  Soft electrohydraulic actuators known as HASEL actuators have attracted
widespread research interest due to their outstanding dynamic performance and
high output power. However, the displacement of electrohydraulic actuators
usually declines with time under constant DC voltage, which hampers its
prospective application. A mathematical model is firstly established to not
only explain the decrease in displacement under DC voltage but also predict the
relatively stable displacement with oscillation under AC square wave voltage.
The mathematical model is validated since the actual displacement confirms the
trend observed by our model. To smooth the displacement oscillation introduced
by AC voltage, a serial elastic component is incorporated to form a SE-HASEL
actuator. A feedback control with a proportion-integration algorithm enables
the SE-HASEL actuator to eliminate the obstinate displacement hysteresis. Our
results revealed that, through our methodology, the SE-HASEL actuator can give
stable and smooth displacement and is capable of absorbing external impact
disturbance simultaneously. A rotary joint based on the SE-HASEL actuator is
developed to reflect its possibility to generate a common rotary motion for
wide robotic applications. More importantly, this paper also proposes a highly
accurate needle biopsy robot that can be utilized in MRI-guide surgical
procedures. Overall, we have achieved AC-driven series elastic electrohydraulic
actuators that can exhibit stable and smooth displacement output.
