FEATURE DRIVES, CONTROLS & MOTORS
To support the development of miniature linear motion applications, the miniature motor to be used needs to be specified correctly to ensure maximum application performance, as Clémence Muron, application engineer at Portescap, explains
How to specify the right miniature motor for your linear application
L
inear motion is used for a wide range of applications – from
filling syringes and medical devices, to robotic applications in the operating theatre. There are, however, a number of considerations when specifying the motion solution. Firstly, the design engineer needs to consider 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. This 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 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, removing the need for an additional feedback system. Resolution can be managed in full, half or micro steps. With a special optimised ball bearing assembly, it is possible to eliminate the axial play, improving positioning accuracy as well as repeatability of motion; and, with stepper technology, the motor has a detent torque. It can therefore hold its position when the power is removed. Furthermore, the nut can be over-moulded
Motor and lead screw example
in the rotor assembly with special material, minimising friction and consequently increasing efficiency and lifetime.
CUSTOMISATION For a high-optimised linear solution, a customised package can deliver the 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 a gearbox for optimised torque performance. Designing the optimal motor
assembly means understanding the application’s power demand as well as the motor’s power generation. The desired output force and linear speed vary depending on the application’s requirements. Power is generated by the motor’s torque and rotational speed and it can be calculated by using the expected output power and by taking into account motor efficiency and the lead screw parameter, including the efficiency and pitch. As an example, let’s take a look at the development of a laboratory
30 JUNE 2021 | DESIGN SOLUTIONS
medical device for low volume liquid transfer, where a single motor package limited to a maximum diameter of 20mm controls a multi-pipette channel. The filling stage must take less than 2.5 seconds and the pipettes then travel 50mm in four seconds where they are emptied in 30 sub-steps. This requires a high-resolution system and a good
repeatability to consistently provide the same amount of liquid for each sub-step. For this kind of application, a standard
digital linear motor with a lead screw will usually fulfil the requirements, with no special development necessary. A can stack stepper motor enables pipette filling control as a result of the multi-step resolution over liquid delivery into sub-volumes, and thanks to an optimised ball bearing assembly, the axial play is removed, ensuring high repeatability. Another example is a battery-powered medical
device. Handled by a doctor during an operation, this requires efficient power usage while being lightweight and compact. This requires a solution with a maximum diameter of just 13mm and, for this application, coreless brush DC motors ensure high efficiency. For size optimisation, the mini motor should also be paired with a gearbox. For the geared motor selection, the engineer will take into account the duty cycle and, in this case, the medical device will be used over several minutes in continuous duty. To determine the required input power (torque and speed) generated by the motor, some calculations are necessary. First, the conversion of the linear motion
(force and linear speed) requested by the application into rotative motion (torque and rotational speed). This depends on the lead screw parameters (pitch and efficiency). To know the necessary power at the motor level, you will need to consider the ratio and efficiency of the gearbox. To ensure that the motor is powerful enough in continuous use, the required motor torque should be lower than the rated torque specified by the manufacturer. When the motor and gearbox demands have been ascertained, the power requirement and efficiency of the solution can be calculated. To help with the development of linear motion
applications, Portescap can support engineers with standard and customised solutions.
Portescap
www.portescap.com / DESIGNSOLUTIONS
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