May 2021

38 n LINEAR MOTION May 2021 www.drivesncontrols.com Choosing mini motors to deliver linear motion T ypical miniature linear motion applications range from filling syringes to robotic applications in operating theatres. The first consideration when designing such applications is how the rotary motion of a miniature motor is transferred into linear motion. The most common way is to mount a screw-and-nut system on the motor shaft. This operates on rolling contact between the nut and screw, and can provide low friction, high efficiency and high load capabilities. The disadvantage is the cost and the time needed to design such systems, especially for applications which don’t need high load-handling capabilities. A more cost-effective means of achieving linear motion is to use a motor with an integrated leadscrew. In particular, digital linear actuators (DLAs) use a can stack stepper motor combined with a screw. The stepper technology controls its own positioning and is both accurate and cost- effective, avoiding the need for an additional feedback system. Resolution can be in full, half or micro steps. With an optimised ball bearing assembly, axial play can be eliminated, improving positioning accuracy as well as repeatability. Stepper motors also have a detent torque and can hold their position when power is removed. The nut can be over-moulded in the rotor assembly using low-friction materials, thus increasing efficiency and lifetime. Designs can be customised to maximise performance and to deliver characteristics to suit the application. For the screw section, this could include aspects such as dimensions, pitch, materials, and the choice of ball or lead screws. The can stack stepper motors can be replaced by alternatives such as low-inertia disc magnet stepper motors to achieve high acceleration rates. Another possibility is to use a brushless DC motor to maximise power density. For applications requiring high efficiency, such as battery-powered devices, coreless brushed DC motors can offer advantages. Control devices such as encoders can be added to provide high- resolution positioning feedback, or gearboxes to optimise torque performance. Optimising the design of the motor assembly means understanding the application’s power demand as well as the motor’s power output. The desired output force and linear speed depend on the application. Power is generated by the motor’s torque and rotational speed and can be calculated by using the expected output power and taking into account motor efficiency and the leadscrew parameters, including efficiency and pitch. Take, for example, a medical device for low-volume transfer of liquids, in which a single motor with a maximum diameter of 20mm controls a multi-pipette channel. Filling must take less than 2.5 seconds and the pipettes then travel 50mm in 4 seconds where they are emptied in 30 sub-steps. This application would require a high- resolution system with a good repeatability to provide the same amount of liquid consistently for each sub-step. A standard digital linear motor with a leadscrew would usually meet the needs of this type of application without needing special development, thus cutting costs. A can stack stepper motor can provide pipette filling control by resolving multi- step liquid delivery into sub-volumes. By using an optimised ball bearing assembly, axial play can be removed, ensuring high repeatability. Another example is a battery-powered medical device that needs efficient use of power. It also has to be lightweight and compact, with a maximum diameter of just 13mm. For this application, coreless brushed DC motors could ensure high efficiency. To optimise its size, the motor should be paired with a gearbox. When choosing the geared motor, you will need to take in account duty cycles – especially for medical devices that need to be used continuously for several minutes. To determine the required input power (torque and speed) generated by the motor, some calculations are necessary. First, converting the linear motion (force and linear speed) into rotating motion (torque and rotational speed) depends on the leadscrew parameters (pitch and efficiency). To determine the necessary motor power, you will need to consider the gearbox’s ratio and efficiency. To ensure that the motor is powerful enough in continuous use, the motor torque should be less than the rated torque specified by the manufacturer. When the motor and gearbox demands have been ascertained, the required power and efficiency can be calculated. n Tomaximise performance inminiature linear motion applications, it’s vital that the motor matches the application. Clémence Muron, an engineer with Portescap, offers advice on chosing the best technology. There is considerable demand for miniature linear motion technologies for medical applications Motors with integrated leadscrews offer a cost-effective means of achieving linear motion