GB2367197A - Sensorless switched reluctance motor control - Google Patents

Abstract

A method for controlling a motor includes the steps of providing current to a first phase coil 32 of a motor during a first conduction interval and subsequently during a timing interval. During the timing interval, the time for the current to rise between two predetermined levels is measured. Because current rise time is proportional to phase inductance, and therefore, rotor position, current is supplied to one of the first phase coil 32 and a second phase coil in response to the measured rise time in order to bring subsequent conduction intervals into phase with the position of the rotor.

Description

SENSORLESS SWITCHED RELUCTANCE MOTOR CONTROL

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to switched reluctance motor controls, and, more particularly, to a method and a circuit for controlling a switched reluctance motor through indirect sensing of rotor position within the switched reluctance motor.

2. Disclosure of Related Art A conventional switched reluctance motor (SRM) includes a stator having a plurality of pairs of diametrically opposed stator poles and a rotor having a plurality of pairs of diametrically opposed rotor poles. Windings or coils are typically disposed about the stator poles and the windings around any two diametrically opposed stator poles may be connected in series or in parallel to define one motor phase of the multiphase SRM. The windings associated with. a motor phase may be referred to as a phase coil.

By generating current through the phase coil, magnetic fields are established about the stator poles and a torque is produced that attracts a pair of rotor poles into alignment with the stator poles.

The current in the phase coils is generated in a predetermined sequence in order to produce a constant torque on the rotor. The period during which current is provided to the phase coil--and the rotor poles are brought into alignment with the stator poles--is

known as the"active stage"or conduction interval of the motor phase. At a certain point--either as the rotor poles become aligned with the stator poles or at some point prior thereto--it'becomes desirable to commutate the current in the phase coil to prevent a negative or braking torque from acting on the rotor poles. Once this"commutation point"is reached, current is no longer generated

in the phase coil and the-current is allowed to dissipate from the phase coil. The period during which current is allowed to dissipate from the phase coil is known as the"inactive stage"of the motor phase.

In order to maintain a relatively constant torque on the rotor--and to thereby optimize motor efficiency--it is important to maintain an"in-phase"relationship between the position of the rotor and the active stage or conduction interval of each motor phase. In other words, it is important that the conduction interval be initiated, controlled, and commutated as the rotor reaches predetermined rotational positions. If the conduction interval is initiated and/or commutated too early or too late with respect to the position of the rotor (i. e. , the conduction interval"leads"or "lags"the rotor), a constant torque on the rotor will not be maintained and the motor will not operate at an optimum efficiency.

Conventional switched reluctance motors have attempted to maintain an"in-phase"relationship between the conduction intervals of the motor phases and the position of the rotor by continuously sensing rotor position and adjusting the control signals that initiate and commutate the conduction intervals in response thereto.

These conventional motors have employed a variety of"direct"and "indirect"methods and means for sensing rotor position.

Conventional direct sensing means have included Hall-effect sensors and optical sensors mounted directly on the rotor or disposed proximate thereto. These direct sensors are disadvantageous because they consume a large amount of space, are relatively expensive and are unreliable. Indirect sensing methods and circuits have overcome some of the deficiencies of direct sensors. However, conventional indirect sensing methods and circuits have often required complex and expensive hardware to implement. Moreover, conventional indirect sensing means are often limited in the range of motor

speeds over which they can successfully operate.

There is thus a need for a circuit and a method for controlling a switched reluctance motor that will minimize or eliminate one or more of the abovementioned deficiencies.

SUMMARY OF THE INVENTION According to the present invention there is provided a method of controlling a motor, comprising the steps of: providing current to a first phase coil of said motor during a first conduction interval ; providing current to said first phase coil during a timing interval after said first conduction interval has ended ; measuring a rise time period for said current in said first phase coil to rise between first and second predetermined current levels during said timing interval ; and, supplying current to one of said first phase coil and a second phase coil of said motor responsive to said rise time period.

A circuit in accordance with the present invention includes means for providing current to a first phase coil of the motor. The providing means may include a switch disposed on either side of the phase coil and a microcontroller that generates control signals for selectively closing the switches and coupling the phase coil to a power source. A circuit in accordance with the present

invention may also include a rise time signal generator that ,. generates a rise time signal indicative of a rise time period for the current in the first phase coil to rise between first and second predetermined current levels. The rise time signal generator may include a pair of comparators that compare a measured current level in the first phase coil to the first and second predetermined current levels and a logic gate, such as an AND gate, that outputs the rise time signal. The circuit may finally include means for supplying current to either the first phase coil or a second phase coil responsive to the rise time signal.

A circuit and method in accordance with the present invention represent a significant improvement over conventional circuits and methods for controlling a motor. The inventive circuit and method utilize indirect or sensorless means for determining

rotor position. Accordingly, the inventive circuit and method are - less costly in terms of size and expense as compared to direct sensing means. The inventive circuit and method also represent an improvement when compared to conventional indirect sensing means, however. Because the inventive circuit and method rely upon current rise time as an estimator of rotor position--rather than more complex position estimators--the inventive circuit and method are less complex and less expensive than conventional indirect sensing means. Moreover, the inventive circuit and method can be implemented over a wider range of motor speeds as compared to conventional indirect sensing means.

These and other features and objects of this invention will become apparent to one skilled in the art from the following detailed description and the accompanying drawings illustrating features of this invention by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an exploded perspective view of a conventional switched reluctance motor.

Figure 2 is a cross-sectional view of a conventional switched reluctance motor.

Figure 3 is a combination schematic and block diagram illustrating a circuit in accordance with the present invention.

Figures 4A-B are timing diagrams illustrating the relationship between current rise time, inductance and rotor position.

Figure 5 is a flowchart illustrating a method for starting a motor incorporating a circuit in accordance with the present invention.

Figures 6A-D are flowcharts illustrating a second embodiment of a method for controlling a motor in accordance with the present invention.

Figures 7A-G are timing diagrams illustrating voltage and current levels in the circuit of Figure 3 over time in accordance with a second embodiment of a method for controlling a motor in accordance with the present invention.

Figures 8A-D are timing diagrams illustrating voltage and current levels over time within a circuit in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings wherein like reference numerals are used to identify identical components in the various views, Figures 1 and 2 illustrate a conventional switched reluctance motor 10. Although the illustrated motor comprises a switched reluctance motor, it should be understood that the invention as

disclosed herein could be-applied to other motors as is known in the art. Motor 10 includes a rotor assembly 12 and a stator assembly 14, both of which may be centered about an axis 16. A representative motor phase 18 is indicated by a dashed-line box, while the other two motor phases are not shown. Although the illustrated embodiment includes three motor phases 18, it will be understood by those skilled in the art that the number of motor phases 18 may vary.

Rotor assembly 12 is provided to move a load (not shown) connected to rotor assembly 12. Assembly 12 includes a shaft 20 and a rotor 22 disposed about shaft 20. Shaft 20 is provided to engage either the load or another means for engaging the load. Shaft 20 extends longitudinally along axis 16 and may be centered about axis 16. Rotor 22 is provided to impart rotation to shaft 20 and is capable of clockwise or counter-clockwise rotation. Rotor 22 may be made from a material having a relatively low magnetic reluctance, such as iron. Rotor 22 may be centered about axis 16 and may include a splint or key (not shown) configured to be inserted within a keyway (not shown) in shaft 20. Rotor 22 includes a plurality of radially outwardly extending rotor poles 24 configured as diametrically opposed rotor pole pairs a-a', b-b'. Each of poles 24 is generally rectangular in cross-section and may include one or more radially outwardly extending teeth as is known in the art. It will be understood by those skilled in the art that the number of poles 24 of rotor 22 may vary.

Stator assembly 14 is provided produce a torque to cause rotation of rotor assembly 12. Stator assembly 14 may comprise a plurality of laminations 26 that are formed from a material, such as iron, having a relatively low magnetic reluctance. Assembly 14 includes a plurality of radially inwardly extending poles 28 configured as diametrically opposed stator pole pairs A-A', B-B', C

C'. Each pair of stator poles 28 is provided to attract a corresponding pair of rotor poles 24 of rotor assembly 12 and thereby cause rotation of rotor assembly 12. Poles 28 are generally rectangular in cross-section and may include one or more radially inwardly extending teeth (not shown) as is known in the art ; Poles 28 may extend along the axial length of stator assembly 14 and define a bore 30 that is adapted to receive rotor assembly 12. It will be understood by those in the art that the number of stator poles 28 may vary.

Rotation of rotor assembly 12 is produced by initiating, and later commutating, in a predetermined sequence, conduction intervals. in phase coils 32, 32', 32", respectively, surrounding each stator pole pair. Phase coils 32, 32', 32"are formed by connecting, in series or in parallel, windings on diametrically opposed stator poles 28. As one of phase coils 32, 32', 32"begins to conduct current, the nearest rotor pole pair is magnetically attracted towards the stator pole pair around which the energized phase coil is wound. By initiating and commutating conduction intervals in phase coils surrounding consecutive stator pole pairs, a relatively constant torque can be produced.

Referring now to Figure 3, a circuit 34 for controlling the current in coil 32 in accordance with the present invention is illustrated. Circuit 34 represents an equivalent circuit for one motor phase 18 of motor 10. It should be understood, however, that portions of circuit 34, such as controller 46, may form part of multiple motor phases 18. Circuit 34 may include means, such as switches 36,38, for providing current to phase coil 32, diodes 40, 42, a drive circuit 44, and a controller 46. In accordance with the present invention, circuit 34 may also include the following elements: means, such as sensing resistor 48 and amplifier 50, for generating a current indicative signal Vsgg, indicative of a level

of current in coil 32 ; means, such as rise time signal generator 52, for generating a rise time signal V ? (A) indicative of a rise time period for the current in coil 32 to rise between predetermined lower and upper current levels; and logic gates 54,56. Although only one motor phase 18 of motor 10 is illustrated in Figure 3, it will be appreciated that the other motor phases 18 of motor 10 may have substantially similar constructions.

Switches 36,38 are provided to selectively couple a power supply 58 to phase coil 32 to energize and deenergize coil 32.

Switches 36,38 are conventional in the art and may take any of a plurality of forms well known in the art. For example, switches 36, 38 may comprise MOSFETs. Switch 36 is connected to a first end of coil 32 in series with coil 32. Switch 38 is connected to a second end of coil 32, also in series with coil 32.

Diodes 40,42 are provided to control the dissipation of current from coil 32 and, in particular, to return the current in coil 32 to power supply 58. Diodes 40,42 are conventional in the art. Diode 40 may be connected in parallel with the series combination of switch 36 and coil 32. Diode 42 may be connected in parallel with the series combination of switch 38 and coil 32. When one of switches 36,38 is open and the other of switches 36,38 is closed, the current in phase coil 32 circulates within control circuit 34 and dissipates relatively slowly. For example, if switch 36 is opened and switch 38 is closed, the current will circulate along the path comprised of switch 38, diode 42 and coil 32. When both of switches 36,38 are open, the current in coil 32 rapidly dissipates as it is returned to power supply 58 along the path comprised of power supply 58, diode 42, coil 32, and diode 40.

Drive circuit 44 is provided to adjust the voltage level of a phase control signal VC in a conventional manner to account for different tolerances and requirements among the components of

circuit 34. Drive circuit 44 may also be provided to control the current within coil 32 between predetermined upper and lower current levels during a conduction interval in coil 32.

Controller 46 is provided to initiate and commutate the .. conduction interval of each motor phase 18. In particular, and in accordance with the present invention, controller 46 is provided to initiate and commutate the conduction interval of each motor phase 18 responsive to measured current rise times in the phase coils 32, 32', 32"of each motor phase 18. Controller 46 is conventional in the art and may comprise either discrete circuits or a programmable microcontroller. Controller 46 may generate phase control signals, such as phase control signal VCtA)'to control the initiation and commutation of the conduction interval in each motor phase 18.

Controller 46 may also generate upper and lower current level signals Vu and VL to be used by comparators 52,54 in the manner described hereinbelow.

Sensing resistor 48 is provided to generate a signal indicative of the level of current in coil 32 and is conventional in the art. Resistor 48 may have one terminal connected to switch 38 and a second terminal connected to ground. It will be understood by those in the art that a variety of conventional current sensors could be employed, including, for example, Hall effect current sensors.

Amplifier 50 is provided to convert the signal generated by sensing resistor 48 into current indicative signal VSENSE (A)' Amplifier 50 is conventional in the art.

Rise time signal generator 52 is provided to generate a rise time signal VT (A) indicative of a rise time period for a current in coil 32 to rise between predetermined lower and upper current levels. Signal generator 52 may include the following elements: means, such as comparator 60, for comparing current indicative

signal SENSE (A) to an upper current level signal Vu and generating a comparison signal Vs responsive thereto ; means, such as comparator 62, for comparing current indicative signal VSENSE to a lower current level signal VL and generating a comparison signal Vcz t. responsive thereto ; and means, such as logic gate 64, for generating a rise time signal VT (AI responsive to phase control signal VCCA) and comparison signals Vc1 and Vc2.

Comparators 60,62 are provided to compare current indicative signal VSENSECA) to upper and lower current level signals Vu and VL, respectively. Comparators 60,62 are conventional in the art. The positive input of comparator 60 is responsive to upper current level signal Vu generated by controller 46 while the negative input of comparator 60 is responsive to current indicative signal VSENSE(A) generated by amplifier 50. Comparator 60 outputs a

comparison signal VCl indicative of whether the level of current in coil 32--represented by current indicative signal less than or greater than a predetermined upper current level-represented by upper current level signal Vu. The positive input of comparator 62 is responsive to current indicative signal VSENSE (A) generated by amplifier 50 while the negative input of comparator 62 is responsive to lower current level signal VL generated by controller 46. Comparator 62 outputs a comparison signal Vs

indicative of whether the level of current in coil 32--represented by current indicative signal VsExsEtAj--s le-ss than or greater than a predetermined lower current level---represented by lower current level signal VL.

Logic gate 64 is provided to generate a rise time signal VT (A) indicative of the time required for the current in coil 32 to rise between predetermined upper and lower current levels Vu and VL.

Gate 64 is conventional in the art and may comprise an AND gate. It should be understood, however, that other gate configurations could

be implemented without departing from the spirit of the present invention. Gate 64 is responsive to phase control signal Vu ; and comparison signals Vs and VC2' Logic gate 54 is provided to generate a combined rise time signal V, indicative of the time required for the current in each of phase coils 32, 32', 32"of motor 10 to rise between predetermined upper and lower current levels. As such, gate 54 is

responsive to rise time signals VT (A) VVT (B)' and V, respectively, generated by each of the three motor phases 18 in the illustrated embodiment. Gate 54 is conventional in the art and may comprise an OR gate. It should be understood, however, that other gate configurations could be implemented without departing from the spirit of the present invention.

Logic gate 56 is provided for use in connection with a second embodiment of the present invention wherein the current rise time may be measured during either the conduction interval or during a timing interval that occurs a predetermined period of time after the conduction interval Gate 56 generates the rise time signal VT responsive to combined rise time signal VT cl and a selection signal Vsgenerated by controller 46. Selection signal Vs ensures that the measured rise time is provided to controller 46 only during a selected interval (e. g., either during the conduction interval or during the timing interval). Gate 56 is conventional in the art and may comprise an AND gate. Again, however, it should be understood that other gate configurations could be implemented without departing from the spirit of the present invention.

Referring now to Figures 4A-B, the principal upon which the inventive circuit and method is based will be described. As is known in the art, phase inductance can be used to estimate rotor position. As shown in Figure 4A, the level of inductance in any motor phase 18 increases linearly as a pair of rotor poles 24

approach a corresponding pair of stator poles 28. Inductance reaches a maximum when the rotor poles 24 and stator poles 28 are aligned and then decreases linearly as the rotor poles 24 move past the stator poles 28. As shown in Figure 4B, a similar relationship exists between rotor position and the time required for a current to rise between two predetermined levels in a phase coil of motor 10.

As a pair of rotor poles 24 approach a corresponding pair of stator poles 28, the time period required for the current to rise between the two predetermined levels increases linearly. When the rotor poles are aligned with the stator poles, a maximum amount of time is required for the current to rise between the two predetermined current levels. As the rotor poles move past the stator poles, the time required decreases linearly.

As shown in Figures 4A-B, the time required for current in a motor phase coil to rise between two predetermined levels is directly proportional to the inductance of the phase coil. As a result, measuring the rise time of current between two predetermined levels provides-an estimate of rotor position. The measured rise time can then be compared to a desired rise time that is indicative of a desired"in-phase"relationship between the conduction interval (or current rise time) of the motor phase and rotor position. If the measured rise time differs from the desired rise time, the conduction interval is lagging or leading the rotor. For example, the point designated A in Figure 4B may represent a desired current rise time for establishing an"in-phase"relationship between the conduction interval of a motor phase 18 and rotor position at a particular operating speed. The points designated B and C may represent measured rise times for current in a phase coil of motor phase 18. As illustrated in Figure 4B, points A, B, and C may fall on either the positive or negative slope of the current rise time profile. As is known in the art, the conduction intervals for the

motor phases 18 of a motor 10 operating a relatively low speed generally begin and end on the positive slope of induction.

Therefore, at low speeds, the desired rise time and the measured rise times will fall on the positive slope of the current rise time profile. A current rise time, such as rise time B, that is less than the desired rise time A will indicate that the current is

rising between the two predetermined levels more quickly than desired and will therefore indicate that the conduction interval is leading rotor position. A current rise time, such as rise time C, that is greater than the desired rise time A will indicate that the current is rising between the two predetermined levels more slowly than desired and will therefore indicate that the conduction interval is lagging rotor position. As is known in the art, the start of the conduction interval must be advanced as the speed of the motor increases. This is accomplished by initiating the conduction interval earlier--on the negative slope of inductance.

As a result, the desired and measured rise times will be found on

the negative slope of the current rise time profile as shown in ,. p Figure 4B. Therefore, once the motor reaches a predetermined speed, the current rise time B will be indicative of the conduction interval lagging rotor position while the current rise time C will be indicative of the conduction interval leading rotor position.

A motor incorporating the inventive circuit described above has at least two operating modes: a starting mode and a running mode. Referring to Figure 5, a method for starting motor 10 will be described in detail. The method may include the steps 66, 68,70 of providing current to phase coils 32, 32', and 32"and

measuring the time periods AA, A, and , respectively, for the current in each phase coil to rise between two predetermined levels, such as lower and upper current levels VL and Vu. The method may further include the step 72 of comparing the sum of current rise

time dz plus a predetermined offset value XA to current rise time A. g. If the sum of current rise time TA plus offset value KA is greater than current rise time #TB, the method may include the step 74 of comparing the sum of current rise time #TB plus a predetermined offset value Kg to current rise time A. rc. If the sum of current rise time A. ra plus offset value Kg is greater than current rise time #TC, the method may include the step 76 of energizing coil 32'. On the

other hand, if the sum of current rise time TB plus offset value Kg is less than or equal to current rise time Asc the method may include the step 78 of energizing coil 32. Returning to step 72, if the sum of current rise time #TA plus offset value KA is less than or equal to current rise time #TB, the method may include the step 80 of comparing the sum of current rise time Arc plus a predetermined offset value KC to current rise time #TA. If the sum of current rise time Llrc plus offset value Kc is greater than current rise time #TA, the method may include the step 82 of energizing coil 32". On the other hand, if the sum of current rise time TC plus offset value Kc is less than or equal to current rise time #TA, the method may include the step 84 of energizing coil 32'. It should be noted that offset values KA, KB, and KC will depend upon the particular motor 10 being operated.

Referring now to Figures 6A-D and 7A-G, an embodiment of a method in accordance with the present invention will be described.

Referring to Figures 6A and 7A, a method for

controlling a motor 10 in accordance with a second embodiment of the present invention may include the steps 108, 110 of providing current to a first phase coil 32 of motor 10 during a conduction interval 112 and providing current to coil 32 during a timing interval 114 beginning a predetermined period of time t after conduction interval 112 ends. As shown in Figure 7A, current may be provided to coil 32 during conduction interval 112 and timing interval 114 when phase control signal VC , generated by controller 46, transitions to a high logic level.

Referring again to Figure 6A, a method in accordance with the present invention may further include the step 116 of measuring a rise time period for the current in phase coil 32 to rise between first and second predetermined current levels VL and Vu, respectively, during timing interval 114. Referring now Figure 6B, step 116 may include the substep 118 of sensing a level of the current in coil 32. As shown in Figure 3, within circuit 34 current may be measured using a sensing resistor 48. Amplifier 50 then generates a current indicative signal VSENSE shown in Figure 7B, responsive to, and indicative of, the current level sensed by resistor 48. Referring again to Figure es, step 116 may further include the substep 120 of comparing the current level in coil 32 to predetermined upper current level Vu land predetermined lower current level YL and generating comparison signals Vc, and Vcz, respectively, in response thereto. As shown in Figure 7C, comparison signal Vs assumes a high logic level whenever the current level in coil 32--as indicated by VSENSE(A)--is less than upper current level Vu. As shown

in Figure 7D, comparison signal V, assumes a high logic level 2 whenever the current level in coil 32--as indicated by VSENSE(A)--is greater than lower current level VL. Referring again to Figure 6B, step 116 may further include the substep 122 of generating a rise time signal Vu, indicative of the rise time period for the

current in coil 32 to rise between predetermined lower and upper - current levels VL and Vu during timing interval 114. As shown in Figure 7E, rise time signal VT assumes a high logic level only during the period in which the current level in coil 32 is rising between the lower and upper current levels VL and Vu and phase control signal VC (A) maintains a high logic level. Because this occurs both during conduction interval 112 and timing interval 114, however, a selection signal V, generated by controller 46 is used in connection with a logic gate 56 (best shown in Figure 3) to ensure that only the measured rise time obtained from timing interval 114 is provided to controller 46. As illustrated in Figure 7F, selection signal Vs assumes a high logic state at the beginning of timing interval 114. Selection signal Vs then transitions to a low logic level responsive to the falling edge of rise time signal VT (A).

It should be noted that, if multiple timing intervals 114 are used between conduction intervals 112, selection signal Vs may be used to select which timing interval 114 to use for rise time measurements.

Referring again to Figure 6A, a method in accordance with the present invention may finally include the step 124 of supplying current to either phase coil 32 or another phase coil--such as phase coil 32'or phase coil 32"-responsive to the rise time period AT (A) indicated by rise time signal VT . Referring now to Figure 6C, step 124 may include the substep 126 of deriving a phase period value P' -responsive to the rise time period AT (A} The phase period value P' represents a period of time associated with one or more motor phases 18 of motor 10. Referring to Figures 8A-D, in one constructed embodiment phase period value P'comprises a phase interval period tl between initiation of a conduction interval 134 in one phase coil, such as phase coil 32, and the initiation of a conduction interval 136 in another phase coil, such as coil 321. In a second constructed embodiment, phase period value P'comprises a phase

interval period A2 between initiation of a timing interval 138 in - one phase coil, such as coil 32, and the initiation of a timing interval 140 in another phase coil. Alternatively, phase period value P'could represent the period between initiation of first and second conduction intervals or timing intervals in the same phase coil or the period between initiation and commutation of a conduction interval or timing interval within a phase coil.

In a constructed embodiment phase period value P'is obtained using the following formula: P'= P + K (A, -ATD) wherein P represents an existing phase period value associated with the motor phase 18 in which current rise time is being measured, K represents a gain selected to stabilize motor 10 and maintain its operation during transient conditions, and AT, represents a desired rise time value. As described hereinabove with reference to Figure

4B, desired rise time llm is indicative of"in-phase"relationship between rotor position and a conduction interval for the motor phase 18 in which current rise time is being measured.

As shown in Figure 6D, and as reflected in the aboverecited formula, substep 126 may include the substep 128 of

comparing the measured rise time period AT (A) to desired rise time period ATI to obtain a rise time error value. In a constructed embodiment, this comparison is accomplished by subtracting the desired rise time period AD from the measured rise time period A,.

Substep 126 may further include the substep 130 of adding the rise time error value to phase period value P to obtain the phase period value P'. Unlike the first method embodiment described hereinabove, the rise time error value will always be added to the phase period value P. This is because conduction interval 112 is normally commutated at or near the end of the positive slope of inductance.

By beginning timing interval 114 a predetermined period of time t

after-the end of conduction interval 112, timing interval 114 can be - made to occur during the negative slope of inductance in coil 32.

Because the desired rise time and the measured rise times will then always occur on the negative slope of the rise time current profile, the rise time error value will always be added to phase period value P.

Referring again to Figures 6A and 6C, step 124 may finally include the substep 132 of controlling current in phase coil 32 or another phase coil responsive to phase period value P'.

Controller 46 will generate phase control signals, such as VC (A)' responsive to phase period value P'in order to initiate and/or commutate conduction intervals within the motor phases and bring the

conduction intervals into an"in-phase"relationship with the position of the rotor. For example, if P represents a phase interval period between initiation of conduction intervals in coils 32 and 32', P'represents a phase interval period between initiation of conduction intervals in coils 32'and 32", and the rise time AX (A) of the current in coil 32 indicates that the conduction interval in

.. coil 32 is lagging rotor position, then phase period value P'will be less then phase period value P using the above-recited formula.

As a result, the conduction interval for coil 32"will be initiated earlier than it otherwise would have and will be brought into phase with the position of the rotor.

While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it is well understood by those skilled in the art that various changes and modifications can be made in the invention without departing from the scope of the invention.

Claims (8)

1. A method for controlling a motor, comprising the steps of: providing current to a first phase coil of said motor during a first conduction interval ; providing current to said first phase coil during a timing interval after said first conduction interval has ended; measuring a rise time period for said current in said first phase coil to rise between first and second predetermined current levels during said timing interval; and, supplying current to one of said first phase coil and a second phase coil of said motor responsive to said rise time period.

2. A method as claimed in claim 1, wherein said timing interval begins a predetermined period of time after said first conduction interval ends.

3. A method as claimed in claim 1 or 2, wherein said measuring step includes the substeps of sensing a level of said current in said first phase coil ; comparing said level of said current to said first predetermined current level ; comparing said level of said current to said second predetermined current level ; and, generating a rise time signal indicative of said rise time period.

4. A method as claimed in any one of the preceding claims, wherein said supplying step includes the substeps of: deriving a first phase period value responsive to said rise time period; and, controlling current in said one phase coil responsive to said first phase period value.

5. A method as claimed in claim 4, wherein said deriving step includes the substeps of: comparing said rise time period to a desired rise time period to obtain a rise time error value; and, adding said rise time error value to a second phase period value to obtain said first phase period value.

6. A method as claimed in claim 4 or 5, wherein said first phase period value comprises a period between initiation of a first conduction interval in said one phase coil and initiation of a second conduction interval in another phase coil.

7. A method as claimed in any one of claims 4 to 6, wherein said first phase period value comprises a period between initiation of a first timing interval in said one phase coil and initiation of a second timing interval in another phase coil.

8. A method as claimed in any one of claims 4 to 7, wherein said controlling step includes the substep of commutating a conduction interval of said one phase coil responsive to said first phase period value.