Attiny85 controlled watch winder for self-winding mechanical watches

In mechanical self-winding movement, the center-anchored Rotating Weight rotates about its axis with physical motions of the hand. It thereby winds the mainspring only when worn and stops during lay-offs. A watch winder re-creates the physical motions to maintain the beat in the caliber, when not worn. The paper proposes a residential tourbillon watch winder for self-winding mechanical calibers. The main objectives of this paper are to minimize the working time of the winder and consequently optimize its power consumption. A significant trade-off between the winding and unwinding ratios of the mainspring is ensured, maintaining a higher former ratio using a single-phase 9 RPM Permanent Magnet Synchronous Motor. The proposed model states the sufficiency of the winder to work only for 60% of the time. The model uses Watchdog timer to rekindle Attiny85 Microcontroller from sleep mode for scheduling the working time of the winder. The disclosed model is deployed to a Seiko (7S26A) caliber and a Citizen (8200A) Caliber for a period of 26 days to validate the working. The results show that the proposed model with the Seiko caliber requires 41.1 h for winding while the commercial models require 70.99 h. Similarly with the Citizen Caliber, the proposed watch winder requires only 28.8 h which is 21.11 h less than the time required by commercial models.


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Thothadri Journal of Electrical Systems and Inf Technol (2022) 9:15 paper proposes a tourbillon-based winder that works for a shorter duration with higher power optimization using Permanent Magnet Synchronous Motor and Attiny85 microcontroller. Attiny85 is a low power micro-controller with advanced RISC design. With 512 bytes of EEPROM, 8 kilobytes of flash memory, 256 bytes of SRAM and 32 registers, Attiny85 has an internal oscillator and an 8-bit timer. The controller has three interrupt sources with a programmable Watchdog timer. Attiny85 works in three modes: power down mode, ADC noise reduction mode and idle mode [4]. Permanent Magnet Synchronous Motor has been chosen for this model as the speed of this motor is unchanged with any change in load [5,6]. Also, by determining the initial rotor position of the Permanent Magnet Synchronous Motor, sensor-less speed control can be achieved [7][8][9]. Further, the Permanent Magnet Synchronous Motors operate at a better power factor that results in reduced losses in the power system [10][11][12]. Considering the low cost and easy-availability of low-speed Permanent Magnet Synchronous motors, a single-phase 9 RPM Permanent Magnet AC Synchronous Motor is chosen for winding the caliber.

Related work
Traditional tourbillon mechanisms use detachable padded mandrel that holds the watch intact. The mandrel is coupled with the shaft of the low-speed electric motor that rotates recursively until the motor is de-energized [13]. Modern-day tourbillon watch winder uses two longitudinal rollers, placed parallel to each other, on which the watch is placed in a cylindrical case. One of the rollers is driven using an electric motor and the watch in the case rotates about its axis due to the frictional forces between the rollers. Resultantly, the whirl motion of the oscillating weight is achieved. A sensor is placed accordingly to compute the number of revolutions and it is displayed to the user using a display panel. This "Tourbillon" effect appreciably maintains the accuracy of the caliber by reducing the impact of gravity [14]. With additional features, controllable watch winder adopts the tourbillon concept with an inter-rotating watch case which is controlled by a microprocessor. The microprocessor counts the number of rotations, records and presets the latency between each rotation [15].

Methods
60% winding rule for the winder introduced this rule is validated using two calibers (SEIKO 7S26A and CITIZEN 8200A) by determining their winding and unwinding ratios. The architecture of this model is then fabricated on PCB for customization. The working schedule of the winder with 9 RPM Permanent Magnet Synchronous Motor is formulated to obtain the results. From the results, the 60% winding rule is proven to work flawlessly with all calibers having 40-h or higher power reserves.

SEIKO7S26A caliber
With yoke winding mechanism, SEIKO 7S26A is a 40-h, 21-jewel day-date caliber with 21,600 beats per hour. Its magnetic resistance is 4800 A/m (60 gauss) and has a lift angle of 53°. It has a pallet lever escapement with nickel annular balance and Etachron regulator. The salient feature of "Magic lever mechanism" enables the winding of the mainspring by bidirectional rotation of the oscillating weight and it has become the key reason for considering this caliber.
In SEIKO7S26A movement, the Oscillating Weight (rotor), First Reduction Wheel, Pawl Lever, Second Reduction Wheel, Ratchet Wheel, the Click and Mainspring Barrel are constituted for winding the mainspring. Click is a thin metallic lock that prevents spontaneous unwinding (Figs. 1 and 2).
(1) The winding and the unwinding ratio In the self-winding watches, both winding and unwinding take place simultaneously. So, it must be ensured to maintain winding ratio higher than unwinding ratio, when placed in the winder. The gear ratio and data related to the winding of the mainspring is physically inspected and stored for calculation.
(2) Gear ratio of oscillating weight to ratchet wheel In SEIKO 7S26A movement, one rotation of the oscillating weight in either direction corresponds to 0.0064 rotation of the mainspring barrel. 154.8 turns of the oscillating weight corresponds to one full rotation of the ratchet wheel, winding the mainspring by 0.16%. For a 40-h power reserve, it takes 8 turns of the ratchet wheel to fully wind the mainspring [16]. Hence, 1238.4 turns of the oscillating weight is required to fully wind the mainspring.
Let N 1 , N 2 , T 1 , T 2 denote the rotations of the driver gear, rotations of driven gear, tooth of driver gear and tooth of the driven gear respectively. Then, Let OW, FRW, SRW, PLandRW denote the Oscillating Weight, First Reduction Wheel, Second Reduction Wheel, Pawl Lever and Ratchet Wheel, respectively.
The following four Gear Ratios are calculated using the basic specifications of SEIKO 7S26A, Eqs. (1) and (2).
From Table 1, the gear ratio of the oscillating weight to ratchet wheel is found to be 154.8:1. (1) Gear Ratio is (4) Time taken to unwind a completely wound main spring From the manufacturer's (SEIKO 7S26A caliber) claim, the time taken to unwind a completely wound mainspring is 40 h. Hence, the power reserve is 40 h [16]. Therefore in 1 min, 0.04% of the mainspring gets unwound.

(5) Ratio between winding and unwinding
With 9 RPM motor, in 137.4 min, the mainspring gets fully wound. In one minute, 0.72% of the mainspring gets wound. Similarly, 100% of unwinding takes place in 2400 min. In one minute, 0.04% of the mainspring gets unwound. Hence, the Ratio is 0.72:0.04. The winding ratio is 18 times higher than the unwinding ratio. Hence, the result validates the use of the 9 RPM synchronous motor for SEIKO 7S26A movement.

CITIZEN 8200A caliber
Identical to MIYOTA 8205, CITIZEN 8200A is a 21-jewel, 42-h automatic movement with day and date. It has a lift angle of 49° with 21,600 beats her hour. It uses yoke winding mechanism with Glucydur balance and pallet lever escapement. The movement can be wound either by manually rotating the crown or by the oscillating weight automatically. The rotor is unidirectional with its rotation restricted to counter-clockwise direction and it has become the key reason for choosing the caliber.
In CITIZEN 8200A movement, the Oscillating Weight, Pawl Winding Wheel, Reduction Wheel, Ratchet Wheel, the Click and Mainspring Barrel are constituted for winding the mainspring.  From Table 2, the gear ratio of the oscillating weight to ratchet wheel is found to be 116.47:1. The Power Reserve of the caliber is 42 h [17].
(2) Time taken to wind a completely unwound main spring The mainspring of CITIZEN 8200A is fully wound by 7.5 rotations of the Ratchet Wheel [17].
No. of turns of the rotor to completely wind the mainspring = 116.47 × 7.5 = 873.5 Winding speed (RPM) = 9 The total time taken is 1.61 h.

(3) Time taken to unwind a completely wound main spring
From the manufacturer's (CITIZEN 8200A caliber) claim, the time taken to unwind a completely wound mainspring (power reserve) is 42 h [17]. Therefore in 1 min, 0.03% of the mainspring gets unwound.

(4) Ratio between winding and unwinding
Total Time Taken = 873.5 RPM × 60 With 9 RPM motor, 100% winding of mainspring takes 96.6 min. In one minute, 1.03% of the mainspring gets wound. Similarly, 100% of unwinding takes place in 2520 min. In one minute, 0.03% of the mainspring gets unwound. The ratio of the winding and unwinding is 1.03:0.03. The winding ratio is 34.3 times higher than the unwinding ratio. Hence, the result validates the use of 9 RPM synchronous motor for CITIZEN 8200A caliber as well.

Validation for 60% winding rule
A mechanical or mechanical-self-winding watch requires a minimum of 15% winding of the mainspring to maintain the accuracy of time and to have an undisturbed run. So, a fully unwound caliber is not placed in the winder in still condition. The watch is usually prewound to 15% and the right time is set in the caliber before it is placed in the winder. So, setting 15% as the threshold, with which the caliber is placed in the winder, the proposed winder winds the mainspring by 60%. With the threshold of 15%, 75% of the mainspring is wound in total. This is sufficient to power caliber for 24 h. Let W , UW , BMandWP denote the winding, unwinding, bare minimum and winding percentage, respectively. Then, the winding percentage for each day is calculated using Eqs. (4) and (5).
After 24 h, the caliber with 40-h power reserve is still in beat with 17.4%. Hence, it is evident to wind the caliber after 24 h with an implication that the daily working time of the winder is reduced by 40%. For consecutive days, there is an increment of 2.4% in the winding and the remaining percentage. The values are set in the winder to work for 25 days before its scheduled 1440 min break. As the 60% winding rule proves to be effective with 40-h power reserve, this rule extents to work good for all movements with higher power reserves.

Attiny85 and the watchdog timer
Known for the efficient and minimal power consumption, Attiny85 consumes ~ 2 mA at 1 MHz clock speed with 5 V input. Furthermore, it works with power consumption as low as 5uA in sleep mode. The watchdog timer interrupt wakes up the Attiny85 from the sleep mode. The maximum time between the consecutive interrupts is ob100001 (sleep bit pattern) which is equal to 8 s. But this sleep time is significantly insufficient. Hence, longer sleep time is achieved by recursively calling the 8-s interrupt until required. Figure 3 shows a flowchart representing the fundamental decision-making sequence in the winder. Based on this decision, periodicity of working is defined using the algorithm given below.

System design
The model uses Attiny85 20SU SMD Microcontroller. It is programmed using the Arduino development board [18] and it is designed to stay in the sleep mode for maximum time. The setup is fabricated on PCB with the microcontroller, 5-V Single Pole Double Throw relay, an NPN transistor to drive the relay, a fly-back diode to curb the back emf developed during the coil's switching and a base resistor to limit the current which is flowing though the base. A 220 V Single-phase AC Synchronous motor is externally wired to the printed circuit board. The synchronous motor changes its direction of rotation for every odd switching. Figure 4 shows the circuit diagram of the proposed winder. Figure 5 depiction is the 2-layer PCB layout of the winder designed with KiCad [19,20].  Figure 6 portrays the 3-D rendered image of the proposed winder. The DC input is connected using male-female jumper cables while the mains current for synchronous motor is directly soldered to the board. To the shaft of the synchronous motor, an enclosure in which the watch is placed is coupled about an axis perpendicular to the axis of the shaft. During scheduled intervals, the relay switches the motor and the shaft along with the enclosure and watch rotates thereby winding the mainspring of the caliber.
The prototype of the model is made using a 9 RPM single-phase Synchronous motor, Attiny85 module and 5-V single-pole double-throw relay module. The watch is fixed intact on the base and the base is screwed directly to the shaft of the motor. Figure 7 depicts the prototype of the model.   Figure 9 clearly picturizes that the 60% winding rule pays off for all calibers with power reserves 40 h or higher depicted as in Tables 3 and 4. Table 5 compares the working hours of the proposed winder to that of the marketavailable models. The proposed model in all cases winds the mainspring only by 60% while the commercial models carry out 100% winding superfluously. Figure 10 depicts the total working hours of the proposed model and the commercial models. It is evident from Fig. 10 that the working time of the proposed winder is significantly less. Hence, the model decreases the unnecessary working time and thereby it increases the efficiency of the winder.   Fig. 9 In beat time and remaining in beat time for power reserves between 40 and 50  modified to provide timed-intervals for the synchronous motor to cool down. The model can be enclosed in a honey-comb structured container which not only has a high strength-to-weight ratio, but also possesses good shock absorbing capacity.