HK1119263B - Timepiece including means for determining the angular position of an analogue indicator of the timepiece - Google Patents

Description

Timepiece comprising means for determining the angular position of an analog indicator of the timepiece

Technical Field

The present invention relates generally to a timepiece provided with an analog display device comprising at least one movable indicator and comprising means for detecting steps taken by the movable indicator. The invention relates more particularly to such a timepiece, wherein the detection of the steps is used to check the angular position of the movable indicator.

Background

Electronic clocks with analog displays are known, which comprise an internal digital timepiece as a complement to the hands rotating above the dial. This internal clock is clocked by the same electronic pulse that controls the forward movement of the hand. In principle, the hands and the timepiece move forward in a synchronized manner. It is also known to use such internal timepieces for multi-function watches, in which the same hand is used for indicating the time or a second piece of information, for example an alarm time. In fact, an internal timepiece is necessary if it is desired to continue counting the elapsed time while the hands are occupied for displaying the second piece of information. When the hands then return to their time display function, the data contained within the internal timepiece will allow them to be reset to the exact time.

However, if an application like that just described is desired to be satisfactory, it must be possible to prevent any difference between the time displayed by the hands of the watch and the time given by the internal timepiece. It is known that this difference may occur, for example, if the watch experiences a shock or because of electromagnetic or even mechanical disturbances (e.g. dust in the gear train). Because of these disturbances, some clock motors lose steps. Each analog clock is therefore apt to show the difference between the count of control pulses and the angular position of the hands. If this difference is not corrected in time, it may increase to the point of causing a completely erroneous indication. Further, in the multifunction clock, the hands must be able to move not only forward but also backward depending on the change in the amount being indicated. Further, when the function is changed, the hands of the multi-function clock must be able to quickly change positions forward or backward. To satisfy this constraint, the hands of the multifunction clock are typically each driven by their own motor. Therefore, instead of the single internal timepiece described above, the multifunction clock generally includes a count/count down circuit for a control pulse of the motor of each hand. Furthermore, it will be appreciated that the motor of the multifunction timepiece must satisfy considerably greater stresses. In this case, the risk of any discrepancy, or in other words the risk of the hands not being synchronised, is also significantly higher for the multifunction clock than for any other clock.

To overcome the problems just described, it is known to perform motor control pulse counting/countdown by detecting the true position of the pointer. In particular, european patent No0841538 discloses a timepiece comprising a watch movement driving an analog display. The watch movement includes a wheel rotatably fixed to one of the hands and a magnetic sensor for detecting the position of the wheel. To allow such detection, the wheel plate is covered with a magnetic film defining a complex pattern at a plurality of locations, so that the magnetic sensor provides a signal of 1 or 0 depending on whether it is opposite to the location covered with the magnetic film. The clock further comprises electronic means for determining from the order of 1 and 0 whether the position of the pointer actually corresponds to the theoretical position. The device disclosed in this latter document has some drawbacks. In particular, detection of a step loss can only occur retrospectively. Further, the operation of covering the wheel plate with the pattern cut into the magnetic film can only increase the manufacturing cost.

European patent No0952426 discloses another solution, which solves the same technical problem. This document also discloses a timepiece movement comprising a wheel rotationally fixed to one of the hands. The wheel is formed by a plate having at least one hole located in an intermediate region between the axis of rotation and the circumference. The timepiece further comprises a device for detecting the angular position of the wheel. This device comprises an inductive or capacitive sensor arranged so that it is located directly above the hole in the plate when the wheel occupies a particular position. The sensor is sensitive to changes in the amount of metal located in close proximity. The amplitude of the signal sensed by the sensor therefore varies depending on whether the position of the sensor is opposite the full segment or, conversely, the hole in the wheel plate. The device further comprises a memory for storing the signal amplitude after each step and electronic data processing means for retrospectively determining from the stored data the moment at which the hole in the plate is located directly above the sensor.

The existing solution just described also has some drawbacks. In particular, the wheel whose angular position is detected rotates with the same rhythm as the pointer. If the wheel plate has only a single hole, the angular position of the wheel or pointer is sensed only once per revolution. It is known that in a typical clock, it is desirable for the pointer to have a minimum of sixty steps per revolution. It may be necessary to wait 60 steps before the position of the pointer can be confirmed. Furthermore, the rotation amplitude of the pointer corresponding to a single step is very limited (up to six degrees). Under such conditions, the sensor detects only a slight change in signal intensity from one step to the next, and cannot reliably detect the moment when the hole passes directly above the sensor. This is why the position determination of the pointer according to the prior art cannot take place in real time but retrospectively.

European patent No1662343 discloses a timepiece comprising an optical sensor for detecting a reference position. Similar to the previously described timepiece, this timepiece comprises a timepiece movement including a wheel rotationally fixed to one of the hands. This wheel meshes with the pinion of the intermediate wheel set, which is itself driven by the stepper motor. The gear ratios are such that the intermediate wheel set completes an integer number of revolutions during each revolution of the pointer. The wheel fixed to the pointer and the wheel of the intermediate wheel group partially overlap. The two wheels are each formed from a plate having an aperture formed in the overlap region of the two wheels. Under such conditions, the holes of the two wheels are in exact overlapping positions once during each rotation of the wheel fixed to the pointer. This periodic coincidence of the positions of the two holes defines the reference position of the pointer. The optical sensor is also arranged at a position where the two holes coincide. The optical sensor is formed by a light source and a light sensor arranged on either side of the two wheels, so that light from the light source cannot reach the light sensor except when the two holes are located in extension of each other. The signal provided by the light sensor is used to determine the time of day corresponding to the pointer passing its reference position.

Unlike in the case of a wheel fixed to the hands, each step of the motor produces a relatively large movement of the middle wheel hole. Under such conditions, the signal strength detected by the sensor transitions from virtually all to none between one step of the motor and the next. It is therefore theoretically possible to detect in real time the exact moment at which the pointer passes its reference position. However, in practice the reference position is only detected once per revolution of the pointer. With the timepiece movement of the aforementioned document, it may therefore be necessary to wait for the motor to complete 60 steps or more before being able to confirm the position of the hands.

The abstract of japanese patent No58131583 discloses a timepiece comprising a timepiece movement with hands. The clock comprises a device for detecting the angular position of one of the hands. The device includes a magnet fixed to the pointer and a series of spaced reed contacts arranged around the circumference of the face plate and spaced from each other by two steps corresponding to the pointer. If it is assumed that the hands complete 60 steps per revolution, the clock will have to include 30 reed contacts. This feature results in an increase in manufacturing costs and is only suitable for relatively large-sized clocks.

The abstract of japanese patent No2003107174 discloses a timepiece in which the stepping motor is provided with electronic detection means for confirming that the motor has actually completed one step after each control circuit pulse. This solution also has some drawbacks. Indeed, the electromagnetic field within the clock motor is generated by both the electrical pulses energizing the coils and the magnets rotating with the rotor. Under such conditions, one step loss experienced by the motor results in only a relatively modest change in the signal detected by the electronics. The solutions proposed in this prior document therefore require the implementation of a detection system that is sufficiently sensitive and therefore relatively complex. Furthermore, any electromagnetic interference that occurs in the vicinity of the watch is prone to cause errors in the detection system.

Disclosure of Invention

The object of the present invention is to overcome the drawbacks just described by providing a clock according to claim 1.

It will be appreciated that in the present invention, the comparison means is able to confirm the true position of the intermediate wheel at least once each complete revolution of the intermediate wheel. Furthermore, it is known that in a reduction gear train, the number of revolutions of the rotor necessary to make the intermediate wheel complete one revolution is less than the number of revolutions necessary to make the movable indicator complete one revolution. Under such conditions, the delay between the accidental loss of one step and the detection of this event is reduced compared to prior art devices.

According to an advantageous embodiment of the invention, the number of rotor revolutions necessary to complete one revolution of the intermediate wheel does not exceed 10. It is therefore characteristic that the detection of any difference between the true position of the intermediate wheel and the state of the counting device occurs with a delay of not more than ten rotor revolutions (almost in real time).

According to another advantageous variant of the invention, the at least one angular reference position is detected using a magnet fixed to the intermediate wheel and occupying an eccentric position with respect to the axis of the intermediate wheel and a magnetic sensor arranged at an opposite point on the trajectory of the magnet to detect the passage of the magnet.

According to yet another advantageous variant of the invention, the clock motor is a stepper motor and the clock further comprises first memory means for storing the signals provided by the detection means during a period corresponding to at least one complete revolution of the intermediate wheel, and the identification and comparison means are provided to establish the correspondence between the specific state of the first counting means and the angular reference position on the basis of the signals stored by the first memory means. Because the number of motor steps necessary to make the intermediate wheel complete one revolution is limited, the memory size necessary to implement this feature is entirely reasonable.

According to yet another advantageous variant of the invention, one complete revolution of the intermediate wheel corresponds to at least three motor steps. In fact, the forward and backward steps can be identified when the motor takes at least three steps per revolution, as opposed to what would happen if one full revolution corresponded to only two motor steps.

According to another advantageous variant of the invention, the constant angle through which the intermediate wheel is driven is equal to 2N per revolution of the motor, where N ≦ 10.

According to yet another advantageous variant of the invention, the motor is a stepper motor comprising a rotor provided for rotating through an angle of 2N/N at each step, so that the intermediate wheel can occupy exactly N × N different angular positions in succession.

According to yet another advantageous variant of the invention, 3. ltoreq. n.ltoreq.10.

According to yet another advantageous variant of the invention, N × N — 4 or N × N — 6.

Drawings

Further characteristics and advantages of the invention will become clear from reading the following description, given purely by way of non-limiting example and with reference to the accompanying drawings, in which:

figure 1 is a cross-sectional view of a timepiece movement forming part of a timepiece according to an embodiment of the invention;

figure 2A is a schematic view of the intermediate wheel of the timepiece movement of figure 1, illustrating the four positions that the magnet carried by said wheel may occupy due to the action of the stepping motor;

FIG. 2B is a table showing the sequence of slave-1 and slave-0 provided by the magnetic sensor in various cases;

fig. 3 is a flow chart of an apparatus for controlling motor steps and an apparatus for compensating for missing steps.

Detailed Description

In the section of fig. 1, the movement of a timepiece is shown, the timepiece comprising an intermediate arbour (arbour)2 around which two tubes (referenced 3 and 4) rotate, said tubes being mounted concentrically to carry, respectively, a minute hand and an hour hand (not shown). In the movement of fig. 1, the two hands are driven independently of each other by two motors (of which only the rotor is shown and referenced respectively 7 and 14 in fig. 1). These stepping motors can be controlled back and forth and the clock movement with which they cooperate is therefore suitable for use in a multifunction clock. The first motor 7 drives the hour hand through a first reduction gear train formed by the motor pinion 8, the intermediate wheel 9 and the pinion 10 of the intermediate wheel 9, the wheel 11 and the pinion 12 of the wheel 11 and the last hour wheel 13. The second motor 14 drives the minute hand through a second reduction gear train formed by the motor pinion 15, the intermediate wheel 16 and the pinion 17 of the intermediate wheel 16, the wheel 18 and the pinion 19 of the wheel 18 and the last minute wheel 20. The following description will focus on the first kinematic chain connecting the motor 7 to the hour hand. However, it will be understood that the same explanation applies also to the second kinematic chain driving the minute wheel.

The motor driving the hour hand is a step motor, the rotor 7 of which is designed to perform one revolution per step. The gear ratio between the intermediate wheel 9 and the motor pinion 8 is 4: 1. The intermediate wheel 9 is thus designed to rotate with a rhythm of one revolution per four motor steps. It should also be pointed out that in the movement described above, both the hour wheel and the minute wheel are designed to complete one revolution per 180 motor steps.

Figure 1 also shows a small magnet 21 fixed to the underside of the plate of the intermediate wheel 9. The magnet 21 is fixed near the outer edge of the wheel but is moved back from the teeth of the wheel. The magnet is thus fixed to the intermediate wheel and it passes directly above the reed microcontactor (indicated hereinafter by the abbreviation MR and by the reference number 22 in the figures) once per revolution. The MR forms part of an electronic component mounted on a printed circuit 23, which is itself fixed under the motor clamp plate. The position of MR22 is selected so that magnet 21 passes over directly opposite MR22 once per revolution of intermediate wheel 9. It should also be pointed out that in an embodiment of the invention, the intermediate wheel 16, which forms part of the gear train driving the minute wheel, carries a second magnet 21 'which is used to activate a second MR referenced 22'.

It should be understood that the MR described in this example must be of small size. However, there is an MR that is small enough to fit such applications. In particular, such as MicroReed-14, developed by ASULAB SA, CH-2074, Marin, Switzerland.

Reed contactors are contacts that are sensitive to magnetic fields. This contact closes in the presence of a sufficiently strong magnetic field, but remains open if the magnetic field does not exceed a certain threshold. The reed contactor therefore lends itself to use as a magnetostatic sensor for detecting the presence of a magnetic field whose strength exceeds a certain value. The timepiece whose movement has just been described therefore comprises two MRs referenced 22 and 22'. It also comprises electronics connected to the MR and forming a magnetic field detection means with the MR. These magnetic field detection means are used to provide a binary signal, the value of which depends on the open/closed state of the contacts. In the following description, the value "1" will be due to the signal provided by the detection device when the contacts are closed, and the value "0" will be due to the signal provided by the detection device when the contacts are open.

According to the aforementioned convention, the information provided by the magnetic field sensing device may be associated with a series of 1's and 0's of sensor states after each step reflecting motor control. Since the intermediate wheel 9 completes one revolution every four motor steps, the sequence of 1 and 0 should be repeated regularly in a period of four steps, assuming no reversal of the direction of travel. Recall that, according to the invention, the detection means provide a signal having a first value when the intermediate wheel occupies a first angular position and a second value different from the first value when the intermediate wheel occupies a second angular position. Fig. 2 shows four positions α, β, γ and δ that the magnet 21 may occupy in an embodiment forming the subject of this example. When magnet 21 occupies position α, it is located directly above MR 22. In this position, the strength of the magnetic field at MR is at a maximum and the contactor must therefore be closed in this position. The value of the signal provided by the detection means is thus "1" when the magnet occupies position α. Conversely, when magnet 21 occupies position γ, the distance separating it from MR22 is at a maximum, and therefore the contactor must be opened in this second position. Thus, when the magnet occupies position γ, the value of the signal provided by the detection means is "0".

As for the values of the signals when the magnet is in two other positions (positions β and δ), there may be some number of variations. Indeed, in these two latter positions, the strength of the magnetic field at the MR is at an intermediate strength between that at position α and that at position γ. A priori, the signal provided by the detection means may thus likewise easily have a value "0" and a value "1". The table of fig. 2B describes three variations of the present invention. In the first of these variations (row a in the table), it can be seen that MR is only closed at position α when magnet 21 is located directly above MR. The MR remains off at the other three locations of the magnet. These positions thus correspond to three consecutive 0 s. Variation a corresponds to the use of a relatively weak magnet that closes the contacts only when in close proximity to the contacts. In the second variant shown (row B), it can be seen that MR is closed at positions α and β of the magnet 21 and that MR is open at positions γ and δ. The strength of the magnetic field at the location of the MR is theoretically the same at the location β and the location δ. However, because of hysteresis, the critical strength of the magnetic field that just allows the MR to close is higher than the critical strength of the magnetic field that just allows the MR to open. This is why for the same magnetic field strength it is sufficient to keep the contacts closed when the magnet is from position α to position β, but not to close the contacts again when the magnet is from position γ to position δ. It should be noted that the table in fig. 2B does not take into account the possibility of reversal of the direction of operation. In such a case, the motor rotates backward, and the modification B can be said to be reversed. In other words, the contact is closed at position δ and open at position β. However, the direction of rotation has no effect on the contact at position α and position γ. Finally, in the third variant shown (row C), it can be seen that the positions δ, α and β of the magnet 21 correspond to three consecutive 1's, and only 0 is associated with the position γ. It will be understood that case C corresponds to the use of a relatively strong magnet.

To make a clock forming the subject of this example, the components are preferably dimensioned so that the detection means provide a signal conforming to variant B. However, some examples of this clock may correspond to variant a or variant C. Indeed, if working with usual tolerances, the example dimensioned according to variant B actually has a reduced probability of having the characteristics of variant a or variant C but not reduced to zero. According to an advantageous feature of the invention, such variations between examples do not constitute a problem. Indeed, the means for detecting the angular reference position of the intermediate wheel may be based on the following principle. In the presence of three consecutive 1's (variant C), the second of these 1's corresponds to the alpha position of the magnet 21. In the presence of two consecutive 1's (variant B), the first of these 1's corresponds to the alpha position of the magnet. In the case of 1 discontinuity (variant a), these isolated 1 s correspond to the alpha position of the magnet.

Fig. 3 is a flow chart of an apparatus for controlling, checking and correcting the position of the intermediate wheel. It can be seen that the timepiece of this example actually comprises means for detecting the position of the two intermediate wheels, one associated with the hour hand and the other with the minute hand. The following explanation is given only with reference to the intermediate wheel 9 in relation to the hour hand. However, it will be understood that the same explanation applies to detecting the position of the other intermediate wheels 16 as well.

The diagram in fig. 3 shows an intermediate wheel 9, which is driven by the pinion 8 of the motor 7. The motor 7 is subjected in a known manner to pulses provided by a control circuit 30, the control circuit 30 comprising a quartz oscillator and a frequency divider 36. The control circuit is used for rotating the motor rotor one step forward or backward. Also shown is a counter/down counter 31 which also receives signals provided to the motor by the control circuit 30. Each time the control circuit generates a pulse that results in a forward step, the counter/down counter 31 is incremented by one unit, and each time the control circuit generates a pulse that results in a backward step, the counter/down counter 31 is decremented by one unit. Counter/down counter 31 is a modulo-4 counter. In this condition, in the absence of any event, there must be a correspondence of a single value between the four possible states of the counter 31 and the four positions occupied by the intermediate wheel 9.

The diagram in fig. 3 also shows a two-state magnetic sensor, which is formed by the reed contactor 22 and the power supply device associated therewith. The magnetic sensor generates a signal formed by a series of 1's and 0's, which corresponds to the closed or open state of the MR after each pulse of the control circuit 30. The signal from the magnetic sensor is sent to a RAM memory referenced 35, which stores the last N values that the signal has. In this example, storage may be limited to two values because the sequence of signals is repeated every four steps. From this information the identification means 32 can identify the step corresponding to the reference position alpha of the intermediate wheel 9. According to the foregoing explanation, the identification means is based on the following principle, for example. When there are three consecutive 1 s, the second of these 1 s corresponds to the alpha position of the intermediate wheel 9. If 1 is not continuous, the isolated 1 corresponds to the alpha position of the intermediate wheel 9. According to an advantageous variant, in addition to the position α, the recognition means 32 are also used to recognize the position γ of the intermediate wheel by analyzing a succession of 0's according to the same principle.

Comparison means 33 are also provided to compare the position of the intermediate wheel 9 determined by the identification means 32 with the position in which the wheel should theoretically be according to the counter/down-counter 31. Any inconsistency between these two pieces of information would indicate that the motor is missing at least one step. Under these conditions, the comparison means 33 will send a signal to the further pulse generator 34 in a manner known to those skilled in the art, so that the motor compensates for the missing steps. From the foregoing description, the identifying means 32 retrospectively identifies the step corresponding to the alpha position. However, it is only necessary to wait for the first 0 after a series of 1's to know to which step the position α of the intermediate wheel 9 corresponds. In the absence of any reversal of the rotational direction of the motor, the recognition delay therefore does not exceed the duration of two steps. In this example, this delay corresponds to a 4 degree positioning error of the hour hand. The identification means thus operates in real time.

The diagram of fig. 3 shows that the clock of the present invention includes a second counter/down counter 37. It is a 180 modulo counter/down counter. Like counter 31, counter 37 is incremented or decremented by 1 each time by each pulse provided by control circuit 30. There is thus a single-valued correspondence between the 180 possible states of the counter 37 and the 180 positions occupied by the pointer wrap. It is also seen that counter/down counter 37 is connected to non-volatile memory 38. The function of the non-volatile memory 38 is to store the position of the hour hand, for example when the battery (not shown) driving the clock is replaced. Thus, whenever the supply voltage provided by the battery falls below the threshold, the counter/down counter 37 passes its content to the non-volatile EEPROM memory 38. Once the battery of the table has been replaced, the counter 37 reads the pointer position within the non-volatile memory 38 and thus returns to the state corresponding to the real position of the pointer. At this stage, the relationship between the position of the pointer and the state of the counter 37 is preserved even if the table is slow. The person wearing the watch therefore only needs to use the control handle in a known manner to reset the watch to the correct time.

This feature is particularly advantageous when, as in the case of the present invention, it relates to a watch in which at least two motors each drive a separate hand. Indeed, for such tables, battery failure may occur when the pointers are desynchronized to display information other than time. In such a case, saving the location of each of the pointers in non-volatile memory allows the table to be reset to the correct time without having to resort to resynchronizing the pointers.

It will be understood by those skilled in the art that in order for the state of the counter/down counter 37 to correspond to the position of the hour hand, the position of this hour hand must first be initialized. However, the initialization need not be repeated after each battery change because the state of the counter/down counter 37 is saved in the non-volatile memory 38. Thus, a single initialization may be sufficient to ensure that the table operates throughout its lifetime.

It should also be understood that the diagram of fig. 3 is a flow chart. The different blocks appearing in the figures therefore do not generally correspond to different devices. In a preferred variant of the described embodiment, all the tasks described with reference to fig. 3 are actually performed by a single microcontroller thus programmed and integrated in the clock.

It will also be appreciated that various alterations and/or modifications as will be evident to those skilled in the art may be made to the embodiments forming the subject matter described herein without departing from the scope of the invention as defined by the appended claims. In particular, instead of incrementing the counter/down-counter 37 by the control pulse provided by the frequency divider 36, the counter/down-counter 37 may be incremented by the identification means 32 as a function of the signal provided by the detection means 22. This variant may guarantee that the state of the counter 37 actually corresponds to the position of the pointer. This is true even if the motor 7 is blocked so that the compensating pulses are not operational.

Furthermore, the present invention is not limited to the use of magnets associated with reed contactors. If the intermediate wheel 9 is made of an insulating material, it is possible, for example, to replace the magnets fixed to the wheel with metal parts and to provide inductive or capacitive sensors to detect the passage of the metal parts over the sensors. Conversely, if the intermediate wheel 9 is made of metal, it is possible to use this same type of sensor in relation to the holes arranged in the plate of the intermediate wheel close to its periphery.

Further, the motor that drives the reduction gear train including the intermediate wheel 9 does not have to be a stepping motor. It will be appreciated by those skilled in the art that any other type of motor, such as a synchronous motor, may be used in place of the stepper motor.

Claims (9)

1. Timepiece comprising a timepiece movement associated with an analogue display including at least one movable indicator, said timepiece movement including at least one motor (7) and control means (30) providing pulses for controlling said motor, said motor being provided for driving said movable indicator through a reduction gear train (8, 9, 10, 11, 12, 13) including an intermediate wheel (9) between the motor (7) and a second wheel (13) fixed to the movable indicator, the reduction gear train being provided for driving the intermediate wheel through a constant angle at each complete revolution of the motor, the timepiece further including checking means (21, 22, 31, 32, 33, 35) to determine whether the actual movement of the movable indicator corresponds to a signal provided by the control means (30), and correction means (34) connected to the checking means, correction means (34) are provided for controlling the motor to correct the position of the movable indicator,

the timepiece is characterized in that the checking means comprise detection means (22) for detecting at least one angular reference position of the intermediate wheel (9), said detection means being provided for supplying a signal having a first value when the intermediate wheel occupies said angular reference position (α) and for supplying a signal having a second value different from the first value when the intermediate wheel occupies the second angular position (γ), the timepiece being further characterized in that the checking means comprise first counting means (31) for counting the pulses supplied by the control means (30), and identification means (32) and comparison means (33) for determining at least once per revolution of the intermediate wheel (9) whether the true position of the intermediate wheel actually corresponds to the state of the first counting means (31) on the basis of the signal supplied by the detection means (22).

2. The timepiece of claim 1 wherein the constant angle through which the intermediate wheel is driven at each complete revolution of the motor is equal to 2N, where N ≦ 10.

3. Clock according to claim 2, characterized in that the motor is a stepper motor comprising a rotor (7) provided for rotating through an angle of 2N/N at each step, so that the intermediate wheel (9) can occupy exactly N different angular positions in succession.

4. The clock of claim 3, wherein 3 ≦ N ≦ 10.

5. The clock of claim 4, wherein N-N-4 or N-N-6.

6. Clock according to claim 1, characterized in that the checking means comprise first memory means (35) provided for storing the signals provided by the detecting means (22) during a period corresponding to at least one complete revolution of the intermediate wheel (9), and in that the identifying means and the comparing means are provided for identifying the state of the first counting means (31) corresponding to the angular reference position (α) on the basis of the signals stored by the first memory means (35).

7. Clock according to claim 6, characterized in that identification means and comparison means are also provided for identifying the state of the first counting means (31) corresponding to the second angular position (γ).

8. Clock according to any one of the preceding claims, characterized in that said means for detecting at least one reference position of the intermediate wheel comprise a magnet (21) fixed to the intermediate wheel (9) and occupying an eccentric position with respect to said wheel, so that each angular position of the intermediate wheel corresponds to a different position of the magnet, and further comprise a magnetic sensor (22) arranged opposite the intermediate wheel, so that the magnetic sensor (22) provides said signal having a first value when the magnet occupies the angular reference position (α), and so that the magnetic sensor (22) provides said signal having a second value when the magnet occupies the second angular position (γ).

9. Clock according to claim 1, characterized in that the clock comprises second counting means (37) for counting the pulses provided by the control means (30), and second memory means (38) for storing the position of the removable indicator, said second memory means comprising a non-volatile memory (38) for keeping a record of the state of the second counting means (37) when the clock is stopped.