38 n PRECISION ENGINEERING AND MOTION CONTROL October 2024 www.drivesncontrols.com Using encoders to close linear actuator feedback loops Stepper motor linear actuators (SMLAs) combine stepper motors, leadscrews and nuts in a compact format, providing a configurable, customisable and robust way of providing linear motion. When used with a motion controller and stepper driver, SMLAs can be programmed to position loads precisely. However, standard configurations do not provide feedback that could tell an operator whether the move has been completed. Although not essential for all applications, feedback can be a powerful tool in more sophisticated linear motion applications. Fitting an SMLA with an encoder can be an effective way to get real-time motion feedback about an application. The three most common SMLA formats are leadscrew, lead nut and linear actuators. Each of these includes a stepper motor, leadscrew and nut at its core, but differ in how it produces motion. For leadscrew systems, the screw attaches directly to the motor shaft, which translates the nut on rotation. Lead nut versions integrate the nut inside the motor shaft which, when rotated, translates the leadscrew instead of the nut. Linear motor configurations are essentially leadscrew devices with extra components that house the leadscrew and nut while also providing support and guidance. Open-loop actuation In open-loop SMLA applications, users typically interact with the system via an HMI, which they use to program the rquired motion sequence into the motion controller. This sequence is sent to a stepper motor drive for conversion and amplification before being transmitted to the SMLA, which makes the moves. The communication is unidirectional – neither the motion controller, nor the HMI, receives any notification that the intended move has been completed. Knowing exactly where a load is located is critical for many precision applications such as medical instruments, measuring devices and laboratory equipment. Also, if an application powers off unexpectedly or has its load forced out of position, it would be impossible to know the exact position of the load without a feedback mechanism. One example of an application that benefits from adding an encoder is an automated pipetting machine (shown in Fig 1). Such systems use an encoder on the horizontal axis to track the location of the dispensing pipette accurately and ensure that fluid is transferred to the correct test tube. Other examples include fluid pumps, 3D printers and XY stages. Many SMLAs can provide feedback by Fitting a rotary encoder to a linear stepper motor can add a host of benefits, as well as improving control and load monitoring. Ojesh Singh, Thomson Industries product manager for the EMEA region, explains. Images: Thomson Industries Stepper motor actuators incorporating optical encoders (from left to right: leadscrew, lead nut and linear actuator versions).
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