HK40121665A - High performance current sensing architecture for brushless motors - Google Patents

Description

High-performance current sensing architecture for brushless motors

Citation of prior patent applications

This patent application:

(i) Claiming the benefit of prior pending U.S. Provisional Patent Application Serial No. 62/758,004 (Attorney's File No. BARRETT-13PROV) filed November 9, 2018, by Barrett Technology, LLC and Claude F. Valle IV et al., concerning High Performance Current Sensing Architecture for Brushless Motors; and

(ii) Claim the benefit of prior pending U.S. Provisional Patent Application Serial No. 62/861,772 filed June 14, 2019, by Barrett Technologies LLC and Claude F. Valle IV et al. (Attorney’s File No. BARRETT-15PROV) for HIGH PERFORMANCE CURRENT SENSING ARCHITECTURE FOR BRUSHLESS MOTORS.

The two (2) patent applications mentioned above are hereby incorporated herein by reference.

Technical Field

The present invention relates generally to motors, and more particularly to permanent magnet synchronous motors and motor controllers for operating the motors.

Background Technology

To understand this invention, it is important to understand the following core concepts:

(i) Basic structure and operation of a three-phase permanent magnet synchronous motor (PMSM);

(ii) Two of the mathematical building blocks involved: Clarke transform and Park transform;

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

(iv) How the pulse width modulation (PWM) mode phases (excitation and decay) relate to the motor model and sampling window.

Three-phase permanent magnet synchronous motor (PMSM)

As shown in Figure 1, a three-phase permanent magnet synchronous motor 5 typically includes a fixed stator 10 with multiple magnetic poles 15 and a rotatable rotor 20 with at least one pair of north and south poles (for clarity, the rotatable rotor 20 is shown in Figure 1 as having only one pair of north and south poles; however, it should be understood that in practice, the rotatable rotor 20 typically has multiple pairs of north and south poles). The rotatable rotor 20 is movably disposed within the fixed stator 10. When the magnetic poles 15 of the fixed stator 10 are appropriately electrically excited, for example by a motor controller 25, a rotating magnetic field can be generated within the fixed stator 10, causing the magnetic poles of the rotatable rotor 20 to be attracted toward or repelled away from the magnetic poles 15 of the fixed stator 10, thereby causing the rotatable rotor 20 to rotate within the fixed stator 10. A drive shaft 30 is attached to the rotatable rotor 20 to provide output power from the three-phase permanent magnet synchronous motor 5.

The three-phase permanent magnet synchronous motor 5 is characterized by three phases A, B, and C, and the motor controller 25 drives the three phases A, _B , and _C by supplying three phase currents _iA , iB, and iC. The three phase currents _iA , _iB , and _iC typically include pulses, wherein the timing of the coordinated pulses causes the rotatable rotor 20 to rotate.

Clark and Parker

For a permanent magnet synchronous motor (PMSM) with sinusoidal windings, each of the three phase current waveforms exhibits a sinusoidal shape as the motor rotates (see Figure 2). The motor torque is a function of these three phase currents. However, controlling a constant current value is easier and more efficient than controlling a set of three sinusoidally varying current values. Therefore, the three sensed winding currents are first converted to a two-phase sinusoidal αβ system using the Clarke transform (see Figure 3), and then converted to a binary dq system using the Park transform (see Figure 4), which eliminates the correlation with the electrical angle θ. The q-axis (orthogonal) current _iq is solely responsible for generating the motor torque, and it is therefore useful to have this current as a single, easily controllable variable.

Three-phase half-bridge, switch status, current path

To use a single DC bus voltage (Vbus) to vary the current in the three motor phases, six identical power metal-oxide-semiconductor field-effect transistors (MOSFETs) 35 are used in the three-phase half-bridge amplifier configuration 40 shown in Figure 5. A microcontroller 42 is used to control the operation of the MOSFET gates 35. It will be understood that the microcontroller 42 and the three-phase half-bridge amplifier configuration 40, together with appropriate support circuitry and logic, form the motor controller 25.

Each MOSFET gate 35 is labeled with the phase it controls (A, B, C) and whether it is on the high side (H) or low side (L) of the bridge (i.e., AH, AL, BH, BL, CH, CL). Each motor phase is labeled (A, B, C), where N is the neutral point of the Y-wound motor. The high-side MOSFET determines the state of the amplifier using the S[ABC] form. Each low-side MOSFET is controlled to the opposite state of its associated high-side MOSFET. This prevents direct conduction from Vbus to ground, also known as shoot-through.

By rapidly switching the three bridges (AH/AL, BH/BL, CH/CL) using a calculated timing pattern called 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 mode shown in the table in the section titled "State Description" below consists of a series of 7 states (i.e., S[000], S[100], S[110], S[111], S[110], S[100], and S[000]), of which 4 states (i.e., S[000], S[100], S[110], and S[111]) are unique.

See Figures 6 to 9.

Status Description

The inductance of the motor significantly affects the current flow through the amplifier. This is first evident in S3. It is logical to assume that current flows into phase B, since phase B is connected to Vbus in this state. However, the current outflow from phase B has already been established during S2, and the current flow cannot be changed immediately because it is governed by the following equation:

in:

• Δi _phase is the change in phase current (A)

Δt represents the time variation (s).

• L _phase is the phase inductance (H)

• u _phase is the phase voltage (V).

Besides inductance, other factors may also affect the current flow through the amplifier and/or the performance of the three-phase permanent magnet synchronous motor, such as thermal effects that cause changes in coil resistance, thermal effects that cause changes in internal friction of the motor, and external loads applied to the motor.

For these reasons, it is important for the motor controller 25 to continuously sense the currents through the three motor stages A, B, and C and adjust the three phase currents _iA , _iB , and _iC as appropriate to generate the desired torque in the three-phase synchronous motor.

The initial state S[000] before the current begins to flow can be seen in the context of the three-phase amplifier diagram shown in Figure 5, that is, the initial state S[000] (before the current begins to flow) is shown in Figure 10.

The current flow in the remaining states of this example can be seen in Figures 11 to 16.

Various sensor architectures

As mentioned above, the inductance of the motor significantly affects the current flow through the amplifier. Furthermore, in addition to inductance, other factors can also affect the current flow through the amplifier and/or the performance of the three-phase permanent magnet synchronous motor, such as thermal effects that cause changes in coil resistance, thermal effects that cause changes in internal friction of the motor, and external loads applied to the motor.

Therefore, for the motor controller 25, it is important to continuously sense the currents through the three motor stages A, B, and C and adjust the three phase currents _iA , _iB , and _iC as appropriate to generate the desired torque in the three-phase synchronous motor.

Various schemes have been developed to sense the current flow through the three motor stages A, B, and C. The three most common architectures are the so-called "three-sensor" architecture, the so-called "single-sensor" architecture, and the so-called "standard dual-sensor (B and C)" architecture.

Three sensors

In the three-sensor design, there is a shunt resistor 45 in the grounded current path for each of the three phases. The microcontroller samples each of the three amplifier outputs (e.g., via operational amplifier 50) during the "decay" phase of PWM mode to obtain a complete picture of the winding current. See Figure 17.

Each sample contains additive white Gaussian noise (AWGN), but the microcontroller is able to sample both inputs simultaneously, so this common-mode noise is canceled out by the Clarke term of _iB - _iC when calculating _iβ . However, _iα still contains AWGN, and this is carried over through the Park transform to produce a q-axis noise factor of 1 (multiplied by the AWGN amplitude).

The control system is then applied to the d-axis and q-axis currents. The inverse Parker transformation is then used to convert the d-axis and q-axis efforts back to a sinusoidal α-β system. Finally, the α-β effort is sent to a PWM generator, which determines the three duty cycles required to achieve the control action. This is the five-step process after sampling: Clark, Parker, control, inverse Parker, and then PWM generation.

Each sensor (e.g., operational amplifier 50) requires two calibration steps to determine its offset voltage and amplifier gain. For three sensors, this results in six calibration steps.

single sensor

The single-sensor design minimizes the size and cost of the hardware. See Figure 18.

However, single-sensor designs cannot match the performance of multi-sensor systems for several reasons:

(i) Due to the minimum sampling time, the PWM mode needs to be distorted. This introduces excessive current in some windings, which needs to be corrected by applying a carefully constructed compensation mode in subsequent modes. The rapid injection/correction of phase current results in undesirable audible noise.

(ii) Mode distortion causes a mismatch between the winding voltage and the motor's natural back EMF voltage. This results in uneven torque throughout each electrical cycle.

(iii) Because two consecutive current samples are required, the AWGN cannot be eliminated from either sample. Therefore, the phase reconstruction mathematics yields a phase with twice the AWGN. After passing this noise through the Clark and Park transform, the _iq current eventually reaches four times the AWGN. This noise negatively impacts the controller's ability to apply constant torque.

(iv) Significant computational overhead is involved in selecting the phase to sample, how to stretch the relevant parts of the PWM mode to allow sampling, and how to compensate for those distortions in subsequent PWM modes.

Standard dual sensors (B and C)

Using two current sensors (e.g., operational amplifier 50) eliminates many of the problems associated with single-sensor designs. See Figure 19.

During the decay phase, the phase current can be sampled between each PWM mode. No mode distortion is required, so the motor does not produce audible noise.

Because there is no PWM distortion, the applied winding voltage can be matched to the motor's natural back EMF voltage. This results in a constant torque throughout each electrical cycle.

The microcontroller can obtain simultaneous samples from two current sensors. This allows for the computation of a clean _iβ term, but the _iα term contains twice the AWGN. This noise is carried over to the _iq current, negatively impacting the controller's ability to apply constant torque.

It is important to note that if we instead measure _iA and _iB , _iC will contain twice the AWGN after reconstruction, and therefore the _iβ term will contain three times the AWGN after the Clarke transform. Therefore, it is best to measure _iB and _iC .

Although the third phase current still needs to be reconstructed from the two sampled phases, the design eliminates all the computational overhead involved in sample selection, PWM mode stretching, and mode compensation.

Given the above, a new current sensing architecture is needed that minimizes noise and computational complexity compared to competing designs.

Summary of the Invention

To create a high-performance micro brushless motor controller, a novel current-sensing architecture was developed, minimizing noise and computational complexity compared to competing designs. Efforts were made to achieve a dual-sensor design, but instead of selecting two of the three motor phases for sampling and then reconstructing the third phase current, a differential amplifier (e.g., a differential operational amplifier) of one sensor was used to generate a direct measurement of the motor's _iβ current. This resulted in a 50% noise reduction at the controller input and allowed computational steps to be skipped in the control loop. See the summary table below:

Summary table: Comparison of various current sensor architectures

In a preferred embodiment of the present invention, a motor controller is provided for controlling the operation of a three-phase permanent magnet synchronous motor, wherein the three-phase permanent magnet synchronous motor is characterized by three phases A, B, and C, and further wherein the three-phase permanent magnet synchronous motor is driven by adjusting three phase currents _iA , iB, and _iC respectively for the three phases A, _B , and C, and the motor controller includes:

A three-phase power supply is used to supply three phase currents _iA , _iB and _iC ;

The first sensor is used to sense the phase current _iA ;

A second sensor is used to sense the cross-phase currents _iB and _iC ; and

A microcontroller is used to control the operation of a three-phase power supply to generate the three phase currents _iA , _iB , and _iC required to operate a three-phase permanent magnet synchronous motor. The microcontroller reads the outputs of a first sensor and a second sensor and adjusts the operation of the three-phase power supply to generate the phase currents _iA , _iB , and _iC that produce the desired torque in the three-phase permanent magnet synchronous motor.

In another aspect of the invention, a method for controlling the operation of a three-phase permanent magnet synchronous motor is provided, wherein the three-phase permanent magnet synchronous motor is characterized by three phases A, B, and C, and further wherein the three-phase permanent magnet synchronous motor is driven by adjusting three phase currents _iA , iB, and _iC respectively for the three phases A, _B , and C, the method comprising:

Three phase currents _iA , _iB and _iC are supplied to drive a three-phase permanent magnet synchronous motor;

Sensing phase current _iA and sensing cross-phase currents _iB and _iC ; and

Adjust the phase currents _iA , _iB , and _iC to generate the phase currents _iA , _iB , and _iC that produce the desired torque in a three-phase permanent magnet synchronous motor.

Attached Figure Description

These and other objects and features of the invention will be more fully disclosed or become apparent from the following detailed description of preferred embodiments of the invention, which will be considered in conjunction with the accompanying drawings, wherein like numerals refer to similar parts, and further wherein:

Figure 1 is a schematic view showing a three-phase permanent magnet synchronous motor;

Figure 2 is a schematic view showing the phase current of a three-phase permanent magnet synchronous motor;

Figure 3 is a schematic view showing the Clark transformation of the phase current of a three-phase permanent magnet synchronous motor;

Figure 4 is a schematic view showing the Parker transformation of the phase current of a three-phase permanent magnet synchronous motor;

Figure 5 is a schematic view showing a three-phase half-bridge amplifier that can be used to drive a three-phase permanent magnet synchronous motor;

Figures 6 to 9 are schematic views showing the single nested PWM mode, phase voltage, phase current, and α-β current of a three-phase permanent magnet synchronous motor;

Figures 10 to 16 are schematic views showing the current flow in various states of a three-phase half-bridge amplifier driving a three-phase permanent magnet synchronous motor.

Figure 17 is a schematic view illustrating a three-sensor current sensing architecture;

Figure 18 is a schematic view illustrating a single-sensor current sensing architecture;

Figure 19 is a schematic view illustrating a dual-sensor current sensing architecture;

Figure 20 is a schematic view illustrating the novel current sensing architecture of the present invention;

Figure 21 is a schematic view illustrating how shunt resistors and operational amplifiers can be selected for some sensor architectures; and

Figure 22 is a schematic view showing how shunt resistors and operational amplifiers can be selected for other sensor architectures.

Detailed Implementation

This invention includes providing and using a novel dual-sensor current sensing architecture that focuses on α and β currents.

More specifically, as shown in Figure 20, this new method involves placing a sensor (e.g., operational amplifier 50) on _iA to monitor the α current and applying another sensor (e.g., another operational amplifier 50) across phases B and C to monitor the β current.

The microcontroller 42 of the motor controller 25 reads the outputs of the sensor on phase A (i.e., the α current sensor) and the sensor across phases B and C (i.e., the β current sensor), and adjusts the operation of the three-phase half-bridge amplifier (i.e., by appropriately controlling the MOSFET gate 35) to generate the desired torque in the three-phase permanent magnet synchronous motor.

Using this architecture, clean _iβ is measured because the operational amplifier 50 sensing across phases B and C is unaffected by common-mode noise, but now only the _iα term contains 1x AWGN. This halves the _iq noise (compared to the standard dual-sensor approach), allowing the current controller to be tuned to be more powerful. Furthermore, the operational amplifier 50 sensing across phases B and C is configured with a gain equal to a multiple of the gain of the operational amplifier 50 sensing phase A, eliminating the Clarke transform step from the calculations, as the transformed value is read directly from the hardware.

Calculation of offset and gain of α and β values

For any specific implementation of this design, an operational amplifier 50 must be provided to convert the voltage signal from the shunt resistor 45 into a voltage signal that satisfactorily spans the microcontroller analog-to-digital converter (ADC) range, thereby effectively setting the current range that may be digitized. For the operational amplifier 50 in the aforementioned three-sensor, single-sensor, and standard dual-sensor designs, this is a simple matter of selecting a resistor for the shunt resistor 45 and selecting a non-inverting operational amplifier 50 such that the maximum and minimum desired currents are amplified to the maximum and minimum ADC values, respectively, according to the schematic and formula shown in Figure 21. This formula shows the ADC readings that will be obtained for a given operational amplifier and shunt current, where _IR1 is the shunt current, ADC _bits is the bit resolution of the ADC, and _VCC is the reference voltage of the operational amplifier. Thus, acceptable values for the resistor and _VCC can be selected to provide the correct range for the specific motor current and data acquisition system being applied.

This circuit analysis can also be applied to the α amplifier (i.e., operational amplifier 50 for sensing _iA ) in this new dual-sensor design. However, this does not apply to the β amplifier (i.e., operational amplifier 50 for sensing across _iB and _iC ). At higher levels, the design of the β amplifier is the same: controlling the offset and gain so that the desired output β voltage range matches the range of the data acquisition system. However, unlike the single-current amplifier, the unweighted differential amplifier cannot have arbitrary offsets set via resistor dividers as in the single-current design. This is because the input impedances of the two amplifier inputs must be matched to avoid a gain difference between the two input signals (R3 must match R4 and R5 must match R6, see Figure 22). When this is implemented, the output bias voltage equals the input bias voltage due to the matching of the two resistor dividers. Dividing the bias voltage by the first and then multiplying it by the second makes them cancel each other out, and if the division ratio of the first divider increases so that the bias at 0 is lower than Vbias (as in the α amplifier), the relative gains of B and C are no longer matched. This means that once the resistor ratio is selected for a given gain, the only remaining way to set the output offset is to change the voltage driven on top of R5 (possibly with a dedicated reference voltage). With this added, the relationship between the ADC value and the input current becomes the equation shown in Figure 22, provided that _R3 and _R4 are the same value, and _R5 and _R6 are also the same value. In Figure 22, current _iB is the current flowing through shunt resistor R2, and _iC is the current through _R1 .

Modifications to the preferred embodiment

It should be understood that those skilled in the art can make many additional changes to the details, materials, steps, and component arrangements that have been described and illustrated herein to explain the nature of the invention, while still remaining within the principles and scope of the invention.

Claims (10)

1. A motor controller for controlling the operation of a three-phase permanent magnet synchronous motor, wherein the three-phase permanent magnet synchronous motor is characterized by three phases A, B, and C, and further wherein the three-phase permanent magnet synchronous motor is driven by adjusting three phase currents _iA , iB, and iC respectively for the three phases A, _B , and _C , the motor controller comprising: A three-phase power supply is used to supply three phase currents _iA , _iB and _iC ; The first sensor is used to sense the phase current _iA ; A second sensor is used to sense the cross-phase currents _iB and _iC ; and A microcontroller is used to control the operation of a three-phase power supply to generate the three phase currents _iA , _iB , and _iC required to operate a three-phase permanent magnet synchronous motor. The microcontroller reads the outputs of a first sensor and a second sensor and adjusts the operation of the three-phase power supply to generate the phase currents _iA , _iB , and _iC that produce the desired torque in the three-phase permanent magnet synchronous motor. 2. The motor controller according to claim 1, wherein the first sensor includes an amplifier. 3. The motor controller according to claim 1, wherein the second sensor includes an amplifier. 4. The motor controller according to claim 1, wherein the microcontroller causes the three-phase power supply to supply three phase currents _iA , _iB and _iC in the form of pulses. 5. The motor controller according to claim 1, wherein the three-phase power supply includes a three-phase half-bridge amplifier. 6. The motor controller according to claim 5, wherein the three-phase half-bridge amplifier comprises: an A-phase portion comprising two transistors arranged in series and operated by a microcontroller; a B-phase portion comprising two transistors arranged in series and operated by a microcontroller; and a C-phase portion comprising two transistors arranged in series and operated by a microcontroller. 7. The motor controller according to claim 6, wherein phase A of the three-phase permanent magnet synchronous motor is connected between two transistors in the phase A portion of the three-phase half-bridge amplifier; wherein phase B of the three-phase permanent magnet synchronous motor is connected between two transistors in the phase B portion of the three-phase half-bridge amplifier; and wherein phase C of the three-phase permanent magnet synchronous motor is connected between two transistors in the phase C portion of the three-phase half-bridge amplifier. 8. The motor controller according to claim 7, wherein the first sensor is connected to the A-phase portion of the three-phase half-bridge amplifier, and wherein the second sensor is connected to the B-phase portion and the C-phase portion of the three-phase half-bridge amplifier. 9. The motor controller of claim 5, wherein the three-phase half-bridge amplifier comprises: an A-phase portion comprising two transistors arranged in series and operated by a microcontroller, wherein one transistor of the A-phase portion is a high-side transistor and the other transistor of the A-phase portion is a low-side transistor; a B-phase portion comprising two transistors arranged in series and operated by a microcontroller, wherein one transistor of the B-phase portion is a high-side transistor and the other transistor of the B-phase portion is a low-side transistor; and a C-phase portion comprising two transistors arranged in series and operated by a microcontroller, wherein one transistor of the C-phase portion is a high-side transistor and the other transistor of the C-phase portion is a low-side transistor. In this configuration, phase A of the three-phase permanent magnet synchronous motor is connected to phase A of the three-phase half-bridge amplifier between the high-side transistor and the low-side transistor; phase B of the three-phase permanent magnet synchronous motor is connected to phase B of the three-phase half-bridge amplifier between the high-side transistor and the low-side transistor; and phase C of the three-phase permanent magnet synchronous motor is connected to phase C of the three-phase half-bridge amplifier between the high-side transistor and the low-side transistor. Furthermore, the first sensor is connected to the low-side transistor of the A-phase section of the three-phase half-bridge amplifier, and the second sensor is connected to the low-side transistors of the B-phase section and the C-phase section of the three-phase half-bridge amplifier. 10. A method for controlling the operation of a three-phase permanent magnet synchronous motor, wherein the three-phase permanent magnet synchronous motor is characterized by three phases A, B, and C, and further wherein the three-phase permanent magnet synchronous motor is driven by adjusting three phase currents _iA , iB, and iC respectively for the three phases A, _B , and _C , the method comprising: Three phase currents _iA , _iB and _iC are supplied to drive a three-phase permanent magnet synchronous motor; Sensing phase current _iA and sensing cross-phase currents _iB and _iC ; and Adjust the phase currents _iA , _iB , and _iC to generate the phase currents _iA , _iB , and _iC that produce the desired torque in a three-phase permanent magnet synchronous motor.