed by a settling onto set-point.
References:
Fundamentals of HVAC Control Systems – SI Edition Ross Montgomery & Robert McDowell 2009 American Society of Heating, Refrigerating, and Air-Conditioning Engineers Inc.
Is Controls Loop Tuning & Night Setback Really That Hard? Greg Munn ATS – Group Operations Manager