Patent ID: 12231071

DETAILED DESCRIPTION OF THE DISCLOSURE

A motor system, generally designated10, in accordance with the present invention, is shown inFIGS.1and2. The motor system10is powered by a DC power supply12, for example a battery, and comprises a brushless permanent magnet motor14and a control circuit16. It will be recognised by a person skilled in the art that the methods of the present invention may be equally applicable to a motor system powered by an AC power supply, with appropriate modification of the circuitry, for example to include a rectifier.

The motor14comprises a four-pole permanent-magnet rotor18that rotates relative to a four-pole stator20. Although shown here as a four-pole permanent magnet rotor, it will be appreciated that the present invention may be applicable to motors having differing numbers of poles, for example eight poles. Conductive wires wound about the stator20are coupled together to form a single phase winding22. Whilst described here as a single phase motor, it will be recognised by a person skilled in the art that the teachings of the present application may also be applicable to multiphase, for example three phase, motors.

The control circuit16comprises a filter24, an inverter26, a gate driver module28, a current sensor30, a voltage sensor32, a position sensor34, and a controller36.

The filter24comprises a link capacitor C1that smoothes the relatively high-frequency ripple that arises from switching of the inverter26.

The inverter26comprises a full bridge of four power switches Q1-Q4that couple the phase winding22to the voltage rails. Each of the switches Q1-Q4includes a freewheel diode.

The gate driver module28drives the opening and closing of the switches Q1-Q4in response to control signals received from the controller36.

The current sensor30comprises a shunt resistor R1located between the inverter and the zero-volt rail. The voltage across the current sensor30provides a measure of the current in the phase winding22when connected to the power supply12. The voltage across the current sensor30is output to the controller36as signal, I_SENSE. It will be recognised that in this embodiment it is not possible to measure current in the phase winding22during freewheeling, but that alternative embodiments where this is possible, for example via the use of a plurality of shunt resistors, are also envisaged.

The voltage sensor32comprises a potential divider R2,R3located between the DC voltage rail and the zero volt rail. The voltage sensor outputs a signal, V_DC, to the controller36that represents a scaled-down measure of the supply voltage provided by the power supply12.

The position sensor34comprises a Hall-effect sensor located in a slot opening of the stator20, although it will be recognised that alternative arrangements, for example where the Hall-effect sensor is positioned adjacent to a positioning magnet on the shaft, are also envisaged. The sensor34outputs a digital signal, HALL, that is logically high or low depending on the direction of magnetic flux through the sensor34. The HALL signal therefore provides a measure of the angular position of the rotor18. Embodiments are also envisaged in which the position sensor34is omitted, and sensorless control schemes are implemented. Such sensorless control schemes are known, and will not be described here for the sake of brevity. In such sensorless schemes, the HALL signal may be replaced by a BACK EMF signal, which is representative of the period of the back EMF.

The controller36comprises a microcontroller having a processor, a memory device, and a plurality of peripherals (e.g. ADC, comparators, timers etc.). In an alternative embodiment, the controller36may comprise a state machine. The memory device stores instructions for execution by the processor, as well as control parameters that are employed by the processor during operation. The controller36is responsible for controlling the operation of the motor14and generates four control signals S1-S4for controlling each of the four power switches Q1-Q4. The control signals are output to the gate driver module28, which in response drives the opening and closing of the switches Q1-Q4.

FIG.3summarises the allowed states of the switches Q1-Q4in response to the control signals S1-S4output by the controller36. Hereafter, the terms ‘set’ and ‘clear’ will be used to indicate that a signal has been pulled logically high and low respectively. As can be seen fromFIG.3, the controller36sets S1and S4, and clears S2and S3in order to excite the phase winding22from left to right. Conversely, the controller36sets S2and S3, and clears S1and S4in order to excite the phase winding22from right to left. The controller36clears S1and S3, and sets S2and S4in order to freewheel the phase winding22. Freewheeling enables current in the phase winding22to re-circulate around the low-side loop of the inverter26. In the present embodiment, the power switches Q1-Q4are capable of conducting in both directions. Accordingly, the controller36closes both low-side switches Q2,Q4during freewheeling such that current flows through the switches Q2,Q4rather than the less efficient diodes. Conceivably, the inverter26may comprise power switches that conduct in a single direction only. In this instance, the controller36would clear S1, S2and S3, and set S4so as to freewheel the phase winding22from left to right. The controller36would then clear S1, S3and S4, and set S2in order to freewheel the phase winding22from right to left. Current in the low-side loop of the inverter26then flows down through the closed low-side switch (e.g. Q4) and up through the diode of the open low-side switch (e.g. Q2).

Where the speed of the rotor18is above a pre-determined threshold, for example above 40 krpm, the controller36operates in a steady-state mode. The speed of the rotor18is determined from the interval between successive edges of the HALL signal, which will hereafter be referred to as the HALL period.

The controller36commutates the phase winding22in response to edges of the HALL signal. Each HALL edge corresponds to a change in the polarity of the rotor18, and thus a change in the polarity of the back EMF induced in the phase winding22. More particularly, each HALL edge corresponds to a zero-crossing in the back EMF. Commutation involves reversing the direction of current through the phase winding22. Consequently, if current is flowing through the phase winding22in a direction from left to right, commutation involves exciting the winding from right to left.

The controller36may advance, synchronise or retard commutation relative to the HALL edges, and hereafter the phase angle at which the controller36commutates the phase winding22will be referred to as the advance angle, irrespective of whether commutation is advanced, synchronised or retarded. The period over which current is driven into the phase winding22is hereafter referred to as the conduction period, and the controller36may vary the advance angle or conduction period to obtain desired operating characteristics.

A method according to the present invention will now be described with reference toFIG.4.

The method100comprises determining102a motor operating target. The motor operating target is determined by a user inputting their motor operating target, or, for example, is determined from a mode selected by a user. The motor operating target is compared104to a set of pre-determined excitation timing parameter relationships, which are stored in memory of the controller36. Each pre-determined excitation timing parameter relationship is established via experimentation or computational simulation. In the present case an excitation timing parameter relationship is a relationship between a motor operating target or response and the excitation timing parameter values that need to be implemented in order to achieve the motor operating target or response. It will be recognised by a person skilled in the art that each excitation timing parameter may itself be determined by a further characteristic relationship, which may be determined by other operating characteristics of the motor14.

In response to the comparison104, the controller30operates106the motor14in accordance with an excitation timing parameter relationship determined from the set of pre-determined excitation timing parameter relationships to provide a motor operating response close to the motor operating target, say within a 25% error margin of the motor operating target.

An actual motor operating response is measured108, and the measured motor operating response is compared110to the motor operating target. If the measured motor operating response does not match the motor operating target, then a correction factor is applied112to an excitation timing parameter of the motor such that the measured motor operating response moves toward the motor operating target.

In such a manner, the measured motor operating response may be closely controlled to match the motor operating target in a closed feedback loop manner. However, simply controlling the brushless permanent magnet motor14in such a closed loop manner may result in unstable and/or inefficient control, for example as a result of large correction factors being required to compensate for errors between the measured motor operating response and the desired motor operating target.

By comparing104the motor operating target to a set of pre-determined excitation timing parameter relationships, and operating106the motor14in accordance with an excitation timing parameter relationship determined from the set of pre-determined excitation timing parameter relationships to provide a motor operating response close to the motor operating target, the closed loop control may be provided with a starting point sufficiently close to the motor operating target that large correction factors to the excitation timing parameters are not required. Hence the method100according to the present invention may provide greater stability and improved motor performance relative to a method which simply uses closed loop control.

It will be appreciated by a person skilled in the art that the method100may be used to control a number of motor operating responses, including, for example, motor input power, motor speed, airflow through the motor, phase voltage, DC link voltage, phase current, supply current, motor pressure, motor temperature, or an error function combining any of the aforementioned parameters. Where the motor operating target comprises the minimisation of an error function, the error function may take the form of the equation shown below:
error=K1*(target input power−measured input power)+K2*(target speed−measured speed)
where K1 and K2 are relative weighting factors of each target. In the above the target speed could be set at a maximum achievable for the motor and the target input power to a minimum thus the control system would seek to reduce target input power for maximum speed in a manner defined by the weighting factors. This would give a peak efficiency for a given set of system constraints.

It will further be appreciated by a person skilled in the art that the excitation timing parameter may comprise a parameter which defines when the motor14is commutated and/or when excitation of the motor14begins and/or ends, and/or how long the motor14is excited for. For example, the excitation timing parameter may be any or any combination of an advance angle, a conduction period, a de-energisation (or freewheel) angle, a de-energisation (or freewheel) period, or a duty cycle.

A practical embodiment of the method100will be described with reference toFIG.5, and refers to an instance where the motor operating target is a motor input power.

A desired motor input power is determined102by a user inputting a desired motor input power, or is determined from a mode selected by a user. The desired motor input power determines a mode of control of the brushless permanent magnet motor14.

For example, as can be seen fromFIG.6, the mode of control of the motor14is dependent on the motor input power. Where the motor input power is at a high level, the motor14is controlled such that commutation occurs in advance of zero-crossings of back EMF, a single excitation period occurs in each electrical half-cycle, followed by a dual-FET freewheel period. This is referred to inFIG.6as Mode1. Where the motor input power is at a medium level, the motor14is controlled such that commutation occurs in advance of zero-crossings of back EMF, two excitation periods occur in each electrical half-cycle, each followed by a dual-FET freewheel period. This is referred to as Mode2inFIG.6. Where the motor input power is at a medium to low level, the motor14is controlled such that commutation is retarded relative to zero-crossings of back EMF a single excitation period occurs in each electrical half-cycle, followed by a turn-off of the bridge of switches Q1-Q4. This is referred to as Mode3inFIG.6. Where the motor input power is low, for example around a minimum input power, pulse width modulation (PWM) of the input voltage occurs, with commutation of the motor14being synchronous with zero-crossings of back EMF induced by the motor14. This is referred to as Mode4inFIG.6.

Thus it can be seen that the desired motor input power determined102by a user impacts on the mode of control of the brushless permanent magnet motor14. Some desired motor input powers may span more than one mode of control, and in this case the controller30will choose the most appropriate mode of control on a case by case basis. The controller30also employs hysteresis control to ensure that the motor14doesn't oscillate between different modes of control.

The mode of control of the motor14also has an influence on the excitation timing parameter relationship which determines the desired motor input power, as can be seen fromFIG.6, with different relationships being present for different modes. In particular, where the motor input power is at a high level, and the motor is controlled in Mode1, the excitation timing parameter relationship may have an advance angle as a primary excitation timing parameter, and a conduction period as a secondary excitation timing parameter. Where the motor input power is at a medium level, and the motor is controlled in Mode2, the excitation timing parameter relationship may have a freewheel period as a primary excitation timing parameter, and a conduction period as a secondary excitation timing parameter, or the excitation timing parameter relationship may have an advance angle as a primary excitation timing parameter, and a freewheel period as a secondary excitation timing parameter. Where the motor input power is at a medium to low level, and the motor is controlled in Mode3, the excitation timing parameter relationship may have a conduction period as a primary excitation timing parameter, and an advance angle as a secondary excitation timing parameter. Where the motor input power is at a low level, and the motor is controlled in Mode4, the excitation timing parameter may have a duty cycle as a primary excitation timing parameter.

Thus it can be seen that the desired motor input power determined102by a user is also linked to an excitation timing parameter relationship. In this regard, the method100comprises comparing the desired motor input power to a set of pre-determined excitation timing parameter relationships, which are, for example, stored in memory of the controller30. Each of the set of pre-determined excitation timing parameter relationships links a motor input power to a set of excitation timing parameters which can be implemented to achieve said motor input power.

The method100comprises operating106the motor14in accordance with an excitation timing parameter relationship determined from the set of pre-determined excitation timing parameter relationships to provide a motor input power close to the desired motor input power, say, for example, a motor input power that is within 25% of the value of the desired motor input power. If the desired motor input power lies within a single control mode, then the two closest excitation timing parameter relationships may be interpolated to obtain an operating excitation timing parameter relationship. If, however, the desired motor input power overlaps multiple control modes, then a single excitation timing parameter relationship that provides a motor input power closest to the desired motor input power is selected as an operating excitation timing parameter relationship.

In practice, and to ensure a practical control scheme, the operating excitation timing parameter relationship is taken as an upper or lower limit for the excitation timing parameter values, such that the excitation timing parameter values can only be altered in one direction, for example increased or decreased, to achieve the desired motor input power. This may allow the relationship to change in a well-defined manner, and may make the end-point value more repeatable.

If the operating excitation timing parameter relationship is significantly removed from a previous operating excitation timing parameter relationship, then the controller30implements a transition mode whereby the motor14is accelerated or decelerated as appropriate to provide a gradual transition to the operating excitation timing parameter.

Once an operating excitation timing parameter is chosen and implemented, the actual motor input power is then measured108, and the measured motor operating input power is compared110to the desired motor input power. If the measured motor input power does not match the desired motor input power, then a correction factor is applied112to an excitation timing parameter of the motor such that the measured input power moves toward the desired motor input power.

In this regard, and as mentioned above, each excitation timing parameter relationship may comprise multiple excitation timing parameters, and excitation timing parameters within an excitation timing parameter relationship may be afforded different priorities. For example, where a desired motor input power is relatively high, an advance angle may be viewed as a primary excitation timing parameter, whilst a conduction period may be viewed as a secondary excitation timing parameter. Applying112a correction factor may involve applying a first correction factor to a primary excitation timing parameter and/or applying a second correction factor to a secondary excitation timing parameter. Alternatively, applying a correction factor may involve, for example, a ratio between the primary and secondary excitation timing parameters, for example one secondary excitation timing parameter increment for each two primary excitation timing parameter increments. The type of correction factor applied will be dependent on the excitation timing parameter relationship implemented, as will be appreciated by a person skilled in the art.

Each excitation timing parameter may be defined by other motor operating characteristics. For example, advance angles and conduction periods may be defined by the relationships set-out below:

Advance Time:
AP00+AP10×V+AP01×S+AP20×V2+AP02×S2+AP11×V×S+AP30×V3±correction
Conduction Time:
CP00+CP10×V+CP01×S±correction
where APXX and CPXX are coefficients determined from simulation and measurement, S is motor rotor speed, and V is the motor input voltage. The results of the above equations are altered by applying112the correction factor.

Where the correction factor is applied112to the excitation timing parameter, a pre-determined limit for the excitation timing parameter is imposed, thereby ensuring optimum operating conditions for the motor14. For example, as can be seen fromFIG.7, motor efficiency may vary with motor input power in a non-linear manner in response to a change in conduction period. Whilst there may be multiple points along the motor input power curve which have the same value, for example the desired motor input power value, each of these points may result in a different motor operating efficiency. By setting a pre-determined limit for the excitation timing parameter, the motor may be prevented from operating at the desired motor input power with a low motor operating efficiency.

If the primary excitation timing parameter hits the pre-determined limit before the measured motor input power reaches the desired motor input power, then a second correction factor is applied to the secondary excitation timing parameter until the measured motor input power reaches the desired motor input power. Of course, it is also possible that the measured motor input power reaches the desired motor input power before the primary excitation timing parameter reaches its pre-determined limit, in which case application of a correction factor to the secondary excitation timing parameter may be unnecessary.

Measurement108of the desired motor input power, comparison110of the measured motor input power to the desired motor input power, and subsequent application112of a correction factor is applied in a closed loop manner to continuously hunt for the desired motor input power.

It will be recognised by a person skilled in the art that the method100will be applicable to other desired motor operating targets with minimal adjustment to the method steps described above.

It will further be recognised by a person skilled in the art that the method100may be modified slightly, as shown inFIG.8to provide a method200of controlling a device which comprises the brushless permanent magnet motor14.

The method200is substantially similar to the previously described method100, and differs only in the additional steps of determining202a device output target, and determining204a motor operating target to achieve the device output target. The other steps of the method200are also present in the previously described method100, and hence like reference numerals are used inFIG.8for consistency.