 Electric Power Steering 1
It is foreseeable that at least all cars will be equipped only with a purely electrically assisted steering someday. As soon as within the series any subsets of autonomous driving are integrated, the electric
power steering is essential anyway.
Why this kind of power steering saves fuel? Because no motor driven pump continuously generates pressure. In principle, no energy is consumed when one e.g. the steering wheel leaves largely untouched.
Certainly, a little bit for the electronic control. But that's also the case with the late versions of hydraulic power steering.
Let yourself not impress by fuel savings 'up to', because the amount of the saving depends greatly on the conditions of use. As a second advantage is striking the lower maintenance effort and the minor repair
risk. No circulation of hydraulic oil, the can become leaky at all possible places. How nice if such an innovation would be possible for the service brake.
Think about the effort of different cable routing when change-over from left- to right-hand drive. However, this is difficult despite fewer components, even with electric power steering. As the picture below shows,
the electrical solution has less construction work overall, but the concentration of parts in one place is higher.
As long as there is no real 'drive by wire' in practice, the mechanical connection between the steering wheel, steering gear and steered wheels must be accommodated in a very specific way. And there is often
not enough space on the right side in the engine compartment. A nightmare for the engineers involved.
Nevertheless, the advantages over the hydraulic system remain here as well. The noise that was so common in the past when a hydraulic power steering system hits the end of its stroke has completely
disappeared. The purely electrical support is also intended to better ward off certain forces that act on the steered wheels. There will also be a certain reduction in weight, and not just in small cars. Later we
have to talk about the increasingly important 'overlay'.
Here we are dealing first with an electric support motor at the top of the steering column. It is very important that we are in the interior with it. There are therefore no costly and time-consuming covering and
sealing measures. After all, with such a system, everything is together, including the computer, which would need significantly more protection in the harsh environment of the engine compartment, also with
regard to the maximum temperature.
The support unit can also be attached to the bottom of the first part of the steering column, as shown in the next picture above. The two arrangements shown so far are common in vehicles in the A and B
segments. There is no clearly definable assignment for the C segment. But it is impressive, however, that, e.g. in small Japanese cars is just enough space for the electric actuator in the narrow space
between the steering wheel and the steering gear.
Here you can see the lower part of the steering column, usually after two cardan joints. An electric steering aid can also be arranged in the piece immediately before entry into the housing of the toothed rack,
especially if it replaces the previously installed control part of a hydraulic system. You will find a picture of this in the next chapter. The gearing between the steering shaft and the gear rack can also be seen
here.
This is the view of the servo unit from below. In the foreground the upper of the two cardan joints, which by the way are needle-bearing, although they look so filigree. We will deal with the transmission of forces
in both directions later. In the background the worm drive is very nicely visible, through which the electric motor influences the steering, beneath the control unit.
This you can see better here, and so can the previously mentioned saved protection. On the left it goes down to the toothed rack, on the top right you can still see part of the steering wheel adjustment, which
is mechanical in this case. Of course, there are also electric ones, but with an enormous effort. Whether the customer really wants everything that the manufacturer offers and can he check that at all if it can
be ordered in the package?
The electric power steering directly on the toothed rack is connected to it via a double pinion or a toothed belt (picture above). For some larger vehicles, an electric motor on the steering column would be too
large. In 2017, the really large SUVs (e.g. Ford F-types) reached a support force of up to 18,000 N. Of course, this includes vans that are more well-known here, such as the Sprinter and Crafter.
Since less than half is necessary for small cars. If they are particularly light, then the servo effect is simply switched off outside of the city and without the need for shunting. The particularly large steering
assistance also causes problems with possible feedback on the steering wheel. Here can be a significant lack of steering feeling. The more direct approach to the toothed rack is a help, even if an additional
sensor on the steering column is then required (picture below).
This is a construction with double pinion from ZF, called 'Servolectric'. It is suitable for heavier cars with an axle load of up to 1200 kg on the steering axle. The force at the toothed rack can be up to 11,000 N.
ZF promises a saving opposite to hydraulic assistance of up to 90 percent. In addition, one emphasizes the possibility of professional work-up and the standardized method in which this steering is
manufactured.
In all cases, the power comes from asynchronous motors because you can then save on the wear-prone brushes. A large transmission ratio is also important, e.g. using a planet or, more simply, a worm drive.
When connected to the steering column, it is in a wide range around 15:1. Mind you, that's not the steering ratio. For finer transmission of chassis conditions to the steering wheel, even the length
compensation can be ball-bearing.
Despite the relative compactness of the entire system, a great deal of effort is made. For example, the situation of the electric motor and the turning speed on the steering wheel are determined and passed on
to the control unit. This measures e.g. the temperature of its own power amplifiers. This of course creates a multitude of conditions for a possible reduction or switching off of the moment support.
In the picture above you can see another probably completely unexpected advantage of the purely electrically assisted steering. It is in fact much easier to integrate in a classic car with stiff steering as a
hydraulic one. It is now offered for many models. However, you should check whether the vehicle still meets all the conditions regarding its originality after the conversion or whether you deliberately do without
it. The question of a possible deconstruction would also be interesting.
Here again the electric motor arranged parallel to the toothed rack, driving it via double pinions. Simple pinion drive: Also arranged in parallel for spur gears, at right angles for bevel gears. Next to it is the short
electrical connection to the control module at the bottom of the steering column. From the package alone, the electric power steering is more sympathetic to the car manufacturers than the hydraulic one.
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