The function of a control system is to convert an operator cornmand
into an electrical signal, to convert the electrical signal into a mechanical
response, and to verify that the mechanical response achieves
and maintains the desired level. A vital element in this interface is the
electro mechanical actuator. This paper examines the requirements of control
actuators and illustrates the unique capabilities of ferroelectric devices
to meet these requirements. When substantial energy is required from an
actuator, stacks of ferroelectric disks are shown to be suitable. They develop
high force, respond extremely fast, can be proportionately charged,
have almost infinite resolution, require no holding current to maintain
displacement, and are neither influenced by nor produce magnetic fields.
They have no mechanically wearing components and are not prone to
corrosion. Reliability to 109 cycles has been established. Problems are
discussed; hysteresis is shown to represent a difficulty in control systems.
A method of treatment is discussed whereby the disk stack is connected
in binary groups with separate positive connection to each of the groups.
A common constant voltage power supply is switched to selected groups
so that only zero volts or maximum volts are applied. In this manner
hysteresis is eliminated, the power supply is simplified, and a repeatable,
almost linear response is obtained. An alternative approach using a hybrid
stack composed of two soft ferroelectric and one hard ferroelectric segments
is shown to be capable of linearity and repeatability within 1%.
Some problems remaining with these approaches are discussed. A very
fast, precise beam steering mirror system is described that employs a split
stack assembly. Its performance is described. Fundamental frequency limitations
are shown to exist in stacked disk actuators. For example, an 18,000
Hz limit applies to a 25 mm long actuator. Brief reference is made to future
developments and to the benefits that may be expected from them.