Optical EngineeringFerroelectric actuators in the electromechanical interface
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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 electromechanical actuator. This paper examines the requirements of control actuators and illustrates the unique capabilities offerroelectric 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.