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Issue 115

Electric Power Steering Systems: The now and the future

Liviu-Vasile Pop
Embedded Software Engineer @ Bosch Engineering Center Cluj



Mihai Souca
Embedded Software Engineer @ Bosch Engineering Center Cluj



Rudolf Mate
Embedded Software Engineer @ Bosch Engineering Center Cluj
PROGRAMMING

When someone thinks of buying a car, the first aspect taken into consideration is the body. Some want an SUV that will take them to the highest mountain peaks, some want a small car that can get through the smallest streets and fit into the tightest parking spots. Then, the next important aspects are the features that the car has: heated seats, parking sensors, cruise control and so on... but these are things that you can check online, reading a spec sheet. The aspect that really makes you buy the car is getting in the driver's seat and taking it for a drive. And here, we all must agree that the steering system plays a key role. Nowadays, most cars use an Electric Power Steering (EPS) that facilitates the driver in doing the steering maneuvers.

The Electric Power Steering (EPS) controls and assists the steering process with the support of an intelligent electric motor. Based on the steering signal from the torque sensor, the control unit calculates the optimal steering support and sends the information to the electric motor to provide the necessary assistance.

The EPS variants cover all requirements for the passenger car classes and even light commercial vehicles. Depending on the variant, the motor torque is transferred in various ways.

Figure 1 Electric Power Steering system types

The Electric Power Steering with servo unit on the steering column (EPSc) controls and assists the steering for vehicles up to the lower mid-size class, while the EPS Single Pinion Servo Unit (EPSp) can cover the mid-sized class. The system with electric motor mounted on a second pinion (EPSdp) can assist vehicles from compact class to SUVs and the axis-parallel system with belt drive (EPSapa) can go all the way to sport cars and light commercial vehicles.

The now

Automated driving with electric power steering

As the technology evolves, the idea of automating various processes gets more prominent. We can see this trend in all fields of engineering and far beyond. Tasks that were usually done by humans are now done by a machine, which is automated in such way that it gets maximum efficiency at the lowest risk. However, the level of independence that the machine has varies depending on the complexity of the task which it handles. This idea can be also transferred to the automotive world, where we already can see functionalities like Automated Parking, in which the car takes parts of the decisions on the maneuvers that it executes in order to park. This degree of freedom in decisions that a car system has is called "Driving Automation Level", where 0 is the lowest (fully manual) and 5 is the highest (fully autonomous).

The higher you go with the automation level, the higher the safety levels that the car should achieve. Considering that higher levels of automations imply steering with no driver action, the safety level of a car is achieved by a combination of software and hardware which offers redundancy and low FIT (Failure In Time) rates.

Variants of the Steering Control Unit (SCU) - solutions for every requirement

Figure 2 Society of Automotive Engineers (SAE) defines 6 levels of driving automation ranging from 0 (fully manual) to 5 (fully autonomous). This figure depicts only the levels that have automation.

The Steering Control Unit Gen3 B3 is suitable for the use of intelligent driver assistance functions up to SAE level 2. This basic variant of the Steering Control Unit is characterized by high reliability.

The Steering Control Unit Gen3 C3/C6 is suitable for the use of intelligent driver assistance functions up to SAE level 2+. In the unlikely event of a failure in the electronics, a second, completely independent back-up electronic circuit takes over the steering functionality of this Steering Control Unit variant. Variants C3 & C6 differ mainly in the use of a 3 and 6 phase motor.

The Steering Control Unit Gen3 D12 is suitable for the use of intelligent driver assistance functions up to SAE level 4. In the Steering Control Unit variant for automated driving up to SAE level 4, a second, completely independent electronic circuit takes over the steering functionality in the unlikely event of a failure in the electronics. This provides optimum assurance of controllability in the event of a failure. At least 50% of the steering assistance will be retained.

Figure SCU D12 (left) compared with SCU C6 (right)

The future

The technology never stops developing, new solutions and concepts arise more and more often in all areas. The automotive industry was always a bit slower compared to other industries, for example, smartphones, but this does not mean that new concepts are not developed behind the scene. In this industry, as mentioned before, safety is more important than any futuristic functionality, therefore brand-new cars are still using five to ten years old technology, because it delivers the wanted level of safety.
As the automotive industry progresses in the direction of "electrical mobility", the way of approaching a car design and production is quite different from the one that we use today. Making space for batteries, removing some weight from the chassis are some of the challenges that lay ahead of automotive companies.

In order to see where the EPS will suffer changes, we shall take a look at the actual construction and working principle. In the next figure, the most important components of this system can be seen.

Figure 4 Abstract illustration of an EPS

Most of the concepts, which describe how the vehicles of the future can

look like, show us a car without a steering wheel or with a steering wheel that is hidden (if not used).

This will be possible only with a steer-by-wire system. By removing the mechanical

linkage between the wheels and the steering wheel, new possibilities regarding design and autonomous driving will be available. The disappearance of the steering column will allow easier packaging, but the main advantage for the end-user will be comfort. In a future car, the driver will be able to become a passenger without any discomfort, and there will be no moving steering wheel in autonomous mode.

The biggest issue with any drive-by-wire system is its safety. A mechanical component is much more reliable than a cable which is transferring some data. Another challenge is to find a way to give proper feedback to the driver. Considering that there will be no mechanical link between the EPS and the steering wheel (through which feedback from the wheels will be transferred to the driver), having a drive-by-wire system would feel like playing a video game. In this scenario, finding a proper way to ensure the safety of the passengers and give proper road feedback can be quite difficult.

By solving the safety and feedback issues, this system would have a lot of advantages. If the steering column can be removed, then vehicle designers will have much more freedom in designing the front part of the car. Functionalities, like variable steering ratio, different feedback modes depending on the driving situation will be easily implemented.

Autonomous driving could offer a totally different experience since the steering wheel does not necessarily have to move if the car is driving on its own. The owner of the car could become a passenger without any compromise regarding comfort. Moreover, the steering wheel could be used for entertainment.

A steer-by-wire system has two main components, the steering rack subsystem and the element that is operated by the driver, which is called the steering wheel subsystem. This can be a joystick, a touch-pad or, in the next generation of the steering systems, it will still be a steering wheel with an actuator. In the next figure we can see the most common and important components of a steer-by-wire system. The biggest difference, compared to a classic steering system, is that there is no steering column. The steering wheel is not directly linked to the steering rack.

Figure 5 Abstract illustration of a Steer-by-Wire system

The most important part of the steering wheel subsystem is the electric motor which is responsible for driver feedback. Basically, without this actuator, the driver would not be able to drive the car safely. Usually, this component is linked to the steering wheel via a transmission. To be able to steer the wheels in accordance with the steering wheel, a steering angle sensor is required as well - this is usually mounted directly on the steering wheel.

The steering rack subsystem is really similar to a traditional steering rack. In the case of this system, the purpose of the electric motor is not to assist the driver, but rather to steer the car by itself, based on the received commands. This motor is actuating the steering rack via a transmission. To be able to determine whether the rack is in the desired position, a rack position sensor is also necessary.

The control of the whole system is usually done by one central ECU (Electric Control Unit). Based on the measured data and the data received from other ECUs, this central controller gives the command for the two actuators. Its purpose is to make sure that the driver has the correct feeling of the car and the wheels of the car are at the correct position, so that the car will go in the correct direction.

Conclusion

Nowadays, the world changes at a fast pace. We see new technologies being developed faster, automation levels for factories getting higher, phones becoming smarter, cities becoming smarter, and ultimately cars becoming smarter. To keep up with the pace, the automotive industry is being challenged to develop safer cars, which also means safer parts. In this process, the EPS plays an important role. The SCU (Steering Control Unit) is now challenged with obtaining higher automation levels, this implying redundancy, and lower FIT rates. This change comes both from the design of the hardware used and from the software. In the context of steer-by-wire, new functionalities need to be developed to properly synchronize the system components, provide a proper feedback to the driver, and also ensure end-to-end safety. As observed, ECUs play the main role in the big change to "electrical mobility". Making them smarter, faster, and more durable paves the way to assisted and automated driving. This means that the new generation electric power steering systems will become the key technology for automated driving.

Sources:

[1]G. Reimann, P. Brenner, and H. Buring, Steering Actuator Systems. Springer International, Publishing Switzerland 2015.

[2] M. Harrer and P. Pfeffer, Steering Handbook. Springer International Publishing, 2017.

[3] R. Mate, Control concept for steer-by-wire systems, 2021

[4] BOSCH Mobility solutions [ONLINE], Available: https://www.bosch-mobility-solutions.com

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VIDEO: ISSUE 109 LAUNCH EVENT