The present invention relates to a device for the detection of the presence of a driver at the driver's seat in a vehicle, for a trip by land, sea or in the air, comprising at least one sensor.

Such devices are known since a long time. They usually comprise an electric switch, that directly or via a computer act upon the signal current when a driver does not load a driver's seat. Such switches, mechanical or electronic have many drawbacks. Salt water make the switch contacts quickly rust and oxidize. As switches are binary, it is very hard to decide, with the aid of such ones, if a driver is "semi- sitting" at a driver's seat. Electronic switches are expensive and have to be synchronized with computer collection. Mechanical switches are cheaper, but have shorter life.

The object of the present invention is to achieve a device of the art mentioned introductorily. According to the invention, such a device is characterised primarily in that this sensor is arranged to measure continuously frequency and amplitude of the vibration of the driver ^' s seat in both directions of the horizontal plane, whereas the device comprises means for the continuous registration of the damping, i e the reduction of at least one characteristic resonance frequency, which damping forms an indication of the presence of a driver.

If the driver ^' s seat is provided with a triaxial accelerometer this is utilized. In one advantageous embodiment of the invention the sensor is embedded, protected in the driver ^' s seat. In this way the life of the sensor will be substantially prolonged, as it is not exposed to weather and wind, which is especially important in connection with vessels at sea.

The device can be used at driver ^' s seats of different designs, but it is to be preferred, that the driver ^' r seat is supported by a substantially centrally arranged support element. The invention also comprises a method to detect the presence of a driver in a device of the art mentioned. The invention shall now be described more closely, reference being made to the enclosed figures, of which: figure 1 schematically shows a drivers's seat without a driver, and figure 2 a diagram of the amplitude at natural frequencies in the driver's seat without and with a driver.

In figure 1 a drivers's seat is denoted by 1, supported by a central support element 2, which is anchored in a vehicle or vessel 3, which is not shown more closely. A sensor 4 is embedded, protected in the driver's seat 1. In this case, the sensor 4 is formed by an accelerometer, measuring in two directions at rigd continuously the natural frequencies measured by the sensor.

In the diagram shown the x- axis shows the resp. natural frequencies, with an unbroken curve without a driver and with broken curve with a driver, whilst the y-axis shows the amplitudes of the natural frequencies in volt (root mean squares).

To estimate the time a driver has been present at the driver's seat, the time natural frequencies have been damped, e g from c:a x = 13,2 Hz/y = 0,82 V to x = 4,6 Hz/y = 0,51 V. Of course it must be taken into account that a driver, when present at the driver's seat may be loaded by things, that may displace the natural frequency somewhat..