Automotive

1-2000


Innovation Feature

Double cylinder delivers a smoother ride

A novel hydraulic suspension system for on- and off-road vehicles copes with all kinds of problems and obstacles. Tom Shelley reports

A vehicle suspension based on hydraulics rather than conventional springs and shock absorbers has none of the complexity or large scale pumping of previous attempts at active suspension control.

Rapid fluctuations, bumps, potholes and rough terrain are all accommodated by different aspects of the system.

There are potential applications in all kinds of vehicle, civilian, military or mobile construction equipment, with a rigorous testing programme now under way.

The ‘Fluid Spring’, prototyped and patented by engineer Dennis McNeely of Michigan, US, originally came out of his work as a land development consultant, advising civil engineers, land developers and builders.

It was designed to accommodate extreme load variations and rugged terrain in construction equipment. In particular, it offers a way to allow front end loaders and scrapers to set their cutting edges at an angle different to that determined by the angle of the axles relative to each other. Similarly, operators of backhoes and excavators ordinarily have to bench the side of a hill to ensure stability. The fluid spring, on the other hand, offers the possibility of working level with the wheel or track chassis at an angle. It can also improve the suspension of cars and other road vehicles.

In is basic concept, each wheel is attached to a double hydraulic cylinder on linked but separate circuits. One cylinder is described as the ‘lift’ cylinder, and is used for overall command movements to raise and lower the suspension and to cope with very short term fluctuations in interactions between wheel and running surface – such as on cobblestones. The other cylinder, the ‘spring’ cylinder, copes with larger bumps and potholes.

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The lift cylinder is connected to the overall hydraulic supply though an externally commanded valve and to a hydraulic accumulator.

The spring cylinder is connected to the overall hydraulic supply through a valve built into its top end. The spool is acted on by hydraulic pressure at its lower end and by a return spring and oil pressure from the lift cylinder circuit at its upper end.

The cylinder hydraulic accumulator absorbs perturbations with time constants that are too short for the spring cylinder-valve combination to react to.

If the wheel encounters a raised bump in the road, the pressure in the spring cylinder momentarily exceeds the pressure in the lift cylinder circuit, forcing the valve spool up, releasing fluid from the relief port and allowing the piston in the spring cylinder to rise. As the wheel crests the top of the bump and begins to track down the other side, the pressure in the spring cylinder drops below the pressure in the lift cylinder circuit. The pressurised fluid in the lift cylinder forces the valve spool down, admitting fluid to the spring cylinder from the pressure port and forcing the spring cylinder piston to extend.

Potholes are handled differently. If the vehicle is moving slowly, the spring cylinder will be forced to extend in the manner just described as the wheel enters the hole. If sensors indicate that the vehicle is moving at a medium to high speed – but not accelerating, braking or cornering – McNeely envisages that an additional solenoid in the pressure line to the spring cylinder should close. The closure denies fluid flow to the pressure port, effectively carrying the tyre over the pothole. This occurs despite the downward movement of the valve spool within the fluid spring which would otherwise admit oil from the pressure port. As soon as specified design criteria are met (acceleration, yaw etc) the valve opens and the tyre is forced to track through depressions. Unlike a conventional suspension, the fluid spring at the diagonally opposite corner of the car does not extend and tip the car as the wheel passes through the pothole.

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The device may be operated as an active suspension system, a semi active suspension system or a passive system. Pressurised fluid can be delivered either from a pump or a central reservoir. Initial computer modelling suggests that the reservoir should be pressurised, which greatly reduces or even eliminates fluid power requirement.

CitroŽn has been using such a concept for years, but with its own particular hydro-pneumatic suspension systems. The CitroŽn system uses a hydraulic cylinder, a restricted orifice and an accumulator in series to raise and lower the car on an air suspension. The suspension does not differentiate between bumps and holes, does not allow the wheel to be carried over potholes and does not tailor the support of the car to apparent weight at each moment.

Modelling suggests that stability while cornering, braking or accelerating should be increased in McNeely’s system. He believes the vehicle should remain flat as the suspension compensates for swaying, squatting and diving forces. Neither exotic sensors nor extensive electronics are required for immediate and accurate compensation. This same inherent quality in the fluid spring should resist rollovers which are otherwise caused by a driver swerving and counter steering in a collision avoidance manoeuvre. Shock absorbers, springs, torsion bars, upper and lower control arms, trailing arms, rubber frame stops, automatic levelling devices and a variety of other suspension components are all eliminated.

Other advantages of a hydraulic suspension include the ability to raise or lower the whole vehicle. A sports car can be maintained as close as possible to the ground to reduce air resistance and enhance ground effect when going fast, and then raised up to assist drivers and passengers to get in and out. Conversely, a four wheel drive pickup can be raised up to go over particularly rough terrain and lowered to ease loading and unloading. Access to drive up windows and ATM machines could also be similarly enhanced by the ability to raise and lower a vehicle.

There has already been considerable interest in the concept, despite the early stage of the development. McNeely says that the US Department of Defense has indicated an interest for possible inclusion in future fleet deployment; the National Fluid Power Association has requested the presentation of a paper at its technical conference, as has the Society of Automotive Engineers for its Automotive Dynamics and Stability Conference.

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He anticipates mounting the fluid spring on a test vehicle in the second quarter of this year for dynamic testing. The development is fully covered by patents, and extensive details and animations can be found on the Web ( www.site-solutions.com/QED.htm ).

Design Pointers

A new hydraulic suspension system should have all the advantages of active suspension without the hydraulic power requirements, high cost and complex controls

A double hydraulic cylinder and accumulator combination copes with all the different situations experienced by a suspension

It treats bumps and holes differently and includes the ability to step over potholes at high speed

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