Tom Cheuk-In Wong

Hydraulic Power Steering System Design and Optimization Simulation

Hydraulic rack and pinion power steering system is a high This increases the differential pressure acting on the power bandwidth servo with stringent performance requirements piston and yields the desirable hydraulic power assist force. on accuracy, reliability, and cost. Design of such a system An optional speed sensitive steering controller modulates can be best achieved by using a validated and user friendly the fluid flow rate drawn by the RSV from the supply line, computer simulation program. Hydraulic integrated power using signals from wheel speed sensors and wheel torque steering ( HIPS ) program has been developed using basic sensor. The driver’s steering effort is increased by lowering concepts from science and engineering. HIPS provides a the hydraulic power assist during highway driving, hence design and test environment for the integrated steering and improving driver’s feel of the road during city and highway suspension system subjected to disturbance forces, which driving. The steering torque is obtained when the rack force may be induced by pump flow oscillations and tire loads. is applied to an off-center joint on the knuckle, which turns Two real-world automotive hydraulic steering systems are about the kingpin at the suspension strut, including a coil simulated with HIPS. The simulation results agree closely spring and damper assembly. with the dynamometer test results. The application of HIPS for design optimization is also demonstrated. The hydraulic power assist level, which is proportional to the diverted amount of fluid flow rate from the supply line INTRODUCTION by the RSV, is controlled by the relative angle between the steering wheel and pinion angles. This valve relative angle, Figure 1 shows a hydraulic rack and pinion power steering which is also called valve error angle, is equal to the twist system, which is a high bandwidth nonlinear servo capable angle of the torsion bar. The torsion bar is connected to the of generating a rack force of 4,000 N for cars and 6,000 N steering wheel intermediate shaft and the pinion at its top for small trucks. About 80% of the rack force comes from and bottom ends respectively. Torsion bar stiffness yields hydraulic power assist and the remaining 20% comes from the driver’s steering effort, and is designed to let the driver driver’s effort. At on-center position of the steering wheel, to turn the steering wheel with ease, and at the same time, a vane pump, which is driven by the engine, circulates the to give a memory function for the RSV in order to reduce fluid in a closed -loop hydraulic circuit which includes a the error angle towards zero, after the end of the driver’s reservoir, vane pump with flow control and pressure relief, steering demand. Since the RSV is a servo valve, during supply and return lines, and the rotary spool valve ( RSV ). on-center operation, it generates an opposing rack force to The flow control valve which connects the discharge and power piston motion subjected to tire shock and vibration inlet manifolds of the vane pump, regulates the pump flow loads. rate into the supply line at 2.1 gpm for cars and 3.5 gpm for small trucks, for engine speeds above 900 rpm. When Figure 2 shows a simplified schematic of the RSV and its the steering wheel is turned by the driver, RSV diverts the valve profile curves for two operating conditions. In valve supply line fluid flow to either side of the power piston for operating condition 1, occurring during parking, the valve a right or left turn of the vehicle. At the same time, RSV error angle is large, and valve gain, which is proportional passes an equal amount of fluid flow from the other side to the slope on the profile curve is high. As shown on the of the power piston through the return line to the reservoir. related valve profile curve, any oscillatory rack motion resulting from oscillatory pump flow rates induces a high Two real-world examples are studied with HIPS. Example 1 amplitude oscillatory pressure wave in the power cylinder is related to the front-end mechanical noise generated when of the steering gear. When this pressure wave excites the a car is driven at low speed on a bumpy road. Example 2 is structural modes at the support frame, steering shudder related to the steering wheel dither generated when a small occurs. Test results clearly show that the higher the slope truck with an unbalanced tire is driven at a certain speed on of the valve profile curve at the operating point, the more the highway. the intensity of shudder. In valve operating condition 2 occurring during city and highway driving, any oscillatory MODELING rack motion caused by tire shock and vibration loads will be opposed by the hydraulic force resulting from the servo The most important requirement in modeling a system is the action of the rotary spool valve. This causes the formation complete understanding of the performance specifications, of oscillatory pressure waves in both power cylinders. But physical and operational characteristics of each component the valve gain is low, and hence this pressure wave may in the system. The hydraulic rack and pinion power steering not have sufficient intensity to generate shudder. system of an automotive vehicle consists of hydraulic fluid lines, RSV, vane pump including flow control and pressure The steering system noise, vibration, and harshness(NVH) relief valves, power actuator, inner and outer tie rods, lower related problems occur from dynamic interaction between and upper control arms, suspension struts, front and rear roll the steering gear and suspension systems subjected to the stabilizers, disturbances such as engine torque pulsation and disturbances, such as engine torque pulsation, tire shock tire loads. A logical modularization of the integrated system and vibration loads. These NVH problems include: into steering gear and suspension component modules, and 1) Steering shudder; resulting from the excitation of the establishment of state variables with initial conditions in each fundamental frequency at the frame support by a fluid module, are determined to avoid integration blow-up and periodic flow force caused by the engine-driven vane achieve correct interpretation of simulation results. This is a pump. required method, since the integrated model of the real-world 2) Steering wheel nibble; resulting from the excitation of system under consideration consist of nonlinear components the fundamental frequency of the rack and pinion gear with discontinuous behavior. Figure 3 displays the modeling mechanism by a periodic rack force induced by brake architecture of the HIPS simulation program. disk roughness, during braking at highway speeds. 1) Steering Gear Model. 3) Mechanical front-end noise; resulting from the dynamic Figure 4 shows the hydraulic power steering model which interaction between the rack and steering gear housing, consists of the following component models: caused by tire shock and vibration loads generated by a) Closed-loop hydraulic circuit; including vane pump, tires riding on the bumps, stones, and pot holes on the flow control valve with pressure relief, tuned supply road during low speed driving. and return lines, cooler, reservoir, and RSV. 4) Steering wheel dither; caused by dynamic interaction b) Power actuator; including the steering wheel, torsion between the steering gear and front suspension struts bar, pinion gear, rack spring preload, power piston, subjected to a periodic vertical tire load induced by a tie rods, knuckle, tires, and housing. tire high spot at a certain highway speed. c) Driver commands for applying steering angle, steering rate, and engine rpm profiles. Present solutions for steering system NVH problems d) Disturbances; including pump flow rate oscillations and mentioned above, are usually achieved by using tuned tire shock and vibration loads, which occur when driving hoses, shorter or longer hoses in the hydraulic lines, on a rough terrain or with tire high spots on a highway. passive and active dampers, and reducing the RSV gain. Since, all of the above solutions are based on empirical The steering gear model is obtained by applying fundamental rules, their adaptation to new car platforms would be laws from fluid dynamics, heat transfer, and dynamics. The time consuming and expensive. But, these solutions may state variables are described by linear and nonlinear ordinary cause an increase in power losses, and a reduction in the ordinary differential equations, with discontinuous behavior steering system bandwidth. The steering system NVH and temperature-dependent parameters. related problems are solved best, by using a validated 2) Suspension Model. and user-friendly computer simulation program. The Figure 5 shows an 4-wheel independent suspension system hydraulic integrated power steering ( HIPS ) simulation consisting of the following component models: program consists of two modules: Module 1 contains a) The sprung mass frame, with three degrees of freedom; the steering gear model, and Module 2 contains the heave, roll, and pitch. suspension model. b) Four unsprung masses, each with heave motion. c) Four suspension struts. Each strut consists of a c) Optimization Test. coil spring and a nonlinear damper. Each strut is The system parameters representing torsion bar stiffness, connected between the movable end of the lower rack spring preload, and bushing stiffness at the housing control arm and a corner of the frame or the upper supports of the steering gear have been changed, one at control arm. Each control arm is pivoted about a a time, and corresponding simulation runs were carried rubber bushing connected to the frame. This gives out to obtain responses for XRK3MM and ZRK3MM. heave motion degree-of-freedom to the tire. Each The main goal of this study was to reduce the amplitude strut is tilted with respect to vertical direction, as of above responses, without degrading the steering fe