Stepper motor linear actuators (SMLAs) combine a stepper motor, precision lead screw and nut in one compact envelope, providing a highly configurable, customizable and robust solution for linear motion. With the help of a motion controller and a stepper driver, all SMLAs can be programmed to position a load to a precise location. However, standard configurations do not provide a feedback mechanism that tells the operator whether the move is completed or not. Although not essential for many applications, feedback can be a powerful tool to utilize in more sophisticated linear motion systems. Because of this and the increasing demand for precise load positioning information, outfitting an SMLA with an encoder can be an effective and simple solution to get real-time motion feedback about your application. SMLA core configurations Figure 1 (above) shows three of the most common SMLA constructions: a motorized lead screw (MLS), motorized lead nut (MLN) and motorized linear actuator (MLA). Each of these constructions has a stepper motor, lead screw and nut at its core, but differ in how it obtains motion. For MLS units, the lead screw attaches directly to the motor shaft, which translates the nut on rotation. MLN units integrate the nut inside the motor shaft, which when rotated, translates the lead screw instead of the nut. MLA configurations are essentially MLS units with additional components that house the lead screw and nut while also providing integrated support and guidance. Open loop linear actuation Figure 2 depicts open-loop SMLA architecture that applies to all configurations. Users interact with the system through a human machine interface (HMI). They program the desired motion sequence into the motion controller, which sends it to the stepper motor drive for conversion and amplification before transmitting it to the SMLA, which makes the moves. The communication pathway is unidirectional; neither the motion controller nor the HMI ever receives any notification that the intended move has been completed successfully.

Knowing exactly where a load is positioned is critical for many high-precision applications such as medical instruments, measuring devices and laboratory equipment. Also, certain applications can power off unexpectedly or have their load forced out of position. In such scenarios, it would be impossible to know the exact position of the load without using a feedback mechanism. A good example of an application that benefits from the functionality of an encoder is automated pipetting machine. (Figure 3) These devices utilize an encoder on the horizontal axis to accurately track the location of the dispensing pipette and ensure fluid is transferred to the proper test tube. Other examples include fluid pumps, 3D printers and XY stages.