Modelling, identification and experimental validation of a hydraulic manipulator joint for control

In this paper, modelling and identification of a hydraulic servoactuator system is presented. The development of the model is important for further understanding the system and for developing a robust force controller. A systems approach is used to model the various subsystems including the servovalve dynamics, fluid dynamics and the vane and load dynamics. Included in the model are line losses, leakage, and hysteresis. System parameters are identified using the elbow joint of the SARCOS slave experimental hydraulic manipulator. Specialized hardware was designed and constructed for this purpose. The model was validated by comparing simulation and experimental results. The correlation between model and actual system response proved to be very good. Hence, the developed model predicts well system dynamic behavior and will prove useful in the development of a robust force controller. 1: Introduction Many tasks that require humans to interact with their environment, either through direct contact or via a tool, may pose a danger to complete due to precarious work location or environmental conditions. For example, some applications include hazardous waste management, underwater operations, fire-fighting and live-line maintenance. Teleoperation or automation of such tasks would distance humans from dangerous sites reducing risk of injury and would increase efficiency. However, the control of manipulators interacting with an environment is very complex due to several factors. The manipulator may be located on a vehicle or on a long boom and the manipulator itself may demonstrate some degree of flexibility due to actuator, sensor and link dynamics. Also, a particular task may require large forces to be transmitted to the environment. Of special interest are manipulators with hydraulic actuators, due to their high force output to weight ratio, their inertance to fire hazards and the availability of hydraulic power in mobile applications. Hydraulic actuators further complicate the control of manipulators in contact with their environment. Unlike electrical actuators in which a current produces a torque, a current input to a hydraulic actuator modulates valve resistance. Thus, the direct control of torque is not as easily accomplished. In order to provide an effective and robust control of the interaction between a hydraulic manipulator and its environment, a dynamic model of the robot's joints is required. In turn, a model will be useful in the development, simulation and implementation of a controller for a hydraulic manipulator. The majority of prior work in automation or teleoperated control of manipulators deals with electrically actuated manipulators. In terms of hydraulic actuators, comparatively less work has been done. Previous research has spanned both modelling and control of hydraulic actuators. With respect to modelling, some works deal with the traditional spool valve, for which the orifice areas are generally linear with respect to the valve position. In contrast, the servovalve used in this work is of the jet-pipe/suspension type which is more complex. One advantage of this type of servovalve is that there is no contact between surfaces as there is between the spool and the spool housing. Another advantage is that these valves can be very fast due to their small moving mass. For the jet-pipe servovalve, a detailed model is proposed and studied in [4] and [12]. In terms of control of hydraulic actuators, modelling physical effects is important. In previous works, position and force control have been studied. A linearized model was used for position control of a spool-valve and rotary actuator system, [9]. A model was used in a feedforward simulation filter for control of a hydraulically actuated flexible manipulator, [10]. In force control, several algorithms in the hybrid position/force control have been developed using a model-based approach, [3], [6], and [14]. Explicit force control algorithms for hydraulic actuators have been demonstrated in [4] and [11]. The impedance control law, which is model-based, was applied to a hydraulic manipulator in [8]. Although the focus is on control, modelling is essential in understanding the system to be controlled. One way of obtaining a faithful and robust controller is to include a model-based portion in order to reduce control effort. This paper deals with the accurate modelling of the elbow joint of a small slave SARCOS dexterous manipulator with the goal of using this model for control purposes. This model includes hysteresis, orifice areas, damping, and leakage. To date, no model of a hydraulically actuated joint which includes servovalve and rotary actuator dynamics is available in the literature. The paper is organized as follows: in Section 2 the physical characteristics of the joint's subsystems are discussed. System equations are given. Section 3 describes the experimental setup and discusses the parameters identified and the procedures used. Section 4 compares experimental results with simulation results, validating the model. Finally, conclusions are given in Section 5. 2: System Modelling Due to the highly non-linear nature of this hydraulic system, the possibility of instability and limited performance in control is high. Thus, a model of each subsystem will allow an accurate characterization of the system and a good prediction of the system’s behavior. The hydraulic joint system is composed of a one-stage suspension type servovalve, hydraulic lines, a rotary actuator and a load. Each subsystem is shown in Figure 1. The bond graph of this system is given in a previous technical article [1]. In this section, the equations describing the dynamics of these subsystems are discussed. The end result is a set of first order nonlinear equations of the form ̇ , x f(x,u) y g(x,u) = = (1) First, consideration is given to the hysteresis effects in the servovalve. Second, the valve tip dynamics are described. Fluid dynamics are then discussed, along with the vane and load system equations. Figure 1. Joint and Valve Schematic. 2.1: Servovalve Hysteresis An important phenomenon in the servovalve is hysteresis. In order to include the hysteresis model, an additional differential equation is added which requires, as input, the actual current and produces as output a virtual current, ihys, which modulates the valve position. To mathematically represent the phenomenon of hysteresis, a model based on the Jiles-Atherton theory for magnetization of ferromagnetic material is used, [5]. The model is suitable for insertion into system simulation. It accounts for major and minor loops with knowledge of only the switching point. For modelling purposes, we consider a one-to-one hysteresis between the current after hysteresis, ihys, and the actual current. With the notation used in this paper, the formulation, derived from [5], is