Controller of a stepping motor, control method for the motor and timing device

Abstract: A power-saving controller for controlling a stepping motor mounted for example in an electronic wristwatch. A pulse signal generator in the controller combines two pulse signals GPi and GPi+2 having different duty factors using a compositing signal CP, based on a reference signal BP, thereby compositing a pulse signal GPi+1 having an intermediate duty factor. Even if the frequency of the reference signal BP is lowered, a step size for controlling the duty factor of a drive pulse is prevented from expanding, thereby avoiding the driving power required by the stepping motor from increasing. The lowering of the frequency of the reference signal BP reduces the power consumption of the circuit of the wristwatch and, thus of the controller.

Description: The present invention relates to a controller of a stepping motor and its control method and, in particular, to a power-saving type controller which is useful for driving a stepping motor in an electronic timepiece, and a control method of the stepping motor. Stepping motors, also called pulse motors or digital motors, are driven by a pulse signal and find widespread use as an actuator in digital control devices. Compact electronic devices and information handling apparatuses, appropriate for portable use, have been recently developed, and miniature and light-weight stepping motors are widely employed in these devices. Timing devices such as an electronic timepiece and timing switch are typical of such electronic devices. In the timing device, an oscillator circuit employing a crystal oscillator supplies a reference pulse, which is then converted into a timing signal having a frequency appropriate for timing purposes, for example, 1 Hz. A drive pulse is supplied to the stepping motor in synchronization with the timing signal to drive a second hand in the timing device. Since a power supply for use in such a portable electronic device is subject to space and other limitations, it is important to reduce power consumed by a stepping motor and the like for a reliable and long-time operation of the device. For this reason, in an electronic timepiece employing a stepping motor, the root-means-square value of drive pulse supplied to the stepping motor is automatically set to an appropriate value to match a condition unique to each electronic timepiece or operational conditions. The power consumption by the stepping motor is thus reduced. Several methods for controlling the root-mean-square value of drive pulse are available. In one method, the drive pulse is controlled in its pulse width or pulse height. In another method, the drive pulse is formed of a plurality of sub-pulses, and the root-mean-square value of the drive pulse is controlled by changing the duty factor of the sub-pulses. EP 0704774A discloses an electronic timepiece of a related type in which the output from the power source is detected and the stepping motor drive pulses are selected from one of a plurality of pulse generators in dependence upon the detected power source output. In addition to the reduction in the power consumption by the stepping motor, every attempt is made to reduce the overall power consumption by the electronic device. It is also contemplated today that the power consumed in an oscillator circuit is reduced by lowering the oscillation frequency of a reference pulse (reference signal) output by the oscillator circuit employing a reference oscillator such as a crystal oscillator. By lowering the frequency of the reference pulse, the component count of a frequency divider, for example, is reduced, and the reduction in the operational frequency in the circuit in turn reduces the power consumption in the circuit of the device. A controller, which controls the root-mean-square value of the drive pulse according to the duty factor of the sub-pulses, suffers a drop in the control resolution of the duty factor of the sub-pulses when the frequency of the reference pulse supplied by the oscillator circuit is lowered. Specifically, when the frequency of the reference pulse is 32 kHz, the duty factor of a pulse signal of 1 kHz can be controlled in step sizes of 1/32 (resolution). When the frequency of the reference pulse drops to half, namely, to 16 kHz, the duty factor is controlled in step sizes of 1/16, and the control resolution is substantially degraded. This makes it difficult to control the root-mean-square value of the drive pulse to a small but sufficient current matching the operational state of the stepping motor. To prevent a timepiece hand from running in an erratic fashion under insufficient power of the drive pulse, the electronic timepiece is supplied with a drive pulse of high root-mean-square value rather than narrow step-sized drive pulse based on a high-frequency reference pulse. This increases the power consumption by the motor, and the timing device fails to take advantage of the power saving feature with a low-frequency reference pulse. US 4340946 discloses a system for controlling a stepping motor of an electronic timepiece in accordance with the load applied to the stepping motor. The drive power can be provided at a high, intermediate or low power level by selecting a drive pulse signal from one of these predefined drive pulse signals, each having a different duty cycle. The high power level consists of continuous pulses, the intermediate power level signal consists of a burst of pulses of predetermined duty cycle, and the low power level signal consists of drive pulses comprising pulse bursts of lower duty cycle than the intermediate drive pulses. US 5103462 discloses a pulse duration modulator for converting an analog input signal into a DC output signal proportional to the input signal. The input signal is A to D converted and two number signals are derived, one representing the whole number and the other representing the remainder arising from quantisation of the input signal. The whole number signal is modulated to provide a first pulse sequence whose pulses have a duration proportional to the whole number. The two number signals are also added and modulated to provide a second pulse sequence whose pulses have a duration proportional to the addition of the two number signals. The first and second pulse sequences are combined in a specified way to provide a combined pulse sequence having a mean pulse duration based upon the durations of the first and second pulse sequences. The combined pulse sequence is then averaged to provide the DC output signal. Accordingly, it is an object of the present invention to provide a controller of a stepping motor and a control method of the stepping motor, which control the duty factor of a drive pulse in a resolution practically higher than the oscillation frequency of a reference signal, through a simple method in an simple construction, to reduce the power consumption in a timing device. It is another object of the present invention to provide a controller of a power-saving type stepping motor for use in a portable device and a control method of the stepping motor. According to the present invention, pulse signals having different duty factors are composited at an appropriate ratio. The controller generates a pulse signal having an intermediate duty factor with respect to a pulse signal having a duty factor normally derived from the reference signal, and maintains the control resolution of the root-mean-square value of the drive pulse at a conventional level or raises it above the conventional level, even when the oscillation frequency of the reference signal is lowered. Accordingly, according to a first aspect of the present invention, there is provided a controller of a stepping motor comprising: a pulse signal generator arranged to receive an input pulse signal from a reference source and for generating therefrom a first pulse signal having a first duty factor and a second pulse signal having a second duty factor; characterised by a selector circuit arranged to receive a compositing signal and the first and second pulse signals, and for selectively outputting the first and second pulse signals having the first and second duty factors at a predetermined ratio as determined by the compositing signal, thereby to provide a drive pulse signal having a duty factor which can be selectively varied in accordance with the predetermined ratio as determined by the compositing signal; and a drive control unit for feeding the drive pulse signal to said stepping motor. According to a second aspect of the present invention, there is provided a method of controlling a stepping motor comprising: receiving an input pulse signal from a reference source and generating therefrom a first pulse signal having a first duty factor and a second pulse signal having a second duty factor; a compositing step of providing a compositing signal and selectively outputting the first and second pulse signals at a predetermined ratio as determined by the compositing signal, thereby to provide a drive pulse signal (DP) having a duty factor which can be selectively varied in accordance with the predetermined ratio as determined by the compositing signal; and a driving step of feeding the drive pulse signal to said stepping motor. In the controller and the control method of the stepping motor, of the present invention, a selector selects between the first pulse signal and the second pulse signal, based on a compositing pulse signal having a duty factor of 50%. In this way, a pulse signal, in which the first pulse signal and the second pulse signal appears at an equal ratio, is composited. The compositing pulse signal, if averaged over its period, gives an intermediate duty factor between the first and second duty factors. The duty factor of the composite pulse signal is not limited to 50%. A compositing pulse having any other duty factor may be used so that its resulting duty factor is any one between the first and second duty factors. The drive pulse is formed of a plurality of sub-pulses. When the drive control unit powers the stepping motor, the duty factor of the sub-pulses is controlled based on the first and second pulse signals and a composited third pulse signal. Based on the third pulse signal, a sub-pulse having a duty factor between the first and second duty factors is generated in addition to the first and second pulse signals. Even when the first and second pulse signals derived from the reference signal present a low control resolution with a large pulse width of the drive pulse, the use of the third pulse signal heightens the resolution of the root-mean-square value of the drive pulse, namely, narrows the step size of the drive pulse. According to the present invention, the electronic device, with a low oscillation frequency of the reference signal, still works on the drive pulse of a narrow step size supplied to the stepping motor. In this way, the stepping motor is provided with a low root-mean-square drive pulse matching the operational state of the stepping motor. The power-saving feature resulting from the use of lower oscillation frequency of the reference signal is thus maintained rather than being canceled. By allowing the controller of the present invention to control a hand-driving stepping motor, the root-mean-square value of the drive pulse is finely controlled even though the oscillation frequency of the reference signal is lowered. In the timing device, the power consumption of the stepping motor is not raised, and the power consumption in its electronics is reduced while the frequency of the reference signal in the oscillator circuit is lowered. Given an unchanged oscillation frequency, the controller of the present invention for the stepping motor controls the root-mean-square value of the drive pulse even more finely so that the root-mean-square value of the drive pulse supplied to the stepping motor becomes a minimum power still capable of driving the stepping motor. The power consumption of the stepping motor is thus reduced. Embodiments of the present invention will now be described by way of further example and with reference to the accompanying drawings, in which:- FIG. 1 is a block diagram showing a timing device incorporating a controller of the present invention; FIG. 2 is a schematic diagram of a compositing circuit that composites pulse signals having different duty factors, in a pulse generator circuit in the controller of FIG. 1; FIG. 3 is a flow diagram showing the process in which the controller of FIG. 1 generates a drive pulse of a different root-mean-square value; FIG. 4 is a timing diagram showing how the compositing circuit shown in FIG. 2 composites the drive pulse from a pulse signal using the pulse signals of different duty factors; and FIG. 5 is a timing diagram showing how a pulse signal of a different duty factor is composited based on a compositing signal different from the one shown in FIG. 4, and how the drive pulse is composited based on that pulse signal. Referring to the drawings, an embodiment of the present invention is now discussed. FIG. 1 is a block diagram generally showing a timing device 1 of the present invention. The timing device 1 comprises a stepping motor 10, a controller 20 for controlling the stepping motor 10, a wheel train 50 for transmitting the rotation of the stepping motor 10, and a second hand 61, a minute hand 62, and a hour hand 63 driven by the wheel train 50. The stepping motor 10 comprises a driving coil 11 that generates a magnetic force in response to a drive pulse supplied by the controller 20, a stator 12 excited by the driving coil 11, and a rotor 13 that rotates within the stator 12. The rotor 13 is a two-pole, disk-like permanent magnet, and constitutes a PM-type (permanent-magnet rotary-type) single-phase, stepping motor 10. The stator 12 is provided with magnetically-saturated portions 17 so that different poles are respectively generated under the magnetic force of the driving coil 11, at phases 15 and 16 around the rotor 13. To control the direction of rotation of the rotor 13, the stator 12 has an internal notch at an appropriate position on its inner circumference. With this arrangement, a cogging torque takes place to stop the rotor 13 at its appropriate position. The rotation of the rotor 13 of the stepping motor 10 is transmitted to each hand through the wheel train 50 that includes a fifth wheel and pinion 51 which is meshed with the rotor 13 via a pinion, a fourth wheel and pinion 52, a third wheel and pinion 53, a second wheel and pinion 54, a minute wheel 55 and an hour wheel 56. The second hand 61 is connected to the shaft of the fourth wheel and pinion 52, the minute hand 62 is connected to the shaft of the second wheel and pinion 54, and the hour hand 63 is connected to the shaft of the hour wheel 56. The time is indicated by these hands moving in step with the rotor 13. A transmission mechanism (not shown) may be connected to the wheel train 50 to indicate the year, the month and the day. The timing device 1 indicates the time as the stepping motor 10 rotates. The stepping motor 10 is supplied with a drive pulse in synchronization with a timing signal of a predetermined frequency (1 Hz). The controller 20 for controlling the stepping motor 10 comprises an oscillator circuit 22 that outputs a reference pulse BP having a reference frequency using a reference oscillation source 21 such as a crystal oscillator, a frequency divider 23 that frequency-divides the reference pulse to output pulses WP1 - WPm (m is an integer) having different frequencies, a pulse signal generator (waveform shaper) 25 that outputs a pulse signal that is used to generate the drive pulse to be fed to the stepping motor 10, based on a group of pulses 24 supplied by the frequency divider 23. The pulse signal generator 25 outputs a plurality of pulse signals. In this embodiment, the pulse signal generator 25 outputs, for example, a pulse signal P1 for generating the drive pulse DP to be fed to the stepping motor 10, a pulse signal (auxiliary pulse) P2 that is used to feed a sufficiently large drive pulse to the stepping motor 10 for rotation when the stepping motor 10 fails to start rotating, and a pulse signal P3 for generating a demagnetizing pulse PE to demagnetize the magnetic force that is resident in the driving coil 11 subsequent to the supply of the large drive pulse. The controller 20 also comprises a drive control circuit 30 that controls CMOS transistors 32 and 33 in a drive circuit 31 in response to these signals supplied by the pulse signal generator 25. The drive circuit 31, controlled by the drive control circuit 30, feeds the drive pulse to the driving coil 11 in the stepping motor 10. The stepping motor 10 thus rotates. Connected to the wiring that conducts the drive pulse from the drive circuit 31 to the driving coil 11 is a detector 39 which senses the rotation of the rotor 13 by picking up an induced voltage in succession to the supply of the drive pulse. The drive control circuit 30 is supplied with a sense signal by the detector 39 so that the drive pulse of a more appropriate root-mean-square value is fed to the stepping motor 10 in a more appropriate timing. The controller 20 generates the drive pulse DP constructed of a plurality of sub-pulses having narrow pulse widths, and the root-mean-square value of the drive pulse DP is controlled by changing the duty factors of the sub-pulses. To this end, the pulse signal generator 25 outputs pulse signals GP1-GPn (n is an integer) for generating the sub-pulses having the different duty factors. Referring to FIG. 2, the pulse signal generator 25 generates the pulse signal GPi having a different duty factor. The pulse signal generator 25 in this embodiment includes a plurality of generator circuits 27, each generating a pulse signal GPi having a resolution (step size) of duty factor of 1/16, based on the reference frequency BP of 32 kHz and a pulse signal WPh of an appropriate frequency, for example, 2 kHz, into which the reference frequency BP is divided. The pulse signal GPi generated in the generator circuit 27 is fed to the drive control circuit 30. The pulse signal generator 25 in this embodiment includes a selector circuit 28 that receives pulse signals GPi and GPi+2, with a duty factor difference of 1/16 therebetween, output by the generator circuit 27. The selector circuit 28 is a compositing circuit that outputs alternately the pulse signal GPi and the pulse signal GPi+2 having the different duty factors, at a predetermined ratio (50%, for example) and composites a pulse signal GPi+1 having a duty factor different from that of the pulse signal GPi by 1/32. The pulse signal GPi+1 is also fed to the drive control circuit 30. The selector circuit 28 includes two AND gates 29a and 29b, the outputs of which are connected to each other. The AND gate 29a receives the pulse signal GPi and a compositing pulse signal CP, while the AND gate 29b receives the pulse signal GPi+2 and a compositing signal CP at its inverted input. The selector circuit 28 outputs the pulse signal GPi when the compositing signal CP is at a high level, and outputs the pulse signal GPi+2 when the compositing signal CP is at a low level. When the duty factor of the compositing signal CP is 50%, the pulse signal GPi and the pulse signal GPi+2 are output at equal durations. The selector circuit 28 thus outputs a pulse signal having an intermediate duty factor between the pulse signals GPi and GPi+2 having a step width of 1/16, namely, the pulse signal GPi+1 having a step width of 1/32. The pulse signal GPi+ 1 as well as the pulse signals GPi and GPi+2 are fed to the drive control circuit 30 and are used to control the root-mean-square value of the drive pulse DP. FIG. 3 is a flow diagram showing the process in which the controller 20 generates the drive pulse DP of an appropriate root-mean-square value. In step ST1, the oscillator 22 outputs the reference pulse BP. In step ST2, the plurality of generator circuits 27 in the pulse signal generator 25 generate a plurality of pulse signals GPj having different duty factors. Now let GPj represent an odd-numbered pulse signal generated by the generator circuit 27. In step ST3, a pair of adjacent odd-numbered pulse signals GPj are input to the selector circuit 28. The odd-numbered pulse signals input to the selector circuit 28 are alternately output in accordance with the compositing signal CP and an even-numbered pulse signal GPk between the odd-numbered pair is composited. In step ST4, the odd-numbered pulse signals GPj and the even-numbered pulse signal GPk, namely, all pulse signals GPi (i= 1 through n) generated by the pulse signal generator 25 are fed to the drive control circuit 30. The drive control circuit 30 generates the drive pulse of a predetermined root-mean-square value, based on a pulse signal having an appropriate duty factor among these pulse signals GPi and feeds it to the stepping motor. FIG. 4 is a timing diagram showing how the selector circuit 28 generates a pulse signal having an intermediate duty factor. The 2 kHz pulse signal GPi and 2 kHz pulse signal GPi +2 (the odd-numbered pulse signals GPj shown in FIG. 3), derived from the reference pulse BP, have respectively duty factors of 8/16 and 9/16. By compositing these pulse signals in accordance with a 1 kHz compositing signal CP having a duty factor of 50%, the pulse signal GPi and the pulse signal GPi+2 appear alternately. With this arrangement, the pulse signal GPi+1 having a duty factor of 17/32 (the even-numbered pulse signal GPk shown in FIG. 3) is thus obtained. The drive control circuit 30 composites the pulse signal P1 having a pulse width of 3 ms and the pulse signal GPi+1 to result in the drive pulse DP constructed of the sub-pulse having a duty factor of 17/32, which is then fed to the drive circuit 31. Referring to FIG. 5, the pulse signal GPi+1 is composited using a 0.5 kHz compositing signal CP' different from the above frequency. In this case, as well, the composited pulse signal GPi+1 is constructed of the pulse signal GPi and the pulse signal GPi+2, both of which alternately appear every half cycle of the compositing signal CP'. This results in the pulse signal GPi +1 which has a duty factor of 17/32, if averaged over the period of the compositing signal CP'. Since the pulse width of the pulse signal P1 fails to agree with an integer multiple of the period of the compositing signal CP', a combination of the pulse signal GPi+1 and the pulse signal P1 having a pulse width of 3 ms results in a drive pulse DP constructed of a sub-pulse SP having an average duty factor of 25/48. By changing the duty factor of the compositing signal CP, a pulse signal GP having an intermediate duty factor may be output. For example, by supplying compositing signals of CP having a duty factor of 25 %, 50% and 75%, a pulse signal GP of a duty factor with a step size of 1/64 may be obtained. The controller 20 of the timing device 1 combines the pulse signals GPi having the different duty factors, derived from the reference signal BP, using a compositing signal CP, for example, at a ratio of 50%, and results in a pulse signal GPi+1 having a duty factor intermediate between the duty factors of these pulse signals GPi. Using the pulse signals GPi and GPi + 1, the controller 20 controls the duty factors of the sub-pulses in a step size narrower than the step size directly derived from the reference signal BP, and therefore controls the root-mean-square value of the drive pulse DP in a resolution higher than that of the reference pulse BP. Given a frequency of the reference signal lower than the frequency of the conventional reference signal BP, the controller 20 still can control the root-mean-square value of the drive pulse DP at a control accuracy comparative to the one conventional one. Even with a lower-frequency reference signal BP, an appropriate drive pulse of an appropriate root-mean-square value keeps the power consumption of the motor from increasing. By changing the duty factor of the compositing pulse CP, the control step size is further narrowed, further reducing the power consumption of the motor. Lowering the frequency of the reference signal BP reduces the power consumption of the oscillator 22 and the number of stages of frequency division in the frequency divider 23. The power consumption of the circuit of the device is thus reduced. In the device 1, the use of the low frequency reference signal BP reduces the overall power consumption. By controlling the duty factor of the compositing signal, the frequency of the reference signal BP is lowered even further, leading to a further reduction in power consumption. Given the same oscillation frequency, the present invention controls the root-mean-square value of the motor drive pulse in a finer fashion. According to the controller and the method of control of the stepping motor, the root-mean-square value of the drive pulse supplied to the stepping motor is finely controlled to a minimum power still capable of driving the stepping motor. The power consumption of the stepping motor is thus reduced. The present invention has been discussed in connection with the two-phase stepping motor preferably used in the timing device. The present invention effectively works in multi-phase (two-phase or higher) motors. The driving method of the stepping motor is not limited to a 1-phase excitation. The stepping motor may be driven in 2-phase excitation or in 1-2 phase excitation. The stepping motor, controlled by the controller and the control method of the present invention, is not limited to the PM-type. The present invention may be applied to VR-type and hybrid type motors. According to the controller and the control method of the present invention, the pulses having the different duty factors, derived from the low-frequency reference signal, are combined to composite a pulse signal having an intermediate duty factor. Even with the reference signal frequency lowered, the duty factor of the drive pulse is controlled in a step size equal to or finer than the one in the conventional art. The root-mean-square value of the drive pulse supplied to the stepping motor is finely controlled to a minimum power still capable of driving the stepping motor. The power consumption of the stepping motor is thus reduced. The lowering of the reference signal frequency results in the reduction in the power consumption of the circuit of the device and the power consumption for driving the stepping motor is thus reduced. Since the present invention reduces the power consumption of the controller for controlling the stepping motor, the controller and the control method are suited for a portable device that today features a more compact design and multi-functions. For example, a portable device, such as an electronic wristwatch, consumes more power with a multi-function feature while its compact design makes the use of a large battery difficult. Instead of a battery, some wristwatches employ a generator such as a solar cell. Even in such a timing device with a small power generation capacity, the control method and the controller of the present invention serve the power-saving purpose. Long-time and reliable timing operation is assured. A variety of other functions available on the electronic wristwatch are also performed.

Claims: A controller of a stepping motor (10) comprising: a pulse signal generator (25) arranged to receive an input pulse signal (BP) from a reference source (21, 22) and for generating therefrom a first pulse signal (GPi) having a first duty factor and a second pulse signal (GPi+2) having a second duty factor; characterised by a selector circuit (28) arranged to receive a compositing signal (CP) and the first and second pulse signals (GPi, GPi+2), and for selectively outputting the first and second pulse signals (GPi, GPi+2) having the first and second duty factors at a predetermined ratio as determined by the compositing signal (CP), thereby to provide a drive pulse signal (DP) having a duty factor which can be selectively varied in accordance with the predetermined ratio as determined by the compositing signal (CP); and a drive control unit (30) for feeding the drive pulse signal (DP) to said stepping motor (10). A controller of a stepping motor according to Claim 1, wherein said selector circuit (28) selects between said first pulse signal (GPi) and said second pulse signal (GPi+2), based on a compositing signal (CP) having a duty factor of 50%. A controller of a stepping motor according to Claim 1 or Claim 2 wherein the pulse signal generator (25) comprises a plurality of generator circuits (27). A controller of a stepping motor according to Claim 3 wherein each of the plurality of generator circuits (27) is arranged to provide a respective pulse signal (GPi, GPi + 2, GPi+4) having a respective duty factor. A method of controlling a stepping motor (10) comprising: receiving an input pulse signal (BP) from a reference source (21, 22) and generating therefrom a first pulse signal (GPi) having a first duty factor and a second pulse signal (GPi +2) having a second duty factor; a compositing step of providing a compositing signal and selectively outputting the first and second pulse signals (GPi, GPi+2) at a predetermined ratio as determined by the compositing signal (CP), thereby to provide a drive pulse signal (DP) having a duty factor which can be selectively varied in accordance with the predetermined ratio as detemined by the compositing signal (CP); and a driving step of feeding the drive pulse signal (DP) to said stepping motor (10). A method of controlling a stepping motor according to Claim 5, wherein said compositing step selects between said first pulse signal (GPi) and said second pulse signal (GPi+2), based on the compositing signal (CP) having a duty factor of 50%. A method of controlling a stepping motor according to Claim 5 or Claim 6 comprising generating the first and second pulse signals (GPi, GPi+2) by a plurality of generator circuits (27). A method of controlling a stepping motor according to Claim 7 comprising arranging each of the plurality of generator circuits (27) to generate a respective pulse signal (GPi, GPi+2, GPi+4). A method of controlling a stepping motor according to any one of Claims 5 to 8 comprising: providing the input pulse signal (BP) from an oscillator as the reference source (21, 22); and driving a timepiece hand of a timepiece by the stepping motor. A timing device comprising a controller according to any one of Claims 1 to 4 and further comprising: an oscillator as the reference source (21, 22) for generating the input pulse signal (BP); and a timepiece hand that is driven by said stepping motor. A method of operating a timepiece comprising controlling a stepping motor according to any one of Claims 5 to 9, and driving a timepiece hand by the stepping motor.

Inventors: KOJIMA HIROYUKI, YABE HIROSHI, KOJIMA, HIROYUKI, YABE, HIROSHI
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1998
Main classification: G04C3
IPC codes: G04C3