INTRODUCTION The main function of an automotive suspension system is to provide comfort to the passengers by isolating the vehicle motion from the road irregularities, and to ensure the road holding and handling in any driving condition. According to the level of damping force, the automotive suspension systems can be classified in passive, semi-active and active. The passive suspension systems are the most used in commercial vehicles, the damping properties are designed mechanically; however, the constant damping force can have many limitations for ensuring the comfort and safety under risk driving conditions. On the other hand, the semi-active and active suspension systems can regulate the vehicle motions through a control system over the damping force. The framework of this kind of suspensions can be magnetic, hydraulic or pneumatic. The active suspension achieves a better comfort performance but it is necessary to have an external power system for supplying/dissipating energy from the tire and chassis motions; its practical implementation is more expensive than a semi-active suspension system. Thus, a semi-active damper allows to control the force for reducing the relative movement between the sprung and unsprung masses in avehicle corner; however, the power requirement is lower than an active damper. Commercially, there are two types of semi- active dampers: mono-tube and double-tube, the mono-tube dampers are the most used; the Magneto-Rheological (MR), electro-hydraulic and pneumatic dampers are commercial mono-tube dampers. The main advantages of the MR damper versus the other semi-active dampers are: (a) fast time response (20-40 ms) and, (b) long bandwidth of control. The MR damper is a non-linear device with dissipative capability used in an Automotive Suspension Control System (ASCS), where the damping coefficient varies according to the applied electric current. This damper allows to achieve good comfort, to emphasize road holding and to keep a safety suspension deflection. In an experimental evaluation, it has proved that the semi-active dampers, with up 25 operating states, only activates two states for an average of 92% of the driving time [ 1], concluding that an on-off control strategy offers sufficient potential benefit to evaluate the semi-active property. In the practical side, the model-free on-off controllers Sky-Hook (SH) technique [ 2], hybrid technique [3], Mix-1-sensor (Mix-1) strategy [ 4], and Frequency Estimation-Based (FEB) control [ 5] have and efficient performance in comfort. The hybrid controller is an improvement of the SH control because the Ground-Hook 2012-01-0979 Published 04/16/2012 Copyright © 2012 SAE International doi:10.4271/2012-01-0979 saecomveh.saejournals.org Efficiency of On-Off Semiactive Suspensions in a Pick-up Truck Juan C. Tudon-Martinez, Jorge Lozoya-Santos and Ruben Morales-Menendez Tecnologico de Monterrey ABSTRACT A comparative analysis between three different semi-active suspension control techniques in a full vehicle model of a pick-up truck is presented. Each independent corner of the vehicle uses a Magneto-Rheological ( MR) damper model. The MR damper model includes nonlinearities, hysteresis, and transient response between the manipulation and actuation. The damper model parameters have been obtained from experimental data provided from a BWI™ MR damper. Different tests were used to evaluate the semi-active control strategies by using the CarSim® software. The evaluated controllers, which are based on switching manipulation between the low and high damping force, are: 1) the hybrid controller, 2) Mix 1- sensor (Mix-1) and 3) Frequency Estimation-Based (FEB) controller. These selected controllers share the next features: the use of few sensors, free of models, and they do not require anti-saturation mechanisms. Simulation results showed that the FEB controller had the best performance for passengers comfort in the bounce sine sweep test (9% less of movement in th