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Feature   February 1, 2016   by Jeff Taylor

Going in the right direction


Power steering systems are evolving. Yes, the carmakers want to reduce the weight of the components so they can improve vehicle mileage and decrease emissions. But when it comes to steering, they’re also motivated by safety concerns and customer convenience.

Steering-PIC-1

An example of a Ford Edge Belt Drive EPS Rack. The belt drive housing and motor is mounted on the steering rack.

In its earliest hydraulic version, power steering was first used on the 1951 Chrysler Imperial. It is now used on a wide variety of vehicles, from backyard ATVs to jumbo jets.

Of course the vehicles we service have had it for quite some time. But the technology behind it is changing… and fast!

Everything is changing – from the way it works, to the way it’s controlled – and these changes are starting to show up in our bays with unique issues.

The move towards electric power steering (EPS) or electric power steering assist (EPSA) started slowly but it’s gaining steam. Fuel economy and emission control were the initial drivers of this change. But in recent years, OEMs have embraced the notion of self-driving cars and this brings new demands for automated steering, evasive maneuvering, and parking assist. EPS is central to these objectives.

Second pinion design showing both pinion locations on a Chevy Cruze.

Second pinion design showing both pinion locations on a Chevy Cruze.

The earliest complaint about EPS revolved around lack of ‘road feel,’ road vibrations being felt in the hands of the driver, software issues, and component problems. (Remember the 2004-2010 Chevy Malibu steering column rattle?) Most of these issues have been addressed and today’s versions of EPS work and perform really well – a real accomplishment considering they didn’t have the 60-plus years of development that the older, more familiar hydraulic systems had.

Many of todays EPS systems work on similar principles. An electric motor has replaced the hydraulic pump that was driven by the internal combustion engine. This is an important development. The hydraulic pump constantly consumes engine power. That power requires excess fuel and creates more emissions. As we’ve learned, vehicle manufacturers are under enormous pressure to curb both of those concerns.

The hydraulic system initially had a static turning ratio, offered poor assist under low RPMs, and less road feel at highway speeds. These bugs were worked out, but the system still relied on a number of components – like belts, hoses, and pumps – that eventually wear out. That just doesn’t happen as fast or as often with electrical systems.

EPS reduces the warranty and maintenance costs, and manufacturers can program just about any steering ratio they want without having to redesign the whole system. Road feel and steering force can also be tweaked with software changes. No wonder carmakers embraced this technology!

Remote mounted EPS electric pump on a Mazda.

Remote mounted EPS electric pump on a Mazda.

Perhaps they’re most excited by the fact that the EPS system doesn’t really add any additional manufacturing cost, aside from making sure the charging system, wiring, and battery are adequate to meet the additional demand. (Which, according to Ford engineers, is marginal at best.)

Three types

In the simplest terms, there are three different types of EPS available at this time. The two most common involve the use of an electric motor attached either to the input shaft on the steering rack (column mounted), or to the rack itself in a number of different ways (rack mounted). The third, and somewhat less common type, features a small on-demand pump that works a traditional hydraulic power steering rack. It still has fluid and hoses, but there’s no belt taking constant power from the engine.

All of the EPS systems have integrated electronics and modules that provide the feedback needed to perform accurately and safely. There will be some sort of failsafe mode, and a number of sensors that will supply data to the steering systems control module: steering input torque, rotational speed, steering loads, and a host of other values.

The steering module will inevitably be tied in to the rest of the vehicle’s electronic systems on the high-speed or CAN network system so it can share data from other systems. Among the data it looks for are vehicle speed from the ABS or PCM (to manage road feel and ratio control), G sensors (for stability control), and the vehicle’s radar camera system (for automatic parking).

These represent just the tip of the iceberg as far as what the EPS system can do or control. The type of system that is used depends on a number of factors – the most important of which is the weight on vehicle’s front axle. This is a key factor in figuring out what type of EPS unit is best suited to a particular vehicle.

Smaller vehicles

Vehicles with a gross vehicle weight of less than 1000 kg on the front axle, require less force or torque to steer the front wheels. The ideal EPS system will be column-mounted. The motor may be assimilated into the steering column, or it may be remotely mounted under the dash in line with the steering shaft.

This design may have all the needed components in one neat package… and that includes the EPS steering ECU and torque sensor. Vehicle design, safety concerns, and steering column packaging will dictate the EPS motor location. This system will transmit the power steering assist into the intermediate shaft that connects to a standard mechanical steering rack.

As with all EPS systems, the electronics can easily be tailored to make, model, and option packages, as well as any safety controls or other features the manufacturer or customer desires. (This weight class may also use the hybrid remote electro-hydraulic system, but the totally electric EPS system is quickly taking over.)

Mid-size vehicles

Vehicles with a front axle weight of 1200 to 1600 kg require more torque to steer and maneuver, so the EPS system has to be a little more robust. There are a number of designs that are being used that all provide the needed road feel and performance that the manufacturer and the customer desire.

Two major designs that are used currently are the second pinion design and the belt-driven ball screw design.

The second pinion design has two pinions that are in contact with the steering rack: one that is attached to the steering columns intermediate shaft for driver input, and one that is connected to the electric motor to give the steering assist.

The other design uses a toothed belt-driven ball screw mechanism driven by an electric motor to provide the needed steering assist.

In both situations, the EPS motor is rack mounted, and the input torque sensors are mounted to the input steering shaft.

One other note about this style: because the EPS motor is not contained in the driver’s compartment, the vehicle’s interior crash safety increases, and this is something the manufacturers like.

Full-size vehicles

Cars and trucks with more than 1600 kg of front axle weight will use the toothed belt-driven ball screw design, very similar to the design used in lighter cars. The components used are built bigger and stronger for use in heavier applications. This style is now very popular in the pickup truck segment.

The EPS systems are designed with safety and redundancy in mind. They will have modes built into the software that will limit the amount of current that the EPS electric motor can draw, and may even limit the power steering available if the system is being abused, overloaded, or overheated.

The system will have some form of warning system that will alert the driver to failures, limited service (overheated electric motor, for example) or an issue that will require service and diagnostics. But almost all of today’s systems still have a mechanical link from the steering wheel to the rack, allowing for manual power steering in the event of total EPS failure.

Most of the issues of noises, road feel and other complaints have been addressed using the proper EPS system for the vehicle’s design, weight, and software, but there are still going to be issues with bad connections (Ford Fusion’s recall for bad connections to the EPS motor) and improper boosting (GM has issued a bulletin talking of blown EPS fuses from improper boosting). There are also occasional customer-caused issues that the engineers couldn’t have anticipated.

Diagnosing the EPS systems that are on today’s vehicles will require a scan tool, but it may not have to be a factory tool, as a number of aftermarket tools will perform quite well.

All of the current EPS systems require very little in the way of service, but there are certain situations that you may need to make adjustments or recalibrations. For example, if an alignment is being performed, the steering system is going to have to recognize that the actual vehicle and the steering wheel are in the straight position (Toyota calls this the torque sensor zero point).

Most of these types of adjustments are straightforward and done electronically following the instructions of your information system or the scan tool.

There are other times that the system may have to be told that the steering wheel is in the straight-ahead position when performing unrelated services such as G sensor or ABS services or clearing codes in those systems. But don’t be surprised if you’re asked to do the resetting of a steering system angle for something totally unrelated such as an active cruise repair because so many vehicle systems are now very closely interfaced.

Is EPS near its end? I don’t think so. The technology continues to move forward. Infiniti already has a vehicle that is steer by wire: the Q50 (yes, it was recalled for a steering issue). It has a number of redundant systems and a mechanical link (a clutch in the steering shaft) in the event of total failure steering.

But one thing’s for sure. The completely automated vehicle is coming… and electric power steering will definitely be part of the equation.

Jeff Taylor is a former ACDelco Technician of the Millennium and Canadian Technician of the Year. He’s the senior tech at Eccles Auto Service in Dundas, Ont.

 


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