Device and method for detecting back electromotive force phase and device and method for controlling excitation

1. A device for detecting a back electromotive force phase of a motor coil of a two-phase excitation stepper motor that is driven with constant-current PWM, the device comprising:

a timing generating unit for generating a timing signal indicating a start timing, an intermediate timing and an end timing of a 180-degree electrical angle period in a detection target phase, from excitation pulse signals indicating respective current passage directions of two current passage phases; and

a difference calculating unit for receiving the timing signal, and calculating a difference between a total PWM control period of the detection target phase during a first-half 90-degree period defined by the start timing and the intermediate timing, and a total PWM control period of the detection target phase during a second-half 90-degree period defined by the intermediate timing and the end timing.

2. The back electromotive force phase detecting device of claim 1 , wherein

the difference calculating unit receives an excitation control signal related to the detection target phase, and uses a period during which the excitation control signal is at a predetermined logic level, as a PWM control period of the detection target phase.

3. The back electromotive force phase detecting device of claim 1 , comprising:

a PWM control period detecting unit for outputting a signal having a predetermined logic level during a time that an absolute value of a shunt resistance voltage related to the detection target phase is larger than zero,

wherein the difference calculating unit uses a period during which an output signal of the PWM control period detecting unit is at the predetermined logic level, as a PWM control period of the detection target phase.

4. The back electromotive force phase detecting device of claim 1 , wherein the difference calculating unit includes:

a power supply unit for outputting a constant voltage whose polarity is reversed between during the first-half 90-degree period and during the second-half 90-degree period;

an analog integrator for integrating the output voltage of the power supply unit during the PWM control period of the detection target phase; and

a sampling/holding unit for sampling and holding an output of the analog integrator.

5. The back electromotive force phase detecting device of claim 1 , wherein the difference calculating unit includes:

an oscillator for generating a clock signal having a frequency higher than that of the excitation control signal related to the detection target phase;

a counter for performing one of counting up and counting down of the clock signal during a PWM control period of the detection target phase during the first-half 90-degree period, and performing the other during a PWM control period of the detection target phase during the second-half 90-degree period; and

a memory for storing a count value of the counter.

6. The back electromotive force phase detecting device of claim 1 , comprising:

an overcurrent detecting unit for outputting a signal having a predetermined logic level during a time that a current flowing in the detection target phase exceeds a threshold,

wherein the difference calculating unit ignores a PWM control period of the detection target phase during a time that the output signal of the overcurrent detecting unit is at the predetermined level.

7. An excitation control device for controlling a motor drive device for driving a stepper motor by two-phase excitation with constant-current PWM, the device comprising:

a control pulse generating unit for supplying a control pulse indicating a rotation angle of the stepper motor to the motor drive device;

a reference voltage generating unit for supplying to the motor drive device a reference voltage that is a reference for the constant-current; and

a control unit for instructing the reference voltage generating unit to change the reference voltage based on an output of the back electromotive force phase detecting device of claim 1 .

8. The excitation control device of claim 7 , wherein

the control unit instructs the control pulse generating unit to change the control pulse generation timing based on the output of the back electromotive force phase detecting device.

9. The excitation control device of claim 8 , wherein

the control unit ignores the output of the back electromotive force phase detecting device immediately after changing the control pulse generation timing.

10. A method for detecting a back electromotive force phase of a motor coil of a two-phase excitation stepper motor that is driven with constant-current PWM, the method comprising the steps of:

generating a start timing, an intermediate timing and an end timing of a 180-degree electrical angle period in a detection target phase, from excitation pulse signals indicating respective current passage directions of two current passage phases; and

calculating a difference between a total PWM control period of the detection target phase during a first-half 90-degree period defined by the start timing and the intermediate timing, and a total PWM control period of the detection target phase during a second-half 90-degree period defined by the intermediate timing and the end timing.

11. The back electromotive force phase detecting method of claim 10 , wherein

the PWM control period of the detection target phase is a period during which an excitation control signal related to the detection target phase is at a predetermined logic level.

12. The back electromotive force phase detecting method of claim 10 , wherein

the PWM control period of the detection target phase is a period during which an absolute value of a shunt resistance voltage related to the detection target phase is larger than zero.

13. The back electromotive force phase detecting method of claim 10 , comprising:

selecting a constant voltage whose polarity is reversed between during the first-half 90-degree period and during the second-half 90-degree period;

integrating the selected voltage during the PWM control period of the detection target phase; and

sampling and holding a result of the integration at the end timing.

14. The back electromotive force phase detecting method of claim 10 , comprising:

generating a clock signal having a frequency higher than that of the excitation control signal related to the detection target phase;

performing one of counting up and counting down of the clock signal during a PWM control period of the detection target phase during the first-half 90-degree period, and performing the other during a PWM control period of the detection target phase during the second-half 90-degree period; and

storing a result of the counting operation at the end timing.

15. The back electromotive force phase detecting method of claim 10 , wherein

in the difference calculating step, the PWM control period of the detection target phase is ignored during a time that a current flowing in the detection target phase exceeds a threshold.

16. An excitation control method for controlling a motor drive device for driving a stepper motor by two-phase excitation with constant-current PWM, the method comprising the steps of:

supplying a control pulse indicating a rotation angle of the stepper motor to the motor drive device;

supplying to the motor drive device a reference voltage that is a reference for the constant-current; and

changing the reference voltage based on a result of detection by the back electromotive force phase detecting method of claim 10 .

17. The excitation control method of claim 16 , comprising:

changing the control pulse generation timing based on the detection result of the back electromotive force phase detecting method.

18. The excitation control method of claim 17 , wherein

in the reference voltage changing step and the control pulse generating timing changing step, the detection result of the back electromotive force phase detecting method immediately after changing the control pulse generation timing is ignored.