Light detection and ranging systems are able to measure conditions at a distance in front of wind... more Light detection and ranging systems are able to measure conditions at a distance in front of wind turbines and are therefore suited to providing preview information of wind disturbances before they impact the turbine blades. In this study, preview-based disturbance feedforward control is investigated for load mitigation. Performance is evaluated assuming highly idealized wind
measurements that rotate with the blades and more realistic stationary measurements. The results obtained using idealized, "best case" measurements show that excellent performance gains are possible with reasonable pitch rates. However, the results using more realistic wind measurements show that without further optimization of the controller and/or better processing of measurements, errors in determining the shear local to each blade can remove any advantage obtained by using preview-based feedforward techniques.
The cost of wind energy can be reduced by controlling the power reference of a turbine to increas... more The cost of wind energy can be reduced by controlling the power reference of a turbine to increase energy capture, while maintaining load and generator speed constraints. We apply standard torque and pitch controllers to the direct inputs of the turbine and use their set points to change the power output and reduce generator speed and blade load transients. A power reference controller increases the power output when conditions are safe and decreases it when problematic transient events are expected. Transient generator speeds and blade loads are estimated using a gust measure derived from a wind speed estimate. A hybrid controller decreases the power rating from a maximum allowable power. Compared to a baseline controller, with a constant power reference, the proposed controller results in generator speeds and blade loads that do not exceed the original limits, increases tower fore‐aft damage equivalent loads by 1%, and increases the annual energy production by 5%.
Compressed Sensing for Atomic Force Microscopy is a newer imaging mode that requires the piezo st... more Compressed Sensing for Atomic Force Microscopy is a newer imaging mode that requires the piezo stage be driven rapidly between measurement locations. In contrast to raster scanning applications, this translates to a setpoint tracking problem. This paper considers the setpoint tracking performance of a piezo nano-positioning stage subject to rate-of-change limitations on the control signal, which we derive from the current limit of the power amplifier. To compensate the vibrational dynamics of the stage, we consider a model predictive control scheme (MPC) and a linear quadratic Gaussian (LQG) controller which saturates the control increment. In both cases, hysteresis and drift are compensated via dynamic inversion. We analyze the robustness of both schemes using classical methods. We conclude that model predictive control is of limited practical utility because selecting the control weights such that model predictive control is nominally beneficial results in degraded robustness, and...
Controls research plays an important role in wind energy. Advances in controls are making wind tu... more Controls research plays an important role in wind energy. Advances in controls are making wind turbines more efficient, more reliable, and more cost-effective. Wind turbines have evolved from passively controlled machines to actively controlled machines, and more recently, to distributed machines controlled collectively (wind farms). With this open session, we invite researchers to present their latest results in wind energy control. The attendees of these sessions will learn how controls research can make substantial contributions to wind energy, and they will also get an overview of the latest developments and open issues. Example contributions include: ‘smart’ rotor control, lidar-based control, control of floating turbines, wind farm control, and active power control.
Acquiring Atomic Force Microscope images using compressed sensing requires the piezo X−Y stage to... more Acquiring Atomic Force Microscope images using compressed sensing requires the piezo X−Y stage to track a sequence of step commands. To achieve fast tracking of such commands requires a precise system model. We show that once such a model is obtained, standard linear feedback can be used to achieve excellent tracking of step inputs. The system under consideration has a significant amount of time delay, and we develop a computationally efficient state estimator for this scenario. We demonstrate that beyond a certain step size threshold, the control law attempts to violate actuator slew-rate limits, resulting in a severely deteriorated response. We then show how this can be overcome by tracking an optimal trajectory obtained by solving a constrained, finite horizon Linear Quadratic Regulator problem and demonstrate the feasibility of this approach experimentally.
2017 IEEE Conference on Control Technology and Applications (CCTA), 2017
Random sub-sampling imaging methods in Atomic Force Microscopy require the piezo X-Y stage to tra... more Random sub-sampling imaging methods in Atomic Force Microscopy require the piezo X-Y stage to track a sequence of step inputs. Control slew-rate limits combined with linear feedback methods have been shown to limit achievable performance in this scenario. Due to its natural ability to account for actuation constraints, we consider the application of Model Predictive Control. By recasting the problem in an incremental form, the arising quadratic program takes a form that can be solved efficiently. Specifically, we solve an input constrained Model Predictive Control problem with 50 states, a control horizon of 12 samples, and a sample frequency of 25 kHz using the Fast Gradient Method. We present experimental results using the method applied to a nano-positioning stage.
We compare two common model inversion ar- chitectures, plant inverse (PI) and closed-loop inverse... more We compare two common model inversion ar- chitectures, plant inverse (PI) and closed-loop inverse (CLI), by evaluating their ability to achieve settle time performance improvements. The plant models of interest are discrete- time, single-input single-output (SISO), linear time-invariant (LTI), nonminimum phase (NMP), and uncertain. We use a simple algebraic analysis to show that PI and CLI yield the same desired to actual output dynamics if the plant is minimum phase. Using a stable inverse approximation when the plant is certain but NMP, the same algebraic analysis shows that CLI achieves superior settle time performance relative to PI when the settle boundaries are tight. Simulation and experimental data are used to derive conclusions when the plant is NMP and uncertain. We show that CLI has superior performance over PI for our plant dynamics of interest when low frequency parametric uncertainty is present. For higher frequency unstructured uncertainty, the distinction between the...
This article provides an overview of utility grid operation by introducing the fundamental behavi... more This article provides an overview of utility grid operation by introducing the fundamental behavior of the electrical system, explaining the importance of maintaining grid reliability through balancing generation and load, and describing the methods of providing ancillary services using conventional utilities. This article also introduces the basic structural components of wind turbines, explains the traditional control systems for capturing maximum power, and highlights control methods developed in industry and academia to provide active power ancillary services with wind energy. As the penetration of wind energy continues to grow, the participation of wind turbines and wind farms in grid frequency stability is becoming more important. The future of wind energy development and deployment depends on many factors, such as policy decisions, economic markets, and technology improvements. Improvements through research and development in areas such as forecasting, turbine manufacturing p...
Light detection and ranging systems are able to measure conditions at a distance in front of wind... more Light detection and ranging systems are able to measure conditions at a distance in front of wind turbines and are therefore suited to providing preview information of wind disturbances before they impact the turbine blades. In this study, preview-based disturbance feedforward control is investigated for load mitigation. Performance is evaluated assuming highly idealized wind
measurements that rotate with the blades and more realistic stationary measurements. The results obtained using idealized, "best case" measurements show that excellent performance gains are possible with reasonable pitch rates. However, the results using more realistic wind measurements show that without further optimization of the controller and/or better processing of measurements, errors in determining the shear local to each blade can remove any advantage obtained by using preview-based feedforward techniques.
The cost of wind energy can be reduced by controlling the power reference of a turbine to increas... more The cost of wind energy can be reduced by controlling the power reference of a turbine to increase energy capture, while maintaining load and generator speed constraints. We apply standard torque and pitch controllers to the direct inputs of the turbine and use their set points to change the power output and reduce generator speed and blade load transients. A power reference controller increases the power output when conditions are safe and decreases it when problematic transient events are expected. Transient generator speeds and blade loads are estimated using a gust measure derived from a wind speed estimate. A hybrid controller decreases the power rating from a maximum allowable power. Compared to a baseline controller, with a constant power reference, the proposed controller results in generator speeds and blade loads that do not exceed the original limits, increases tower fore‐aft damage equivalent loads by 1%, and increases the annual energy production by 5%.
Compressed Sensing for Atomic Force Microscopy is a newer imaging mode that requires the piezo st... more Compressed Sensing for Atomic Force Microscopy is a newer imaging mode that requires the piezo stage be driven rapidly between measurement locations. In contrast to raster scanning applications, this translates to a setpoint tracking problem. This paper considers the setpoint tracking performance of a piezo nano-positioning stage subject to rate-of-change limitations on the control signal, which we derive from the current limit of the power amplifier. To compensate the vibrational dynamics of the stage, we consider a model predictive control scheme (MPC) and a linear quadratic Gaussian (LQG) controller which saturates the control increment. In both cases, hysteresis and drift are compensated via dynamic inversion. We analyze the robustness of both schemes using classical methods. We conclude that model predictive control is of limited practical utility because selecting the control weights such that model predictive control is nominally beneficial results in degraded robustness, and...
Controls research plays an important role in wind energy. Advances in controls are making wind tu... more Controls research plays an important role in wind energy. Advances in controls are making wind turbines more efficient, more reliable, and more cost-effective. Wind turbines have evolved from passively controlled machines to actively controlled machines, and more recently, to distributed machines controlled collectively (wind farms). With this open session, we invite researchers to present their latest results in wind energy control. The attendees of these sessions will learn how controls research can make substantial contributions to wind energy, and they will also get an overview of the latest developments and open issues. Example contributions include: ‘smart’ rotor control, lidar-based control, control of floating turbines, wind farm control, and active power control.
Acquiring Atomic Force Microscope images using compressed sensing requires the piezo X−Y stage to... more Acquiring Atomic Force Microscope images using compressed sensing requires the piezo X−Y stage to track a sequence of step commands. To achieve fast tracking of such commands requires a precise system model. We show that once such a model is obtained, standard linear feedback can be used to achieve excellent tracking of step inputs. The system under consideration has a significant amount of time delay, and we develop a computationally efficient state estimator for this scenario. We demonstrate that beyond a certain step size threshold, the control law attempts to violate actuator slew-rate limits, resulting in a severely deteriorated response. We then show how this can be overcome by tracking an optimal trajectory obtained by solving a constrained, finite horizon Linear Quadratic Regulator problem and demonstrate the feasibility of this approach experimentally.
2017 IEEE Conference on Control Technology and Applications (CCTA), 2017
Random sub-sampling imaging methods in Atomic Force Microscopy require the piezo X-Y stage to tra... more Random sub-sampling imaging methods in Atomic Force Microscopy require the piezo X-Y stage to track a sequence of step inputs. Control slew-rate limits combined with linear feedback methods have been shown to limit achievable performance in this scenario. Due to its natural ability to account for actuation constraints, we consider the application of Model Predictive Control. By recasting the problem in an incremental form, the arising quadratic program takes a form that can be solved efficiently. Specifically, we solve an input constrained Model Predictive Control problem with 50 states, a control horizon of 12 samples, and a sample frequency of 25 kHz using the Fast Gradient Method. We present experimental results using the method applied to a nano-positioning stage.
We compare two common model inversion ar- chitectures, plant inverse (PI) and closed-loop inverse... more We compare two common model inversion ar- chitectures, plant inverse (PI) and closed-loop inverse (CLI), by evaluating their ability to achieve settle time performance improvements. The plant models of interest are discrete- time, single-input single-output (SISO), linear time-invariant (LTI), nonminimum phase (NMP), and uncertain. We use a simple algebraic analysis to show that PI and CLI yield the same desired to actual output dynamics if the plant is minimum phase. Using a stable inverse approximation when the plant is certain but NMP, the same algebraic analysis shows that CLI achieves superior settle time performance relative to PI when the settle boundaries are tight. Simulation and experimental data are used to derive conclusions when the plant is NMP and uncertain. We show that CLI has superior performance over PI for our plant dynamics of interest when low frequency parametric uncertainty is present. For higher frequency unstructured uncertainty, the distinction between the...
This article provides an overview of utility grid operation by introducing the fundamental behavi... more This article provides an overview of utility grid operation by introducing the fundamental behavior of the electrical system, explaining the importance of maintaining grid reliability through balancing generation and load, and describing the methods of providing ancillary services using conventional utilities. This article also introduces the basic structural components of wind turbines, explains the traditional control systems for capturing maximum power, and highlights control methods developed in industry and academia to provide active power ancillary services with wind energy. As the penetration of wind energy continues to grow, the participation of wind turbines and wind farms in grid frequency stability is becoming more important. The future of wind energy development and deployment depends on many factors, such as policy decisions, economic markets, and technology improvements. Improvements through research and development in areas such as forecasting, turbine manufacturing p...
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measurements that rotate with the blades and more realistic stationary measurements. The results obtained using idealized, "best case" measurements show that excellent performance gains are possible with reasonable pitch rates. However, the results using more realistic wind measurements show that without further optimization of the controller and/or better processing of measurements, errors in determining the shear local to each blade can remove any advantage obtained by using preview-based feedforward techniques.
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measurements that rotate with the blades and more realistic stationary measurements. The results obtained using idealized, "best case" measurements show that excellent performance gains are possible with reasonable pitch rates. However, the results using more realistic wind measurements show that without further optimization of the controller and/or better processing of measurements, errors in determining the shear local to each blade can remove any advantage obtained by using preview-based feedforward techniques.