GB2578627A

GB2578627A - A control system for an electric motor

Info

Publication number: GB2578627A
Application number: GB1817894.7A
Authority: GB
Inventors: Hui Jiang; Kudanowski Maciej; Brett Williams Connel
Original assignee: TRW Ltd
Current assignee: TRW Ltd
Priority date: 2018-11-01
Filing date: 2018-11-01
Publication date: 2020-05-20
Anticipated expiration: 2038-11-01
Also published as: DE102019216703A1; GB2578627B; GB201817894D0

Abstract

A control system for a permanent magnet electric motor 50 having a plurality of phase windings 112, 114, 116, particularly of an electric power steering system. The control system comprises a control stage arranged to generate a current demand signal in response to a torque demanded of the motor, a drive stage comprising an inverter 121 having a plurality of drive stage switches 128A, 128B, 130A, 130B, 132A, 132B and a driver which generates drive signals for the switches of the inverter in response to the current demand signal. The switches comprise Gallium Nitride field effect transistors (FET) and the control stage is further arranged to overlay a high frequency, at least 20kHz, signal onto the current demand signal supplied to the drive stage. The position of the motor is determined using a sensorless motor position determining means that extracts the corresponding high frequency component of the current flowing in the motor which in use is modulated as function of the position dependent saliency of the motor.

Description

A CONTROL SYSTEM FOR AN ELECTRIC MOTOR

This invention relates to control systems for electric motors, and in particular to the position sensorless control of an electric motor in an electric power steering system (EPS).

It is known to use electric motors in a wide range of applications, with one specific application being the use of an electric motor to generate an assistance torque that helps the driver of a vehicle to turn the steering wheel of the vehicle. in such an application the motor will be required to operate over a very wide range of speeds including down to zero speed. A multi phase permanent magnet DC motor is often used in automotive applications as it can operate from a DC battery supply. The motor is typically driven from an inverter which includes a set of switching elements that can be opened and closed to connect each phase to a positive battery supply of to ground. Rapidly opening and closing the switches in a defined pattern enables AC waveforms to be applied to each phase from the DC battery supply. A typical inverter bridge employed in an EPS system comprises a plurality of silicon metal oxide (MOSFET) semiconductor switching elements which control the current delivered to the motor and thus the rotation of the motor.

A closed loop current system for a electric motor typically includes a control stage that uses 'ieetor control in response to a requested Earn,ct current to generate a Voltage that is applied to each phase winding of the meter by the inverter. The resultant generated currents are measured and then fed into a current controller, typically a PI controller, to control to the requested phase voltages to achieve the target current. For correct control of phase voltage waveforms it is important that the position of the rotor is known. A position sensor could be provided although this adds costs and bulk to the system and is not preferred. A number of different motor control methods have been proposed that estimate the motor position by observing the currents or voltage in the motor rather than using a position sensor.

There are two main approaches to the position sensorless measurement of motor position. A first approach is applicable for high motor speed and a second is applicable for low speed, including zero motor speed (detail). Most techniques have advantages in only one of the low speed or high speed motor ranges, and have disadvantage or may not work at all in the other range. For many applications, in particular the use of the motor in an electric power steering application, there is a need to combine the high and low speed sensorless approaches due to the wide range of operational speeds of the motor.

One known method for high speed sensorless position measurement uses the motor back-EMF which is known to vary with motor rotor position. However, this does not work well at low speeds, and not at all at zero motor speed.

One of the most promising methods used in low speed sensorless motor control is based on the injection of a high frequency (HF) signal onto one or more phases of the motor. It is known that the saliency of a motor causes a motor rotor position dependent variation in the inductance of the motor. By superimposing a high frequency sinusoidal (HF) signal, usually several of several hundred HZ to a few KHz onto the fundamental components of the machine, a response to the varying inductance is revealed as an HF component of the motor current whose amplitude is modulated by the saliency of the machine. This technique is favoured due to its simplicity and the fact that it can be easily applied to existing motor control without additional hardware.

The applicant has appreciated that a drawback to existing position sensorless motor control based on the injection of high frequency signals is that they generate noise in the audible frequency range if the frequencies used are in the audible range. For many applications this may be acceptable but in a vehicle that noise is distracting to the driver and therefore unflavourcd by vehicle manufacturers who are striving to constantly improve the refinement of their vehicles.

An object of the present invention is to ameliorate the problems of noise in prior art motor systems.

BRIEF SUMMARY OF THE INVENTION

Accordingly the present invention provides a control system for a permanent magnet electric motor having a plurality of phase windings, particularly of an electric power steering system, the control system comprising: a control stage arranged to generate a current demand signal in response to a torque demanded of the motor, A drive stage comprising an inverter having a plurality of drive stage switches and a driver which generates drive signals for the switches of the inverter in response to the current demand signal, the switches comprising Gallium Nitride field effect transistors (FET); and in which the control stage is further arranged to overlay a high frequency signal onto the current demand signal supplied to the drive stage and to determine the position of the motor using a sensorless motor position determining means that extracts the corresponding high frequency component of the current flowing in the motor which in use is modulated as function of the position dependent saliency of the motor, and further in that the high frequency signal has a frequency of at least 20KHZ.

The applicant has appreciated that GaN FET devices as a result of their physical and electrical characteristics facilitate higher frequency switching than Silicon MOSFETs, whilst maintaining acceptable switching losses. This facilitates the high frequency signal injection of 20Khz or above which places the HF signal outside of the audible range of the driver.

The position determining means of the control stage may be active for determining the position of the motor over all speeds, and preferably active at speeds down to zero.

The operation of the position determining means may be modified over at least one speed range of the motor. For example, at high speeds the control means may stop applying the high frequency component and the position determining means may in turn stop determining position by extracting the HF component.

At the higher speeds, where HF signal injection is not used, the motor position may be determined by monitoring the back emf of the motor which is dependent on rotor position.

In embodiments according to the invention, the switching elements may be Gallium Nitride field effect transistors (FET) and, optionally or preferably, are enhancement mode Gallium Nitride FETs.

GaN switching elements are conventionally utilised in higher voltage applications, and not in automotive applications such as electric power steering systems. Embodiments of the invention utilise GaN devices suitable for use in automotive 12V and 48V applications.

In some embodiments within the scope of the invention, the drive stage of the electric power steering system may comprise at least two inverters. In normal use one inverter may be used to drive the motor with the other inactive, in the event of a fault in one bridge or the motor windings associated with the one inverter the other inverter being used to drive the motor.

By using GaN-based devices, as noted above, a switching of the devices is possible which enables a high frequency signal to be superimposed that is in excess of 20 kHz which is above the audible range. Most preferably, the control circuit may be arranged in use to inject a signal having a frequency in excess of 100 KHz. Again, this is achievable using GaN devices in the inverter bridge. This HF signal comprises an AC signal that is constructed by suitable modulation of the inverter switches using a PWM strategy. The actual switching speed of the inverters switches will in practice be far greater than the HF signal that is to be constructed.

The system may include a frequency generator which in use generates the HF signal that is injected onto the motor fundamental. The control system may include a mixer, or adder block, that mixes the high frequency component with the current demand signal output from the control stage to produce a modified current demand signal for the motor that is applied to the drive stage.

The position determining means may include a demodulator which receives as an input the output of the current measurement circuit and outputs the high frequency component present in the motor current that is the high frequency injected signal which will be amplitude modulated according to motor rotor position. The position determining means may include a signal processing circuit which in use executes a program or algorithm which receives as an input the high frequency signal output from the demodulator and outputs a position signal indicative of the rotor position. The demodulator may use a heterodyne or homodyne demodulation scheme The position determining means may be adapted to use an up reference frame injection method in which the HF signal is superimposed onto the stationary AB reference frame.

Alternatively, the position determining means may be adapted to use a d-axis pulsating signal injection method in which a pulsating HF signal is superimposed on to the d axis only of the current demand signal. This may be advantageous in that a signal superimposed on the d-axis cannot induce a torque in the motor.

Other signal injection methods can be used within the scope of the present invention, provided that a sufficiently high frequency above 20 KHz is used.

Other advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings.

There will now be described, by way of example only, one embodiment of the present invention with reference to and as illustrated in the accompanying drawings of which:

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows an exemplary power steering system according to an embodiment of the invention; Figure 2 is a diagram of an electric motor of the systcm of FIG. 1 and the associated drive stage inverter; Figure 3 is a general diagram of a motor control system for a motor that may be used in the power steering system of Figure I; and Figure 4 is a more detailed partial diagram of the overall control system of Figure 3 illustrating the injection of a HF signal into the motor for the purpose of measuring the motor rotor position.

DETAILED DESCRIPTION OF THE INVENTION

Referring to Figure 1, a steering system 10 comprises a steering shaft 20 connected at one end to a steering wheel 30 and at the other to a steering rack 40 that is in turn connected to the road wheels (not shown) of the vehicle. As the driver turns the steering wheel 30 the steering shaft 2-rotates which in turn moves the rack 40. Power assistance is provided by means of an electric motor 50 which is connected to either the steering shaft or the rack through a gearbox 60. It is within the scope of the present invention that there may be no mechanical connection from the steering wheel 30 to the road wheels, a so called steer-by-wire electric steering system, the motor 50 providing the sole means for turning the steering rack or working along with a secondary source of power for turning the steering rack such as a second motor or hydraulic circuit. A position sensor for measuring the angular position of the steering wheel would of course then be needed.

Referring to FIG. 2, the motor 50 is a three phase motor and more specifically, may comprise an electrically commutated sinusoidal AC brushless permanent magnet synchronous motor which comprises a rotor having, for example, six magnets mounted on it arranged so as to provide six poles which alternate between north and south around the rotor. The rotor therefore defines three direct or d axes evenly spaced around the rotor and three quadrature or q axes interspaced between the d axes. The d axes are aligned with the magnetic poles of the magnets where the lines of magnetic flux from the rotor are in the radial direction, and the q axes are spaced between the d axes where the lines of magnetic flux from the rotor are in the tangential direction. As the rotor rotates, the directions of the d and q axes clearly rotate with it.

The motor stator may comprise, for example, a nine slot copper wound element having three groups of three teeth, each group of teeth having a common winding forming a respective one of the three phases. There are therefore three electrical cycles in each full rotation of the rotor, and the three teeth in any phase are always in the same electrical position as each other.

The three motor windings 112, 114, 116, generally designated as phases A, B and C, are connected in a star network. In other embodiments, other arrangements, such as delta networks, can be used. The phase windings are coiled around the stator teeth.

One end of each coil is connected to a respective terminal 118, 120, 122 of an inverter bridge. The other ends, of the coils are connected together to form the star centre 124.

The inverter 121 of the exemplary embodiment comprises a three-phase inverter bridge with one phase for each motor phase. A DC voltage is applied by a battery 126 between a supply rail and a ground rail. The inverter 121 comprises three parallel arms (A, B, C), each of which has a pair of Gallium Nitride field effect transistors (FET) and, optionally or preferably, are enhancement mode Gallium Nitride FETs 128A, 128B, 130A, 130B, I32A and 132E connected in series between a supply rail and a ground rail. The terminals 118,120,122 connecting the motor phases A, B. C to the bridge are located between each pair of FETs. As such, FETs 128A and 128B are connected to a first phase A of the motor 50, FETs 130A and 130B are connected to a second phase B of the motor 50, and FETs I32A and I 32B are connected to a third phase C of the motor 50. The pairs of FETs associated with each phase (arms A, B, C) are connected in parallel to each other and to the battery 126.

The gate of each FET is connected to a gate driver of a drive stage to switch each FET ON or OFF. Each gate driver receives control signals from a control stage. More specifically and referring to Figures 3 and 4 0f the drawings the motor 50 is controlled by a closed loop motor current control system according to an embodiment of the invention, which comprises a current sensing system 134, a position sensing system 136, a drive stage 138 and a current controller 140.

The current sensing system may comprise a resistor R in the ground line between the motor 50 and ground which enables measurements to be made of the total current flowing though all of the windings 112, 114, 116. In order to measure the current in each of the windings the total current has to be sampled at precise instants within the PWM period where the voltage applied to each terminal of the winding (and hence the conduction state of a particular phase) is known. As is well known, in order for the currents in each of the windings to be measured in any one PWM period, the drive circuit needs to be in each of at least two different active states for a predetermined minimum time.

The current sensing system measures the currents Ec in the three phases of the motor. wfrich con prise stationary 6,, :ridings_ and outputs a. signal Fndicau ce of the current vector in the stationary coordinates having a. and r, components. These measurements are then processed within a signal processing unit which comprises a coordinate transformation block arranged to convertcurrent vector from the a and j3 components in the stationary reference frame, to D and Q components ii, andio defining the current vector in the rotor reference frame, which rotates relative to the fixed 'vindings, with the Q axis current being the torque generating component and the 1) axis cu neat being non-torque generating. A comparator (not shown) receives the measured D and Q currents from the current sensing system 12 and compares them with demanded D and Q current components to generate a current error.

The current controller 140 receives the current error and outputs a demanded y vecior,, in the form of a D and Q axis voltage demand iron calculated to reduce the current error so that the measured current vector fop approaches the demanded current vector ipo*. A further transformation block receives the voltage demand from the current controller and converts it to a and Ii component; Vapycitiely are input to the drive stage 131 which controls the switches to apply voltages to the phase windings of the motor in a PWM eattern which produces the net voltage in the windings haying a magnitude and direction corresponding to the voltage demand vector.

The current request received by the control stage is typically a function of a torque demand signal 142 indicative of the level of assistance torque to be applied by the motor. A torque sensor 144 measures the torque carried by the steering shaft 20. This torque demand signal may be generated by a torque controller 146 which receives as an input the torque signal from the torque sensor measuring the torque applied at the steering wheel. The torque controller 146 will typically use the torque signal to look up a torque demand signal using a look up table or boost curve stored in a memory of the system.

In use, the drive stage applies voltage signals to the gate of each GaN FET to switch them ON and OFF rapidly in a predefined sequence, thus controlling the voltage applied to each phase of the motor and current flowing through the windings. This in turn controls the strength and orientation of the magnetic field produced by the windings, and hence the torque and speed of the motor. By using a sufficiently rapid pulse width modulation (PWM) switching pattern, a phase drive waveform can be applied that approximates the ideal sinusoidal waveform required to rotate the motor smoothly.

in the exemplary circuit the motor position is determined by position determining means 136 that is arranged to monitor the output of current measuring device. In particular this is achieved by monitoring a HF signal that has been injected onto the fundamental of the motor in a so called position sensorless control strategy. This position is used by the controller to determine the appropriate timing of the PWM waveforms In the embodiment of the present invention a signal generator 144 is provided which generate a HF signal having a frequency of several 100 KHz. This is added to the current demand signal that is input to the drive stage. The signal has a fixed amplitude and frequency, and the current flowing in the motor is monitored in order to extract this injected HF signal. Figure 4 shows a position determining means 136 that comprises a signal processing unit that performs this monitoring function. This will typically include a demodulator that extracts the high frequency information from the current measurement output from the current sensor and feed this demodulated HF signal into the signal processing unit.

From the amplitude modulation of the HF signal the content coupled with motor saliency is further extracted, which is known to be dependent on the motor rotor position. Motor position is then derived from this signal.

This is then fed back to the current controller. The demodulation of the HF signal to determine position is known in the art, albeit at much lower frequencies than those proposed by the applicant. For example, several schemes of this type are described in the document Audible noise reduction in the high frequency injection based sensorless torque control for EPS applications. PhD thesis, Jiang, Hui (2012) University of Nottingham. The teachings of that document are incorporated herein by reference.

The reader will appreciate that the present invention can be used as the main/only motor position estimation means for a motor, or may be as one of a multiplicity of means in conjunction with a physical sensor for example, either to facilitate a cross check or for fault tolerance improvement.

Claims

CLAIMS1. A control system for a permanent magnet electric motor having a plurality of phase windings, particularly of an electric power steering system, the control system comprising: a control stage arranged to generate a current demand signal in response to a torque demanded of the motor, A drive stage comprising an inverter having a plurality of drive stage switches and a driver which generates drive signals for the switches of the inverter in response to the 10 current demand signal, the switches comprising Gallium Nitride field effect transistors; and in which the control stage is further arranged to overlay a high frequency signal onto the current demand signal supplied to the drive stage and to determine the position of the motor using a sensorless motor position determining means that extracts the corresponding high frequency component of the current flowing in the motor which in use is modulated as function of the position dependent saliency of the motor, and further in that the high frequency signal has a frequency of at least 20KHZ.
2. A control system according to claim 1 in which the position dctcrmining means of the control stage is active for determining the position of the motor over all speeds, and preferably active at speeds down to zero.
3. A control system according to claim 1 or claim 2 in which the switches comprise enhancement mode Gallium Nitride FETs.
4. A control system according to any preceding claim which further includes a frequency generator which in use generates the HF signal that is injected onto the motor fundamental.
5. A control systems according to any preceding claim further comprising a mixer, or adder block, that mixes the high frequency component with the current demand signal output from the control stage to produce a modified current demand signal for the motor that is applied to the drive stage.
6. A control system according to any preceding claim in which the position determining means includes a demodulator which receives as an input the output of the current measurement circuit and outputs the high frequency component present in the motor current.