Supplier Technology

Neues CDC System von ZF Sachs ist viel sneller als ein Augenblick!

Papenburg, Germany, is the site of a Damlier/Chrysler owned proving ground and we were there recently to test automobiles fitted with the ZF Sachs continuous damping control (CDC).

This system debuted in 2001 as an option on upmarket cars such as the Maserati 3200 Coup?, Ferrari Modena and the BMW 7 Series, then spread to the Audi A8, the Porsche Cayenne, VW Phaeton and Touareg, but now it has come down to earth as an option on the 2004 Opel Astra, a high volume automobile.

It’s expected that about 8 percent of Opel Astra buyers will select the CDC option this year and penetration is projected at 12.4 percent next year. That means ZF Sachs will be putting approximately 250,000 CDC systems on vehicles in 2005 for all applications.

We drew a nice clear day in late March for test driving. A rain or snow laden track might have been more interesting in some ways, but there was still plenty of excitement reacting to various road surfaces, a tight slalom course and a challenging double lane change.

The proportioning valve on this CDC damper (front strut) is located externally in the bypass line of the displacement unit.

Most of the automobiles mentioned above, 17 in all, were available, some with CDC and some without, so comparisons could be made readily. Some of the Brazilian journalists were far better drivers than I so I learned as much or more as a passenger as I did while driving. Picture this venue which lasted about five hours. A typical vehicle test run lasted about 20 minutes.

The venue even included a couple turns round the track with retired Formula 3 and 1999 LeMans champion Joachim Winkelhock at the wheel. It’s fascinating to be thrilled and intimidated by the same experience.

Before turning to the technology behind the CDC system, let me give you my impression of the testing. The best handling car for me on the rough or smooth road surfaces was the BMW 7 Series, but I wasn’t any good in the slalom or double lane change with this auto because the steering wheel was so big and required too much input. I couldn’t react quickly enough; someone else probably could.

Although no match for the CDC equipped BMW or Phaeton on road surfaces, I was best with the CDC equipped Opel Astra and could carry more speed in the slalom and lane change because of the smaller steering wheel, requiring less input. The smaller mass of the Astra and wheelbase also simplify maneuvering.

ZF Sachs talks about the “skyhook” effect, the idea being it’s as though the car hangs from an invisible rail in the sky and the wheels hang down to meet the uneven road surface from that stabilized platform. The vehicle body is kept as calm as possible as the shocks react instantly to the road surface and driving conditions. And the ride did kind of feel like that.

Journalists generally agreed the CDC equipped cars had markedly reduced pitching and rolling motion in all road conditions, superior ride, handing and maneuvering characteristics and, of the cars available for testing, the most improvement was found on the CDC equipped Touareg versus the standard suspension Touareg.

The heart of CDC: The proportioning valve adjusts its operating diameter within just a few milliseconds. This action alters damping force. The change in damper characteristics is this continuously variable.

All vehicles equipped with CDC give the driver a selection of two or three suspension tuning options that can be selected on the fly while driving. In the Astra you can select a ride for comfort mode or a sporty mode — and the system adjusts from softer to harder just like that.

It’s a question of providing bias in one direction or the other. We tried most test cars in all damping modes and you can feel the change instantly. This ushers in a new era of adaptive tuned suspensions. And it lets drivers pick the mode they want instantaneously. If an emergency situation is detected, the bias is eliminated and the system proceeds independently to adjust damping. ZF Sachs talks about the “lightning speed” of the CDC system and let us put some perspective to that to appreciate what the system accomplishes.

First consider that the blink of an eye, ein Augenblick to my friends in Papenburg, is given at 300 milliseconds (ms). This CDC control unit makes a calculation every 2 ms and adjusts the value sent to each of the four proportional control valves in the CDC dampers at each wheel, softening or stiffening the damping action.

Now appreciate that a vehicle traveling at 60 mph moves 88 feet per second. So in 2 ms it travels only 2.1 inches. For reference, in the blink of an eye, the vehicle would travel 13 feet, nearly the overall length of the Astra. Now that simply wouldn’t get the job done, not even close.

Owing to different application requirements, ZF has developed two types of damper shock cylinders for the systems, one with an integrated proportional valve and the other with an external valve mounted at the base. But the operating principle is the same for both.

A solenoid in each valve meters the position of a cone in and out of an orifice to restrict or increase the flow of fluid in the cylinder which produces a softer or harsher response movement of the shock to road input. The system will default to a harsh ride in case of failure. A current of only 0-1.6 amps controls the solenoid. Nominal system power consumption is approximately 15 Watts.

Because of the desired response sensitivity of the system, special care has to be taken to assure the correct bearing resilience especially where either end of the damper connects with the axle and the vehicle body.

This point came up when asking Thomas Kutsche, manager of product development for variable damping systems, what kind of suspension geometry CDC works best with.

“It really doesn’t matter what type of suspension system is used. On the Astra we have McPherson struts on the front and a beam axle at the rear. The critical thing, however, is that the friction in the system is as low as possible in the shocks and ball joints so the CDC can accurately sense the road and driving conditions and react accordingly. You want a very nimble suspension system, that’s all,” Kutsche told us.

The current CDC control unit uses a 16-bit microprocessor to process information from accelerometer sensors at each wheel, a pitch/yaw sensor and information from the vehicle’s CAN (controlled area network) bus to come up with the correct input to each shock. It also provides system diagnosis and can store errors in memory if needed.

On the Astra application the CDC system is married into Opel’s Interactive Driving System (IDS), which links it together to an electronic stability system (ESP) and ABS system, electronic power steering system and shift control unit using CAN bus architecture.

With all these systems tied in, and with the CDC system keeping the wheels firmly on the road, it is also possible to optimize wheel contact forces and significantly reduce braking distances. No U.S. built autos offer the CDC system yet, but it probably isn’t too far away. My bet is it will be an SUV application that comes first, probably an up-market model where the cost of the system becomes less significant. But then again if Opel can offer it in an Astra level car, there are certainly numerous possibilities.