EP0161438B1 - Time piece comprising at least a chronograph function - Google Patents

Abstract

The timepiece of the invention includes at least a seconds hand arranged to respond to a chronograph mechanism. It comprises a single motor which receives at least two pulses per second so as to cause the seconds hand to advance by at least two steps per second. The system as disclosed enables an improvement in readability of the precision of time intervals on the one hand and on the other hand assures certain operation of the chronograph while using a stepping motor dimensioned to drive a simple timekeeper.

Description

The present invention relates to a timepiece comprising an electronic oscillator as a time base, a frequency divider and a stepping motor controlling both a timepiece display and at least one chronograph hand obeying a mechanism by which the chronograph hand can be started, stopped, and then returned to its starting point.

Mechanical chronograph watches generally have a central seconds hand which, when it is started, is coupled to the gear train and therefore advances step by step at the frequency which is imposed by the balance-spring. If this frequency is 18,000 vibrations per hour, the seconds hand will progress in steps of 18,000: 3,600 = 5 per second, i.e. at an angle of 1.2 ° per step. If the hand moves on a dial of the order of 30 mm in diameter, the stepwise movement of its tip on the hour circle is easily visible (arc of 0.3 mm). In this case, the smallest period that can be read is one fifth of a second.

We have already sought to produce chronograph watches of the electronic type where the balance-spring is replaced by a quartz time base which controls, via a frequency divider, a stepping motor which progresses at the rate of one step per second.

This is the case of the construction described in patent US-A-3 884 035 which however provides at least two drive motors, one for displaying the timepiece and the other for displaying the the chronograph function. In an alternative embodiment, the frequency divider circuit has two outputs of different frequencies, one at 1 Hz to control the watch motor and the other at at least 8 Hz to control, via an engagement switch and an auxiliary frequency divider, the chronograph motor (s). When the switch is turned on, the auxiliary divider begins to count the pulses at 8 Hz received from the main frequency divider to deliver at its output a control pulse for eight pulses received at its input. In this way it will be understood that the display error can reach the value of -1 second if the chrono function is triggered just before the arrival of the eighth pulse. It should be noted in passing that this error could be ± 1 second at most if an auxiliary divider was not provided and if the chronograph motor was controlled directly by pulses at 1 Hz from the main divider.

The fact remains that for a chronograph function even the error of -1 second remains significant with respect to the reading of the fifth of a second that one could obtain with a conventional mechanical chronograph. To remedy this drawback, the aforementioned American patent provides, as a variant, the use of another motor driven at a frequency. greater than 1 Hz which then displays fractions of a second. First we realize that the complication of such a system is a serious obstacle to its commercial realization. Then, by the admission of the author of the invention, the use of several motors can significantly increase the current consumption to such an extent that an additional power source may be necessary.

The chronograph watch described in document GB-A-2 028 545 also uses two stepping motors, one of which actuates the train indicating the current time, and the other the chronograph train. The chronograph motor is driven by pulses which exit the divider circuit at a frequency of 10 Hz. The central seconds hand which is part of the chronograph display mechanism, is driven at a rotational speed of ten times the speed normal, so that it makes a dial turn in six seconds. In this way it is possible to read the tenth of a second. However, the reduction ratios that must be provided in the chronograph train to drive a minute counter and an hour counter are then such that the size of the movement of the chronograph watch becomes significant. It should also be said that the reading of periods of inferior duration. greater than one minute, but greater than six seconds is therefore difficult.

Other chronograph watches equipped with two motors are described in documents GB-A-2,067,798 and FR-A-2,203,108 where one of the motors controls the display of the time and the other the display of the chronograph. It will be understood that these constructions not only consume energy, but also are bulky.

In mechanical chronograph watches it is in common use that the chronograph mechanism, in almost all, is carried by an independent frame which is removably attached to the frame of the movement as taught for example by the CH 225 patents. 004 and CH 527 462. This procedure can be carried out without the need to increase the mechanical power contained in the engine equipping the basic movement, said engine having enough reserve to cause the additional load presented. by the chronograph mechanism.

This same principle has been applied more recently to a basic quartz movement and stepper motor. However when it came to relate the chronograph mechanism on the basic movement controlled by a single stepping motor normally resulting in a simple movement of timepieces, we realized that the energy supplied by the motor was at the limit of acceptable, which is understandable ble since the motor must drive an additional load for which it was not calculated. It resulted from this situation of operating insecurities appearing especially during particular external constraints. To overcome these drawbacks, it has been thought to increase the power to be supplied by the stepping motor, which has led to an increase in its size, and therefore to substantially modify the thickness and arrangement of the entire basic caliber. We thus distance ourselves from the primary goal taught by the cited Swiss patents which allowed the use of a master key suitable for being used in a simple watch with minimum thickness or in a chronograph watch.

It will also be noted that the known chronograph watch cited in the above paragraph is equipped with a stepping motor which progresses at the rate of one step per second. As explained above, the measurement error can extend to ± 1 second, which in many cases may seem unacceptable.

The chronograph watch described in document GB-A-2 008 291 can operate, at the user's choice, with wide steps (1 Hz) or with narrow steps (10 Hz). If wide steps are used, a simple timepiece motor will not be enough to drive a chronograph mechanism that requires additional torque. If narrow steps are used, consumption becomes prohibitive as will be mentioned in the last paragraph of this description. It can therefore be seen that there is an operating slot for a chronograph watch which has never been used and which consists in driving the mechanism at a frequency between 2 and 5 Hz.

• - la figure 1 est une vue en plan de dessus d'une forme d'exécution de la pièce d'horlogerie, et
• - la figure 2 est un schéma bloc de l'alimentation du moteur pas à pas équipant ladite pièce d'horlogerie.

The present invention overcomes the above-mentioned drawbacks by making use of the means which appear in claim 1. It will now be understood from the following description and with the aid of the drawing in which, and by way of example,

• FIG. 1 is a top plan view of an embodiment of the timepiece, and
• - Figure 2 is a block diagram of the power supply of the stepping motor fitted to said timepiece.

The timepiece shown in Figure 1 comprises a housing 1 which contains all the mechanisms and circuits necessary for the display which appears on a dial 2 above which several hands move. In the example chosen, the timepiece is a chronograph watch. The current time is displayed using an hour hand 3, a minute hand 4, a small second hand 5 and a date indicator 12. The time is set using of the crown 6. The chronograph for its part comprises a hand with a large second centrafe 7, a minute counter 8 and an hour counter 9. The chronograph is controlled by an engagement and stop button 10 and by a button reset 11.

Thus the timepiece described comprises at least one second hand obeying a chronograph mechanism with the aid of which said hand can be started, stopped, then brought back to its starting point. In the timepiece object of the invention, the hands are controlled by a single stepping motor 15 itself controlled by a frequency divider 16 in turn excited by a time base 17 as shown in Figure 2 The axis of the stepping motor controls the chronograph display mechanism 18. In the case of the chronograph watch shown in FIG. 1, the motor 15 also controls a timepiece display mechanism 19 with its hand small second 5. Note that the motor 15 is preceded by a shaping circuit which receives the signals from the divider 16, a circuit which is not shown in the drawing.

Compared to the prior art cited above, the invention is distinguished by the fact that the frequency divider 16 is arranged to supply the motor 15 with at least two driving pulses per second, which will result in advancing the seconds hand 7 at least two steps per second. This appears in the division in front of which the second hand 7 of FIG. 1 moves, where, between two indexes 20 indicating the seconds, an additional index 21 indicating the half second is arranged. If the time base 17 is a quartz oscillator delivering 32,768 vibrations per second, the divider 16 will include 14 stages-dividers by two to deliver at its output a frequency of 2 Hz. Similarly a divider 16 comprising 13 stages-dividers will output a frequency of 4 Hz at its output.

This unprecedented way of proceeding until today has at least two advantages: Compared to a chronograph hand beating the second, the invention makes it possible to improve reading accuracy at least twice, the error for a progression of two steps per second being at most ± 0.5 seconds. It will be understood that this error will decrease further if the number of steps per second increases.

At 5 Hz, it will only be ± 0.2 seconds. So much for the first benefit.

The second advantage is that by increasing the number of pulses per unit of time, the motor can cause higher loads if the reduction ratio between the rotor axis and the needle axis increases in the same proportions. Thus a stepping motor calculated to drive a simple timepiece display mechanism can be used to drive a chronograph mechanism requiring more power and even a chronograph mechanism mounted in bypass on a timepiece mechanism, this without increasing the size of said motor.

We will take an example to fix the ideas. The stepping motor chosen is that calculated to drive a timepiece movement with large second hand. The motor advances at two steps per revolution and the seconds hand advances by 6 ° every second. Under these conditions, the reduction ratio is 30 and the torque measured on the axis of the second hand is 6 µm. If now the same engine is powered so that the hand advances by 3 ° every half second, the gear ratio will be increased to 60 and the torque available on the second hand will increase to 12 wNm. It is therefore understood that the proposed device allows to cause a movement requiring a higher torque with an engine whose size does not vary. This is important, especially for sizes which we want to keep a small thickness.

This reasoning could be extended to a simple timepiece movement originally planned with a second hand progressing by one step per second. Theoretically, according to what has been said above, it would be possible to reduce the size of the motor if this same mechanism was employed at double frequency, since the torque to be supplied by the motor is two times lower. However, it should not be forgotten that most of the time the engine is not loaded and runs almost empty. However, to progress safely, it is necessary to provide the rotor with a positioning torque between well-defined pitches, which requires that the motor be given a mutual magnet-coil torque _. sufficient to overcome the positioning torque. In addition the mutual torque must be high enough to overcome internal friction in the engine itself. Finally, the construction of the motor must be such that it resists external parasitic fields. The requirements which have just been listed mean that generally the size of the motor is already reduced to the minimum of reasonable and it is hardly conceivable to reduce it further without taking risks which would affect the smooth running of the movement of the watch.

où Cm = consommation moyenne (A) ;

• Cci = consommation du circuit intégré (A) ;
• Cmot = consommation du moteur à 1 Hz ;
• n = nombre d'impulsions par minute.

It remains to be seen what happens to the current consumption when the supply frequency according to the invention is increased. We know that in a watch movement the average consumption in amps is given by:

where Cm = average consumption (A);

• Bcc = consumption of the integrated circuit (A);
• Cmot = motor consumption at 1 Hz;
• n = number of pulses per minute.

If the consumption of the integrated circuit can generally be considered as a constant, the above formula shows that the average consumption increases with the number n of pulses per minute. By setting Cci = 0.3 µA, Cmot = 1 µA, we find an average consumption Cm of 1.3 µA for n = 60 (seconds hand advancing in 1 sec steps = 6 °) and 2.3 µA for n = 120 (seconds hand advancing in 0.5 sec steps = 3 °). The consumption at 1 Hz of 1 µA corresponds to that of a simple known watch movement. We can see that the average consumption goes from 1.3 to 2.3 µA when we double the frequency. This increase would go from 1.3 to 5.3 µA if we wanted to advance the second hand by five steps per second, which is still admissible from the point of view of the life of the battery that powers engine.

It should also be noted that such consumption can be greatly reduced if the motor is supplied with pulses whose width is controlled by the load which it is called upon to drive, for example by means of the system described in the document FR -A-2 521 799. If we are dealing with a chronograph watch, it will be understood that wide pulses will only be necessary when the chronograph is engaged, while in normal operation, the watch is content with 'reduced energy. It appears from this that the values of 2.3 µA and 5.3 _li A advanced above could be reduced in appreciable proportions if a controlled system was envisaged.

Advancing the second hand beyond five steps per second does not seem advisable because, on the one hand, we increase the consumption of the timepiece and, on the other hand, we advance the hand by no longer visible, in any case for a dial whose diameter is less than 30 mm.

Claims (4)

1. Timepiece including an electronic oscillator (17) serving as a time base, a frequency divider (16) and a stepping motor (15) controlling both a timekeeping display (3, 4, 5) and at least one chronograph-hand (7) arranged to respond to a mechanism through operation of which the chronograph-hand may be started, stopped and returned to its initial position, characterized by the fact that the frequency divider is arranged to supply the motor with from two to five pulses per second thereby to advance said chronograph-hand two to five steps per second.

2. Timepiece according to claim 1, characterized by the fact that the stepping motor drives additionally to the chronograph-hand (7) a minutes counter (8) and an hours counter (9).

3. Timepiece according to claim 1, characterized by the fact that the timekeepjng display comprises a second hand (5), a minutes hand (4) and an hours hand (3).

4. Timepiece according to claim 1, characterized by the fact that the width of the motor (15) drive pulses is slaved to the load to be driven by said motor.

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