Lavet type stepping motor
When Lavet stepper motor is a single-phase stepper motor, electro- mechanical quartz watches and radio serves as a drive and is responsible for the "jumping seconds " of the second hand. Miniaturization he comes in wristwatches, a field that requires very little power, so one years of battery operation is possible. As the inventor of this motor refers to the French engineer Marius Lavet, who described the principle of operation in 1936 patent FR823395.
Design and operation
Like other single-phase motors knows the Lavetmotor only one direction of rotation, which is determined by the design of the stator. The rotor consists of a permanent magnet in the adjacent figure whose poles are shown in red or green.
Essential for the functioning of the engine are the grid points of the rotor, which can be realized for example by notches in the stator. These two possible grid points are shown in the adjacent detail sketches in position a and c. The position of the rotor when current flows through the stator coil is different from the de-energized condition and in the two diagrams b and d.
The rotor is initially at rest in a position, a transition to the position B by the angle made by the current pulse of appropriate polarity α. For an energy- saving operation of the current pulse through the coil as possible should be of short duration, but also long enough so that the rotor is rotated far enough and is in the capture range of the next rest point instead of falling back. As soon as the current pulse is turned off is produced in position B through the notches in the stator, an uneven distribution of magnetic flux, whereby a according to another of the reluctance torque acts on the rotor, and the rotor by the angle β in the stable state C is further moved. The stator forms a magnetic circuit, the magnetic resistance is increased in the region of the indentations compared to the areas with no notch. In position c, as well as in the starting position a, the magnetic resistance of the circuit for the rotor is minimal, resulting in the two locking points.
To convert the rotor position d to the position c, a further current pulse with the opposite current direction is required, the coil is driven bipolar. Wherein the current pulse is switched off in the position d of the rotor then moves to the initial position A and the cycle can begin again.
This special shape of the stator is created a kind of auxiliary phase, similar to the split-pole motor, the positions of the notches in the stator define the direction of rotation. In the usual design a Lavetmotor requires two steps for one revolution. Lavet the stepper motor is controlled in quartz watches by pulses every second, which are supplied by a quartz watch with a frequency of 32,768 Hz. From this frequency, a bipolar pulse per second can be technically simple to carry 15 times the frequency halving in an integrated circuit form. An attached to the rotor mechanical transmission and translation occurs in the clock drive of the second, minute and hour hands.
In addition to classic watches engine, there are several variants derived this concept. Engines according to this principle, for example, used in modern vehicle instruments.