  In the compact variable-gap undulator with Hydraulic-Assist Driver, miniature
hydraulic cylinders compensate for ~90% of magnetic forces and mechanical
actuators are used to accurately control the undulator gap. This method of
hydraulic assist enables design simplicity and compactness of the undulator.
Proof-of-concept experiments and test results of the undulator prototype in
step-by-step motion mode indicated excellent accuracy and stability properties
of the driver. In this note we present test results of hydraulic-assist driver
in continuous motion mode. Results indicate that hydraulic-assist driver can
provide smooth continuous undulator gap change, keeping the undulator harmonic
energy variation within the required 1eV/sec, and speed and RMS normalized
energy errors at 1.8e-4 level (or less). That satisfies any application planned
at CHESS.
