There are known electromechanical watches having hands wherein the hour hand and the minute hand of the current time display are driven either by the same electric motor, or by distinct electric motors which move the wheels of the watch mechanism forward in steps. In both cases, it may happen that motor steps are lost because of shocks experienced by the watch, the presence of electromagnetic fields or other external disturbances. As a result, although the internal clock of the watch provides a correct indication of the current time, the hour and minute hands provide an incorrect current time indication, because the motors have jumped several steps under the effect of the external disturbance applied to the watch. It is therefore necessary to resynchronise the position of the hour and minute hands, either at the demand of a software application, or at the user's demand.
A known solution for detecting the position of a timepiece mechanism wheel for an electromechanical watch having hands consists in arranging, on either side of the wheel plate, a light source emitting a light beam and a light detection system. The light source and the light detection system are arranged facing each other and a hole is arranged in the wheel plate on the path of the light source and of the light detection system. When the hole reaches the vicinity of the light source, the light beam passes through the hole and falls onto the light detection system, which allows a precise indication to be provided as to the position of the wheel.
This type of device for detecting the position of a timepiece mechanism wheel for an electromechanical watch having hands has several drawbacks. The first of these drawbacks lies in the fact that the light source and the light detection system are arranged in a stepped manner in an essentially vertical direction, which makes the detection device bulky and requires recesses to be provided towards the top and bottom. The second problem becomes critical in the case where it is sought to detect the position of two coaxial wheels as is the case of an hour wheel and a minute wheel. Indeed, in that case, it is, for example, necessary to arrange the two light detection systems between the two wheels and to place the light sources respectively above and below the assembly of the two coaxial wheels. This type of arrangement not only requires a lot of space but also requires two of the detection device elements, for example the two light detection systems, to be placed between the wheels, which makes it practically impossible to automate the manufacture of these timepiece movements.
Another solution for detecting the position of a timepiece mechanism wheel for an electromechanical watch having hands is disclosed in EP Patent Application No 1493935. More specifically, this document discloses a device for detecting the position of at least a first wheel of a timepiece mechanism for an electromechanical watch having hands, the first wheel extending in a horizontal plane. The detection device includes at least one light source emitting a light beam and at least one light detection system, said detection device also including a first light reflector element, the light source and the light detection system being arranged so that, in a determined position of the first wheel of the timepiece mechanism, the light beam emitted by the light source is reflected by the first reflector element towards the light detection system.
This solution advantageously allows detection of the position of two wheels of the timepiece mechanism by means of a single light source and a single reflector element. However, the light source and the light detection system on one hand, and the reflector element on the other hand are arranged on either side of the wheel whose position is required to be detected, which increases the thickness of the timepiece mechanism and makes it more difficult to integrate in a watch case.
Yet another solution for detecting the position of a timepiece mechanism wheel for an electromechanical watch having hands is disclosed by EP Patent Application No 2626752 in the name of the Applicant. More specifically, this document discloses a device for detecting the position of at least a first wheel of a timepiece mechanism for an electromechanical watch having hands, said first wheel extending in one plane. The detection device includes at least one light source emitting a light beam and at least one light detection system. A first light reflector element projects from one of the top or bottom surfaces of the first wheel of the timepiece mechanism, the light source and the light detection system being arranged so that, in a determined position of the first wheel of the timepiece mechanism, the light beam emitted by the light source is reflected by the first reflector element towards the light detection system.
This other known solution provides a detection device wherein the light beam emitted by the light source is reflected by a reflector element which stands on the surface of a timepiece mechanism wheel towards the light detection system. Rather than being arranged on either side of the wheel, the light source and the detection system may be arranged at the periphery of the wheel, which results in a considerable space saving. However, the device has to be integrated in the timepiece mechanism, which makes the timepiece mechanism more complex and thus more expensive to manufacture.
Yet another solution for detecting the position of an electromechanical watch hand is disclosed by JP Patent Application No 2006-275803. In this document, the hand position detection device includes two light sources and two light detection systems. A first pair formed of a light source and a light detection system detects the position of a first hand, while a second pair formed of a light source and a light detection system detects the position of a second hand. The two pairs of light sources and light detection systems are arranged underneath the watch dial which is therefore translucent.
The watch hand position detection device briefly described above detects the position of a watch hand by means of components mounted underneath the surface of the watch dial. This therefore avoids the need to assemble the detection device within the volume of the timepiece mechanism. Further, since the components of the detection system are mounted underneath the surface of the watch dial, it is hardly unnecessary or unnecessary to increase the height of the hand relative to the surface of the dial, so that the detection device according to the invention is compact. However, a notable drawback of this type of detection device lies in the fact that it requires two light sources and two detection systems in order to be able to detect the position of two distinct hands. This type of detection device is thus expensive both as regards the components required and as regards assembly time.