Real-time force control for servo-hydraulic actuator systems using adaptive time series compensator and compliance springs

Servo-hydraulic actuators have been widely used for experimental studies in engineering. They can be controlled in either displacement or force control mode depending on the purpose of a test. It is necessary to control the actuators in real time when the rate-dependency effect of a test specimen needs to be accounted for under dynamic loads. Real-time hybrid simulation (RTHS) and effective force testing (EFT) method, which can consider the rate-dependency effect, have been known as viable alternatives to the shake table testing method. Due to the lack of knowledge in real-time force control, however, the structures that can be tested with RTHS and EFT are fairly limited. For instance, satisfying the force boundary condition for axially stiff members is a challenging task in RTHS, while EFT has a difficulty to be implemented for nonlinear structures. In order to resolve these issues, this paper introduces new real-time force control methods utilizing the adaptive time series (ATS) compensator and compliance springs. Unlike existing methods, the proposed force control methods do not require the structural modeling of a test structure, making it easy to be implemented especially for nonlinear structures. The force tracking performance of the proposed methods is evaluated for a small-scale steel mass block system with a magneto-rheological damper subjected to various target forces. Accuracy, time delay, and resonance response of these methods are discussed along with their force control performance for an axially stiff member. Overall, a satisfactory force tracking performance was observed by using the proposed force control methods.