Murata-pioneered navigation sensors

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22 September 2004 Passive Components

Information from Avnet Kopp

Ref: z2717151m

Satellite navigation systems are great when they can 'see' the satellite, but they need a temporary reference when they cannot. One of the best available solutions currently is Murata's ceramic bimorph gyroscope technology. However, car navigation systems require higher accuracy than, say, video cameras and the like. Therefore, Murata has increased the processing accuracy of the bimorph vibrator and devised a new structure that produces more stable vibration.

With tuning bar piezoelectric gyroscopes, the bar is vibrated by piezo-electric ceramics. When rotational motion is applied to the centre axis of the vibrator, a Coriolis force is generated perpendicular to the original direction of vibration. This force is detected by more piezoelectric ceramics.

The principle used for detection is as follows: taking the Foucault pendulum, (shown in Figure 1) when angular velocity 'w' is applied to the axis of an object of mass 'm' swinging at velocity 'V', a Coriolis force expressed by formula: F = 2m V X w is generated.

Figure 1

This force is perpendicular to the reciprocating motion of the pendulum, being largest at the highest velocity point. Generally, the magnitude of angular velocity can be measured by electrically detecting the Coriolis force while keeping 'V' constant. Murata's original structure was triangular, with numerous micron-diameter lead wires making interconnections.

This has been changed to a rectangular cross-section needing no interconnections, reducing size while improving production efficiency and performance. The latest version is smaller, with a plate inserted into the bottom of the vibrator section to provide more stable vibration, giving better frequency stability.

Electrical bonding and mechanical support have recently been improved by a new process. Figures 2 and 3 show the principle of operation.

Figure 2

Figure 3

The net result of this work is the ENV-05G, a bimorph gyroscope that is ideal for car navigation systems. The new design is much more accurate and easier to produce. In a 12,4 by 7,6 mm package, 18 mm high, it has an operating temperature range of -40°C to 85°C, scale factor of 25 mV/°/s and offset drift of 9°/s. This sensor is specifically intended for measuring the position of moving objects, especially motor vehicles.

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