the quartz watch
Always have wrist exact time, almost to the second, is something so common today that no one pays more attention. However, this laboratory precision, unthinkable only twenty years ago, it has the quartz, whose operation is much more mysterious than a pendulum clock . the exception of some enthusiasts who still have good mechanical wrist or a Rolex Longines, everyone wears a watch today quartz electric, whose long-term accuracy is considerable On average, the drift not exceeding more than three seconds per month, barely more than half a minute in the year. Suffice to say that in practice it never needs to put his watch on time - except for the stupid time change at the beginning and the end of spring. This stunning
stability is so quick entry into the manners that no one pays more attention. However, there are only twenty years, there were only mechanical watches sprung balance with better versions, fully paid, still drifting ± 2 seconds per day. Standard models, meanwhile, took easily fifteen seconds per day, hence the need to put the shows on time every week.
In return, the functioning of these mechanical watches was known to all, because pendulum clocks were still common and rare ones who in their childhood, had not opened or disassembled an alarm clock. we went back the spring, the spiral balance, the anchor, the gears were familiar things. It is quite otherwise with the quartz we imagine most often a reflection of what nature provides a transparent crystal prism terminated by a small pyramid, gender or size diamond pendant chandelier.
How this crystal gives there time? This remains an unfathomable mystery to many. To remove the mystery, he goes back to 1817, when the abbot Haüy, a founder of crystallography, discovers that certain crystalline bodies have on their faces as soon as electrical loads are subjected to constraints, ie as soon as we pulled or pushed. Yet it took until the communication of Peter and Paul Jacques Curie in the Academy of Sciences in 1880 for this phenomenon called piezoelectricity, is really known and quantified by a law, the electric charge collected is proportional to the force.
The following year, Gabriel Lippmann showed that the process is reversible and that the electric fields applied to certain surfaces produce a deformation of the crystal. Among the piezoelectric body, the most common and most interesting is quartz, which is none other than crystalline silica (silicon dioxide SiO2). This is the most abundant mineral on Earth since the sand, gneiss, granite, sandstone is silica. The appearance of quartz as it present naturally in rocks is known: it forms transparent hexagonal prisms, terminated by a sharp face. But, once cut and polished, it can take any desired shape, since the cube to the sphere. For specialists, the crystal lattice of quartz benchmark by three axes, according to which its physical properties are different (it is said anisotropic): an optical axis Z in the length of the prism, an axis perpendicular to a mechanical torque faces, and electrical axis parallel to these faces and then passing through two opposite edges. It was this power that will interest us here. As
all solid, quartz is elastic, as well as glass, steel and porcelain. It may therefore be the seat of vibration when it departs from its equilibrium position and to be abandoned, like a steel spring which is pulled abruptly. But like the quartz piezoelectric distortion gives rise to an electric field (the one related to the charges that appear on the sides), and vice versa. However, vibration is but a sequence of alternating periodic deformations, which will generate periodic alternating current - that is, alternating currents. Moreover, being a quartz crystal very hard, its vibration frequencies are high: tens of thousands of hertz, which is the sound generated by these vibrations at the ultrasonic.
When a voltage is applied on both sides of a quartz , it is deformed in flexion, extension or shear depending on the arrangement of electrodes. When you suddenly cut the power, it oscillates, like a leaf spring which has been shot and is released at once. But because the quartz is piezoelectric, these oscillations will generate an alternating current of same frequency as the vibrations.
By coupling the appropriately input current, which will serve to maintain the vibrations of quartz (as it maintains the oscillations of a swing by giving a little push every round), and the output current, which will be used to cut the time it obtains a quartz oscillator whose oscillations have a remarkable consistency. It only remains to count these oscillations to have a clock.
We must realize that we indeed can not measure time continuously, as would a heavy top in full swing, as no rotational phenomenon is stable at our scale. It is therefore reduced, since the beginnings of watchmaking, to split time and counting the fractions in the hope that they are all exactly equal, which of course is never really the case. The pendulum clocks - which the oscillations are maintained by a system of pawls acting on a ratchet wheel driven by a spring or weight - cut the time in roughly equal fractions provided the temperature does not change (the balance s lengthens or shortens depending on whether it is hot or cold) that remain constant friction of the gears, etc.. The spiral balance
watches and clocks is subject to the same problems and, except for laboratory equipment, the oscillations are far from all of equal duration. As time is measured by adding all these oscillations, the differences are also involved in the total and the final count deviates more and more real time (as defined by the astronomical movements or, more recently, by the oscillations of atoms).
With pendulum clocks installed in the basements of observatories, free from the slightest twitch, the smallest variation in temperature or pressure, we could reduce these differences to the point of an hour specifies that the rotation of the Earth itself. But with a wristwatch, rocked and rocked all day at the end of arm, the sum of differences always exceeded one second after twenty-four hours.
The engineers who designed the clock movements have long known that the only way to reduce these gaps was to increase the frequency of the sprung balance, which was mechanically very difficult: a wheel oscillating around its axis can not make thousands to go and re-turn per second. However, a simple tune a piano tuner, vibrating at 440 Hz, would have been a much greater control.
In fact, despite the inherent difficulty in converting a rotational vibration, there were clocks and even tuning fork watches. But from the twenties, the researchers thought that quartz was beginning to be used to adjust the frequency of radio transmitters, and whose vibrations are 100 to 1 000 times faster than tuning forks. From 1930, the Germans and Scheibe Adelsberg devised laboratory clocks with the regulatory body was a bar of quartz. Vibration at 100 000 Hz were maintained by triode tube circuits. Counting oscillations posed major problems at that time, yet the comparison of clocks with Scheibe best pendulum clocks observatories showed as quartz gave the same accuracy.
Now, the pendulum was at the peak of his art, while the quartz was in its infancy. The Second World War interrupted the research, but from 1950 onwards, all the astronomical observatories equipped themselves with quartz clocks that had progressed significantly and whose accuracy was significantly higher than that of pendulums. The invention of transistors and integrated circuits that would allow miniaturization of this technique and give it a fantastic rise: clocks, alarm clocks, watches are now almost all quartz.
must say that the device is remarkably simple: a single piece of quartz, set into vibration (usually to 32 768 Hz, or 32,768 oscillations per second), whose current output is counted by integrated circuits, which are similar to those operations in calculators, and displayed by liquid crystals or needles controlled by a stepper motor step. In
watches, quartz, still tiny, is a pitch-derived tune Karolus presented in 1954. In the clock or clocks, it has the form of a thin wafer. In all cases, it is enclosed in a vacuum container in a small copper-nickel. The branches of the fork, or the faces of the disc is metallized by vacuum deposition of gold, aluminum or chromium. This metal film serves as an electrode for conducting current to the quartz.
The design of the electrodes is calculated on the computer because it plays an essential role in the quality of the resonator, quartz used for both insulation and mechanical oscillator capacitor whose plates are metal deposits. We must realize that what changed between the tuning fork watch, as existed briefly around 1965, and the quartz watch now is the counting mode oscillations.
In the first case, a pawl attached to a metal part of the pitch pushed a tooth of a ratchet wheel at each oscillation. In the second, the mechanical vibration of the crystal silica is converted into alternating current by the piezoelectric effect, and an electronic circuit has alternations. But in both cases, it is an elastic material made to vibrate. A mechanical watch uses elsewhere as the oscillations of a small spiral elastic spring set, the anchor and escape wheel serving as the return of a tiny router connected to the spring.
The gain in precision is mainly due to increased frequency the oscillating system: 2.5 to 5 Hz for the spiral balance, 300 Hz for the pitch metal, 32 768 Hz tuning fork quartz. This is the equivalent of a pendulum which oscillates even after a month without having restored to him any momentum.
A performance due to its crystalline nature, which explains its exceptional regularity: in labboratoire, quartz guarantees a variation not exceeding one second in 275 years
Legend:
In A, the miniature battery as an energy source
In B, the quartz resonator, vibrating at a stable and accurate frequency 32768 Hz
In C, the integrated circuit divides the frequency by 2 15 times in succession to reach a frequency of 1 Hz is equal to one second.
Finally a micro electric motor in D, is "excited" by the signals obtained and it turns stepping pulses seconds precision movement which actuate the wheel needles E.
