FIELD OF THE INVENTION

This invention relates to motors in general, and more particularly to permanent magnet synchronous electric motors and motor controllers for operating the same.

BACKGROUND OF THE INVENTION

To appreciate the invention, it is important to understand some core concepts:

(i) the basic construction and operation of a 3-phase permanent magnet synchronous electric motor (PMSM);

(ii) two of the mathematical building blocks involved: Clarke and Park transforms;

(iii) 3-phase half-bridge operation: switching states, current paths, diode circulation; and

(iv) how the pulse-width modulation (PWM) pattern moments (energize and decay) relate to the motor model and the sampling windows.

Three-Phase Permanent Magnet Synchronous Electric Motor (PMSM)

As shown inFIG. 1, three-phase permanent magnet synchronous electric motor5typically comprises a fixed stator10comprising a plurality of poles15, and a rotatable rotor20comprising at least one pair of North and South poles (for clarity of illustration, rotatable rotor20is shown inFIG. 1as having only one pair of North and South poles, however, it should be appreciated that in practice, it is common for rotatable rotor20to comprise many pairs of North and South poles). Rotatable rotor20is movably disposed within fixed stator10. When poles15of fixed stator10are appropriately electrically energized, e.g., by a motor controller25, rotating magnetic fields can be created within the interior of fixed stator10so that the poles of rotatable rotor20are sequentially attracted toward, or repelled from, poles15of fixed stator10, whereby to cause rotation of rotatable rotor20within fixed stator10. A drive shaft30is attached to rotatable rotor20so as to provide output power from three-phase permanent magnet synchronous electric motor5.

Three-phase permanent magnet synchronous electric motor5is characterized by three phases A, B, C, and motor controller25drives the three phases A, B, C by supplying three phase currents iA, iB, iC. The three phase currents iA, iB, iCtypically comprise pulses, with the timing of the pulses being coordinated so as to cause rotation of rotatable rotor20.

Clarke and Park

For a sinusoidally-wound permanent magnet synchronous electric motor (PMSM), each of the three phase current waveforms takes a sinusoidal shape as the motor turns (seeFIG. 2). The motor torque is a function of these three phase currents. But it is easier and more efficient to control a constant current value than a set of three sinusoidally-varying current values, so the three sensed winding currents are first converted to a 2-phase sinusoidal αβ system using the Clarke transform (seeFIG. 3), then they are converted to a 2-element dq system that eliminates the dependency on electrical angle θ using the Park transform (seeFIG. 4). The q-axis (quadrature) current iqis solely responsible for generating a motor torque, so it is useful to have this in a single, easily-controllable variable.

3-Phase Half-Bridge, Switching States, Current Paths

To vary the current in the three motor phases using a single DC bus voltage (Vbus), six identical power metal-oxide semiconductor field-effect transistors (MOSFETs)35are used in a three-phase half-bridge amplifier configuration40shown inFIG. 5. A microcontroller42is used to control operation of MOSFET gates35. It will be appreciated that microcontroller42and three-phase half-bridge amplifier configuration40, together with appropriate supporting circuitry and logic, form motor controller25.

Each MOSFET gate35is labeled with the phase it controls (A, B, C) and whether it is the high-side (H) or the low-side (L) of the bridge (i.e., AH, AL, BH, BL, CH, CL). Each motor phase is labeled (A, B, C), with N being the neutral point of the wye-wound motor. The high-side MOSFETs determine the state of the amplifier which takes the form S[ABC]. Each low-side MOSFET is commanded to be the opposite state as its related high-side MOSFET. This prevents direct conduction from Vbus to ground, also known as shoot-through.

By switching these three bridges (AH/AL, BH/BL, CH/CL) on and off rapidly using calculated timing patterns known as pulse-width modulation (PWM), the amount of current flowing in each motor phase (A, B, C) can be controlled to achieve the desired torque. For example, the PWM pattern in the table shown below in the section entitled “Description Of The States” consists of a series of 7 states (i.e., S[000], S[100], S[110], S[111], S[110], S[100] and S[000]), 4 of which (i.e., S[000], S[100], S[110] and S[111]) are unique.

SeeFIGS. 6-9.

Descriptions of the States

StateABCDescriptionS1000Initial state, no current flows(i.e., S[000])S2100Phase A excitation, current flows into A, and(i.e., S[100])out through B & CS3110Phase A & B excitation, current flows into A(i.e., S[110])and out through C while the phase B currentis reduced to zero over timeS4111Freewheeling, motor current decays through(i.e., S[111])upper FETsS5110Phase A & B excitation, current flows into(i.e., S[110])A & B, and out through CS6100Phase A excitation, current flows into A, and(i.e., S[100])out C while the phase B current is reduced tozero over timeS7000Freewheeling, motor current decays through(i.e., S[000])lower FETs

The motor's inductance dramatically affects the flow of current through the amplifier. This is first evident in S3. It is logical to assume that current is flowing into phase B because it is connected to Vbus in this state. But the outflow of current in phase B was already built up during S2, and the flow of current cannot change instantly because it is governed by the equation:

Δ⁢⁢iphase=Δ⁢⁢tLphase⁢uphase
where:

Δiphaseis the change in phase current (A)

Δt is the change in time (s)

Lphaseis the phase inductance (H)

uphaseis the phase voltage (V)

In addition to inductance, other factors can affect the flow of current through the amplifier and/or affect performance of the three-phase permanent magnet synchronous electric motor, e.g., heat effects causing changes in resistance in the coils, heat effects causing changes in friction within the motor, external loads applied to the motor, etc.

For these reasons, it is important for motor controller25to continually sense the flow of current through the three motor stages A, B, C and adjust the three phase currents iA, iB, iCas appropriate in order to produce the desired torque in the three-phase synchronous electric motor.

The initial state S[000] before current begins to flow may be visualized in the context of the 3-phase amplifier diagram shown inFIG. 5, i.e., the initial state S[000] (before current begins to flow) is shown inFIG. 10.

The flow of current in the rest of the states of this example is visualized inFIGS. 11-16.

Various Sensor Architectures

As noted above, the motor's inductance dramatically affects the flow of current through the amplifier. And, in addition to inductance, other factors can affect the flow of current through the amplifier and/or affect performance of the three-phase permanent magnet synchronous electric motor, e.g., heat effects causing changes in resistance in the coils, heat effects causing changes in friction within the motor, external loads applied to the motor, etc.

As a result, it is important for motor controller25to continually sense the flow of current through the three motor stages A, B, C and adjust the three phase currents iA, iB, iCas appropriate in order to produce the desired torque in the three-phase synchronous electric motor.

Various schemes have been developed for sensing the flow of current through the three motor stages A, B, C. Three of the most common architectures are the so-called “Triple-Sensor” architecture, the so-called “Single-Sensor” architecture and the so-called “Standard Dual-Sensor (B & C)” architecture.

Triple-Sensor

In a triple-sensor design, there is a shunt resistor45in the current's path to ground for each of the three phases. The microcontroller samples each of the three amplifier outputs (e.g., via operational amplifiers50) during the “decay” phase of the PWM pattern to get a full picture of the winding currents. SeeFIG. 17.

There is additive white Gaussian noise (AWGN) in each sample, but the microcontroller is able to sample two inputs simultaneously, so the common-mode noise cancels when calculating iβdue to the iB-iCClarke term. But iαstill contains the AWGN, and this is carried through the Park transform to yield a q-axis noise factor of 1 (times the AWGN amplitude).

Then the control system is applied to the d-axis and q-axis currents. Then the d-axis & q-axis efforts are transformed back into the sinusoidal alpha-beta system using the inverse Park transform. Finally, the alpha-beta efforts are sent to a PWM generator that determines the three duty cycles required to implement the control effort. This is a 5-step process after sampling: Clarke, Park, control, inverse Park, then PWM generation.

Each sensor (e.g., operational amplifier50) requires two calibration steps to determine its offset voltage and amplifier gain. For three sensors, this yields six calibration steps.

Single-Sensor

A single-sensor design minimizes the size and cost of the hardware. SeeFIG. 18.

However, a single-sensor design is unable to match the performance of multi-sensored systems for a few reasons:(i) The PWM patterns need to be distorted due to minimum sample times. This introduces excess currents in some windings which need to corrected by applying carefully-constructed compensation patterns in subsequent patterns. The rapid injection/correction of phase currents results in undesirable audible noise.(ii) Pattern distortion results in winding voltages which do not match the motor's natural back-EMF voltages. This results in uneven torque production throughout each electrical cycle.(iii) Because of the need to take two sequential current samples, there is no way to eliminate the AWGN from either sample. So the phase reconstruction math results in one phase with 2× the AWGN. After passing this noise through the Clarke and Park transforms, the iqcurrent ends up with 4× the AWGN. This noise negatively affects the controller's ability to apply a constant torque.(iv) There is substantial calculation overhead involved in choosing which phases to sample, how to stretch the relevant parts of the PWM pattern to allow for sampling, and how to compensate for those distortions in subsequent PWM patterns.

Standard Dual-Sensor (B & C)

Using two current sensors (e.g., operational amplifiers50) eliminates many of the problems with single-sensor design. SeeFIG. 19.

The phase currents can be sampled between each PWM pattern, during the decay phase. No pattern distortion is necessary, so the motor generates no audible noise.

Because there is no PWM distortion, the applied winding voltages can be made to match the motor's natural back-EMF voltages. This results in a consistent torque throughout each electrical cycle.

The microcontroller can take simultaneous samples from the two current sensors. This allows calculation of a clean iβterm, but the iαterm contains 2× the AWGN. This noise is carried through to the iqcurrent, negatively affecting the controller's ability to apply a constant torque.

Note that if iAand iBare measured instead, iCwould contain 2×AWGN after reconstruction, and the term iβwould therefore contain 3×AWGN after the Clarke transform. Thus it is better to measure iBand iC.

While it still requires reconstructing the third phase current from the two sampled phases, this design eliminates all the calculation overhead involved in sample selection, PWM pattern stretching, and pattern compensation.

In view of the foregoing, there is a need for a new current-sensing architecture which minimizes noise and computational complexity compared to competing designs.

SUMMARY OF THE INVENTION

In an effort to create a high-performance miniature brushless motor controller, a novel current-sensing architecture was developed that minimizes noise and computational complexity compared to competing designs. A dual-sensor design was pursued, but instead of choosing two of the three motor phases to sample and then reconstructing the third phase current, one sensor's difference amplifier (e.g., a difference operational amplifier) was leveraged to yield a direct measurement of the motor's iβcurrent. This results in a 50% noise reduction at the inputs to the controller and allows a calculation step to be skipped in the control loop. See the summary table below:

SUMMARY TABLEComparison of various current sensor architecturesComputationalQ-axis noiseCalibrationsTorqueAudibleSensorscomplexityfactorrequiredSizeCosterrornoiseTriple516LargeHighLowLowSingle842SmallLowHighHighB + C624MediumMediumMedLowInvention:414MediumMediumLowLowAlpha + Beta

In one preferred form of the invention, there is provided a motor controller for controlling the operation of a three-phase permanent magnet synchronous electric motor, wherein the three-phase permanent magnet synchronous electric motor is characterized by three phases A, B, C, and further wherein the three-phase permanent magnet synchronous electric motor is driven by regulating three phase currents iA, iBand iCfor the three phases A, B, C, respectively, the motor controller comprising:

a three-phase power supply for supplying the three phase currents iA, iBand iC.

a first sensor for sensing the phase current iA;

a second sensor for sensing across the phase currents iBand iC; and

a microcontroller for controlling the operation of the three-phase power supply so as to produce the three phase currents iA, iBand iCneeded to operate the three-phase permanent magnet synchronous electric motor, wherein the microcontroller reads the outputs of the first sensor and the second sensor and adjusts operation of the three-phase power supply so as to produce phase currents iA, iBand iCwhich produce the desired torque in the three-phase permanent magnet synchronous electric motor.

In another form of the invention, there is provided a method for controlling the operation of a three-phase permanent magnet synchronous electric motor, wherein the three-phase permanent magnet synchronous electric motor is characterized by three phases A, B, C, and further wherein the three-phase permanent magnet synchronous electric motor is driven by regulating three phase currents iA, iBand iCfor the three phases A, B, C, respectively, the method comprising:

supplying the three phase currents iA, iBand iCso as to drive the three-phase permanent magnet synchronous electric motor;

sensing the phase current iAand sensing across the phase currents iBand iC; and

adjusting phase currents iA, iBand iCso as to produce phase currents iA, iBand iCwhich produce the desired torque in the three-phase permanent magnet synchronous electric motor.

