Gyroscopes are well known as sensors to detect the revolution movements or angular velocities of objects. The gyroscope is a sensor with a wide range of applications that is not affected by distance between its installation position and the centre of revolution.
Gyroscopes using piezoelectric vibrators are called piezoelectric vibrating gyroscopes. Recently, piezoelectric vibrating gyroscopes have been downsized and made surface-mountable. Precision was also improved, so that their application areas are expanded. This article focuses on the basic principle of piezoelectric vibrating gyroscopes, including technical trends in size reduction and high-density mounting.
Tuning bar vibrating gyroscopes
The gyroscope utilises a physical phenomenon called Coriolis force. Foucault's pendulum is frequently used to explain this force. As shown in Figure 1, when a mass (m) is vibrating with a velocity and when it is given a revolution as expressed by an angular velocity of w, Coriolis force is generated, as expressed by the following equation: F = 2m V x w
Coriolis force operates in a direction perpendicular to the direction of the motion of the pendulum and is proportional to its velocity. The piezoelectric vibrating gyroscope has its tuning bar vibrator by use of piezoelectric ceramic - this corresponds with the pendulum's vibration - and if this vibrating system is given a revolving angular velocity, Coriolis force is generated in a direction perpendicular to the original vibration. Since this gyroscope uses a piezoelectric ceramic, Coriolis force can be detected and transformed to electric signals by the basic principle of piezoelectric ceramic.
Triangle pole vibrators
Murata introduced its Gyrostar using the triangle pole vibrator in 1989. This Gyrostar was entirely distinct from the conventional square pole vibrators. The cross-section of the triangle pole vibrator shows an equivalent triangle, and it is characterised by the fact that its driving force is not at a right angle relative to the detected force, but at a slanted angle. As a result, the circuits were substantially simplified, the adjustment of frequencies was made easier, and the sensitivity of the triangle pole product improved compared with conventional gyroscopes. Thus, the Gyrostar came to be used extensively for car navigation systems and video movies as angular velocity sensors that were excellent in precision, and lower in cost compared with conventional ones.
However, there were some problems to be solved. Since the product depended on the joining of metal with a ceramic, its performance as a sensor tended to depend on the conditions of joining of metal and ceramic. Furthermore, as the processing method and wiring involved three-dimensional processes, it required a more complex production system.
Meanwhile, car navigation systems became multifunctional and video movies were improved in picture quality and downsized as a result of the development of digital video cameras. Naturally, smaller and cheaper angular velocity sensors are needed. Murata developed a ceramic bimorph vibrator whose productivity was greatly improved, to solve these problems of the metal triangle pole type. Thus, in 1997, it put in the market the small-sized Gyrostar ENC-03J series gyroscopes, mainly intended for the detection of involuntary hand movement in the use of video cameras. Later, in 2000, the company introduced the ENV-05F series of high precision type products, mainly for use in car navigation systems.
Ceramic bimorph vibrators
The structure of a ceramic bimorph vibrator is shown in Figure 2. It is structurally characterised by a ceramic plate attached to another, and these two plates are arranged so that their polarised directions are reversed. When voltage is applied to these electrodes, a curvature movement is effected, due to one of the plates expanding while the other shrinks.
In the case of the ceramic bimorph vibrator, voltage is applied to the right and left electrodes formed on the flat upper surface to drive the vibrator. Vibrations are detected by the right and left electrodes on the upper surface of the vibrator, in the same way as the vibrator is driven. The ceramic bimorph vibrator is structurally the same as the conventional triangle pole type, in that the same electrodes are used for driving and detection, and this feature is also incorporated in the circuitry of the new type.
Furthermore, as for the supporting method of the vibrator, it adopts supporting pins that function as electric joints, in addition to their function as mechanical supports, making it possible to do away with the extremely fine wire that was used for the triangle pole type.
The manufacturing method of this ceramic bimorph vibrator is as follows:
* Electrodes are formed on both sides of a flat ceramic plate called a unit and are polarised by impressing high voltage on them.
* Two polarised piezoelectric ceramic plates are bonded in such a way that the poles of the ceramic plates are reversed.
* The bonded piezoelectric plate device is cut into individual vibrator sizes by a dicer.
* At the same time, the centre of the vibrator is grooved to separate the electrodes on the upper surface into right and left ones. Thus, three electrodes are formed, including the rear side.
A vibrator is completed in this way. This structure, which is flat with no electrodes on either side, not only facilitates the processing of the vibrator but also contributes to the simplification of its assembling.
ENC-03M series downsized
As explained earlier, the Gyrostar using this ceramic bimorph vibrator is extensively adopted for car navigation systems and for the prevention of involuntary movement of movie cameras. In 2002, the company developed a new series to follow the ENC-03J. The basic structure of the new super-small size series, the ENC-03M, is shown in Figure 3. The vibrator, circuit board, leads (supporting pins) and the package of the preceding series, were all reviewed with a view to downsizing, without sacrificing performance.
The ENC-03M has an IC for vibrator driving and angular velocity detection, mounted on the circuit board as a bare chip. This substantially reducing the occupancy area, allowing more space for other parts on the board. As a result, it has become possible for the vibrator connected to the support pins - which also function as lead wires - to be mounted directly on the circuit board. The new series adopts a cap-based structure in which the circuit board is protected by a metal cover, doing away with a resin part that covered conventional products previously.
In the past, vibrating gyroscopes were mainly pin-type products, so that a lot of manual soldering work was needed to mount parts. However, the ENC-03M is surface-mountable and permits high-speed and high density mounting by automatic mounters. It is resistant to high temperatures, allowing Pb-free reflow soldering.
The new ENG-03M series measures 12,2 x 7 x 2,6 mm (a volume of about 0,2 cc), and is reduced by about 40% in volume compared with the previous ENC-03J series. Generally speaking, the downsizing of angular velocity sensors tends to reduce their sensitivity and results to deterioration of their S/N ratios. However, ENC-03M's basic performance is equal to that of the ENC-03J. Moreover, its response characteristic is improved due to the smaller vibrator.
So far, the widespread use of angular velocity sensors has been due largely to the downsizing and improved precision of piezoelectric vibrating gyroscopes. In the future, they will be used and applied in many different fields. As for their use for consumer electronics, for which the ENC type gyroscopes introduced here are mainly intended, they will be used more extensively in digital cameras, for which the market is growing fast, as well as for movement control of various kinds of robots and radio-controlled models, for pointing devices and head-mount displays, as well as be applied to technologies for human interface with electronic equipment.
In the future, Murata intends to further develop its downsizing and production technologies through the manufacture of the ENC-03M, and merge these technologies with the high precision technology it has acquired through the development and manufacture of the ENV series, in order to develop angular velocity sensors that will be used in more extensive applications.