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Dr. Lanchester, or How I Learned to Stop Worrying and Love Electric Power Steering

electric_power_steering

Parallel parking is a skill that many new drivers have a hard time accomplishing smoothly (and many older drivers too). A great deal of entertainment …

Parallel parking is a skill that many new drivers have a hard time accomplishing smoothly (and many older drivers too). A great deal of entertainment has been had at the expense of a driver getting totally befuddled while attempting to parallel park. But now, manufacturers, are building cars that will parallel park themselves.

A perky TV reporter exited from a Ford Escape at the Kansas City Auto Show after doing a ride-along on a parallel self-parking demonstration. She was accosted by your humble scrivener and asked, rhetorically, if she knew what the enabling technology for what she just witnessed was. It’s electric power steering (as well as a bunch of lines of code and some rugged computer hardware). It can’t be done with a standard hydraulic steering gear or a manual steering gear; it’s the electrically assisted steering rack that allows the vehicle to steer itself. Scary, isn’t it. And think about all of those lines of code that enable inter operation between vehicle subsystems (ECU, transmission, ABS, park sensors, and power steering, among others).

Dr. Lanchester, or How I Learned to Stop Worrying and Love Electric Power Steering

Ford Self Parking

But self-parking is merely icing on the cake for what electrically assisted power steering brings to manufacturers. Primarily, electric power steering is used because of its positive contribution to fuel economy. The passenger car fleet average MPG (an average for all of a manufacturer’s passenger cars) is supposed to be 39.5 by 2016 in order to avoid fines from the EPA.

In Europe, carbon dioxide levels are also regulated and, like MPG, benefit from smaller engines, lower weight and minimal fuel consumption. That is why we see direct injection and turbochargers on small displacement engines. These standards are tough and they are getting tougher for manufacturers to meet. So an up to a one MPG fuel consumption savings by adapting electric power steering makes corporate sense.

BMW EPS

However, if you ask the enthusiast, they’ll tell you electric power steering is terrible. It has no ‘tactile’ feel up into the hands. And that’s true, there are some terrible electric power steering implementations. The question is, what prevents electrically assisted power steering from delivering good steering feel. And the answer is pretty convoluted, unfortunately. But a good place to start is at the beginning.

The steering wheel is used to turn the front wheels (with rare exceptions – think forklifts and combines) from side to side. The steering ratio is an expression of the number of degrees the steering wheel has to be turned to turn the wheels one degree. For instance, the steering ratio 16:1 means the steering wheel has to be moved off center by 16 degrees of rotation to turn the wheels 1 degree. The lower the ratio, the ‘quicker’ the steering.

The total steering deflection (the total number of degrees the wheels can be turned from one extreme to the other) times the steering ratio divided by 360 (a complete circle) will give the turns of the steering wheel ‘lock to lock’. But just because a car has fewer turns ‘lock to lock’ doesn’t mean its steering is necessarily quicker. It may mean that its wheels cannot be deflected as far as another’s, a trait of front wheel drive cars for instance.

So those are some of the basic parameters that are used when talking about steering. The closer the ratio is to 1:1 the harder the car will be to steer without some assistance. In fact older heavy cars relied on big ratios (20:1 +) and large diameter steering wheels (to multiply the leverage the driver delivers) to make the steering manageable. Think of the steering effort needed to turn a 1965 Chevy Impala station wagon without power steering and you understand why the driver had to really crank the wheel to get the tires to turn and it looked like the steering wheel had been stolen from the pilothouse of a Mississippi river steamboat.

Steering gears can be divided into two basic types, Pitman arms and rack & pinion. Pitman arm steering gears include worm & sector, worm & roller, worm & nut (also known as recirculating ball), and cam & lever. They rely on a worm gear at the end of the steering column to move a Pitman arm. The worm gear would operate a sector gear, a roller, a ‘nut’, or a lever, which ‘twisted’ the Pitman arm. Pitman arm type steering gears are rarely used in modern passenger cars (but the recirculating ball variant was used heavily in cars in the recent past).

Dr. Lanchester, or How I Learned to Stop Worrying and Love Electric Power Steering

Recirculating Ball Illustration from Wikipedia

The knock on Pitman arm steering gears was that they tended to have a bit too much lash (otherwise called ‘slop’) and for enthusiasts, something with better ‘feel’ was desired. Rack and pinion steering was the answer.

Rack and pinion steering is a pinion gear attached to the end of the steering column that moves a rack (a shaft with gear teeth cut along the length of the shaft). There is less slop and fewer linkages in a rack and pinion steering unit but also less mechanical leverage than a recirculating ball steering gear. But the tactile feel that a good manual steering rack feeds back into the driver’s hands is what enthusiasts crave.

Rack & pinion – WRC

And a manual rack is the ticket up to about 1000 kilograms (2200 pounds). The heavier the vehicle the less ‘quick’ the steering ratio could be and still maintain reasonable steering effort. So, manual racks were replaced by power assisted rack & pinion steering once cars started getting significantly heftier.

The power assist was via hydraulic pressure that exerted it’s force, in both directions, on a piston enclosed in a cylinder attached to the steering rack. The amount of assist was controlled by a complex valve setup attached to the pinion. The power steering pump was attached as an accessory drive from the engine. The pressure had to be specially regulated given the fact that it would otherwise vary greatly depending on engine RPM.

As good as some hydraulically assisted power steering racks are, others are, and were, pretty bad. Depending on the manufacturer’s desires, racks could be isolated by softer mounts, the rack mounting location could be compromised, suspension geometry could be slanted towards minimal feedback, and/or inferior tires could be specified. In other words, you can remove feel from a steering rack.

But the question is not why hydraulically assisted steering racks feel good and electrically assisted racks not. It’s the wrong question. The question should be, when will an electrically assisted rack feel as good as the better hydraulically assisted racks. And the answer is that they will, eventually. However, truth be told, not all electrically assisted racks will.

ZF makes a number of electrically assisted steering racks under its Servolectric range. And a sampling of their competitors shows a similar set of technologies employed in a variety of electrically assisted steering racks.

There are three basic types of electrically assisted racks, column mounted assist, second-pinion assist, and rack drive assist. Of the three, the column mounted assist is the least likely to provide good ‘tactile’ feedback. It employs a power assist unit mounted on the steering column, usually in the interior of the car. This type of electrically assisted power steering rack is more likely to show up on smaller and less expensive cars. The assist motor is mounted on the steering column, smack dab in between your hands and the steering rack.

Dr. Lanchester, or How I Learned to Stop Worrying and Love Electric Power Steering

Column Mounted Electrical Assist / source: ZF

Further, the steering column, below the assist motor, has to be strengthened to handle the additional torque the assist motor provides. It’s not an easy recipe for providing good steering feel. Unfortunately, this is a very common solution at the low end of the market. Further, because these cars are price sensitive, the other components that can effect steering have probably been optimized for cost over feel.

The type of electrically assisted steering rack that holds the most promise for steering feel is the rack drive assist. ZF makes a very nice rack drive assist rack called, ZF Servolectric/Paraxial. This is the electrically assisted steering rack solution that an enthusiast will eventually look for. In this solution, there is a straight connection between the steering wheel and the rack.

Dr. Lanchester, or How I Learned to Stop Worrying and Love Electric Power Steering

ZF Servolectric/Paraxial Electrically Assisted Rack / Source: ZF

The assist unit is mounted in parallel to the end of the rack away from the pinion and provides assist through a toothed belt and a recirculating ball gear (hmmm, everything old is new again). One telling line in ZF’s product paper is the following, “[t]he low noise generation of the recirculating ball gear allows rigid connection of the steering gear to the subframe of the vehicle, which gives a very direct steering feel.”

Dr. Lanchester, or How I Learned to Stop Worrying and Love Electric Power Steering

Source: ZF

From an illustration in the ZF material: “To transform the rotational movement of the electric motor into a linear movement of the steering rack (1), ZF Servolectric with paraxial drive uses a drive concept consisting of toothed- belt drive (2, 3, 4) and recirculating ball gear (5, 6, 7). Both drive stages work with a very high efficiency.

The recirculating ball gear is a system in which the ball chain (7) is returned through a channel (6) integrated in the ball recirculating nut (5).”

So there’s reason to hope that an electric rack drive assist can deliver good steering feel. But there is a lot of tuning involved. Remember that hydraulically assisted steering racks used a valve setup to regulate the amount of assist. In an electrically assisted steering rack that is accomplished with software. A lot of software. And a lot of parameters which can be changed in that software.

Huibert Mees, Chassis Fellow for Tesla Motors, commented to BMWblog at this year’s NAIAS, that there are over 500 parameters that can be set over various operating modes and driving conditions of the electrically assisted rack. That can take some time to sort out even if sorting it out can be done on the fly, unlike recutting an appropriate valve setup for a hydraulically assisted rack. There was an interesting blog post on tuning an electrically assisted rack ‘on the fly’ at PickupTrucks.com that’s worth a read: http://news.pickuptrucks.com/2010/05/driving-a-pickup-with-electric-power-steering.html

While there is hope that we will get decent feel from the best of the electrically assisted racks, it may take time to sort through all of the parameters to find a sweet spot. And remember, tires, suspension geometry, and the quality of other components in the chain can and will effect steering feel. One space to watch, to see if some gearheads will buy into using electrically assisted steering racks, is the upcoming F10 version of the M5. If it doesn’t use an electrically assisted rack, like its 5 series siblings do, then we can conclude that electrically assisted power steering racks are not ready for prime time . . . yet.

While I haven’t learned to love electric power steering, my worrying has subsided. There’s no getting around the fact that manufacturers are scrambling for every tenth of a gallon they can save to apply towards the coming fuel economy and emissions standards. The trick for an enthusiast will be finding a manufacturer that still cares about them. Good steering feel can be delivered in an electric steering rack, but it won’t be at the bottom end of the market.

And why does the title reference a Dr. Lanchester, rather than Dr. Strangelove? Dr. Lanchester was an English engineer and inventor who is pretty much under appreciated these days. He worked in the very early years of the automotive industry and contributed a lot to it. He also took out a patent on a power steering assist system in 1902. http://en.wikipedia.org/wiki/Frederick_W._Lanchester

  • Nnnn

    Verrrry interesting article, Hugo. Very good explanations and visuals.

    One thing I still don’t understand (and have never understood) is how any of the assist types are triggered. For example, if the pinion is attached to the rack, and rotation of the pinion actuates the hydraulics (or electrics), then you have to overcome the inertia yourself until the assist can sense your motive and assist, until you overcome the assist to indicate stoppage of rotation. Actually, from my description, it couldn’t work at all. So it’s clear how drive-by-wire would work (actuate wheels to that position), or a pure hydraulic system would work (leverage), but I don’t understand how it can work when the steering wheel is mechanically coupled to the steering and wheels.

    The other thing I was wondering was about the fuel economy. Why does a hydraulic power assist always need to be running? Could it not be variable like the electric system?

    • http://www.facebook.com/profile.php?id=100000477069969 Hugo Becker

      A hydraulic rack has hydraulic pressure in equilibrium in the assist cylinder when the steering wheel is ‘on-center’, and since the pump is always running (it’s belt driven off the engine – or is cars like the Mazda RX-8 the hydraulic pump is powered constantly by an electric motor) pressure is always there. So once you move the steering wheel, hydraulic fluid is diverted to the appropriate side of the piston to assist in turning the steering rack.

      And it’s the weight of the pump, the fluid, the hoses, the hydraulic assembly on the rack, and the fact that the pump is always eating HP that otherwise could be put to the drive wheels that has prompted manufacturers to go to electric racks.

      With an electrically assisted steering rack, there is no assist when the steering is held ‘on-center’. The electric motor is not consuming electrons. It comes into operation when the steering wheel is deflected off-center. And it has to do so virtually instantaneously. If it doesn’t you get this weird fraction of a second hesitation – and some of the early implementations of electrically assisted power steering had that ‘feature’. ;-)

      • Nnnn

        If the assist is related to the position of the steering wheel and not the motion….. then I guess I’m misunderstanding the force that’s being overcome. I had assumed that centripetal forces would resist the direction of the wheel being changed, thus you would need to amplify the force in any change to the steering position (as you experience holding a bicycle wheel as it spins). But what you’re describing suggests that there’s much more “centripetal” resistance in the wheel turning as part of the car changing direction. Am I on the right track here?

        So, the hydraulic valve always applies force to the left when the steering wheel is left of center. Is that a constant force regardless of position, or does it increase as the wheel is turned further left? Which might mean little or no assist when steering is centered. AND it would mean that the hydraulic pump could do this by running at variable speeds, including zero when centered.

        Having a sharp transition at dead center seems…. glitchy. Was that “feature” a delay, or was it an awkward transition between right/left assist?

        • http://www.facebook.com/profile.php?id=100000477069969 Hugo Becker

          Assist is related to the position of the steering wheel (or how the steering wheel uncovers the valving in the valve body for hydraulic racks). The pressure needed to move the assist piston in its cylinder is modified by how the valving is uncovered. Pressure from the pump should be kept constant, and the valving can increase or decrease flow to either side of the piston as needed.

          Speed dependent variable assist can also be accomplished with specialized valve bodies (and variable ratios can be done by fudging with the spacing of the gear teeth on the rack).

          When the wheel is centered there is equal force being applied to both sides of the piston so it basically cancels out in a hydraulically assisted rack.

          There is no sharp transition ‘off-center’ in a hydraulic rack – and depending on how good the steering subsystem was there may or may not be ‘slop’, or crappy feel, on-center. With an electrically assisted rack there is no pump applying pressure to two sides of a piston. The assist is off, instead of being always on like a hydraulic rack. The trick with a electric rack is to not have any noticeable delay in the transition from no-assist to assist.

  • Nnnn

    and…. when’s our article on Fear and Loathing on Run-Flats?

    • http://www.facebook.com/profile.php?id=100000477069969 Hugo Becker

      I ditched the run flats tout de suite on my 135 and replaced them with Conti ExtremeContact DW tires. I put a Conti emergency kit (a compressor and a can of slime) in the trunk for peace-of-mind.

      If you ever get a chance to see the inside of a run-flat tire, you’ll know why ‘loathing’ is appropriate.

  • http://www.millsford.net/ Used Cars Orange County

    I like nice cars but I’m not like a die hard enthusiast so power steering is fine for me. All my cars have had power steering, I don’t think I’ve ever driven a manual one and did I probably couldn’t tell the difference.

    • http://www.facebook.com/profile.php?id=100000477069969 Hugo Becker

      Remember we’re talking primarily about the difference between rack & pinion steering that has power assist by either hydraulics or electricity.

      I’m not sure if there are many (if any) cars you can buy new today that don’t have power assisted steering. But I’ve driven a whole bunch of cars in my life that had no power assisted steering. That just shows you how old I am. ;-)

      • http://www.facebook.com/DeviateAE Lee Kaldenbach

        My Miata has no power steering. The Jeep may as well not have it since it’s been thus far impossible to keep fluid in the reservoir despite replacing it 3 times in the last year.

        Personally, I’m not a fan of the steer-by-wire stuff. Z4’s had it since its inception, and while it’s mechanically solid it just lacks the feel of even a power-assisted manual rack.

        To “Used Cars”, you would feel a difference in most cars. Something as small as a Miata with small tires, you may not (I do, personally). If you were to hop into a full-sized older car that didn’t come with it, or even an E46 with the notorious LF20 power steering pump issue, you WILL feel the difference. Think about the effort required to pick up a couple cinder blocks tied to a rope and translate that into turning the steering wheel. Night and day difference.

        • Nnnn

          What year is your Miata?

          You know, I wonder if —with advanced, lightweight materials — we could see a return to pure mechanical steering. We already have carbon fiber wheels, composite brakes, kevlar-belted tires — if it weren’t for 19″+ rims, and 150+mph top speeds, that might be enough.

          On the other hand, they’re probably also making sacrifices in front suspension design because they can reintroduce “feel” using these highly configurable electric assist devices. It would be great to see front wishbones again, but that’s not likely to happen now.

  • Nnnn

    Huibert Mees mention of the 500 parameters for adjusting the electric steering – that’s an awful lot for one actuator operating in one axis of motion. If there are a small number of inputs (vehicle speed, steering position) then it has to be control points plotting some curve of assist force. Or it could be matrix or map of the sheer number of inputs they’re using to calculate the assist.

    What bothers me about this system is the potential for “faking it”. They could use this system to tune out torque steer in a FWD car, for example. Maybe it doesn’t matter, I don’t know.

    • http://www.facebook.com/profile.php?id=100000477069969 Hugo Becker

      “They can use this system to tune out torque steer in a FWD car. . . ‘. Why yes, come to think of it, that is something they could do. You’re really thinking this through. Very good!!

  • http://www.ittflowcontrol.com/midland-acs Hydraulic Valves

    Hey, nice post. Manufacturers are building cars that will parallel park themselves.
    Big deal of entertainment has been had at the expense of a driver getting completely puzzled while attempting to Parallel Park.

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