Automotive industry analysts in Europe and North America predict that higher adoption of electric power steering (EPS), steer-by-wire (SbW) and rear-axle steering / all-wheel-steering (AWS) will be some of the biggest drivers in the fast-evolving automotive steering systems vertical. For the period 2017-2020, a compounded annual growth rate of 8-10 % is predicted for the global steering systems market, which will see major developments coming in due to new requirements specified by electric vehicles and partially/fully autonomous vehicles. Here, we take a look at some technologies that we believe will drive development in the automotive steering systems space over the next few years.
ELECTRIC POWER STEERING
Over the last few years, OEMs have increasingly moved from hydraulic to electric power steering systems, in the interests of reduced cost and complexity, and increased efficiency. Hydraulic systems use a pump that’s powered by the car’s engine, with a control valve varying the amount of pressure (depending on steering input and vehicle speed) generated by the pump. Hydraulics are pretty well suited to amplifying steering effort, hence making it easier for the driver to manoeuvre the vehicle in heavy traffic, at slow speeds, or while parking the vehicle. Tuned properly, hydraulic systems a capable of providing optimum ‘feel’ and steering feedback, but they do sap some amount of power from the engine and can hurt performance as well as fuel economy.
To overcome the disadvantages of hydraulic systems, OEMs have now increasingly started moving to electric power steering systems, which use electric motors rather than a hydraulic pump to provide steering assistance. Electronic sensors calculate the amount of steering force being applied by the driver and using electric motors (that may be placed on the steering rack or the steering column), that force is amplified to make steering easier. Most EPS systems are speed-sensitive, providing more assistance at lower speeds for easier manoeuvrability, and scaled-back assistance at higher speeds, for better stability.
Another major advantage offered by electric power assist systems is full compatibility with ‘driving modes’ that are increasingly being offered by OEMs, across all vehicle segments. So, for example, EPS systems can provide gentler steering response in ‘Comfort’ mode and sharper, more aggressive response in ‘Sports’ mode – something that would be difficult to achieve with older hydraulic-based systems.
On the other hand, EPS systems can be devoid of ‘feel’ or steering feedback, which enthusiast drivers often demand, especially on performance-oriented cars. While the hydraulic fluid used by older systems did provide better feel, electric motors help boost efficiency, since they do not use power from the engine (instead, they get their power from an alternator) and can hence even help with improving fuel economy by up to 2-3 %. Also, with some years of development and refinement, some EPS systems can now rival hydraulic systems for steering feedback as well. Older EPS systems only took into account steering wheel movement initiated by the driver, along with torque sensors taking data from the front wheels. Some newer, more advanced systems on the other hand use a complex set of data streams coming from yaw and pitch sensors, as well as data from the car’s ABS and traction control systems. The system uses this data in conjunction with the driving mode selected, to provide optimum steering response at all times, in all road and weather conditions.
As mentioned earlier, reduced cost, weight and complexity are also some of the advantages offered by electric power assist steering systems, since these do not require any hydraulic piping and are able to provide variable assistance by simply varying the amount of charge created by the electric motors. EPS systems can also potentially help increase safety, since these can be fully integrated with a vehicle’s other electronics systems and driver aids (ABS, traction control, stability control and many others), and can react faster when they detect understeer/oversteer, or even an imminent roll-over in case of extreme steering inputs. Additionally, EPS systems can work with automotive radar/LIDAR for lane assist systems, autonomous parallel parking and, in the future, navigation systems for autonomous cars.
The global automotive power steering systems market registered revenues of more than $ 17 bn in 2015 and is expected to cross $ 40 bn by 2020, with an estimated annual growth rate of close to 16 %. EPS systems already comprise about 50 % of all automotive steering systems currently being used, and their reach is expected to go up to 70-80 % by the year 2025, due to the advantages they offer in terms of improved fuel economy, reduced emissions, better integration with a vehicle’s suite of electronics and higher compatibility with the new 48 V electrics currently being developed for automotive usage.
With the focus on replacing mechanical linkages with electronic ones, and with drive-by-wire (which entails full microprocessor-controlled throttle management) already fast becoming the norm across most vehicle segments, it’s only a matter of time before by-wire functionality comes to steering systems as well. SbW systems do away with mechanical/physical linkages between the steering wheel and the front wheels, using electric signals instead, which control actuators that actually steer the vehicle. In the future, SbW systems are expected to provide more flexibility in terms of altering the steering ratio on-the-fly (useful for variable-rate slow- and high-speed steering response, or for altering steering response when moving from tarmac to an off-road driving environment), and when autonomous vehicles become a reality, even steering wheel placement, since there is no steering-related mechanical paraphernalia to connect to.
Since steering is a ‘mission critical’ function, where failure of an electrical/electronic system can potentially cause serious harm to a vehicle’s occupants, OEMs have by and large restricted the use of SbW on prototypes and concept vehicles only. One notable deviation from the norm comes from Japanese luxury-car manufacturer, Infiniti, which uses SbW on the current Q50 luxury sedan. Infiniti’s ‘direct adaptive steering’ SbW system has no physical linkages between the steering wheel and the steering rack, but the system has been heavily criticised in the media, for its complete lack of feel and steering feedback.
However, while current SbW steering systems may not be fully competent yet, OEMs and tier-1 technology suppliers continue to work on developing these systems, as in the future they are expected to contribute to weight reduction, better compliance with semi- and fully-autonomous systems and significant improvements in safety. In the context of the latter, to take just one example, SbW can help by doing away with the steering rod, which can intrude into the passenger compartment in the case of a head-on collision.
Another significant element that is likely to encourage increased SbW adoption, is the ease of integration with a car’s other on-board electronics and driver-aids like traction and stability control systems. Steering movement and response is critical to the functioning of these systems, and since SbW is fully computer-controlled, it has the potential to be safer, since it removes or at least minimises the element of human error or oversight.
SbW will be a critical element of fully autonomous, self-driving cars of the future, and European analysts estimate that the market for SbW systems will grow at an annual rate of close to 30 % from the year 2020.
REAR WHEEL STEERING
While the steering function has traditionally been restricted to the front wheels only, some OEMs have experimented with four-wheel-steering (4WS) or all-wheel-steering (AWS) systems over the last few decades, with mixed results. Mitsubishi used 4WS on its Gallant VR-4 Turbo in the late-1980s, Honda used 4WS on the 1980s Prelude, and Nissan used its HICAS (High Capacity Actively Controlled Steering) rear-wheel-steering system on the 300ZX and Skyline GT-R in the late-1980s and early-1990s.
Systems that utilise the rear wheels as well (in addition to the front wheels), for steering, can help reduce the turning circle at low speeds, thereby making the vehicle more easily manoeuvrable, and at the same time, they can also help increase vehicle stability at high speeds. Of course, the degree of movement available for the rear wheels is much smaller than that for the front wheels, which continue to handle most of the steering function.
At low speeds, 4WS/AWS systems make the rear wheels turn (only by a few degrees) in the direction that’s opposite to the direction in which the front wheels are turning, as this helps with low-speed manoeuvres, while at higher speeds, the rear wheels turn in the same direction as the front wheels, which promotes stability.
While OEMs did experiment with 4WS/AWS systems in the 1980s and 1990s, these systems fell out of favour after that due to their higher cost and complexity. Now, however, due to the dynamic advantages that they offer, such systems are once again being developed by leading tier-1 technology suppliers (not just for cars but also for commercial vehicles) and one can find 4WS on some high-end cars produced by Ferrari, Porsche, BMW and a few others.
In each application case, electronic sensors that monitor various parameters (steering wheel movement initiated by the driver, vehicle speed, gear position, suspension movement, pitch and yaw data etc.) decide the degree to which the rear wheels move, in accordance with the front wheels, for best possible handling. Consequently, all-wheel-steering systems have the potential to boost safety, since they increase the effectiveness of anti-lock braking and traction control systems. At higher speeds, especially, AWS systems provide inherently higher levels of traction to begin with, which positively affects a vehicle’s handling characteristics. Furthermore, in the case of emergency braking or steering, with increased and/or sudden driver inputs, the position of the wheels can help ABS and TC systems do their job better. Going forward, recognising the potential improvements in handling and safety that AWS systems can bring to their cars, more OEMs are expected to use such systems, especially on high-end vehicles
TEXT: Sameer Kumar