INDUSTRY FOCUS MEDICAL & PHARMACEUTICAL
Miniature motors used in medical pumps have very specific requirements, and the selection of the right product for the application is essential. Dr. Norbert Veignat from Portescap looks at the design considerations
WHY MEDICAL MOTOR SELECTION IS FAR FROM A TRIVIAL TASK...
D
esigned to deliver carefully metered and timed doses of medicine, the introduction of
small, portable, infusion pumps heralded a new chapter in medical care by opening up new possibilities for delivering continuing care at a patient’s home. Meanwhile the introduction of ambulatory infusion pumps powered by batteries have given patients even more freedom by delivering the likes of insulin, nutritive supplements and anti-cancer drugs. Such infusion pumps mean patients can be safely discharged from hospital for extended periods of time, significantly increasing their quality of life. However, such systems need to be highly reliable, which places stringent
requirements on the key components involved. For the design engineer, a key area of focus is the electric motor which drives the pump. Performance, in particular the ability to deliver the required torques over a speed range, size and lifespan are all prime considerations; while miniaturisation is also often crucial, particularly with portable equipment. Furthermore, efficiency is especially important in battery powered equipment, providing longer time between charging; and the patient should not be disturbed by the pump noise, requiring the use of a very quiet motor. While the primary goals of the motor are clear, the design engineer
must select from a number of different technologies, where each might offer decisive advantages in some areas of the performance specification 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.
BRUSH DC MOTOR TECHNOLOGY An ironless brush DC motor is a good fit for a battery-operated portable pump application because it has no iron losses and is therefore highly efficient. Indeed, precious metal commutation allows motor efficiencies up to 90%, while evolving magnet technologies enable the motors 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, impacting the lifespan of the product. So, if this is a decisive consideration, then the design engineer may want to look instead at a brushless DC (BLDC) motor for the infusion pump.
BRUSHLESS MOTOR TECHNOLOGY In a BLDC motor the coils are fixed and the magnet is part of the rotor, with commutation in the coils done electronically. Usually the external tube closing the magnetic field of the magnet is fixed, 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 efficiency. Where
30 JULY/AUGUST 2020 | DESIGN SOLUTIONS / DESIGNSOLUTIONS
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 efficiency 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 TECHNOLOGY The final option is the stepper motor, which has the strong 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. The disadvantage of a stepper, however, is that it is not able to run at high speed, due to the inductance combined with the commutation frequency, and due to iron losses. There are a number of different stepper motor technologies, including
permanent magnet, hybrid and disc magnet. For battery applications, disc magnet technology is a good fit as it carries lower inertia and lower iron losses than other steppers, resulting in higher efficiency. 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 sufficient to ensure no movement from outside forces. For higher speeds, the design engineer should consider whether the motor has to run only intermittently or to 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 efficiency 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, with each motor offering both advantages and disadvantages. Portescap offers a full range of motors covering all of the primary technologies and encourages design engineers to work with such a specialist in order to take the entire system into consideration.
Portescap Ironless brush DC Motor design and features
www.portescap.com
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