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Torque (power steering)



Function

This page describes how the control device arrives at its most important measurement value, the torque which is necessary for the steering. This is important for the calculation of additional steering force support. Therefore, this page also points out which conditions are valid if other values come into play. In this case e.g. - perhaps as an input during the production - the vehicle weight and the speed queried through the CAN-Bus.

How it works

Here, only the principle of the steering moment measurement should be explained. Basically such a measurement is possible if before, and behind a defined weak spot in the steering column, a disc with a sophisticated aperture scheme exists. In this case, light-emitting diodes on one side, and photo-diodes on the other, work together in an axial direction. However, both discs can also be interlaced as a larger and a smaller pot into each other so that the light-emitting-diode and photo-diode are arranged on a radial line. It is important that both ascertain even the finest differences in the rotary angle of the two discs and pass them on as a qualitative measure to the control device.

The processing in the control device takes place with the help of a characteristic map, from which, in the above figure 3, only a few characteristic curves are shown. In this case it's only about the two parameters, driving speed and vehicle weight. If you wish to test yourselves, you can now, without reading further, allocate only the following 5 situation terms to the single figures at the ends of the curves:

- Vehicle weight low, vehicle speed low
- Vehicle weight high, vehicle speed low
- Vehicle weight low, vehicle speed high
- Vehicle weight high, vehicle speed high
- Proportional increase of the steering force support.

Here are a few tips:

The heavier vehicle of course, needs a little more steering force support (in each case the upper curve). The slower vehicle also needs more support (upper two curves). However, the difference between a light and a heavy vehicle is by far not as large as it is between a fast and a slow vehicle. And in between them, runs the proportional curve (straight line). This can also have been adjusted to the vehicle weight during the production. If it was complemented with straight line to the other incline, this could also take into consideration various driving speeds. It could also have been adjusted to the vehicle weight in the factory. However it cannot reduce the support at higher speed as strongly, nor can it quickly increase the support at low speeds. The result is a linear connection. 07/09

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