Conventionally, variable-speed electro-hydraulic linear actuators utilize the speed control loops of electric machines and associated drives to control the pump flows, thereby realizing primary control functions. An alternative control approach is secondary control, which realizes a pressure coupling between the motor/pump and cylinder via the electromagnetic motor torque, with a flow reaction. Secondary controls in such drives has theoretically been shown to enable significantly higher control band-widths compared to primary control approaches. The theoretical bandwidth improvement is possible as the secondary control approach utilizes the faster dynamics of the electric machine, whereas the primary control approach revolves about the slower hydraulic dynamics present in the speed control loop. This paper considers the design and implementation of a secondary control function in a variable-speed electro-hydraulic actuator test bench, in order to validate the properties of such controls. Initial results show that the proposed secondary control approach is highly sensitive to measurement noise, which proves to be a limiting factor for the achievable control bandwidth, if smooth operation of the system is to be maintained. To attenuate the noise impact an extended Kalman filter is proposed in conjunction with the secondary control approach. Results demonstrate that the inclusion of the extended Kalman filter significantly reduces the impact of signal noise on the internal drive states, thereby enabling increased bandwidth and expanding the application range for this control method.

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