HAP 0921

HYDRAULICS Requirements for linear motion, miniature applications range from filling syringes through to medical devices, or robotic applications used in the operating theatre. The first consideration for a design engineer is the method used to transfer the rotary motion of a miniature motor into linear motion. The most common way of achieving this is by mounting a screw and nut system on the motor shaft. It operates on rolling contact between the nut and a screw, which can provide low friction, good efficiency and high load capability. The disadvantage however is the cost and the time to design such a solution, especially for applications which don’t require high load handling. A more cost-effective means of achieving linear motion can be reached with a standard linear motorised solution by choosing a motor with an integrated lead screw. In particular, a digital linear actuator (or DLA) utilises a can stack stepper motor combined with a screw. Inherent with stepper technology, the motor controls its own positioning and is both an accurate and cost-effective solution, doing away with the requirement for an additional feedback system. Resolution can be managed in full, half or micro steps. With a special optimised ball bearing assembly, the axial play can even be eliminated, improving positioning accuracy as well as repeatability of motion. In addition, with stepper technology, the motor has a detent torque. As a result, it can hold its position when the power is removed. The nut can be over-moulded in the rotor assembly with special material, minimising friction and consequently increasing efficiency and lifetime. Customised package For a high-optimised linear solution, a customised package can deliver the maximal performance and characteristics most suited to the application if the motorised solution is well designed. Taking the screw section, this could include considerations over dimensions, pitch, material, ball or lead screw. Regarding the miniature motor, the can stack stepper motor can be replaced with various choices dependent on requirement. As an example, a low inertia disc magnet stepper motor ensures the highest acceleration with the benefit of the stepper technology, providing ease of control, positioning capability and detent torque. Alternatively, a brushless DC motor maximises power density. For applications which demand energy efficiency, such as battery powered devices, a coreless brush DC motor can be advantageous. Control devices can also be added, such as an encoder for high resolution positioning feedback or Optimising compact linear motion applications with mini motors 3 2 HYDRAULICS & PNEUMATICS September 2021 www.hpmag.co.uk To achieve maximum performance from a miniature linear motion application, it’s vital to make sure the motor’s specification meets the requirements. Efficiency, high power, optimised dimensions and acceleration are among the criteria for selection to optimise demanding lead screw applications. Clémence Muron, application engineer at Portescap, explains how to make the right choice of linear motion technology. Requirements for linear motion, miniature applications range from filling syringes through to medical devices, or robotic applications used in the operating theatre.