  The superconducting cavities operated at high Q level need to be precisely
tuned to the RF frequency. Well tuned cavities assure the good field stability
and require a minimum level of RF power to reach the operating gradient level.
The TESLA cavities at XFEL accelerator are tuned using slow (step motors) and
fast (piezo) tuners driven by the control system. The goal of this control
system is to keep the detuning of the cavity as close to zero as possible even
in the presence of disturbing effects (LFD - Lorentz Force Detuning and
microphonics). The step motor tuners are used to coarse cavity tuning while
piezo actuators are used to fine-tuning and disturbance compensation. The
crucial part of the piezo control system is the piezo driver. To compensate LFD
the piezo driving with relatively high voltage (up to 100V) and high current
(up to 1A) is needed. Since the piezo components are susceptible to destruction
with overvoltage, overcurrent, and also overtemperature, one has to pay special
attention to keep the piezos healthy. What makes things worse and more
critical, is that the piezo exchange is not possible after the module is
assembled. Therefore the special hardware must be assisting the power
amplifier, detecting the dangerous conditions and disabling piezo operation
when needed. It must be fail-safe, so even in a case of failure the piezos
shall survive. It must be also robust and it must not disturb or disable normal
operation. Due to many channels (16 for master/slave RF), the hardware solution
must be well scalable. The paper discuss the design of XFEL's piezo driver
together with PEM (Power and Energy Monitor) supervising the driver operation
and preventing piezos from destruction. The achieved results and operation of
the complete system are demonstrated.
