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US-12618584-B2 - Gain-scheduling system and method for improving fan speed control in air handling units

US12618584B2US 12618584 B2US12618584 B2US 12618584B2US-12618584-B2

Abstract

The present disclosure describes a system comprising a gain-scheduling control strategy which improves its nonlinear control performance. A control-oriented model, which does not require numerous physical parameters and extensive test data, has been developed to address the nonlinearity of the fan system. Based on theoretical model and experimental verifications, the issue of an aggressive response with a conventional fixed-gain controller is caused by the fact that the system gain is proportional to the ratio of the duct static pressure to the fan speed. To address the issue, a scheduling function of the measurable duct static pressure and fan speed is included in the conventional fixed-gain controller to compensate for the fan system gain variation. The gain-scheduling control strategy approximately maintains the identical control performance under all operation conditions. The gain-scheduling control strategy can be readily implemented on a processor without intensive computation and additional measurements.

Inventors

  • Gang Wang
  • Li Song
  • Zufen Wang
  • Rodney D. Hurt

Assignees

  • THE BOARD OF REGENTS OF THE UNIVERSITY OF OKLAHOMA
  • UNIVERSITY OF MIAMI

Dates

Publication Date
20260505
Application Date
20230207

Claims (19)

  1. 1 . An air handling unit (AHU), comprising a pressure differential sensor, a supply fan having a motor controlled by a variable frequency drive, a gain-scheduling controller configured to receive a duct static pressure measured by the pressure differential sensor, and a fan speed from the variable frequency drive, and which comprises a scheduling function based on the duct static pressure and the fan speed determined by an equation: g ⁡ ( H sp , N ) = H sp , r H sp · N N r where H sp is the duct static pressure, and N is the fan speed, under a first reference condition, and where H sp,r and N r are, respectively, a duct static pressure and the fan speed under a second reference condition, and wherein the gain scheduling controller is configured to provide control signals based upon the scheduling function to the variable frequency drive coupled to the motor to control the motor thereby resulting in a desired control performance of the motor.
  2. 2 . The air handling unit of claim 1 , further comprising a duct, the supply fan having the fan speed and being configured to deliver air to the duct, and a pressure differential sensor configured to measure the duct static pressure within the duct.
  3. 3 . The air handling unit of claim 2 , wherein the pressure differential sensor is positioned between an upstream portion of the duct and a downstream portion of the duct, and wherein the duct static pressure reflects a pressure loss of the downstream portion of the duct.
  4. 4 . The air handling unit of claim 1 , wherein the fan and the duct are modeled by a fan duct gain being a ratio of a duct static pressure change within the duct to a fan speed change of fan speed, and wherein the scheduling function is a gain-scheduling function that is reversely proportional to the fan duct gain.
  5. 5 . The air handling unit of claim 4 , wherein the fan duct gain is calculated at a stable operation condition of the fan.
  6. 6 . The air handling unit of claim 1 , further comprising a variable frequency drive coupled to an induction motor of the fan, and wherein the gain-scheduling controller provides control signals to the variable frequency drive to control speed of the fan.
  7. 7 . The air handling unit of claim 1 , wherein the first reference condition is a predetermined amount of hertz.
  8. 8 . The air handling unit of claim 1 , wherein the second reference condition is a predetermined amount of pascals.
  9. 9 . The air handling unit of claim 8 , wherein the second reference condition is a static pressure setpoint rather than a measured value.
  10. 10 . The air handling unit of claim 1 , wherein the duct static pressure sensor is an existing duct static pressure sensor.
  11. 11 . A method, comprising: receiving duct static pressures within a duct of an air handling unit; receiving fan speeds of a supply fan providing air into the duct; and controlling by a gain-scheduling controller providing control signals to a variable frequency drive coupled to the supply fan, the gain-scheduling controller comprising a scheduling function running on a component, the scheduling function adjusting the control signals based upon the duct static pressures and the fan speeds and wherein the scheduling function generates the control signals with an equation: g ⁡ ( H sp , N ) = H sp , r H sp · N N r where H sp is a duct static pressure selected from a group consisting of a measureable duct static pressure, which is measured by a duct static pressure sensor; and a duct static pressure setpoint, and N is the fan speed, which can be obtained from a fan speed feedback of a variable frequency drive (VFD), under a reference condition, and where H sp,r and N r are, respectively, a duct static pressure and a fan speed under a reference condition; providing control signals, by the gain-scheduling controller, based upon the scheduling function to the variable frequency drive coupled to the motor to control the motor.
  12. 12 . The method of claim 11 , wherein receiving the duct static pressures are defined further as receiving the duct static pressures from a duct static pressure sensor in the duct.
  13. 13 . The method of claim 11 , wherein the fan speeds are a fan speed feedback provided by the variable frequency drive coupled to the fan.
  14. 14 . The method of claim 11 , further comprising modeling the fan and the duct by a fan duct gain being a ratio of a duct static pressure change within the duct to a fan speed change of fan speed, and wherein the scheduling function is a gain-scheduling function that is reversely proportional to the fan duct gain.
  15. 15 . The method of claim 14 , further comprising calculating the fan duct gain at a stable operation of the fan.
  16. 16 . The method of claim 11 , further comprising receiving, by the gain scheduling controller, control signals in a form of variable frequency drive frequency commands from a fixed gain controller, and adjusting the variable frequency drive frequency commands to generate the control signals.
  17. 17 . The method of claim 16 , wherein the gain scheduling controller multiplies each of the variable frequency drive frequency commands by a respective gain value to adjust the variable frequency drive frequency command and thereby generate the control signals.
  18. 18 . A controller, comprising: a fixed-gain controller operable to receive a first control signal indicative of a difference between a duct static pressure setpoint and a duct static pressure, the fixed-gain controller comprising circuitry operable to calculate a motive force frequency of a motive force generator based at least in part on the first control signal, the fixed-gain controller being further operable to provide a second control signal indicative of the motive force frequency; and a gain-scheduling controller operable to receive the second control signal, a third control signal indicative of the duct static pressure, and a fourth control signal indicative of a motive force speed of the motive force generator, the gain-scheduling controller comprising circuitry operable to calculate an adjusted motive force frequency based at least in part on a scheduling function: g ⁡ ( H sp , N ) = H sp , r H sp · N N r where H sp is the duct static pressure selected from a group consisting of the measured duct static pressure and the duct static pressure setpoint, N is the motive force speed, H sp,r is a reference duct static pressure, and N r is a reference motive force speed, the gain-scheduling controller being further operable to provide a fifth control signal indicative of the adjusted motive force frequency for controlling the motive force generator.
  19. 19 . The controller of claim 18 , wherein the motive force generator is selected from the group consisting of a fan operable to deliver air and a pump operable to deliver a liquid.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS This patent application claims priority to U.S. Ser. No. 63/307,386, filed on Feb. 7, 2022, the entire content of which is hereby incorporated herein by reference. STATEMENT OF GOVERNMENT INTEREST This invention was made with government support under Contract Number DE-EE0008683 awarded by the US Department of Energy (DOE). The government has certain rights in the Invention. BACKGROUND ART Ventilation energy, which is mainly consumed by the fans in heating, ventilation and air-conditioning systems, accounts for approximately 15.9% of the electricity consumption in commercial buildings in the U.S. (DOE 2011, Zhou, Harberl et al. 2017). Variable air volume (VAV) systems, with adjustable supply fan speed, have gained popularity in air handling units (AHUs) because of their potential to help achieve energy efficiency. Traditionally, the supply fan speed control system in an AHU is a single control loop system in which the supply fan speed is modulated by the variable frequency drive (VFD) to maintain the duct static pressure setpoint. A nonlinear relationship among the fan speed, airflow rate, and duct static pressure has been identified in numerous experiments (Zaheer-Uddin and Zheng 1994, Mei and Levermore 2002, Wang and Wang 2008), indicating a nonlinear feature of the fan speed control system. On the other hand, the duct static pressure reset in VAV systems has proven to be energy efficient with an energy savings of 30%-50% by supply fans compared with the constant static pressure control strategy (Taylor 2007, Shim, Song et al. 2014) and is required for systems with direct digital control (DDC) at the zone level (ASHRAE 2010). As a result, the dynamical reset of static pressure setpoints will significantly change the fan operation conditions. Moreover, the increasing deployment of occupancy sensors in commercial buildings increases the dynamical reset of the terminal box (TB) minimum airflow setpoints (Esrafilian-Najafabadi and Haghighat 2021), resulting in dynamically variable TB damper positions. Variable TB damper positions will, in turn, affect the duct static pressure, creating disturbances on the fan speed control system and on variable operation conditions. Due to its nonlinear feature, the frequently variable operation conditions and external disturbances, coupled with system interactions, make the fan control system design and operation a significant challenge (Mei and Levermore 2000). To achieve good operation performance in addition to low energy cost, it is critical to apply more effective control strategies to the fan speed control system to handle these nonlinearities. To do so, the fan speed control system model needs to be developed and the main factors that cause the nonlinearities have to be identified. Zaheer-Uddin and Zheng (Zaheer-Uddin and Zheng 1994) built a fan speed control system model through theoretical analysis by using nonlinear time-varying equations. However, the theoretical model is complicated, and numerous system operation parameters are required, which is generally not feasible in practice. Mei and Levermore (Mei and Levermore 2002) developed a nonlinear dynamic fan model by combining the artificial neural network (ANN)—trained fan performance curves and a first-order equation with different time constants at various operation conditions. While the model is able to reproduce the trend in fan control performance, the accuracy, especially for closed-loop control systems, is limited and large datasets have to be collected to train the model, which is costly and time consuming. Similarly, Raisoni, Raman et al. (Raisoni, Raman et al. 2018) developed a low order control-oriented model to predict the fan airflow rate in response to changes in fan speed and damper positions by introducing the dynamics into the steady-state functions. The control-oriented model is aimed at real-time control computations. While the model can be used for control system design, a variety of tests have to be conducted to calibrate the unknown parameters. Wang and Wang (Wang and Wang 2008) pointed out the difficulty in establishing the theoretical fan speed control system model. They obtained the relationship among the fan speed, airflow rate and duct static pressure through experimental methods by measuring the duct static pressure variation as the fan airflow rate is changed under different fan speeds while the time constant of the system response is neglected. In conclusion, the fan speed control system models built above are computationally complicated and time-consuming. In addition, none of the studies have explicitly pointed out the component gain variations under various operation conditions, and the key factors that impact the system nonlinear control performance still remain unknown. On the other hand, the controller designed for a traditional fan speed control system is a Proportional-Integral-Derivative (PID) controller with fixed gains. With such a