Electronic Timepiece

The present invention relates to an improvement in an electronic watch in which the power consumption of a stepping motor is reduced.

The present invention will be explained on the basis of an embodiment applying the principles of the present invention to an analogue type electronic watch.

The display mechanism of an analogue type crystal watch heretofore used is generally constructed as shown in FIG. 1. The output of the motor comprising a stator 1, a coil 7 and a rotor 6 is transmitted to a fifth wheel 5, a fourth wheel 4, a third wheel 3 and a second wheel 2. Although not shown, the output is then transmitted to a cylindrical member, a cylindrical wheel, and a calendar mechanism whereby a second hand, a minute hand, an hour hand and a calendar are driven.

In the case of wrist watch, a load seen from the stepping motor is extremely small except for the time for switching the calendar, so that a torque of 10 g/cm in the second wheel is sufficient for the driving. However, when switching the calendar, a torque which is several times greater than this is required. The time required for switching the calendar within twenty-four hours operation per day is only at most about six hours. However, for the reasons described above in the mechanism according to the prior art, there is a problem that electric power which enables the calendar driving mechanism to be operated in a stable condition must always be supplied from a power supply.

FIG. 2 shows an electronic watch circuit construction according to the prior art. The signal of 32.768 KHZ from an oscillation circuit 10 is converted to one second signal by a frequency dividing circuit 11. The one second signal is converted to a signal having a width of 7.8 m sec and a period of 2 seconds by a pulse width combining circuit 12, and the signal along with a signal having the same period and pulse width but being dephased by one second are applied to the inputs 15 and 16 of inverters 13a and 13b. As a result, an inverted pulse which changes the direction of the current is applied to a coil 14 once every second, so that the rotor 6 magnetized in two poles rotates in one direction. FIG. 3 shows the current waveform therefor. In this manner, the drive pulse width of the present day electronic watch is set by the required maximum torque as its standard. Therefore, in the time interval which does not require a large torque, electric power is wasted. This has prevented a lower power consumption for the watch.

In order to overcome this noted difficulty and insufficiency, in the electrode watch according to the present invention, a motor is driven by a drive circuit having a resistor in series for reducing current, and afterwards a detected pulse is applied to a coil so as to determine rotation of the rotor, and the rotation of the rotor is detected by a voltage level across the resistor connected in series with the drive circuit and if the rotor fails to rotate, a correction is effected by driving the motor for increasing the current by short circuiting the resistor.

FIG. 1 shows a display mechanism of an analogue type cyrstal watch,

FIG. 2 shows a circuit construction of an electronic watch,

FIG. 3 shows a current waveform of the conventional stepping motor,

FIGS. 4, 5 and 6 show operations of the stepping motor,

FIG. 7 shows the current waveforms of the rotor of normal rotation and non-rotation,

FIG. 8 shows an overall block diagram of one embodiment of the electronic watch according to the present invention,

FIG. 9 shows a circuit construction of one embodiment of the present invention,

FIG. 10 shows a time chart of a circuit.

The principle of the rotation of a stepping motor for use in the electronic watch according to the present invention is described as follows with respect to FIGS. 4-10.

Referring to FIG. 4, numeral 1 shows a stator constructed in one integral body having a magnetic path or circuit 17a, 17b which is easily saturable. The stator is magnetically engaged with a magnetic core with the coil 7. In order to determine the direction of the rotation of the rotor 6 with two magnetic poles provided in the direction of the diameter, a notch 18a, 18b is provided in the stator. In FIG. 4, the condition is shown in which electric current has just been applied to the coil 7. When no current is applied to the coil 7, the rotor 6 remains stationary at the position of approximately 90 degrees in angle between the notch 18a, 18b and the magnetic poles of the rotor.

In this condition, when, in the coil 7, the current flows in the direction of the arrow mark, the magnetic poles are produced in the stator 1 as shown in FIG. 4, so that the rotor 6 rotates clockwise by repulsing action of the poles with respect to each other. When the current flowing through the coil 7 is interrupted, the rotor 6 will station itself of the reversed condition in the magnetic poles against the previous condition in FIG. 4. Afterwards, the rotor 7 keeps sequentially rotating in the clockwise manner by the current flowing in the opposite condition. Since the stepping motor used in the electronic watch according to the present invention is constructed in one integral body having saturable portion 17a, 17b the current waveform flowing through the coil 7 presents a characteristic with the slow rising curve as shown in FIG. 3.

The reason for this is that before the saturable portion 17a, 17b of the stator 1 saturates, the magnetic resistance of the magnetic circuit seen from coil 7 is very small, so that the time constant .tau. of the series circuit of the resistor and the coil becomes very large. The equation of this condition can be expressed as follows:

Therefore, the following equation is established:

where L denotes the inductance of the coil 7, N is the number of turns of the coil 7 and Rm is magnetic resistance.

When the saturable portion 17a, 17b of the stator 1 is saturated, the permeability of the portion becomes the same as that of the air. Accordingly, Rm increases and the time constant .tau. of the circuit becomes small and the waveform of the current rises abruptly as shown in FIG. 3.

According to the present invention, the detection of the rotation or non-rotation of an rotor 6 for use in the electronic wristwatch is found by the difference of the time constant of the circuit consisting of the resistor and coil connected in series. The reason for producing a difference in the time constants will now be explained hereinafter.

FIG. 5 shows a magnetic field at the time of current flow through the coil 7. In the figure, the rotor 6 is in the position which is rotatable against the magnetic poles. The magnetic fluxes 20a, 20b are the ones which are derived from the rotor 6. The magnetic flux which intersects the coil 7 also exists practically, however, this is neglected here. The magnetic fluxes 20a and 20b are shown as being derived from the saturable portions 17a and 17b of the stator 1 and they are directed as shown by the arrow mark. The saturable portion 17a, 17b is, in most cases, not in the saturated condition. In this condition, the current flows in the direction of the arrow mark of the coil 7 so as to rotate the rotor 6 clockwise. The magnetic fluxes 19a and 19b produced by the coil 7 are added to the magnetic fluxes 20a and 20b produced by the rotor 6 within the saturable portion 17a and 17b, so that it rapidly saturates. Afterwards, the magnetic flux which is sufficient for rotating the rotor 6 is produced. However, this is omitted in FIG. 5. FIG. 7 shows the waveform of the current flowing through the coil, as numeral 22.

FIG. 6 shows the condition of the magnetic flux in which the current is flowing through the coil 7 at the time when the rotor 6 could not be rotated for some reason and is returned to the original point. Generally, in order to rotate the rotor 6, the current must be flowing in the coil 7 in the opposite direction against arrow mark, i.e. in the same direction as that as shown in FIG. 5. However, in this case since as alternating inverted current is applied to the coil 7 for every rotation, this condition occurs whenever the rotor 6 can not be rotated. Since the rotor 6 could not be rotated in this case, the direction of the magnetic flux producing from the rotor 6 is the same as the one shown in FIG. 5. In this case, since the current is flowing in the opposite direction against FIG. 5, the direction of the magnetic fluxes become 21a and 21b. In the saturable portion 17a and 17b, the magnetic fluxes produced respectively from rotor 6 and the coil 7 cancel each other out, so that in order to saturate that portion of the stator 1, a longer time is required.

FIG. 7 shows this condition as waveform 23. In this embodiment, the time interval "D" before the portion 17 of the stator 1 saturates in FIG. 7 was 1 m sec on the condition that the diameter of the coil is 0.23, number of turns is 10000, the coil resistance is 3K.OMEGA., the diameter of the rotor is 1.3 and the minimum width of the saturable portion is 0.1. As is apparent from the waveforms 22 and 23 of the two currents in FIG. 7, the inductance of the coil is small when the rotor 6 is rotating within the range of "C" in FIG. 7 while it is larger at the time of non-rotation. In the stepping motor as described above, the equivalent inductance in the range of "D" was chosen as L=5 henry at the current waveform of 22 when rotating, and was chosen as L=40 henry at the waveform 23 during non-rotation. For instance, when the resistor "r" as a passive element for the detection and the coil series resistor "R" are connected in series to the inductance through the power supply VD, the change in inductance is easily detected by the voltage appearing across the resistor element for detecting the threshold value Vth of a MOS inverter, i.e. 1/2 Vn voltage.

As it is apparent from the above description, rotation or non-rotation of the rotor 6 is to be determined by applying a detection signal, so that it is possible to drive the rotor in a low torque by applying a pulse with a small current as well as to amend the driving in high torque by a pulse with a normal current to effect rotation of the motor.

FIG. 8 shows the block diagram of an overall electronic watch. A crystal oscillating circuit 51 oscillates a signal which is used as a standard signal of the watch. A frequency dividing circuit 52 is constructed by multi-stage flip flops which can divide up to one second for the oscillating signal required for a watch. A pulse width combining circuit 53 combines each flip flop output of the frequency dividing circuit, to produce a normal drive pulse signal with the pulse width necessary for the driving, a drive pulse signal for the correcting drive, a detection pulse signal with a duration necessary for the detection, a time interval setting signal between the normal drive pulse and the detecting pulse, and a time interval setting signal between the detecting pulse and the correcting drive pulse, etc.

A drive circuit 54 supplies the normal drive pulse, the detecting pulse, or the correcting drive pulse as an inverted pulse to the stepping motor.

The rotor of the stepping motor 55 is rotated by the application of the normal drive pulse when the load is low. However, the rotor is not rotated when the load is high, so that it is possible to detect either the rotating condition or the non-rotating condition of the rotor from the difference of the coil depending on the above condition by applying the detecting signal to the detection circuit 54. Accordingly, when the load of the motor increases for some reasons and the rotor is not rotated at the time of applying the normal drive pulse, either the rotating or non-rotating condition of the rotor is detected by applying the detecting pulse immediately after the drive pulse has been applied. In this case, when the rotor is not rotated, the resistor which is connected to the drive circuit in series is short circuited, the correction drive pulse is applied to the rotor from the control circuit 56 for the correction drive. In the embodiment of the electronic watch according to the present invention, the direction of the detection pulse is set in the same direction as that of the drive pulse, but it is also possible for the direction of the drive pulse to be reversed.

In the present embodiment, the pulse width combining circuit 53 can be easily constructed by the direct use of the pulses, such as 1 m sec, 3.9 m sec, 7.8 m sec and 31 m sec which is obtainable from the output of the crystal oscillates at circuit 51 which oscillates at 32.768 KHZ. A detailed circuit thereof is omitted.

FIG. 9 shows an embodiment of the motor control circuit 100. The drive circuit 54 consists of NAND gates 64a and 64b, a flip flop 65, drive inverters 66a, 66b and 67a, 67b. The motor 55 is provided with the coil 72. The detection circuit 57 comprises inverters 70a, 70b and gate 73, a transistor 74 as a switching element and a resistor element 68. The control circuit 56 is constructed by a flip flop 71 and an OR gate 63. FIG. 10 shows a timing chart of each portions of FIG. 10.

To terminals 60, 61 and 62 is timely applied the normal drive pulse, the detecting pulse and the correction drive pulse as shown in FIGS. 10a, 10b and 10c. These signals are combined by OR gate 63 and also phases thereof are selected by flip flop 65 and NAND gates 64a and 64b. These signals are applied to the terminal of the coil 72 through the drive inverters 66a, 66b and 67a, 67b as shown in FIGS. 10d and 10e. The rotor is rotated normally one step by the drive pulse 71a.

When the voltage at "f" overcomes a threshold value of the inverter 70a when the normal drive pulse is applied to the coil 72, however, since one input of AND-gate 73 is maintained at the 0-level as indicated in the terminal "b", the a voltage of "g" is not changed. The detection circuit is constructed so as to operate only when a detection signal has appeared at the terminal "b". When the detection pulse 72a is applied there to, the relationship of the magnetic pole is as indicated in FIG. 6, whereby a voltage of "f" does not rise to a threshold value of the inverter 70a and the terminal "S" of the flip flop 71 is not changed. Accordingly, the correction drive pulse is not applied to the coil 72. If the rotor was not rotated in the stepping operation for some reason, the relationship of magnetic pole is as indicated in FIG. 5 when the detection pulse 72b was applied thereto, whereby a voltage of "f" becomes the same level of the threshold of the inverter 70a, and the output thereof is reversed. A voltage of output "g" of AND-gate 73 rises to the "1" level, a set signal 75a is applied to the flip flop 71. Further the output "Q" of the flip flop 71 rises, a correction drive pulse h is applied to the coil 72 through NOR gate 63, further transistor 74 turns ON by the output "Q", whereby the resistor 68 is shorted. It is then possible to flow a larger current than the normal drive condition to the coil 72 according to the above noted operation.

According to the present invention, the resistor 68 is used as a passive detection element and a current reduction element, the transistor 74 and is used as a switching element, it is possible to replace these elements by a MOS transistor. In this case, one is able to omit the resistor 68 of FIG. 10 by designing the ON-resistance of a MOS-transistor to nearly zero and OFF-resistance of MOS-transistor to 2K.OMEGA..

Therefore, the location of a rotor is easily acknowledged without using an amplifier whereby a power consumption is remarkably reduced.

Further, in spite of the kind of motor, an electronic timepiece including a motor have a difference of inductance of a coil of a rotor is completely involved to the scope of the present invention.