FEATURE Drives, Controls & Motors
Miniature motors for medical pumps Dr. Norbert Veignat, Portescap, who looks at the design considerations of miniature motors for medical pumps
T
he development of small, portable infusion pumps heralded a new chapter in medical care. Capable of
delivering carefully-metered and timed doses of medicine, they opened up new possibilities for delivering continuing care at the patient’s home. In addition, ambulatory infusion pumps powered by batteries gave patients even more freedom, delivering the likes of insulin, nutritive supplements and anti-cancer drugs. It goes without saying that the highest levels of reliability are required for these systems, placing stringent requirements on the key components involved. For the design engineer, a key area of focus is the electric motor which drives the pump. Size, lifespan and performance – in particular the ability to deliver the required torques over a speed range – are all prime considerations. Miniaturisation is often crucial to assure convenience for the user, particularly with portable equipment. So is effi ciency, which is especially important in battery-powered systems, providing longer time between charging. Further, the patient should not be disturbed by the pump noise, either at rest or in social surroundings, requiring a very quiet motor.
While the primary goals of the motor
are clear, the choice of product is often less so. The design engineer must select from a number of diff erent technologies, where each might off er decisive advantages in some areas of the performance specifi cation but imply compromises in others. The choice of miniature motor for infusion pump products comes down to three major technologies: a brushed DC ironless motor combined with a gearbox and encoder; a brushless DC motor with a gearbox and sometimes an encoder; or a stepper motor, either direct drive or with a gearbox, and sometimes an encoder where closed loop operation is desirable.
DC motor technologies An ironless brush DC motor is a good fi t for battery-operated portable pumps because it has no iron losses and is therefore highly effi cient. Indeed, precious metal commutation allows motor effi ciencies up to 90%, while evolving magnet technologies enable the motors
18 February 2021 | Automation
Ironless brush DC motor design and features
to deliver high torque at a given speed. Ironless motors also have a very low inductance and a commutation with small contact surface and pressure, resulting in a low electrical resistance and very little friction.
The primary disadvantage of a brushed DC motor, of course, is that its brushes will wear, which shortens the product’s lifespan. If this is to be avoided, look instead at a brushless DC (BLDC) motor for the infusion pump. In a BLDC motor the coils are fi xed and the magnet is part of the rotor. Commutation in the coils is done electronically. Usually, the external tube closing the magnetic fi eld of the magnet is fi xed, generating iron losses while the magnet is rotating. In applications where inertia is not critical, the tube and the magnet can rotate together, removing iron losses. There are two categories of BLDC motor: slotless and slotted. The slotless design has the advantage of no cogging or detent torque, while also having less iron loss than a slotted design and therefore higher effi ciency. Where size is a primary consideration, advancing high energy magnets make slotless motors a good option.
Compared with a DC ironless motor, the BLDC motor will have lower effi ciency due to the iron losses and lower torque for the same size unit. One way to compensate for the losses is to use the BLDC motor at higher speeds, taking into account this parameter when selecting a gearbox.
Stepper motor technologies The fi nal option is the stepper motor, which has the great advantage of having many stable positions (steps) per revolution, so providing a high torque
for a given size compared with either the BLDC motor or brush DC motor. A stepper, however, is not able to run at high speed, due to the inductance, combined with the commutation frequency and iron losses. There are several stepper motor technologies, including permanent magnet, hybrid and disc magnet. For battery applications, disc magnet technology is a good fi t as it carries lower inertia and lower iron losses than other steppers, resulting in higher effi ciency. For small, portable pumps, stepper motors are a strong selection if they can be used in full step mode at low speed, and the detent torque is suffi cient to ensure no movement from outside forces. For higher speeds, it should be considered whether the motor has to run only intermittently or operate frequently. If the high-speed requirement is only intermittent (for example for a syringe change) then the motor can be driven as a standard stepper. If the motor needs to operate many times at high speed, the effi ciency can be increased by closing the commutation loop with position feedback like a standard servo motor. In some applications, a stepper solution with a gearbox may be the most economical design, since no encoder is required. In addition, at stall position no energy will be needed if the detent torque is strong enough to maintain the position. When it comes to medical devices such as infusion pumps, there is no single motor technology that provides an ideal solution for every application. In any given situation, each motor choice will have its own set of advantages and disadvantages.
CONTACT:
Portescap
www.portescap.com
automationmagazine.co.uk
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