The invention relates to a method of controlling the torque developed at a belt shaft of a belt retractor coupled to an electric motor. DE 100 18 972 discloses a belt retractor whose belt shaft is driven by an electric motor rather than by a coil spring. The electric motor is coupled to the belt shaft by a toothed belt. Assuming the electric motor to be supplied with a constant amount of current, friction and the flexing work of the toothed belt result in a hysteresis of the seat belt forces, being different when the seat belt is wound up and when it is unwound.
It is an object of the invention to essentially avoid hysteresis forces in order to keep the pressure exerted by the seat belt on the body of the vehicle occupant essentially constant.
In the method according to the invention, torque developed at a belt shaft of a belt retractor coupled to an electric motor is controlled by detecting a direction of rotation of the belt shaft and, depending on the direction of rotation as detected, changing electric current supplied to the electric motor from a first value to a second value that is different from the first value by an amount required to compensate for hysteresis.
Accordingly, to equalize the seat belt forces the amount of electric current supplied to the electric motor is adjusted in such a way that it is increased in the winding direction and decreased in the unwinding direction, resulting in substantially equal torque at the belt shaft and hence essentially equal seat belt forces when the seat belt is wound up and unwound.
In accordance with a preferred embodiment of the invention, a standstill of the belt shaft is also detected. The seat belt forces needed to keep the pull-out length of the seat belt constant are smaller than the forces needed to unwind it. Accordingly, to compensate for the smaller forces, a higher amount of electric current is supplied to the electric motor.
In case of an emergency, when strong deceleration forces act on the vehicle, inertia forces tend to move the body of the vehicle occupant away from the seat, entraining and unwinding thereby the seat belt at a speed that is higher than during normal movement of the vehicle occupant. In order to protect the vehicle occupant, such a rapid unwinding of the seat belt has to be avoided.
In accordance with another preferred embodiment of the invention, rotational speed of the belt shaft is measured in an unwinding direction and when a predetermined first threshold value of rotational speed is exceeded, the resistance against pulling out the seat belt is increased by increasing the amount of electric current supplied to the electric motor, the rate of increase being constant. Only when a second threshold value of rotational speed, corresponding to a threshold value of the amount of electric current, is reached, a mechanical locking mechanism on the belt retractor is activated. Otherwise a restriction of the seat belt is achieved by means of the electric motor only.
A preferred use of the inventive method is in a retractor of the type comprising a frame, a belt shaft rotatably fixed in the frame and an electric motor coupled to the belt shaft by a drive belt, wherein the direction of rotation can be detected and, depending on the direction of rotation as detected, the motor current is adjusted.