A team of engineers at the U.S. Army Tank Automotive Research, Development, and Engineering Center (TARDEC) has been working with a suspension OEM to assess the possible improvement and feasibility of integrating a magneto-rheological (MR) semi-active damper suspension into the Stryker family of vehicles. Filled with MR fluid, MR dampers are controlled by a magnetic field, usually an electromagnet, and by varying the power of the electromagnet, the damping characteristics of the shock absorber can be continuously controlled.
Due to their flexibility to adapt to a range of damping needs using custom control systems, MR dampers have been studied by many manufacturers to develop optimal ride solutions. Many studies in the past demonstrated their usefulness either by solely conducting modeling and simulation (M&S) or through full-vehicle testing. While a pure M&S approach is efficient, it lacks subjective feel. And though full-vehicle testing presents higher confidence, it is prohibitively expensive in many cases. Real-time driving simulation presents a reasonable compromise between M&S and testing where a driving simulator simulates a vehicle dynamics model while providing the desired subjective feedback to the driver.
To help demonstrate the feel of the baseline system and compare it to the MR system, a driving simulation model was developed. The simulation was conducted on TARDEC’s Ride Motion Simulator (RMS), a six-degree-of-freedom (6-DOF) hexapod simulator. A “black-box” Simulink model for the MR damper, as provided by the OEM, was integrated into the Stryker passive dynamics model.
The hydraulically driven RMS consisted of LCD screens for an out-the-window view of the environment, a speaker system for vehicle sounds, a steering wheel, and pedals. The RMS is a 40-Hz single-occupant simulator capable of producing linear accelerations of ±2 g (lateral, longitudinal, vertical) and angular accelerations of ±1150°/sec² (roll, pitch, yaw). It can be used to reproduce the ride of both wheeled and tracked military ground vehicles.
One of the major challenges faced by simulation engineers is the limitation of 6-DOF driving simulators to recreate the sustained lateral or longitudinal accelerations found in real-world driving due to the limited motion envelope. The RMS has a linear range of ±20 in (508 mm) and an angular range of ±20°. To make up for the limited motion envelope, simulation engineers utilize washout/tilt-coordination filtering, which augments lateral and longitudinal accelerations with roll and pitch displacements.
To drive the simulator, real-time dynamics models of the two Stryker variants were developed using SimCreator from Realtime Technologies Inc. The simulated MR damper control component, developed by the suspension OEM, used the same control logic as the actual vehicle system.
The Stryker vehicle model consists of four independent axles: two in the front and two in the rear. All axles are driven and consist of MacPherson strut suspensions with the front two axles as steering axles. All suspension kinematics are represented parametrically using ride and camber curves. Suspension stiffness and damping were represented at the wheel using representative motion ratios. The vehicle model was tuned to match corner weights and weight distribution, with the representative inertial properties.
The maximum lateral acceleration was achieved with a steady-state cornering procedure by driving the vehicle model at a constant speed of 40 mph (64 km/h) and slowly increasing the steering angle until it began to slide. The vehicle model achieved the maximum lateral acceleration of 0.43 g, which seemed reasonable based on the historical test data. A NATO lane-change maneuver was simulated at 40 mph (64 km/h) and compared against test for the roll rate. Both test and simulation indicated the maximum roll rate between 10-12°/sec. Though the terrain and vehicle did not precisely reproduce the ride seen from the field data, the difference between the passive and MR variants was deemed close enough to be a useful assessment tool.
Also integrated into the simulation was a virtual terrain for participants to drive on in real time. The participants drove the vehicle on two different simulated terrains. The first one was a simple linear bump course consisting of a line of two sizes of triangular bumps. The first was 0.26 m tall, 6 m long, and 10 m wide (0.85, 20, and 33 ft). The second was 0.13 m tall, 6 m long, and 10 m wide (0.43, 20, and 33 ft). The bump course was meant to give the occupant a simple input to compare the suspensions while they got used to how the simulator behaved during driving. The second course was a simulated version of the U.S. Army’s Aberdeen Proving Grounds Munson Test Area. Within Munson, occupants drove over a portion of the Munson Gravel course and all of the Belgian Block course.
The Munson test area was determined to be a suitable terrain to test the ride of the MR suspension. However, to develop a terrain that will allow the simulation to run in real time as well as be visually and physically accurate, the terrain database needed to be simplified. For simplification, the terrain database was separated into two files. One file contained the visual database, which consisted of the polygonal representation of the terrain and the overlay textures. The other contained higher resolution data of the courses being driven.
Initially, the modeled test courses did not contain enough resolution to produce a vehicle response that would adequately highlight the difference between the passive suspension and the semi-active MR suspension. For graphics to run smoothly in real time, the complex road details were replaced with largely flat polygons that describe the general shape and large bumps on the road. While this may be suitable for some experiments, it is not acceptable for a suspension evaluation. Preliminary drives on the terrain produced rigid body mode frequency response below 2 Hz. To obtain higher frequency road roughness, a technique known as texture bump mapping was applied to the roads.
A team of engineers from TARDEC, TACOM, and the OEM took the opportunity to drive a simulated passive and MR damped Stryker. Though this wasn’t a formal test to gather data, some ride data was collected by measuring command accelerations to the simulator. Also, participants gave their subjective opinions of the ride of the Stryker with and without the MR suspension as a blind evaluation.
On the linear bump course, all participants could tell which vehicle was the passive suspension or the MR suspension and unanimously preferred the MR damper. On the Belgian Block course, it was suggested to drivers to maintain a vehicle speed of 25 mph (40.2 kph) to best feel the difference in the two suspensions. However, many drivers did not maintain 25 mph due to simulator discomfort, which resulted in many of the drivers not being able to feel the difference between the two variants. Also, it was determined that some participants couldn’t get used to the washout algorithms that are used to move the simulator. When the simulator has to produce a sustained linear acceleration but hits the end of its travel, the simulator tilts to make up for the lack of linear space. The washout algorithms may have caused some participants to believe the vehicle was pitching/rolling more than it actually was. However, the objective analysis of accelerations during those maneuvers showed demonstrable benefit of the MR damper over the passive one. The MR suspension significantly reduced the magnitude vertical accelerations and lowered the absorbed power of the ride.
This article is based on SAE International tech paper 2012-01-0303 by Michael G. Megiveron and Amandeep Singh of the U.S. Army Tank Automotive Research, Development, and Engineering Center.