What is servotronic and its principle of operation

Servotronic EGC: what is it and how does it work?

Electric power steering Servotronic is an element of car steering, which creates additional force during rotation of the steering wheel by the driver. In fact, electric power steering (EHP) is an advanced power steering system. Electric power steering has an improved design, as well as a higher level of comfort when driving at any speed. Let’s consider the principle of operation, the basic components, and the advantages of this element of steering control.

Design and operation of Servotronic

The Servotronic system is divided into three main segments: informing, calculating and executing. Each of these consists of certain nodes and components. The data acquisition segment consists of a number of sensors:

• power steering (steering wheel angle);
• Hall sensor on the crankshaft;
• speedometer;

Servotronic sensors collect information about the speed of the car, the angle of rotation of the steering wheel in relation to the wheels and other parameters of the car. The collected data goes to the computing unit, where it is interpreted into instructions for the actuators.

The actuators segment includes a chamber with a reverse piston. The chamber contains a solenoid valve, controlled by signals from the evaluation unit. The valve and piston connected to the power steering spool and perform the main functions of the system.

When the steering wheel is turned, the spool opens and allows hydraulic oil to flow into the power steering cylinder. At the same time, the solenoid valve receives data interpreted by the calculation unit and fills the return chamber.

As the pressure in the power steering cylinder is reduced, the piston in the return chamber, in turn, blocks the spool. The force on the steering wheel increases along with the steering comfort.

In the case of the electric power steering, the signals from the calculation unit are fed to the servo actuator, which is connected to the steering rack via a planetary unit. The mechanical connection between the steering rack and the rack is then retained. In the event of a failure, the servo actuator is blocked and the ability to steer the vehicle is retained.

During straight driving, hydraulic oil circulates in the main line between the steering rack pump and the storage tank. Turning the steering wheel serves as a signal to change the circulation path. Depending on the side of the turn, the fluid flows into one of the power cylinder chambers. From the opposite chamber it is sent to the storage vessel. The result is a pressure difference which transmits the force to the steering rack. The latter in its turn presses on the steering rods and the steering occurs.

The efficiency of hydraulic power steering is the highest at low speeds, for example, when maneuvering in the city or parking. This is due to the inverse relationship between the speed of the hydraulic pump and the speed of the vehicle. The lower the latter, the higher the former.

SAFETY

Even in the event of a failure of the vehicle’s electrical system or the failure of some of its components, the steering drive remains fully functional. In an emergency situation the Servotronic 2 functions by means of a forced mechanical opening of the converter valve with maximum hydraulic countermeasure (characteristic at high driving speed).

In case of an unexpected lack of signals during driving, e.g. if the contact is broken or the tachometer fails, the high-performance microprocessor in the electronic control unit is able to calculate the flow rate on the basis of the last signals received. This ensures error-free steering until the engine is switched off. The next time the engine is started, maximum hydraulic resistance is ensured in accordance with the characteristic at high speed.

SPECIAL EQUIPMENT

In order to optimize the performance of the steering system and adapt it to different kinematic conditions and different powertrains, the Servotronic 2 can be equipped with a range of optional features.

VARIABLE RATIOS

In addition to a constant gear ratio, the Servotronic 2 can have a variable gear ratio. The teeth of the rack can vary in shape and angle of engagement. This means that, on the one hand, the reaction of the steering wheel remains the same when driving straight ahead. On the other hand, the transmission ratio decreases and the steering becomes more active when the steering wheel is turned a significant angle (to the right and to the left). The difference between the maximum and minimum ratios should not exceed 35%. This results in an extremely low ratio of steering rotation (up to two full turns) and rack gearing movement.

Servotronic 2 with variable ratios is used to equip midsize cars, light trucks as well as sports cars. The system provides accurate, responsive steering at high speeds, eliminates the risk of “over-steering” and ensures optimum control when parking and turning in tight road sections and safe passage through tight corners.

END-OF-STROKE DAMPING

If deemed necessary by vehicle manufacturers, the Servotronic 2 system may have polymer elements as part of the end-of-stroke dampers. The operation of the damper before the end of stroke is felt in the form of noise at maximum wheel spin.

Advantages and disadvantages

Firstly about the advantages of ESC R: compact design; comfortable driving; operation with disconnected / not working engine; easy maneuvering at low speeds; accurate control at high speeds; economy, reduced fuel consumption (turns on at the right moment).

Disadvantages: risk of EGM failure due to delay of wheels in extreme position for a long time (oil overheating); reduced information capacity of steering at high speeds; higher cost. Servotronic is a trademark of the AM General Corp. Servotronic EHUR can be found on cars of such companies as: BMW, Audi, Volkswagen, Volvo, Seat, Porsche. Servotronic electrohydraulic power steering certainly makes life easier for the driver, making automobile trips more comfortable and safer.

Design and principle of operation of Servotronic

Electric power steering Servotronic is an element of car steering, which creates additional force during rotation of the steering wheel by the driver. In fact, electric power steering (EHP) is an advanced power steering system. Electric power steering has an improved design, as well as a higher level of comfort when driving at any speed. Let’s consider the principle of operation, the basic components, and the advantages of this element of steering control.

Operating principle of Servotronic EGUR

The principle of operation of the electrohydraulic booster is similar to that of the hydraulic power steering. The main difference is that here the power steering pump is driven by an electric motor, not the internal combustion engine.

If the car is moving straight ahead (the steering wheel is not turned), the fluid in the system simply circulates from the power steering pump to the tank and back. When the driver turns the steering wheel, the circulation of the fluid stops. Depending on the direction of steering, it fills a certain cavity in the power cylinder. The fluid from the opposite cavity flows into the reservoir. The working fluid then presses the steering rack via the piston and the effort is transferred to the steering linkage and the wheels are turned.

The hydraulic power steering functions at its best at low speed (tight turns, parking). At this point, the electric motor rotates faster and the power steering pump works more efficiently. The driver does not need to apply much effort when turning the steering wheel. The higher the speed of the car, the slower the electric motor runs.

Construction and main components

The Servotronic electronic hydraulic control system has three main components: an electronic control system, a pump unit and a hydraulic control unit.

The pumping assembly of the electrohydraulic booster consists of a reservoir for the working fluid, a hydraulic pump and an electric motor for it. This component is equipped with an electronic control unit (ECU). It should be noted that there are two types of electric pump: gear pump and paddle pump. The first type of pump is characterized by simplicity and reliability.

The hydraulic control unit includes a power cylinder with a piston and a torsion (torsion bar) with a distribution sleeve and spool. This component is integrated with the steering mechanism. The hydraulic unit is the actuator for the booster.

Servotronic electronic control system:

• The input sensors are speed sensor, torque sensor on the steering wheel. If the car is equipped with ESP, a steering angle sensor is used. The system also analyzes the engine crankshaft speed data.
• Electronic control unit. The ECU processes the signals from the sensors, and after analyzing them, sends a command to the actuator.
• Actuator. Depending on the type of electro-hydraulic booster the actuating device may be an electric motor of pump or an electromagnetic valve in the hydraulic system. If an electric motor is installed, the performance of the booster depends on the capacity of the motor. If a solenoid valve is installed, the capacity of the system depends on the size of the flow area.

Differences from other types of amplifiers

As noted earlier, unlike conventional power steering, the Servotronic has an electric motor that drives the pump (or other actuator – electromagnetic valve), as well as an electronic control system. These constructive differences allow the electro-hydraulic booster to regulate the power depending on the speed of the car. This provides comfortable and safe driving at any speed.

We should separately mention the ease of maneuvering at low speeds, which is inaccessible to conventional power steering. At higher speeds, the level of amplification decreases, which allows the driver to control the car more accurately.

Advantages and disadvantages

First about the advantages of the EGUR:

• compact design;
• convenience of driving;
• Functioning when the engine is off/not running;
• ease of maneuvering at low speeds;
• Precise control at high speeds;
• Economy, reduction of fuel consumption (activated at the right moment).

• Risk of EGUR failure due to delay of wheels in extreme position for a long time (oil overheating);
• Reduced information capacity of steering wheel at high speeds;
• higher cost.

Servotronic is a trademark of the AM General Corp. Servotronic EHUR can be found on cars of such companies as: BMW, Audi, Volkswagen, Volvo, Seat, Porsche. Servotronic electrohydraulic power steering certainly makes life easier for the driver, making automobile trips more comfortable and safer.