Lavet stepper motor
The Lavet stepper motor is a single-phase stepper motor that drives electro-mechanical quartz and radio-controlled clocks and is responsible for the " jumping second " of the second hand. As a result of miniaturization , it is used in wristwatches and requires very little power, which means that it can be operated on batteries for years. The French engineer Marius Lavet , who described the functional principle in patent FR823395 in 1936, is considered to be the inventor of this motor .
Structure and functionality
Like other single-phase motors, the Lavet motor only knows one direction of rotation, which is determined by the design of the stator . The rotor consists of a permanent magnet , the poles of which are shown in red and green in the illustration opposite .
The locking points of the rotor, which are implemented in the stator, for example by notches, are essential for the function of the motor . These two possible locking points are shown in the adjacent detailed sketches in positions a and c . The position of the rotor when the stator coil flows through it differs from that in the currentless state and is shown in the two representations b and d .
If the rotor is initially in the rest position in position a , the current pulse of the corresponding polarity causes a transition to position b by the angle α. For energy-saving operation, the current pulse through the coil should be as short as possible, but also long enough so that the rotor rotates far enough and gets into the catch area of the next latching point instead of falling back. As soon as the current pulse is switched off, the notches in the stator in position b result in an uneven magnetic flux distribution , which means that a further torque due to the reluctance forces acts on the rotor, and the rotor continues to move by the angle β into the stable state c . The stator forms a magnetic circuit , the magnetic resistance of which is increased in the area of the notches compared to the areas without a notch. In position c , as in the starting position a , the magnetic resistance of the circuit for the rotor is minimal, which results in the two locking points.
To move the rotor from position c to position d , another current pulse with the opposite direction of current is required - the coil must therefore be controlled in a bipolar manner. When the current pulse is switched off in position d , the rotor then changes to the starting position a and the cycle can begin again.
This special form of the stator creates a kind of auxiliary phase similar to that of the shaded pole motor , whereby the positions of the notches in the stator determine the direction of rotation. In the usual design, a Lavet motor requires two steps for one revolution. The Lavet stepping motor is driven in quartz watches by pulses with a repetition rate of 1 Hz from a frequency of 32.768 Hz ( watch crystal ) by a 2 15 - frequency divider are generated. The clock circuit containing this divider also generates the bipolar pulse train, which alternately sends a positive and a negative voltage to the coil for a short time every 0.5s. The generation of the bipolar pulses would also be possible by differentiating a 1 Hz square wave signal by means of a capacitor ( high pass ), but such a capacitor cannot be integrated into the circuit, so it would take up a large volume and cause additional costs. A toothed wheel drive moved by the rotor drives the seconds, minutes and hours hands.
In addition to the classic watch motor, there are various derived variants of this concept. Motors based on this principle are also used, for example, in display instruments for motor vehicles.
literature
- Patent FR823395 : Perfectionnements aux systèmes et appareils de commande électrique à distance, notamment aux moteurs et horloges synchrones. Registered September 28, 1936 , published January 19, 1938 , applicant: Hatot, inventor: Marius Lavet (French).
- Patent US4550279 : Step-by-step motor unit. Registered on September 7, 1983 , published October 29, 1985 , applicant: Fabriques D'horlogerie De Fontainemelon SA, inventor: Eric Klein (explanation of the operating principle in English).
Web links
Individual evidence
- ↑ A brilliant concept . March 29, 2005, Neue Zürcher Zeitung
- ↑ Thanassis Speliotis et al .: Micro-motor with screen-printed rotor magnets . Journal of Magnetism and Magnetic Materials, Volume 316, Issue 2, September 2007, pp. 120-123
- ↑ Control and Analysis of Low Inertia Miniature Synchronous Motors , accessed October 5, 2010
- ↑ Hermann Winner, Stephan Hakuli, Gabriele Wolf: Handbook Driver Assistance Systems : Basics, components and systems for active safety and comfort . Vieweg + Teubner, 2009, p. 337