Torque sensor for electric power-assisted steering systems

#The invention relates to a sensor for detecting the torque on a shaft such as a steering shaft.

#Torque sensors for steering shafts are already known. For example, DE 198 34 897 C1 discloses a sensor of this type used in power-assisted steering systems of modern motor vehicles to measure the forces acting on the steering assembly. The measured values obtained in this way are then supplied as controlled variables to a control unit which calculates a signal for a preferably electric or electrohydraulic power-assisted steering system. These systems typically operate such that when the steering actuating force is below a predetermined torque threshold, power assist is minimized so the driver has to apply most of the steering force. Mechanical feedback is thus imparted to the driver. Above the predetermined torque threshold, the control unit provides power assistance and therefore uniform steering actuating force, so the steering actuating force to be applied by the driver is always within a comfortable range. The “spongy driving sensation” frequently criticised previously in power-assisted steering systems can therefore be avoided.

#The sensor shown in DE 198 34 897 C1 is used to detect the rotational forces in a portion of the steering shaft so that the forces introduced into the steering shaft are transmitted by a torsion bar having a measuring cell. The steering force introduced therefore causes torsion which is detected by the measuring cell and measured, for example, as a strain by strain gauges (DMS). For safety reasons, a torsional angle limit is provided in the known sensor and this limits the difference in the twisting angle between the two portions of the steering shaft connected the sensor, so that malfunctioning of the measuring cell, for example by overloading and rupture of the torsion bar, does not lead to complete failure of the steering assembly.

#The invention will now be described by way of example with reference to the accompanying figures of which:

#FIG. 1 is a lateral section of a sensor according to the invention,

#FIG. 2 is a cross sectional view of the sensor taken along the line X—X of FIG. 1,

#FIG. 1 shows a sectional side view of a sensor substantially consisting of the two portions 1 and 2 of a steering shaft, the movement converter 4, the carrier 5, the movement guide 6 and the displacement sensor element 8. A steering wheel (not shown) of a motor vehicle is connected to the portion 2. The carrier 5 surrounding the shaft 1 and 2 like a sleeve is floatingly mounted with a fit 11 on the shaft 1 and 2. The carrier 5 can consequently be moved laterally, in other words along an axis of the shaft 1 and 2. The relative lateral position of the carrier 5 with respect to a measuring device arranged outside of the shaft 1 and 2, in particular a displacement sensor element 8, is evaluated for the measurement of the torque. A magnet 7, for example, can be arranged in a defined position on the carrier 5 for interaction with the displacement sensors.

#Reference is now made to FIGS. 3 to 5 showing three essential individual components of the sensor, each in perspective view. The disc-shaped movement converter 4 is shown in FIG. 3 with a central aperture provided for fastening to the first portion 1 of the shaft. In addition, three recesses 9 are uniformly distributed over the periphery. One respective side of each recess 9 is designed as an oblique and bent-in or helical running face 14.

#The carrier 5 in FIG. 4 consists of the central sleeve-like portion 19 formed with an extension 15 at one end into which the movement converter 4 can be fitted in such a way that the pins 10 engage in the recesses 9. The other end of the carrier 5 is designed as a connecting piece 16 comprising elongate extensions 17 projecting in the axial direction and guided in corresponding grooves 18 of the movement guide 6 (FIG. 7) shown in FIG. 5.