Link readies semi-active cab suspension for intro on select 2020 models
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To perform a side-by-side comparison, Link purchased a 2016 Peterbilt 579 80-in sleeper cab. With the 579, Link can observe the vehicle’s original ride features and compare them to the enhanced characteristics of its new semi-active cab suspension. (image: Link)

Link readies semi-active cab suspension for intro on select 2020 models

Link Manufacturing targets heavy-duty vehicles that transition from on- to off-road terrain, in particular, for what it claims will be the industry’s first intelligent semi-active cab suspension. The benefits of such a system are many, according to the company’s VP of Engineering, Bill Ott. Chief among them is an improved work environment for operators, which could ultimately help with driver retention, he said.

Less stress on the cab, its structure and the driver also means lower maintenance costs, better vehicle handling and reduced driver fatigue, which could result in increased productivity. For these reasons—and the not-insignificant matter of reasonable cost—Ott believes the time has finally come for market acceptance of this technology. 

“Not surprisingly, it will cost more than our traditional Cabmate due to the added components and complexity. The level of integration into the OEM electronics will also have a large impact on the final cost. When all the dust settles, I believe our system will be less than the cost of installing two premium seats in a sleeper cab,” he told Truck & Off-Highway Engineering.

Link began exploring semi-active cab suspensions prior to 2006 but shelved the project because the technology was cost-prohibitive. The company’s engineering team revisited the idea in 2014.

“Our skunkworks team started by determining that we could simplify the dampers and control systems for a cab suspension because the cab suspension is itself suspended by a primary system,” Ott said. “We’re taking this into account to reduce the cost and complexity of our semi-active Cabmate.”

In-house-developed system
The project team determined that most of the system’s development could be accomplished in-house. Link created its own control system, writing proprietary algorithms to govern system performance. The team continues to work with its shock absorber suppliers to perfect dampers that will be exclusive to the semi-active Cabmate application. Both magnetorheological (MR) dampers and hydraulic actuation are being explored.

“MR damping and hydraulic controls are used in primary suspensions in the automotive world. They both have their strong points and their weak points,” Ott said. “We are testing both. If both types perform well on fleet testing, it may boil down to economic considerations.”

In addition to the proprietary control system and shock absorbers, the system consists of an electronic control unit (ECU), accelerometers, and position sensors, as well as assorted brackets and lateral control rods. 

The ECU was the only off-the-shelf component that Link needed to buy, according to Ott. While the ECU adds an initial 1 lb (0.45 kg) to the weight of a cab, Ott notes that once OEMs begin to integrate the new Cabmate semi-active system into their equipment builds, even that minimal weight gain will be eliminated by loading the licensed software onto their resident ECUs.

“Our biggest challenge has been engineering the control system and then adding the commercial and customer interface aspects to the software,” Ott told TOHE.

The system works by taking in information via two accelerometers, positioned on either side of the cab, that continuously measure acceleration at the top of the suspension. Data also comes from two position sensors, also located on both sides of the vehicle, that constantly monitor the position and velocity of the vehicle’s primary suspension.

The system’s ECU reads information from the accelerometers and position sensors and determines the optimum settings for the shock absorbers to maximize ride comfort. The ECU also uses the information to determine whether it needs to fill or exhaust air from the system’s air springs. The ECU’s adjustment command signals are output directly to the shocks and air valves. These signals and the resulting adjustments occur hundreds of times per second.

With the MR approach, voltage from the ECU causes the fluid in the shock to thicken very quickly, making the shock stiffer when needed. The hydraulic method employs an orifice that rapidly adjusts its size to either admit or impede fluid migration as needed to mitigate the activity of the frame rails, Ott explained. Neither system requires more than 10 watts to operate.

The Cabmate semi-active suspension also features electronic height control.

Testing proves less vibration exposure
Link has long concerned itself with driver comfort and safety in the commercial vehicle industry, Ott said. 

“A number of years ago, we started using ISO2631 to evaluate driver vibration exposure as a function of acceleration, frequency and time,” much like measurements to sound exposure, he explained. “While there are no regulatory limits in the U.S., the physical effects can be noticeable.” 

The European Union does, however, limit driver whole body vibration exposure, based on ISO2631. Over time, whole body vibration above a certain level can have a deleterious effect on some drivers’ health, Ott noted.

“We are finding in limited testing so far that the vibration exposure from the semi-active system is lower than that from our normal Cabmate—i.e., 8 hours in the normally suspended cab gives the same amount of vibration as 10 hours in the semi-active suspended cab—and this was on pretty smooth roads. We have not evaluated off-road applications yet,” he told TOHE in May.

The Link engineering team purchased a Peterbilt 579 80-in sleeper cab in 2016 to use for semi-active suspension development and testing. The 579 already featured the original Cabmate system.

“We also have a very complete physical test facility where we perform endurance testing as well as vibration, bushing and suspension transmissibility testing,” Ott said. “In addition, Link is owned by a family that also owns a 200-plus truck fleet where we have done testing. And, we are beginning a national field testing program on a number of fleets using different tractors and pulling different loads over different roads.”

Link is currently in discussions with multiple OEMs and expects to see the Cabmate semi-active cab suspension available on select 2020 and 2021 model vehicles. Continue reading »
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