Robert D. Lorenz

Transducerless position and velocity estimation in induction and salient AC machines

This paper presents a viable transducerless rotor position and velocity estimation scheme for PWM inverter driven induction, synchronous, and reluctance machines with the capability of providing robust and accurate dynamic estimation independent of operating point, including zero and very high speeds, light and heavy loading. The injection of a balanced 3 phase high frequency signal (500 to 2 kHz) generated by the inverter, followed by appropriate signal demodulation and processing combined with a closed-loop observer, enable the tracking of rotor magnetic saliencies from the machine terminals. Although rotor magnetic saliency is inherent within reluctance machines, and most synchronous machines, saliency in the induction machine is introduced via a modulation of the rotor slot leakage with minimal detrimental effects on the machine performance. Experimental verification for the induction machine is included.<<ETX>>

A physically insightful approach to the design and accuracy assessment of flux observers for field oriented induction machine drives

Rotor flux observers can provide an attractive means for achieving direct field oriented control of induction machines. The authors present a physics-based design methodology and use it to evaluate open loop observers and to develop a novel closed loop flux observer. It is shown that the new flux observer is a straightforward structure with properties that combine the best features of known methods. A distinction is made between observers which use only integration and feedback summation operations, and those estimation methods requiring approximate differentiation which are, in essence, cancellation methods. Furthermore, a clear distinction is made between accuracy and dynamic robustness of the observer. This distinction is important because the accuracy of flux observers for induction machines is inherently parameter sensitive. A distinctive form of frequency response function analysis similar to that used in classical control engineering is demonstrated to be a useful and insightful tool even though flux observers are multiple-input, multiple-output systems.<<ETX>>

Motion control with induction motors

Induction motors (IM) provide a very wide speed range, mechanically robust and relatively low cost motion control option. An up-to-date summary of the status of induction motor motion control technology is the subject of this paper. The topics which this paper includes are as follows: basic motion control system requirements; field orientation instantaneous torque control principles for induction motors (FO-IM); current regulators for induction motor motion control; flux and torque regulators for induction motor motion control; self commissioning and continuous self-tuning for field orientation. Technology advances based on modern control and estimation theory have the potential to further enhance the capability of this important class of servo drive systems. >

A simplified approach to continuous on-line tuning of field-oriented induction machine drives

A simplified approach to continuous, online tuning of rotor flux feedforward, field-oriented induction machine drives is presented. The procedure offers the advantages of not requiring a special test signal or special test conditions. The approach takes advantage of the stator voltage equations, which allow robust, parameter-insensitive estimation of the electromagnetic torque while operating at nominal speeds for which the stator voltage (IR) drop is negligible ( mu approximately 5 Hz and above). This torque computation is independent of the rotor flux field-oriented control. In essence the approach uses a stator flux controller to detect improper tuning of the feed forward, rotor flux controller. The authors present the theoretical and practical implementation of such a continuous, self-tuning system. Experimental results are based on a rotor flux feedforward, field-oriented induction machine servo drive.<<ETX>>

High resolution velocity estimation for all digital, AC servo drives

Because the position transducers commonly used (optical encoders and electromagnetic resolvers) do not inherently produce a true, instantaneous velocity measurement, some signal processing techniques are generally used to estimate the velocity at each sample instant. This estimated signal is then used as the velocity feedback signal for the velocity loop control. An analysis is presented of the limitations of such approaches, and a technique which optimally estimates the velocity at each sample instant is presented. The method is shown to offer a significant improvement in command-driven systems and to reduce the effect of quantized angular resolution which limits the ultimate performance of all digital servo drives. The noise reduction is especially relevant for AC servo drives due to the high current loop bandwidths required for their correct operation. The method demonstrates improved measurement performance over a classical DC tachometer.<<ETX>>

Initial rotor position estimation of an interior permanent-magnet synchronous machine using carrier-frequency injection methods

This work presents a method using carrier-frequency injection to estimate the initial rotor position and magnetic polarity for an interior permanent-magnet synchronous machine. A nonsaturating inductance model of the machine provides no information about the polarity of the rotor magnet because the position observer based on this model is locally stable at both poles. To distinguish the polarity of the rotor magnet, the magnetic saturation effect can be used. The Taylor series can be used to describe the nonlinear magnetic saturation relationship between the current and the flux linkage in the d-axis rotor reference frame. The second-order term produces the second harmonic component of the carrier frequency, and the sign of its coefficient identifies the polarity of the rotor magnet being tracked. Both simulation and experimental results show good response of the position observer at several rotor electrical positions using either a rotating vector in the stationary reference frame or a oscillating vector in the estimated rotor reference frame.

Sensorless control of interior permanent-magnet machine drives with zero-phase lag position estimation

This paper presents an improved method to estimate rotor motion states for an interior permanent magnet machine drive. This approach is based on the estimation of the saliency-based EMF in a stationary reference frame using a state filter. The spatial information obtained from the estimated saliency-based EMF is used in an observer to estimate the motor motion states. By adding the commanded torque as a feedforward input to the observer, the motion state estimation has zero phase-lag, providing a very high bandwidth estimate.

Experimental identification of friction and its compensation in precise, position controlled mechanisms

The authors present methodologies developed for experimentally determining accurate models for the nonlinear friction inherent in most mechanisms. The second objective is to present alternative closed-loop controller strategies for decoupling the effect of friction in order to improve positioning accuracy. The identification methodology is novel in the manner in which it extracts the nonlinear friction properties from the closed-loop errors via an iterative signal processing technique. The present work is based on both theoretical modeling and a practical position control problem which was substantially resolved in developing the methodologies. The application was a robotic gripper with highly preloaded rack and pinion mechanism. The authors provide both measurement and control design methodologies to help systematically circumvent the problems of nonlinear friction in precise, position controlled mechanisms.<<ETX>>

Carrier-Signal Selection for Sensorless Control of PM Synchronous Machines at Zero and Very Low Speeds

This paper compares pulsating- and rotating-vector carrier-signal injection for self-sensing (or sensorless) control of permanent-magnet synchronous machines (PMSMs). The main focus of this paper is the evaluation of estimation errors due to the physical nonideal attributes of both the machine and the inverter. Initial-position and magnet-polarity estimations are analyzed as well. Transient response and signal processing for both techniques are discussed for completeness. The theoretical analysis is supported by experimental and simulation evidence obtained using interior PMSMs.

Stator- and rotor-flux-based deadbeat direct torque control of induction machines

A new deadbeat type of direct torque control is proposed, analyzed and experimentally verified in this paper. The control is based on stator and rotor flux as state variables. This choice of state variables allows a graphical representation which is transparent and insightful. The graphical solution shows the effects of realistic considerations such as voltage and current limits. A position and speed sensorless implementation of the control, based on the self-sensing signal injection technique, is also demonstrated experimentally for low speed operation. The paper first develops the new, deadbeat DTC methodology and graphical representation of the new algorithm. It then evaluates feasibility via simulation and experimentally demonstrates performance of the new method with a laboratory prototype including the sensorless methods.