Knowledge Building: Semi-Active Ride Control

With the recent introduction of service parts into the aftermarket by Volvo’s Controlled Electronic Suspension (CES) supplier, Tenneco, it seems fitting to look in some detail at how this and other active suspension systems operate.

Technically, the systems that find their way onto passenger cars are “semi-active,” which distinguishes them from more sophisticated systems that can anticipate the approaching road surface; they are, in effect, only reacting to forces rather than anticipating them, although they do this very quickly. (Notably, the system should not be confused with the Nivomat self-levelling system found on Volvo and other vehicles.)

Rather, CES incorporates continuously variable electro- mechanical valves, developed by Öhlins Racing. These components replace conventional piston valves and base valves.

In engineering-speak, the CES design delivers a ratio of 1/20 between the minimum and maximum damping force levels at lower-velocity ranges up to 0.2 m/s. At higher velocities above 1 m/s, the ratio is reduced to 1.5. Moreover, the slope of the low- velocity bleed curve is variable.

These characteristics are extremely important to the overall ride quality of the vehicle. In addition, the large range between minimum and maximum damping force curves ensures that a CES-equipped vehicle has the damping force levels necessary to control the vehicle’s dynamics.

CES valve adjustments occur very rapidly to produce different damping forces. Typically, adjustments take place within 10 milliseconds. This adjustment speed provides effective control of wheel resonant vibrations up to 20 Hz. As a result, wheel hop frequency control is achieved in addition to body frequency control.

An Electronic Control Unit (ECU) lies at the heart of the CES System to exploit the full potential of the electro- mechanical valve system. The ECU processes data sent by a group of sensors placed at key locations on the vehicle. The sensors include three acceleration sensors mounted on the vehicle body and four displacement sensors. These devices feed data to the ECU regarding steering wheel angle, vehicle speed, brake pressure, and other chassis control information.

The ECU utilizes control software that processes the sensor information in real time, and sends signals that independently adjust the damping level of each shock absorber valve. CES dampers allow a large separation between maximum and minimum damping levels, and adjust to assure the optimum in ride comfort and firm, safe vehicle control.

Technical specifications aside, CES achieves the continuous adjustment of damping levels according to road and driving conditions. It switches from levels that are as soft as possible to provide maximum comfort, to levels that are firm enough to provide optimal road holding and handling.

Tenneco’s CES shock absorbers are used in production on the Volvo S60R, V70R, S60, V70, and S80 (including new S80, model 2007), as well as other vehicles.

A similar-looking system from ZF Sachs that it calls Continuous Damping Control is pictured here. Counterpeople should be aware that this system is out there.

Another system that will be entering its aftermarket years soon, if not already, is the Delphi-developed MagneRide system. In contrast to valve-based systems, the MagneRide semi-active suspension system has a monotube design with no electromechanical valves or small moving parts. GM has referred to the system as GM Magnetic Ride Control. (MagneRide is now Beijing West Industries Group of China (BWI), Inc.’s proprietary name for its magneto rheological suspension system. Delphi sold its Brakes and Suspension business to BWI in 2009.)

MagneRide consists of magneto-rheological (MR) fluid-based monotube struts and shock absorbers; a sensor set that consists of a relative position sensor between each control arm and the body, as well as a lateral accelerometer and a steering-wheel angle sensor, which are also part of the Stabilitrak system (a yaw rate sensor is used indirectly by the MagneRide system in active-brake-apply events); an onboard controller; and an optional levelling compressor module.

Delphi and Lord Corp. co-developed the MR fluid, which consists of suspended iron particles in a base fluid of synthetic hydrocarbon, specifically for the shock-absorber application. In its “off” state, the MR fluid is not magnetized, and the iron particles are dispersed randomly; however, in the “on” state, the applied magnetic field aligns the metal particles into fibrous structures, changing the fluid rheology and thus regulating the damping properties of the monotube struts.

Based on the wheel inputs from the road-sensing suspension– as well as driver inputs such as steering and braking–the system’s onboard computer sends electrical currents up to 1,000 times per second to the electromagnetic coils in each damper to change the flow properties of the damping fluid. According to Delphi, the MR fluid can change from a mineral-oil-like consistency for low damping forces to a jelly-like one for high damping within 1 ms. The result is continuously variable, real-time damping.

In its basic mode, the suspension allows for a more comfortable vehicle feel. In sport mode, the vehicle offers a more responsive ride thanks to a push of a button.

Because of the greater emphasis on ride comfort in the basic mode, maximum comfort is assured when driving over long distances or uneven surfaces.

The greater emphasis on precise and responsive handling in the sport mode results in even better control at the limit. It also optimally suppresses the transient rolling tendency of the car’s body in corners.

I drove a Cadillac equipped with a pre-production system some years ago and the ability of the system to transition between a supple ride and a firm cornering situation was noticeable, and has been refined since.

In any case, it is wise for counterpeople to familiarize themselves with which models are equipped with such semi-active ride control systems, as well as where the replacement components can be sourced.

Information from Volvo, Tenneco, ZF Sachs, and Delphi was used in this article.