Creation of small, portable infusion pumps opened a new chapter in medical care. A patient can receive carefully metered and timed doses of medicine without requiring a visit to a medical practitioner, allowing life to be less restrictive. Ambulatory pumps have been developed to deliver insulin, nutritive supplements, and anti-cancer drugs.
Such equipment demands extremely high reliability — any failure is unacceptable — so during the design phase, it’s important to take into account the complete system: pump, motor, driver, feedback, etc.
Miniaturization is crucial for convenience to the end user, especially with portable equipment. The patient should not be disturbed by pump noise, either at rest or in social surroundings. Hence, portable battery-powered pumps need very efficient and quiet motors.
A typical design of a small syringe pump as illustrated in Figure 1 has a piston moved by a lead screw, with the screw or the nut driven with a micro DC motor. When choosing a DC motor, there are numerous advantages and disadvantages, depending on which technology you choose. Today’s design engineers have a choice among three major technologies:
- Brush DC ironless motor with gearbox and encoder,
- Brushless motor with gearbox and sometimes encoder, and
- Stepper motor, either direct drive or with gearbox, and sometimes encoder.
Brush DC Motors
Brush-type motors are available in two designs: iron core and ironless (see Figure 2). For battery-operated portable pump applications where system efficiency must be optimized for battery life, the ironless design offers distinct advantages as they have no iron losses and very low inductance. For better efficiency, the motor should also have the smallest possible R/K2 ratio, which represents power lost by the Joule effect in winding. New maget technology has helped today’s DC ironless motors achieve higher torque by inducing more mechanical power and lowering Joule losses (see Figure 3). In addition, precious metal commutation with small contact surface and pressure results in small electrical resistance and negligible friction even at high speeds. This enhances motor efficiencies up to 90%.
Brushless DC Motors (BLDCs)
Brushless motors have reduced dependency on bearings and brushes with the help of electronic commutation. In a BLDC motor, winding is stationary and the magnet is part of the rotor. Usually, external tube closing of 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 (see Figure 4).
BLDC motors are also available in two designs: slotted and slotless. Slotless design has the advantage of no cogging or detent torque, and has less iron loss than the slotted design. Slotted motors are typically employed in tough environments; for example, for autoclave sterilization applications. New high-energy magnets are making BLDC slotless design the preference in small motors. However, the BLDC motor driver and controller are equally critical for system efficiency.
BLDC slotless design can offer the best efficiencies and reliability necessary for medical pumps due to the double-fold advantage of slotless and brushless features combined with ball bearings. Furthermore, fluid/vial deliveries are sometimes made of short pulses and very small quantities. During these “start-stop” operations, slotless as well as brushless motors are best suited due to their above mentioned advantages.
By definition, a stepper motor is a BLDC with many poles; thus, current in each phase will have to be commuted many times per revolution. For instance, a two-phase stepper having 100 steps/revolutions will need 25 current reversions in each phase to make one full revolution. This design has the advantage of having many stable positions (steps) per revolution, providing high torque for a given size (versus a regular DC or BLDC motor). A disadvantage of the stepper motor is that it is not able to run at high speed, due to inductance combined with commutation frequency, and due to iron losses (current reversed so many times).
Different technologies of stepper motors are available, including variable reluctance, permanent magnet (can stack), hybrid, and disc magnet technology (turbo-disc).
For battery applications such as infusion pumps, disc magnet technology (see Figure 5) is the best, as it carries lower inertia and iron losses than other steppers, resulting in higher efficiency. As with a BLDC motor, a stepper motor driver is critical for system efficiency.
For small portable pumps, stepper motors are the primary selection if at low speed, they can be used in full step mode, and detent torque is sufficient to hold the load. In this case, they are driven like a watch motor — the correct quantity of energy is delivered to move one step to the next, while at the stall position, no current is applied in the phase. At high speeds, there are two options: either the motor has to run at high speed intermittently (syringe change), then driven as a regular stepper; or the motor needs to operate many times at high speed, increasing efficiency by closing the commutation loop like a regular servo motor (thereby adding position feedback).
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 is needed if detent torque is strong enough to maintain position.
Gearboxes and Encoders
A DC motor operating at high speed often requires a gearbox between the motor and the application. Different styles of gearboxes are available, including types with planetary or spur gears, and units with belt drives. Gearboxes are defined (for a given frame size) by output torque needed, gear ratio, and efficiency desired. A spur gearbox has better efficiency than a planetary gearbox for a given size and gear ratio, but the planetary system will be able to handle stronger torque.
An encoder, important for closing position loop, is defined by its resolution and efficiency. Different options exist, such as optical, magnetic with a hall sensor, and magnetic with magneto-resistance (MR). Today, the trend is to use MR due to an extremely high resolution in a tiny package. Dedicated ASICs used in MR encoders are able to interpolate two sine-shaped signals in quadrature.
Selecting Your Motor
Each of these technologies presents some advantages and disadvantages. A stepper motor could be an economical and reliable solution, but at the same time, less efficient. A BLDC is the most reliable and efficient, but is expensive. A brush DC motor solution is constrained due to commutation life and usually is preferred due to its simplicity.
Optimizing a solution with specific criteria requires much expertise and access
to different technologies. Thus, during the design phase, project engineers should work closely with mechanical specialists, motor experts, and electronic designers in order to take into account the entire system.
This article was written by Norbert Veignat, Ph.D., Vice President–Medical Market, and Claude-Alain Brandt, Sales Manager-Europe for Portescap, West Chester, PA. For more information, Click Here