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'Smart' shock absorber needs no electronics
Using a fast-acting hydraulic valve, engineers have created a smart shock absorber
By Charles J. Murray, Senior Regional Technical Editor -- Design News, December 20, 1999
Atlanta, GA-Using a fast-acting hydraulic valve, engineers have created a "smart" shock absorber that can quickly change its cushioning characteristics without input from an electronic controller. The new shock absorber enables automakers to provide a vehicle with greater control during hard cornering, while at the same time supplying a softer ride over bumpy surfaces.
Although most automakers already offer so-called "active" suspensions, all of them employ electronic control (see DN, July 19, 1999, p. 41, 56). In contrast, the new shock absorber is purely mechanical. As a result, it eliminates the need for automakers to embark on costly development programs and write specialized algorithms. Known as the Monroe Reflex shock absorber, it can be used on any size vehicle, ranging from the smallest compact to a full-size truck.
The key to the success of the new shock absorber is its fast-acting valve. The Impact Sensor, as it's known, allows the Reflex to behave like a conventional shock absorber, until it hits a bump that sends 1.5 g's into the wheel. When it does that, the valve provides additional flow to dampen the impact. Because it acts within 12 milliseconds, the Impact Sensor enables the shock absorber to cushion the vehicle's occupants from forces that would ordinarily be felt through the seats and steering wheel.
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| Adjusting hydraulic oil flow between the outer (reserve) tube and inner (pressure) tube provides improved damping characteristics. |
The Impact Sensor consists of three main parts: a powder metal element; a spring; and a valve body. During operation, the powder metal element plays a simple role: It acts as a mass that bears against the spring. When the vehicle drives along a smooth road, the mass and the spring move together. However, when it hits a hard bump-reaching a pre-determined acceleration force of 1.5 g's or greater--the valve body and spring move up, but the mass doesn't. (Engineers sized the powder metal element so its mass would take effect precisely at 1.5 g's.)
As a result, it bears hard against the spring, compressing it. This, in turn, opens the valve's passages. Hydraulic oil then flows through the center of the valve body, out through holes in the bottom of the valve, and into an area located between the shock absorber's outer tube and its pressure tube. Result: The shock absorber damps the impact, enabling the vehicle to strike a balance between control and cushioning.
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| The valve’s powder metal element overcomes the upward force of the spring, causing oil to flow through the center of the valve and out to a location where it can provide additional cushioning. |
The ability to react so quickly is what distinguishes the Reflex from other purely mechanical systems that have tried to achieve the same thing. Because the Reflex acts within 12 milliseconds, it adjusts control of the shock absorber while it is still in compression. That's unique among such systems. Most don't react while the shock absorber is still in compression. Instead, they react after full compression has been reached--in other words, when the shock absorber has begun to extend, or rebound, after the impact. But by then, say Tenneco engineers, it's too late. "You want it to respond while the wheel is coming up, not after it starts going back down," notes Tom Trent, lead development engineer for Tenneco Automotive, designers of the Reflex. "If you respond after the wheel starts coming back down, the driver has already felt the impact. But if you react while the shock absorber is still in compression, you can take the edge off those bumps."
For OEMs, the Impact Sensor offers similarly important benefits. It eliminates the need to run wires between sensors and control modules. It also eliminates significant development time and cost. For those reasons, it has been incorporated on the 2000 Nissan Altima, and will be available for aftermarket use. "All of the previous solutions have been very costly and have limited applicability," Trent says. "This is a much simpler, integrated solution. It's all there in one shock absorber."
Additional details...Contact Tom Trent, Tenneco Automotive, One International Drive, Monroe, MI 48161; Tel: (734)243-8136; Fax: (734)243-8249; E-mail: tom.trent@tenneco-automotive.com
Other Applications
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Minivans
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Trucks
Active Suspension Performance
Active suspension type
Reaction time
Acts in rebound and/or compression
Ultra-sensitive electronic
10-20 milliseconds
Adjustable in both directions
Less sensitive electronic
70-150 milliseconds
Adjustable in both directions
Mechanical
Unknown
Rebound
Monroe Reflex
12 milliseconds
Compression
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