The electrical-mechanical interfaces of servo valves and proportional valves display electromagnetic nonlinearities, such as hysteresis and saturation. In many cases, these nonlinearities have usually unfavorable effects on the performance of the whole control system. For this reason the concept of stage proportional control is put forward in this paper. The fundamental idea of the stage proportional control is to divide the whole stroke of the hydraulic (pneumatic) control valve into a number of stages in which independent proportional control is carried out.In this way, the electromagnetic nonlinearities can be greatly reduced. The stage proportional control can be realized by using a linear or rotary stepper motor controlled in a continuous mode. Experimental results demonstrated the electromagnetic nonlinearities associated with proportional control are indeed reduced and the position stiness of the spool and the resolution accuracy are also enhanced. An application example of the stage proportional control is also presented in this paper.

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The iterative learning control (ILC) learns the unknown information from repeated control operations. The tracking error from previous stages is used as the correction factor for the next control action. Therefore, the ILC controller can make the system tracking error converge. to a small region within the limited numbers of iterations. In this paper, a proportional-valve controlled hydraulic cylinder system with repeated external loads is built to perform the position tracking control experiments. The PID and ILC controllers are implemented in these experiments and the results are compared. We use the P-type update law with delay parameter for its position tracking control. Experimental results show that the ILC controller can effectively control the system to track the repetitive position trajectory, especially in transient response that traditional control methods can't follow appropriately. Therefore, in this application, the control performance of the ILC is superior to the traditional PID controller.

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An electro-hydraulic manipulator with hydraulic actuators has many nonlinear elements, and its parameter fl uctuations are greater than those of an electrically driven manipulator. So it is quite difficult to obtain good robust control performance. In this paper, we propose a two-degree-of-freedom control method with parallel feedforward compensators (PFC). The PFC plays a most important role in the stability of a proposed control system. Through the experimental results applied to a six-link electro-hydraulic serial manipulator, it is shown that the stability and therobustness against disturbances are improved by using the proposed control method. As the results of our previous papers, it was shown that the control method using disturbance observer designed by making use of H-infinity control theory is the most suitable among other robust control methods. Through the comparisons with the results of the disturbance observer, it is ascertained that the proposed control method is also suitable for the control as well as the disturbance observer above mentioned. We examined and established the design criterion of control system using the proposed method. The optimal parameters of the controller can be easily determined by using the design algorithm proposed here. The effectiveness of this proposed control method is verified through experiments of the six-link electro-hydraulic serial manipulator.

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The Field Robot means the machinery applied for outdoor tasks in construction, agriculture and undersea etc. In this study, to field-robotize a hydraulic excavator that is mostly used in construction working, we have developed an automatic excavation system and adaptive control system. A model-reference adaptive controller has been designed on the model that is obtained through off-line System Identification. It is illustrated by computer simulations that the proposed control system gives good performances in the trajectory tracking control and adaptation to parametervariation.

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The minimal controller synthesis (MCS) algorithm is one of the techniques of the model reference adaptive control, which is applicable for a system whose dynamic model is almost unknown. But, from our previous experimental study, we found out two problems in the practical implementation of MCS algorithm. The first is that the error convergence is prescribed by the dynamic characteristic of the reference model. The second is the nonlinearity in which the control characteristic of the system depends on the allocation of the origin of coordinates for the state variables. In this paper, the improved MCS algorithm for solving the problems is proposed. It is experimentally applied to the adaptive impedance control of a hydraulic robot without joint torque sensors and the experimental results illustrate the validity of the proposed algorithm. Although, in the impedance control, the torque control system using joint torque sensors is usually needed, our results confirms that the robustness of MCS algorithm makes it possible to implement the adaptive impedance control for hydraulic robots without joint torque sensors.

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