Force Control Compensation Method with Variable Load Stiffness and Damping of the Hydraulic Drive Unit Force Control System

Each joint of hydraulic drive quadruped robot is driven by the hydraulic drive unit（HDU）, and the contacting between the robot foot end and the ground is complex and variable, which increases the difficulty of force control inevitably. In the recent years, although many scholars researched some control methods such as disturbance rejection control, parameter self-adaptive control, impedance control and so on, to improve the force control performance of HDU, the robustness of the force control still needs improving. Therefore, how to simulate the complex and variable load characteristics of the environment structure and how to ensure HDU having excellent force control performance with the complex and variable load characteristics are key issues to be solved in this paper. The force control system mathematic model of HDU is established by the mechanism modeling method, and the theoretical models of a novel force control compensation method and a load characteristics simulation method under different environment structures are derived, considering the dynamic characteristics of the load stiffness and the load damping under different environment structures. Then, simulation effects of the variable load stiffness and load damping under the step and sinusoidal load force are analyzed experimentally on the HDU force control performance test platform, which provides the foundation for the force control compensation experiment research. In addition, the optimized PID control parameters are designed to make the HDU have better force control performance with suitable load stiffness and load damping, under which the force control compensation method is introduced, and the robustness of the force control system with several constant load characteristics and the variable load characteristics respectively are comparatively analyzed by experiment. The research results indicate that if the load characteristics are known, the force control compensation method presented in this paper has positive compensation effects on the load characteristics variation, i.e., this method decreases the effects of the load characteristics variation on the force control performance and enhances the force control system robustness with the constant PID parameters, thereby, the online PID parameters tuning control method which is complex needs not be adopted. All the above research provides theoretical and experimental foundation for the force control method of the quadruped robot joints with high robustness.

A Parallel-type Load Damping Factor Controller for Frequency Regulation in Power Systems with High Penetration of Renewable Energy Sources

Renewable energy sources(RESs)are rapidly devel-oping and their substitution for traditional power generation poses significant challenges to the frequency regulation in power systems.The load damping factor D characterizes the active power of load that changes with power system frequency,which is an important factor influencing the frequency response.However,the value of D is small,resulting in the limitation in frequency regulation of the power system.This paper proposes a parallel-type load damping factor controller to enhance load damping factor by utilizing static var generators(SVGs)in substations.Additionally,it discusses the configuration method for the relevant parameters of the controller,evaluates its frequen-cy regulation capability,investigates the impact of large-scale application of the controller on static and dynamic loads,and conducts a comprehensive evaluation of the impact of the damping factor control process on the voltage stability of the main grid.The large-scale application of the proposed controller can significantly improve the frequency regulation capability,and almost have no influence on the working status of the load.It can also significantly improve the dynamic performance of system frequency.The proposed controller can provide technical support for the frequency regulation of new power systems with high proportion of RESs.

Hierarchical Frequency-dependent Chance Constrained Unit Commitment for Bulk AC/DC Hybrid Power Systems with Wind Power Generation

As the steady-state frequency of an actual power system decreases from its nominal value,the composite load of the system generally responds positively to lower power consumption,and vice versa.It is believed that this load frequency damping(LFD)effect will be artificially enhanced,i.e.,sensitivities of loads with respect to operational frequency will increase,in future power systems.Thus,for wind-integrated power systems,this paper proposes a frequency-dependent chance constrained unit commitment(FDCCUC)model that employs the operational frequency as a dispatching variable so that the LFD effect-based load power can act as a supplemental reserve.Because the frequency deviation is safely restricted,this low-cost reserve can be sufficiently exerted to upgrade the wind power accommodation capability of a power system that is normally confined by an inadequate reserve to cope with uncertain wind power forecasting error.Moreover,when the FDCCUC model is applied to a bulk AC/DC hybrid power system consisting of several independently operated regional AC grids interconnected by DC tie-lines,a hierarchically implemented searching algorithm is proposed to protect private scheduling information of the regional AC grids.Simulations on a 2-area 6-bus system and a 3-area 354-bus system verify the effectiveness of the FDCCUC model and hierarchical searching algorithm.