What Is a Servo Actuator?

A servo actuator is a device used to provide a wide range of remote, automatic switching, or actuation movement based on feedback signals from the system on which it is used. In contrast, conventional actuators produce a set, finite working motion in response to a single trigger input. The fine level of control possible with a servo actuator is made possible by the inclusion of a servo controller capable of constantly comparing desired results to system conditions on a real-time basis and calculating the difference, if any, between the two. If differences are sensed, the actuator is activated by the controller to achieve the desired result. Servo actuators are used in a wide range of remotely-controlled or automated systems ranging in size from tiny optic auto focus systems to large automated targeting systems on naval guns.

The basic theory of actuation revolves around the concept of remotely providing the impetus, or motion, required to perform a task. This can be a simple back-and-forward motion to activate a switch or an extremely complex, multiple stage rotary movement used to focus a set of lenses. The extent and power of the actuation movement may also be no more than a fraction of an inch and a couple of ounces of pressure to several feet and thousands of pounds of torque. In the case of conventional actuators, the supplied movement is fairly simple and of a pre-set finite direction and extent triggered by a single external source. Applications that require variable actuation movement in response to demanding system demands require more control and require a servo actuator system.

In contrast to single trigger input of simple actuators, the servo actuator supplies its output motion in response to what are known as feedback inputs. These are signals sent out by the actuated system, which define the mechanism's exact state and position on a real-time basis. These signals are fed into a servo controller that compares the real-time data to a set of ideal situation parameters. These may be remote inputs from other sensors and systems or part of a pre-programmed data block.

For example, if the target system of a naval gun receives a set of desired situation parameters consisting of a rotational orientation of 185° and a barrel depression angle of 52° from a targeting system, it will cross check those parameters against the real-time positional signals received from sensors on the turret. If the two differ, an error status is logged by the controller, which then directs rotational and elevation actuators to turn the turret and move the gun barrel up or down. When the desired conditions are met, the error status cancels and the turret locks in place in readiness for a firing signal. This is a rather simplistic explanation of a highly complex system, but is a fair indicator of how a servo actuator operates based on a comparison of desired and existing conditions. The servo actuator is widely used in many applications ranging from heavy installations, such as the naval gun turret control to very fine, light duty examples in lens auto-focus systems.