Control system for electric motor circuit

Abstract

A control system for an electric motor circuit comprises a current controller which produces a set of idealised voltage demands for the motor circuit, an observer which observes the inputs to the motor circuit and the outputs of the motor circuit and which generates from the observations estimates of the voltage disturbances within the motor circuit, the observer being arranged in use to output a first correction signal indicative of the voltage disturbances in the motor circuit, a feed-forward controller which receives as an input a measurement or estimate of the current flowing in the motor and calculates from the input a second correction signal. The first correction signal output from the observer and the second correction signal output from the feedforward controller are combined with the idealised voltage demands output from the controller to provide a set of modified voltages demands that are fed to the motor.

Claims

1. A control system for an electric motor circuit, the control system comprising: a current controller which produces a set of idealised voltage demands for the motor circuit, an observer which observes inputs to the motor circuit and outputs of the motor circuit and which generates from the observations estimates of voltage disturbances within the motor circuit, the observer being arranged in use to output a first correction signal indicative, of the voltage disturbances in the motor circuit, a feedforward controller which receives as an input a measurement or an estimate of current flowing in a motor and calculates from the input a second correction signal, and a compensating means which combines both the first correction signal output from the observer and the second correction signal output from the feedforward controller with the idealised voltage demands output from the current controller to provide a set of modified voltages demands that are fed to the motor wherein the input to the feedforward controller corn rises an estimate of the motor current produced by the observer.

2. The control system for the electric motor circuit according to claim 1 in which the motor current signal input to the feedforward controller is provided from the observer with no sampling delay.

3. A control system for an electric motor circuit, the control system comprising: a current controller which produces a set of idealised voltage, demands for the motor circuit, an observer which observes inputs to the motor circuit and outputs of the motor circuit and which generates from the observations estimates of voltage disturbances within the motor circuit, the observer being arranged in use to output a first correction signal indicative of the voltage disturbances in the motor circuit, a feedforward controller which receives as an input a measurement or an estimate of current flowing in a motor and calculates from the input a second correction signal, and a compensating means which combines both the first correction signal output from the observer and the second correction signal output from the feedforward controller with the idealised voltage, demands output from the current controller to provide a set of modified voltages demands that are fed to the motor wherein the feedforward controller additionally receives as an input a measure of an angular velocity of the motor, either the mechanical or electrical angular velocity.

4. A control system for an electric motor circuit, the control system comprising: a current controller which produces a set of idealised voltage, demands for the motor circuit, an observer which observes inputs to the motor circuit and outputs of the motor circuit and which generates from the observations estimates of voltage disturbances within the motor circuit, the observer being arranged in use to output a first correction signal indicative of the voltage disturbances in the motor circuit, a feedforward controller which receives as an input a measurement or an estimate of current flowing in a motor and calculates from the input a second correction signal, and a compensating means which combines both the first correction signal output from the observer and the second correction signal output from the feedforward controller with the idealised voltage demands output from the current controller to provide a set of modified voltages demands that are fed to the motor wherein the observer is arranged to provide smooth estimates of undelayed d- and q-axis motor currents with no sampling delay.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a diagram of a simple closed loop current control system for an electrical plant, such as an electric motor;

(2) FIG. 2 is a corresponding diagram of an embodiment of a closed loop current control system in accordance with the present invention;

(3) FIG. 3 is a diagram showing the Augmented D-Axis Electric Plant Model for LQE Design; and

(4) FIG. 4 is a diagram showing the Augmented Q-Axis Electric Plant Model for LQE Design.

DETAILED DESCRIPTION OF THE INVENTION

(5) FIG. 2 shows an embodiment of a closed loop motor current control system in accordance with the present invention. In this example the motor 100 comprises an interior permanent magnet motor which forms part of an electric power assisted steering system. However, the invention can be applied to other motor types and systems other than power assisted steering systems.

(6) The current control system comprises a current sensing system 12 and a current controller 14. The current sensing system 12 comprises a current sensor arranged to measure the currents i.sub.A, i.sub.B, i.sub.C in the three phases of the motor, which comprise stationary windings, and output a signal indicative of the current vector in the stationary coordinates having and components. The current sensing system 12 further comprises a coordinate transformation circuit arranged to convert the current vector from the and components in the stationary reference frame, to D and Q components i.sub.D and i.sub.Q defining the current vector in the rotor reference frame, which rotates relative to the fixed windings, with the Q axis current being orthogonal to the D axis current.

(7) The current controller 14 includes a PI controller 16 that receives as a primary input a current error signal I.sub.DQerr which is obtained by combining a demanded current IDS with a measure or estimate of the actual motor current. Typically two methods of phase current measurement can be employed:

(8) 1. Phase current sensors, where a current measurement device is placed in each of the phases. (For a three phase system it may be that only 2 phases are measured as the 3rd phase can be calculated from the 2 measured phases.)

(9) 2. Single current sensor, where the current flowing in the DC link is measured at specific points during the PWM duty cycle to allow the current in the 3 phases to be calculated.

(10) In the example embodiment the current is measured using a single current measurement sensor.

(11) The PI controller 16 outputs an ideal motor demand voltage in the form of voltage vector, specifically in this embodiment in the form of a D and Q axis voltage demand signal V.sub.DQ. The controller calculates a value of VDQ that reduces, ideally to zero, the current error I.sub.DQerr so that the measured current vector approaches the demanded current vector. As will be explained later this ideal voltage demand signal is further modified to produce an actual voltage demand signal which will compensate for disturbances in the motor and motor drive that cause it to behave in a non-ideal manner.

(12) A further transformation circuit (not shown) receives the actual voltage demand signal and converts it to and components V.sub. which are input to the motor and drive circuit. The drive circuit in this example comprises a PWM driver or inverter 22 which is arranged to control a number of switches to apply voltages to the phase windings of the motor in a PWM pattern which produces the net voltage in the windings having a magnitude and direction corresponding to the voltage demand vector. The switches may be arranged in a bridge with a top and bottom switch for each motor phase as shown in FIG. 3.

(13) The modification of the ideal motor demand voltages to the actual motor demand voltages within the control system is performed by two separate sub-circuits which work together to form a compensation system. The two sub-circuits can be broken down into:

(14) Sub-circuit 1: A Feedforward Control 18 for cross-coupling and back-EMF voltage compensation.

(15) Sub-circuit 2: An Observer-Based Feedforward Control including a linear observer 20 for voltage disturbance compensation and for proving the input to sub-circuit 1.

(16) These two sub-circuits produce respectively a first correction signal V.sub.dist,est and a second correction signal V.sub.dist,est. In use the first correction signal is subtracted from the ideal voltage demand signal to compensate for disturbances in the plant based on a predefined model 24 of the motor 100. In ideal operating conditions with a perfect motor that matches the model, this will remove most if not all of the errors. However, in practice some errors will not be removed as the motor will not perfectly match the model.

(17) The second compensation signal is subtracted from this modified voltage demand signal to compensate for disturbances in the plant, thereby correcting the errors that the feedforward control does not correct with a non-ideal motor.

(18) The feedforward control is based on an analytical calculation of the cross-coupling and back-EMF voltages using constant inductance and permanent magnet flux values. Therefore, it provides a highly dynamic compensation of nominal voltage disturbances from cross-coupling and back-EMF. In effect, the feedforward controller guesses what the errors may be according to the model, and generates a suitable compensation voltage.

(19) In the real system, inductances and permanent magnet flux change with the operating conditions (e.g. temperature, load) or have some sort of variation due to Part-2-Part variation. Therefore, error is introduced to the feedforward compensation and uncompensated voltage disturbances remain that negatively impact current control (i.e. the feedforward control is not robust).

(20) The sum of the remaining voltage disturbances and any other voltage disturbances affecting current control (e.g. error due to electrical parameter variation) can be reconstructed by linear observers with integrator disturbance models. Thereby, these disturbances become accessible for feedforward control and can be instantaneously compensated. Hence, robustness is ensured.

(21) An additional benefit of the observer is that it provides smooth estimates of undelayed d- and q-axis current signals. Since measurement delay is in general restricting the performance of feedforward control (using delayed currents for the calculation of cross-coupling and back-EMF voltages gives delayed compensation terms for feedforward control and hence results in an inaccurate compensation in particular at high frequencies), this limitation can be overcome by using undelayed current estimates from the observers as an input to feedforward control. Thereby, the bandwidth of disturbance compensation can be significantly improved by using undelayed current estimates rather than measured currents for feedforward control.

(22) In control terms the motor and motor drive circuit can be considered to be the plant, and may include additional circuitry located between the input for the actual voltage demand signals and the current signals output from the motor to the current sensor. This plant may include, for instance, the transformation circuit.

(23) For the d- and q-axis two separate electric plant models may be defined. In these models the cross-coupling and back-EMF voltages are considered as unknown inputs. Thereby, fully linear time-discrete system models in state-space representation may be derived.

(24) Design of a Suitable Observer

(25) The observer may be synthesized as linear quadratic estimators (LQE) (or optimal observers). They provide optimal estimates for the d- and q-axis currents and reconstruct unknown voltage disturbances based on the voltage control inputs and current measurement outputs.

(26) Looking at FIG. 3 and FIG. 4, the objective of LQE design becomes clearer. The objective of the design is to find optimal estimates for the system plant states and for the unknown disturbance states in the presence of process and measurement noise. For example for a high level of process noise w(k) and a low level of measurement noise v(k), the estimator may rely more on the measurement signals than on the input signals for the state estimation and vice versa.

(27) For the discrete implementation of the optimal observer, the current estimator form may be selected which corrects the state estimates using the most recent measurement samples. For the application of feedforward control this form is to be preferred, because it provides the fastest response to unknown disturbances or measurement errors.

(28) The skilled reader will appreciate, from the teaching of this document, that the embodiment provides a motor controller arranged to provide for voltage disturbance compensation for a current controlled Interior Permanent Magnet Synchronous Motor (IPMSM), especially suitable for use in an electric assisted power steering system (EPS). The controller can in use compensate for voltage disturbances originating from cross-coupling, back-EMF, electrical parameter variation or uncertainty and other unknown sources. The applicant has appreciated that the Voltage Disturbance Compensation of the invention combines the advantages of a feedforward disturbance compensation (high dynamics) and an observer-based disturbance compensation (robustness) and may additionally provide other synergistic advantages.

(29) In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiments. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.