EPS (Electronic Power Steering) is not exactly a new feature on modern vehicles, and all of us have encountered these systems in their various forms, all of which have until comparatively recently, only been capable of providing steering assistance in much the same limited way that old-style hydraulic systems did.
By way of contrast, modern iterations of EPS systems have advanced capabilities that greatly improve the general driving experience. However, from our perspective as mechanics and technicians, the advanced functionalities and safety features that modern EPS systems offer come at the price of greatly increased complexity that includes several layers of redundancies, or back-ups to back-ups to prevent steering system failures. So, if you have not yet encountered a modern EPS system, this article will provide a general overview of what modern EPS systems are capable of, as well as how these systems work. Before we get to specifics though, let us look at-
As with many innovations and advances in automotive technologies, the main reason why hydraulic power steering systems have largely been abandoned by car manufacturers has much more to do with regulatory requirements to reduce harmful exhaust emissions than with a desire by manufacturers to improve driving safety. In the case of improved EPS systems, however, the upside is that these systems do improve driving safety while reducing exhaust emissions at the same time, so the increased complexity that comes with improved EPS systems is perhaps a fair trade-off.
So, what makes modern EPS systems “better” than both older iterations of EPS and/or old-style hydraulic power steering systems? Let us look at some of the advantages of modern EPS systems in detail-
Reduced parasitic power losses
Although hydraulic power steering systems have been shown to absorb (on average) about 5 to 8 kW of an engine's power output during tests in laboratory settings, it is difficult to quantify actual power losses under real-world driving conditions, especially on small displacement engines. Nonetheless, the combination of pumping fluid through a steering system, and the friction of a drive belt on pulleys produces measurable parasitic power losses, which have a much greater negative effect on the fuel economy of small engines than on larger, more powerful engines.
So, since the drag of a hydraulic power steering system has a proportionally greater effect on the fuel economy of small engines, small engines have to burn proportionally larger volumes of fuel to overcome this drag than larger and more powerful engines would have to. Moreover, since the general trend is now to downsize engines, the overall effect of large parasitic power losses on millions of vehicles with small engines is that collectively, these vehicles burn many millions of litres of fuel just to drive hydraulic power steering pumps.
Reduced NVH
While most hydraulic power steering systems operate relatively silently and with no noticeable vibrations, the lack of noise and vibration in passenger compartments at high engine and road speeds is largely due only to the vast amounts of sound-absorbing material and vibration mitigation measures new(ish) vehicles contain.
Some modern vehicles contain up to 80 or so kilograms of sound-absorbing material, so by replacing noisy hydraulic power steering systems with silent electronic power steering systems it is possible to reduce the weight of a vehicle significantly, which translates into improved fuel economy and reduced emissions.
Reduced maintenance costs
All of us have encountered fluid leaks in hydraulic power systems, and while some leaks are as easy to fix as replacing a hose clamp, most of us have replaced hugely expensive hydraulic power steering racks because fixing a leaking steering rack is often more expensive than replacing the defective unit with a professionally rebuilt replacement.
By way of contrast, electric and electronic power steering systems hardly ever fail, and when they do, the failure rarely involves the steering gear itself. Most EPS system defects result from the failures of associated sensors or wiring, which is (almost) never quite as expensive to repair as replacing a leaking hydraulic steering rack or pump.
The above covers the main advantages that electric power steering systems have over old-style hydraulic power steering systems, but the biggest difference between electric and hydraulic power steering systems and modern electronic power steering systems involve the extremely high levels of integration between modern power steering systems and multiple ADAS systems.
As a practical matter, and based on how deeply modern power steering systems are integrated into several modern ADAS systems, it would not be unreasonable to view modern power steering systems as a new addition to the wide range of ADAS systems that are now in common use even on basic entry level vehicles. Having said that, let us take a look at some of the-
When we talk about the capabilities of modern power steering systems, it is important to distinguish between electric systems that merely provide steering assistance at low road speeds, and modern electronically controlled power steering systems that apart from providing on-demand steering assistance, can also assume or exercise autonomous control of some vehicles’ steering in some situations, albeit for only brief periods.
In terms of practicalities, the above is not the same as true autonomous steering capability such as is currently available in several vehicle models that are equipped, if not always allowed, to drive themselves without human intervention on public roads. Nonetheless, the capabilities of modern electronic power steering systems far exceed those of even their most advanced predecessors, so let us look at some of these capabilities in some detail-
Driver attentiveness monitoring
Although systems that monitor the attentiveness level of the driver have been in use for several years, these systems mainly relied on cameras and software that analyse a driver's eye movements, or in more advanced designs, on biometric systems that monitor a driver's breathing and heart rates. Other systems use facial recognition software that can detect the relaxation of a driver's facial muscles as a function or result of decreasing attentiveness or alertness.
While these systems are generally effective, they do not use or monitor steering inputs as a measure of how alert (or otherwise) a driver is. In practice, steering a vehicle even in a seemingly straight line involves a series of corrective steering inputs to counter the effects of crosswinds, changes in road camber, and other factors that act perpendicularly or tangentially to the vehicle's direction of movement.
Put differently, this means that a vehicle does not move forward (even on straight roads) in a dead straight line; what a driver thinks of as a straight line is actually a series of small zigzag movements whose combined effects keep a vehicle roughly centred in a driving lane. Of course, the magnitude and frequency of said zigzag movements vary based on driving conditions, the road surface and camber, road speed, and the driver’s skill level. Most importantly, though, the magnitude of deviations from straight-line movement depends on the vehicle’s mechanical condition, speed, and weight, as well as on the condition and inflation pressure of the tyres, in about equal measures. So what does this have to do with electronic power steering systems?
Simply this; many modern electronic power steering systems actively monitor all steering inputs that deviate from the straight-ahead position via both the steering angle and steering torque sensors, both of which are referenced to the vehicle’s thrust line or longitudinal axis. This data is then compared to inputs from the yaw rate sensors, accelerometers, cameras, microphones, and radar/lidar transponders that support ADAS systems such as Stability Control, Lane Departure Warning/Lane Keep Assist, Proximity Warning, Adaptive Cruise Control, Automatic Headlight Control, Forward Accident Mitigation, and others, such as Automatic/Autonomous Emergency Braking.
Note that not all of the ADAS systems mentioned above are necessarily involved in driver attentiveness monitoring via the electronic power steering system on all modern vehicles. However, the purpose of comparing input data from steering angle and torque sensors with whatever relevant ADAS systems are fitted to any given vehicle is to confirm that the vehicle is moving ahead in a) a relatively straight line while the vehicle is under the driver's control, or b) is being actively steered around bends in the road by an attentive driver.
As a practical matter then, one or more control modules use extremely complex algorithms to process a vast amount of input data to detect whether (or not) a driver is actively steering the vehicle based on the number, magnitude, and frequency of corrective steering inputs. For example, if a driver is not holding the steering wheel, or is not attentive enough to keep the vehicle centred in a driving lane, there will be no corrective steering inputs, which the power steering system will interpret as a driver being inattentive. In some vehicles, the power steering system will work in conjunction with systems like (among others), the Stability Control and Lane Departure Warning systems to steer the vehicle back towards the centre of the driving lane while sounding various types of alarms and warnings to alert the driver to a potentially dangerous situation.
On some advanced systems, the vehicle might be slowed or even brought to a complete stop autonomously if the driver does not react to warnings/alarms. This could also happen in other situations, but the point is that the electronic power steering system (as opposed to any other system) on many vehicles now can assume control over a vehicle's steering under strictly defined conditions.
One other example of autonomous steering control on some Honda vehicles involves a kind of fuzzy logic that allows the power steering system on equipped vehicles to make it almost impossible for a driver to turn the steering wheel in the wrong direction. For instance, if an equipped vehicle enters a slide, skid, or a similar dangerous or potentially uncontrollable situation, the power steering system will largely prevent the steering wheel from being turned in a direction that will make the situation worse, by reducing or eliminating steering assistance in that direction.
While this particular functionality is currently only available on some Honda models, similar steering capabilities will no doubt become a standard (and possibly legally mandated) feature on all new vehicles in the next few years, which brings us to-
Enhanced park assist capability
Park assist systems have been a rather common feature on many, if not most high-end vehicles for many years. In recent years, though, this system has started to appear even in entry-level models, and while the ability of a vehicle to park itself is a nice feature to have, park assist systems have typically only been able to park vehicles in spots where vehicles are arranged nose-to-tail. Moreover, previous iterations of park assist systems have also been unable to drive or steer a vehicle out of a parking spot.
Some modern versions of electronic power steering, however, can not only park a vehicle nose-to-tail with other vehicles, but also diagonally between other diagonally parked vehicles, or parallel between other vehicles that are parked in for, instance, a shopping mall parking lot. While this is an even nicer feature to have, the most noteworthy aspect of this feature is the fact that equipped vehicles can also drive themselves out of parking spots.
This is an important point, since, in most parking situations, both the front and rear of a vehicle move in opposite directions when it leaves a parking spot than when it entered the parking spot, so essentially when an equipped vehicle drives itself out of a parking spot, the Park Assist system has to work in reverse. In addition, other systems such as, among others, Blind Spot Monitoring, Proximity Warning, Rear View Monitoring, and Automatic/Autonomous Braking systems also have to recognise that the Park Assist system is now working in reverse without raising conflicts in one or more implicated control modules or communications networks.
The finer technical details of how all of the above is accomplished fall outside the scope of this article, but suffice it to say that it is only possible because of recent advances in computing technology that can now, unlike previous control strategies, exert precise control over modern power steering systems in real-time. Note also that at present, power steering systems that can drive vehicles out of parking spots are only available on high-end to ultra-high-end vehicles.
Nonetheless, like power steering systems that can monitor a driver’s level of attentiveness based on steering inputs, power steering systems that can drive vehicles out of parking spots will no doubt also become available in lesser vehicles over the next few years, which begs this question-
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Regarding basic operating principles, there is not much difference between how older iterations of electric power steering systems work(ed) and how modern systems function. Both versions use steering angle and steering torque sensors to monitor steering inputs, although modern power steering systems use two steering angle sensors and two steering torque sensors, with one of each acting as a backup to primary sensors to prevent steering system failures should a steering torque or angle sensor fail or malfunction.
However, there are some significant differences between the electric motors of older electric power steering systems and those of modern versions. There are also major differences in control and self-diagnostic functions between older and modern power steering systems, so let us look at some of these differences, starting with-
Electric motor design
Regardless of whether the motor is mounted on the steering column inside the vehicle or directly on the steering rack outside the passenger compartment, all versions of modern electronic power steering systems now use permanent 3-phase DC brushless motors that are fully reversible. In this context, “fully reversible” means that a) these motors develop the same torque in both directions of rotation, and b), the torque the motor develops is not affected by the motor’s direction of rotation. Note also that depending on the application, modern power steering system motors do not necessarily have operating voltages of 12V: while 12V motors are common, it is also common to see power steering motors that operate on voltages ranging from as low as 9V to as high as 16V delivered through PMM (Pulse Width Modulated) signal inputs.
As a practical matter, the biggest advantage of 3-phase DC motors over other types [of motors] is that the motors' torque is not affected by the rate of steering inputs. Put differently, this means that neither steering torque, nor steering response times are affected when the motor is rotating at low speeds, such as might happen when a driver turns the steering wheel slowly.
Moreover, since both steering torque and motor response times remain constant, modern power steering systems can both detect, and compensate for normal wear and tear that may affect the free movement of steering and suspension system components such as ball joints and/or tie rod ends. If such wear and/or binding is detected, the power steering control module will increase steering assistance to preserve the original “feel” of the steering system when it was new. In addition, modern electric power steering systems can also detect and compensate for steering pull caused by underinflated tyres based on inputs from wheel speed sensors, variations in the road's camber based on inputs from various suspension-mounted sensors, as well as for the effects of crosswinds based on inputs from both yaw sensors and wheel speed sensors.
One other noteworthy innovation involves the ability that some electronic power steering systems have to override the steering system’s motor to exceed its learned stop positions. For example, on some 2022 model year GM products, this allows the steering rack to move past its learned stop positions to increase the steering rack’s travel in both directions, which makes it possible to reduce the vehicles’ turning circle significantly in tight spots like overcrowded shopping mall parking lots, which brings us to-
For modern power steering systems to work as intended, the power steering control module must always be aware of both the actual position and the status of the electric motor relative to its programmed/learned/desired dead-ahead position.
Thus, the power steering control module monitors the entire power steering system continuously when the engine is running to detect any faults, malfunctions, failures, or electrical values that fall outside of acceptable ranges. When any such defects are detected by the power steering control module (or another implicated control module) the system will illuminate dedicated warning lights and set appropriate fault codes.
Note, though, that depending on the faults and/or defects detected, the power steering control module may reduce or even eliminate steering assistance. For example, let us say that the electric motor begins to overheat as a result of the steering wheel being held at full lock for an excessively long period. In such a case, the power steering control module will reduce the current flowing to the motor to lower the electrical load on it, and should the driver move the steering wheel away from the full lock position, the control module may also reduce or eliminate steering assistance until the motor has cooled down to an acceptable temperature.
Other critically important self-diagnostic checks include (among many other operational parameters) service/reset/calibration data on both the steering angle and steering torque sensors, as well as data on the steering motor’s current draw and temperature. Some of this data may be available as freeze-frame data accompanying fault codes, but regardless of how fault data is presented, the point is that it may be impossible to extract fault data from modern power steering systems without dealer-level diagnostic equipment, which may also be required to perform-
Servicing and calibrations
As with many older electric power steering systems, modern electronic power steering systems also require the calibration of the steering angle sensor with a suitable scan tool after wheel alignment procedures, after some types of mechanical repairs had been performed on steering and/or suspension components, or after a defective battery had been replaced.
However, depending on the vehicle, some modern power steering systems can largely calibrate themselves. On these systems, the process involves turning the steering wheel from full lock to full lock in both directions with the engine running and then doing a short test drive to provide the power steering system with input data from the vehicle speed sensor. This process will usually reset the system and extinguish illuminated power steering system-related warning lights and/or alarms.
Note, though, that in some cases, such as on many new Toyota products, a suitable scan tool is required to also recalibrate or reset yaw sensors and one or more accelerometers to zero-point values after all wheel alignment procedures. However, be aware that while this service information can be obtained from several aftermarket vendors of service information, service information obtained from some third-party vendors often does not contain software updates, patches, or fixes.
Therefore, when you do need to perform calibrations on these vehicles, the best course of action would be to obtain service information from the vehicle manufacturer to be sure the power steering system operates reliably and safely, which leaves us with this-
As with all new automotive technologies, learning how to diagnose power steering system issues will also be a steep learning curve, but once you understand how these systems work and how deeply they are integrated into multiple ADAS systems, finding and fixing faults and defects on modern power steering systems becomes a whole lot easier.
However, it is unlikely that very many independent workshops have encountered power steering issues on vehicles that are less than about 4 years old, so we hope that this article has given you some new insights into modern power steering systems, and what to expect when you do encounter these systems.
It is almost certain that improved, and therefore, increasingly complex versions of electronic power steering systems will become the norm in the increasing number of cars with self-driving capabilities, which means that learning as much about these systems as possible will be hugely helpful when you encounter issues on modern power steering systems for the first time.
