Developments in the field of electro-hydraulic linear actuators controlled by variable-speed pump drives have been increasing over the past decade. The majority of these benefits from the speed control loops realized in the associated electric drives, hence these variable-speed pump drives have been operated as flow control units. For this reason, control methodologies commonly used in valve operated linear drives can be adopted. Such approaches may be characterized as primary control strategies, meaning that there is a flow coupling between the variable-speed pump drive and the linear actuator, with a pressure reaction. Alternatively, other technologies like hydraulic transformers benefit from secondary control abilities, meaning that there is a pressure/force coupling between the actuating device and the linear actuator, with a flow reaction. This paper presents a study considering the application of secondary control principles in electro-hydraulic linear actuators, where the usage of only the electric motor current control loops allow for this to be realized. It is found that by feeding back the hydraulic force measurement in addition to the current loops, allow for a high force control bandwidth by virtue of state feedback controls. The high bandwidth of the force control enables alternative approaches to motion control of electro-hydraulic linear actuators.

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