Motor Selection

A stationary hospital infusion pump at a patient's bedside.
Fig. 5 - Suggested motor/pump mechanism layout based on pump dimensions.

Due to the required pump dimensions, there is not enough length available for an in-line solution, and the motor must be positioned parallel to the linear peristaltic mechanism with a 180° transmission connecting the motor shaft to the pump axle. (The efficiency of a typical transmission such as this can vary, and the gear ratio can be adjusted as needed by the pump designer. For this example, we will assume 90 percent efficiency and a 1:1 gear ratio (see Figure 5). To achieve 1,200 mL/hr max flow rate, the pump axle must be able to spin at 200 rpm:

  • max pump flow rate × revolutions per mL ÷ 60 min/hr = max pump axle speed.

  • 1,200 mL/hr × 10 rev/mL ÷ 60 min/hr = 200 rpm.

A reasonable operating speed for a DC coreless motor is 5,500 rpm, so using a 27:1 gear ratio would allow the gearmotor to achieve the desired maximum speed:

  • motor speed ÷ gear ratio = gearmotor speed.

  • 5,500 rpm ÷ 27 = 203 rpm.

To ensure that brushed DC is the ideal technology, consider a brushless DC gearmotor as an alternative. However, brushless DC motors are most efficient at much higher speeds (10,000+ rpm), and they require a much higher gear ratio to match the target speed of the application. The higher gear ratio creates more gear contact points, which increases noise and reduces efficiency.

A brushless motor is capable of running at lower speeds. But at low speed, the iron losses are much greater for a brushless than a brushed motor, and efficiency can be 20–40 percent lower. At low speeds, brushless DC motors also require Hall sensors and more complicated and higher cost controllers because their commutation system is electronic (not mechanical like the DC Brushed motor). However, brushless DC motors do have the advantage of longer life due to no brush wear.

Another motor technology to consider is stepper. The speed of 200 rpm is achievable using full or half stepping and the torque is within the capability of a NEMA 11 hybrid stepper motor. The motor has enough torque that it does not require a gearbox. However, the efficiency of a hybrid step motor is around 40 percent, which is too low to allow the pump efficiency target of 25 percent given the efficiencies of the other pump components:

  • motor efficiency × transmission efficiency × pump efficiency = overall efficiency.

  • 40 × 90 × 50 percent = 18 percent.

Table 2. A comparison of miniature motor technology.

The pump battery would have to be made larger to accommodate the lower efficiency motor, adding to size and weight. The hybrid motor itself is also bulkier and heavier than DC coreless and can cause noisy resonance at certain frequencies. Therefore, it is not a good candidate for this mobile application (see Table 2).

In reviewing these considerations, the best solution would be the DC coreless gearmotor as it is the most efficient, is simple to control, and the life requirement is satisfied. A quick check confirms that a 16 mm DC brushed motor with a 27:1 gearhead can provide the required torque output, because the torque reflected on the motor is well below its maximum continuous torque of approximately 6 mNm:

  • torque requirement ÷ gear ratio ÷ (gearhead eff. × transmission eff.) = torque on motor.

  • 40 mNm ÷ 27 ÷ (73 × 90 percent) = 2.25 mNm.

The efficiency of this particular gearmotor can exceed 60 percent, which is more than enough to achieve the 25 percent efficiency requirement for the pump mechanism as a whole:

  • gearmotor eff. × pump eff. × transmission eff. = overall efficiency.

  • 60 × 50 × 90 percent = 27 percent.

An Athlonix™ 16DCT or 16DCP DC coreless motor from Portescap with a 27:1 B16 spur gearhead are two examples of gearmotors that meet the requirement for this particular example. The 16DCT provides a higher torque capability and therefore would require less current during operation, lengthening the time between charges for the pump. The 16DCP, using an Alnico magnet, offers a lower cost structure while still meeting the demands of the application. Both are well-suited for home infusion pump applications.

Noise

Total noise cannot be calculated effectively as it is not cumulative, depends heavily on how the motor and pump mechanism are mounted, and is affected by the insulation in the pump casing. The pump designer will take these factors into consideration, and testing in the pump during the design cycle will determine the noise level achieved.

Feedback

The required feedback of 100 pulses/mL can be achieved by including a 10-line feedback system on the pump axle or at the output of the gearhead:

  • 100 pulses/mL ÷ 10 gearmotor revolutions/mL = 10 pulses/ gearmotor revolution.

Alternatively, a single line encoder can be installed on the rear of the motor shaft. This takes advantage of the gear ratio to achieve 150 pulses/mL, which surpasses the requirement for even better control:

  • 1 pulse/motor rev × 27 motor rev/gearhead rev × 10 gear-head rev/mL = 270 pulses/motor rev.

The unsealed pump case means that dust from the home environment (not as clean as a hospital) can get into the feedback system, so an enclosed magnetic encoder is better than an open optical sensor. For additional safety, a redundant low-resolution sensor on the gearbox shaft or pump axle can be used to provide confirmation that the axle is turning and that the drug is in fact being delivered to the patient.

Conclusion

The sample situation presented in this article illustrates that many design choices are highly codependent. For example, a decision as seemingly unrelated to motor design as the pump case material may ultimately limit the performance of the motor and, in turn, the final pump design. Therefore, it is important to consider the motor used in detail at the earliest stages of concept development. An experienced, capable, and cooperative motor supplier can play an outsized role in the product development process, expanding the limits of pump performance.

This article was written by Brandon Steinberg, Core Market Business Development Manager for Infusion Systems, Portescap, West Chester, PA. For more information, Click Here.