Dynamic load emulation using a resistive control input is a key topic of Hardware-In-The-Loop implementation. Three control configurations are available to design a load emulator. When the bandwidth of the reference system is low enough compared to the bandwidth of the actuator, a one degree-of-freedom (DOF) feedback emulator can be successful. When the reference loop has a very large bandwidth compared to the actuator, the one-DOF feedback design yields poor performance with a possible loss of stability. In this case, if accurate plant model parameters or, alternatively, online adaptation is available, a one-DOF feed forward design can secure stability and improve the performance; otherwise, a two-DOF emulator is the choice to utilize both feedback and feed forward signals and to obtain robust stability and robust performance. To emulate the load dynamics, the usage of a resistance as the control input imposes some fundamental limitations to the closed-loop performance. This paper presents the following: (i) Generalization of load emulation problems and control configurations, (ii) Analysis of performance limitations for resistively controlled systems, and (iii) Design examples including simulation and experimental results.

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