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A linear actuator is a device that creates linear motion, often by converting rotational motion. For instance, an electric linear motor turns a screw that in turn "pushes" a cylinder. By this action, linear motion is created from a rotating shaft.

They are many linear actuator for doggie door opener. You can see their specification on their online websites and choose which best suits your needs.

A linear actuator is an actuator that creates motion in a straight line, as contrasted with circular motion of a conventional electric motor. Linear actuators are used in machine tools and industrial machinery, in computer peripherals such as disk drives and printers, in valves and dampers, and in many other places where linear motion is required. Hydraulic or pneumatic cylinders inherently produce linear motion; many other mechanisms are used to provide a linear motion from a rotating motor?

To control linear actuator by PID controller, use the data from the feedback device. Use the potentiometer to adjust the output until it reaches the target value.