Electrical and Electronic Engineering - Theses
Permanent URI for this collection
Search Results
1 - 2 of 2
-
ItemModel Predictive Controller Tuning by Machine Learning and Ordinal Optimisation( 2021)While for the past several decades model predictive control (MPC) has been an established control strategy in chemical process industries, more recently there has been increased collaboration in MPC research between academia and automotive companies. Despite the promising work thus far, one particular challenge facing the widespread adoption of MPC in the automotive industry is the increased calibration requirement. The focus of the research in this thesis is to develop methods towards reducing the calibration effort in designing and implementing MPC in practice. The research is tailored by application to offline tuning of quadratic-cost MPC for an automotive diesel air-path, to address the limited time-availability to perform online tuning experiments. Human preferences can be influential in automotive engine controller tuning. Some earlier work has proposed a machine learning controller tuning framework (MLCTF), which learns preferences from numeric data labelled by human experts, and as such, these learned preferences can be replicated in automated offline tuning. Work done in this thesis extends this capability by allowing for preferences to be learned from pairwise comparison data, with monotonicity constraints in the features. Two methods are proposed to address this: 1) an algorithm based around Gaussian process regression; and 2) a Bayesian estimation procedure using a Dirichlet prior. These methods are successfully demonstrated in learning monotonicity-constrained utility functions in time-domain features from data consisting of pairwise rankings for diesel air-path trajectories. The MLCTF also constitutes a plant model, yet there will typically be some uncertainty in an engine model, especially if it has been identified from data collected with a limited amount of experimentation time. To address this, an active learning framework is proposed for selection of the next operating points in the design of experiments, for identifying linear parameter-varying systems. The approach is based on exploiting the probabilistic features of Gaussian process regression to quantify the overall model uncertainty across locally identified models, resulting in a flexible methodology which accommodates for various techniques to be applied for estimation of local linear models and their corresponding uncertainty. The framework is applied to the identification of a diesel engine air-path model, and it is demonstrated that measures of model uncertainty can be quantified and subsequently reduced. To make the most of the limited availability for online tuning experiments, an ordinal optimisation (OO) approach is proposed, which seeks to ensure that offline tuned controllers can perform acceptably well, once tested online with the physical system. Via the use of copula models, an OO problem is formulated to be compatible with the tuning of controllers over an uncountable search space, such as quadratic-cost MPC. In particular, results are obtained which formally characterise the copula dependence conditions required for the OO success probability to be non-decreasing in the number of offline controllers sampled during OO. A gain-scheduled MPC architecture was designed for the diesel air-path, and implemented on an engine control unit (ECU). The aforementioned non-decreasing properties of the OO success probability are then specialised to tuning gain-scheduled controller architectures. Informed by these developments, the MPC architecture was firstly tuned offline via OO, and then tested online with an experimental diesel engine test rig, over various engine drive-cycles. In the experimental results, it was found that some offline tuned controllers outperformed a manually tuned baseline MPC, the latter which has comparable performance to proprietary production controllers. Upon additional manual tuning online, the performance of the offline tuned controllers could also be further refined, which illustrates how offline tuning via OO may complement online tuning approaches. Lastly, using an analytic lower bound developed for OO under a Gaussian copula model, a sequential learning algorithm is developed to address a probabilistically robust offline controller tuning problem. The algorithm is formally proven to yield a controller which meets a specified probabilistic performance specification, assuming that the underlying copula is not too unfavourably far from a Gaussian copula. It is demonstrated in a simulation study that the algorithm is able to successfully tune a single controller to meet a desired performance threshold, even in the presence of probabilistic uncertainty in the diesel engine model. This is applied to two case studies: 1) `hot-starting' an online tuning procedure; and 2) tuning for uncertainty inherent across a fleet of vehicles.
-
ItemA Model-based Approach for High Performance Motion Control in Industrial Machines( 2020)Industrial robotics typically consider laser/water cutting, grinding, etc. Within these machines, the motion controller is responsible for the positioning of the end effector. The performance of the motion controller directly influences the quality of the resulting product as tolerance/accuracy are surrogates for machining quality. This is particularly relevant in tracking and contouring applications when the system has structural flexibility, and no direct feedback measurement of the end-effector position is available. Traditional control architectures in machining are unable to explicitly bound tracking and/or contouring errors, and conservative operation is used to ensure satisfactory performance of the overall system. Bounding errors without unduly compromising machine throughput requires advanced control algorithms. The development of such algorithms is the focus of this thesis. Although numerous control methods are proposed, the proportional integral derivative (PID) based cascaded control is still the most prevalent in the industry. Based on this fact, the research starts by objectively assessing the tracking control performance on a single-axis industrial platform. The results provide practitioners with an in-depth understanding of the benefits and limitations of existing control algorithms as well as the motivation to consider advanced controllers as alternatives to the PID-based approach. For the single-axis tracking problem, this research proposes a model predictive based approach that guarantees a desired level of tracking error is met for the cases where the structure is flexible and the end-effector position is estimated. To achieve this, a robust control invariant set is estimated using a computationally tractable algorithm and incorporated into the problem formulation. The applicability of the proposed approach is successfully demonstrated via simulation and experiments conducted on a commercial single-axis system. In terms of biaxial applications, the dual-drive gantry machines are widely used in industry for manufacturing. However, the non-synchronised movement of the dual drive may lead to deterioration in contouring accuracy. In this research, we propose two model predictive based control architectures based on the switched linear time invariant control-oriented models, that is able to guarantee a two-dimensional contouring tolerance in the presence of uncertainty arising from imperfect drive synchronisation. The performance and computational tractability of the proposed approach are demonstrated using high fidelity simulation and experiment.