K.W. Lim

Analysis of direct torque control in permanent magnet synchronous motor drives

This paper describes an investigation of direct torque control (DTC) for permanent magnet synchronous motor (PMSM) drives. It is mathematically proven that the increase of electromagnetic torque in a permanent magnet motor is proportional to the increase of the angle between the stator and rotor flux linkages, and, therefore, the fast torque response can be obtained by adjusting the rotating speed of the stator flux linkage as fast as possible. It is also shown that the zero voltage vectors should not be used, and stator flux linkage should be kept moving with respect to the rotor flux linkage all the time. The implementation of DTC in the permanent magnet motor is discussed, and it is found that for DTC using available digital signal processors (DSPs), it is advantageous to have a motor with a high ratio of the rated stator flux linkage to stator voltage. The simulation results verify the proposed control and also show that the torque response under DTC is much faster than the one under current control.

A direct torque controlled interior permanent magnet synchronous motor drive incorporating field weakening

This paper presents a new control scheme for the wide speed range operation of interior permanent magnet synchronous motor (PMSM) drives, where both torque and stator flux linkage are directly controlled. The proposed scheme possesses some attractive features when compared to conventional current controlled drives. Current controllers followed by PWM or hysteresis comparators and coordinate transformation are not used. This eliminates the delays through these networks and offers the possibility of dispensing with the rotor position sensor for the electronic commutator, if the initial rotor position is known only approximately. The scheme incorporates all the usual control regimes such as the maximum torque per ampere operation in constant torque region, the flux weakening region, and operates the drive within the voltage and current limits of the motor/inverter. The control scheme has been verified by simulation and experimental tests with a prototype interior magnet motor. This paper describes the scheme in detail, followed by results of its implementation.

A direct torque controller for permanent magnet synchronous motor drives

This paper describes an investigation of direct torque control (DTC) for permanent magnet synchronous motor (PMSM) drives. The analysis of PMSMs shows that the increase of electromagnetic torque is proportional to the increase of the angle between the stator and rotor flux linkages and therefore fast torque response can be obtained by increasing the rotating speed of the stator flux linkage as fast as possible. The implementation of DTC in PMSM drives is discussed and the switching table specific for an interior PMSM is derived. The proposed control is implemented on a prototype PMSM, which has a standard induction motor stator, and the experimental results show that the torque response is extremely fast. It is also demonstrated that the position sensor is not essential for the inner torque control loop of PMSM drives with DTC.

Optimal Gain and Phase Margin Tuning for PID Controllers

Simple tuning formulas for the design of the PID controllers to satisfy both robustness and performance requirements are given. Robust control design is an area of intensive research. The PID formulas given in this paper are simple and can be easily adopted by the industry.

Receding horizon recursive state estimation

Describes a receding horizon discrete-time state observer using the deterministic least squares framework. The state estimation horizon, which determines the number of past measurement samples used to reconstruct the state vector, is introduced as a tuning parameter for the proposed state observer. A stability result concerning the choice of the state estimation horizon is established. It is also shown that the fixed memory receding horizon state observer can be related to the standard dynamic observer by using an appropriate end-point state weighting on the estimator cost function.

Direct torque control of induction motor-variable switching sectors

An induction motor drive based on direct torque control (DTC) allows high dynamic performance to be obtained with a very simple hysteresis control scheme. In this paper, the problem of stator flux drooping at low speeds has been investigated. By rotating the original flux sectors defined for conventional DTC near the sector boundaries, it is possible to maintain stator flux within the prescribed hysteresis bands and therefore improve the stator flux in the machine as well as the harmonic contents of stator currents. Moreover, the use of the proposed scheme also results in a reduction of overall inverter switching frequency. The effectiveness of the proposed scheme is evaluated through simulation studies, and is also supported with experimental results obtained from a 1.5 kW induction motor driven in a laboratory environment.

Position estimation in solenoid actuators

This paper describes a novel method of estimating the position of a plunger inside a solenoid. The solenoid is a single phase variable reluctance actuator, with a highly nonlinear magnetic circuit. For the proposed method, position is estimated indirectly through the solenoids incremental inductance in the high current region. It exploits the advantage that motional e.m.f. is negligible under normal operating conditions. The incremental inductance is obtained from the rate of current rise of the PWM waveform, which in turn, is measured by a dedicated current rise measurement circuit. Position is estimated from a two dimensional look up table of incremental inductance and current. The method is simulated and then implemented on a typical industrial solenoid valve. Factors which affect the accuracy of the estimation and methods of overcoming them are also described in the paper.

Technical Communique: Getting more phase margin and performance out of PID controllers

Designing a PID controller to meet gain and phase margin specification is a well-known design technique. If the gain and phase margin are not specified carefully then the design may not be optimum in the sense that there could be larger phase margin (more robust) that could give better performance. This paper studies the relationship between the integral square error performance index, gain margin, phase margin and gives recommendation for gain and phase margin specification to get more phase margin and performance out of PID controllers.

A dual-rate adaptive digital smith predictor

Abstract The limitations of a single-rate adaptive digital Smith Predictor are discussed. The combination of using a larger sampling interval for on-line parameter estimation and a smaller sampling interval for control, leads to the development of a dual-rate adaptive system. The necessary design modifications for the dual-rate system, namely the additional digital filtering of measurements, the conversion of slow model parameters to those of the fast model and the matching of the dc term, are simple to incorporate. The dual-rate system gives superior responses for set-point following and load rejection with a smaller control interval, while it improves the robustness of the model parameter estimation in the presence of unmodelled dynamics with a larger estimation interval. The superior performance of this new design is substantiated by computer simulation and experiments on a heat exchanger pilot plant.

Brief paper: A state space GPC with extensions to multirate control

A state space formulation for generalized predictive control (GPC) is presented. Unlike other state space formulations, we derive both the state feedback controller and state estimator gains using a least squares framework. The present formulation provides a set of consistent design choices for both the controller and the state estimator design. It also encapsulates both single rate and multirate controllers in the same algorithm. It is argued that such a design approach can be advantageous especially when the control algorithm is required to tolerate sensor failure. The generality of the design method is illustrated by synthesizing two well-known multirate control algorithms. A link with the generalized sampled-data hold function is also established.