Structure and principle of operation of the electric power steering column

Today, it is impossible to imagine a car's steering mechanism without an amplifier. And this applies not only to powerful SUVs or business class sedans. Today, even drivers of compact small cars have begun to appreciate the comfort of driving. However, under the bonnet of such cars, there is rarely much room to install a steering rack with hydraulic or even electric power steering. But this is not a problem - you can even place the driver's assistant in the steering column. What are the advantages of such a scheme and how it is implemented, let's look at the example of the popular Volkswagen Lupo 1.2 TDI 3L car produced in 1999-2005.

Functions and benefits of the electric steering column

We would like to point out that an electric steering column cannot be considered only as a compromise option for power steering. This scheme, among other things, has its significant advantages, namely

• monitors input and output signals, as well as the operation of components of other steering components;
• more clearly conveys to the driver the feeling of contact with the road;
• Helps to align the steering wheel in a straight position;
• Enhances steering depending on the actual driving conditions;
• works only when the driver is turning the steering wheel, not consuming energy on straight sections.

The latter factor contributes to the fact that such an electromechanical system is very different in parameters from the traditional hydraulic system. According to the research of Volkswagen specialists, the differences are as follows on the example of the Lupo model 

This is not least because in hydraulic power steering, all the components of its system are installed after the steering driveshaft, while in electromechanical power steering, the power is provided before the driveshaft.

Another significant plus is that the steering amplifier in the speaker is placed in one compact unit, so it does not need long complicated wiring. Finally, and perhaps most importantly for drivers, the power steering motor drives the worm gear right next to the steering wheel, so the driver has a direct feel for the steering wheel, i.e. the same feedback from the road as if there were no power steering.

Structure of the electric power steering column

The main components of the upper part of the steering mechanism in this scheme are shown in the figure

• switches for turn indicators and wiper modes ,
• steering column body,
• worm gear,
• propeller shaft of the steering mechanism.

In addition, very important sensors are installed inside the system: steering position and steering torque.

And this is how the electric steering column looks in the section of the entire front of the car:

Now, let's take a closer look at the components of the steering column, which is equipped with electric power steering. They can be divided into two groups

1. Electric.
2. Mechanical

The first group includes the electronic components of the node:

• electric motor and its clutch;
• electric power steering control unit;
• steering torque and steering wheel position sensors in a common housing.

The second group is used in all, not only electromechanical, speakers. These are

• shafts: steering shaft and universal joint shaft;
• steering column housing, which is height adjustable;
• worm drive with worm and pinion;
• intermediate spindle;
• torsion bar (rod).

This tie rod is the central component of the electrically powered steering column. The torsion bar mechanically connects the intermediate spindle to the worm drive shaft. This allows the two parts to rotate in opposite directions by a small angle. This is sufficient for the system to recognise the start of steering.

The driveshaft consists of two forks connected to a short telescopic arm. It compensates for the length of the height adjustment mechanism and protects the driver and passengers in the event of a frontal collision. If the driver lifts the lever upwards during the adjustment, it becomes shorter, which reduces the distance between the steering wheel and the steering mechanism. If the opposite is true, the telescopic lever lengthens. The adjustment range of the Volkswagen Lupo is 39 mm.

The worm gear is housed in an aluminium gearbox housing together with the electric motor. The worm on the motor shaft engages with a gear on the steering shaft, allowing the steering wheel to be stepped up directly almost where the driver does. The motor shaft is connected to the worm shaft by means of a flexible rubber coupling so that the starting torque of the motor is transmitted as gently as possible. The electric motor has a power of 720 W and develops a torque of 2 Nm. The electronics are designed so that it has an extremely short response time to respond to steering wheel movements as quickly as possible.

Above the gear is a housing for steering wheel position and steering angle sensors. The first one is connected to the worm gear shaft. It registers the current position of the steering wheel or its blockage. The second is connected to the torsion beam. It calculates the angle of rotation of the torsion bar in relation to the intermediate spindle. The control unit calculates the torque based on this signal. If the calculated torque exceeds a value of 0.01 Nm, the control unit decides that power steering is required.

An electronic power steering control unit is also bolted to the worm drive body. It calculates the necessary assistance to the driver based on the data received from the sensors and taking into account the vehicle's speed. The sensor housing is wired to the control unit via a 6-pin connector. If the control unit detects a malfunction in the power steering system, it switches on the warning lamp in the instrument cluster.

How is electromechanical power steering amplified

The driver starts to turn the steering wheel. At the same time, the torsion bar rotates. The torque sensor, which rotates with the torsion bar, sends signals to the control unit indicating the magnitude and direction of rotation of the steering torque. Based on these signals, the control unit calculates the required assistance and activates the electric motor. The sum of the torque acting on the steering wheel and the torque assist is the effective torque acting on the steering gear.

If the driver increases the torque applied to the steering wheel, the electric motor increases the torque. If he reduces it, the torsion bar torsion decreases. As a result, the steering wheel torque sensor sends a smaller signal to the control unit.

When the driver releases the steering wheel while cornering, the tension on the torsion bar is released. At the same time, the electronics deactivate the electric motor because torque assistance is no longer required. However, if the steering position sensor detects that the vehicle is still not moving straight, the electric motor is activated so that the steering wheel is actively returned to a straight position.

Repair of the steering column with electric power steering

Although the described construction is quite reliable, time and active use often lead to malfunctions. The parts most likely to fail are:

The plastic rings of the sensors;

the sensors themselves;

driveshaft crosses.

Although the electric steering column is less vulnerable than other types of power steering because it is located inside the cabin, it too can suffer from corrosion. Sensor and control unit contacts and even the splined joints of mechanical parts can oxidise.