[Current Status]​ Currently, most medical infusion pumps, syringe pumps, and enteral feeding pumps still use PM stepper motors or hybrid stepper motors. This is primarily because stepper motors are relatively easy to control and have lower costs. However, many drawbacks of stepper motors pose significant design challenges for equipment manufacturers. For instance, to ensure quiet operation of the equipment in hospital wards and to mitigate the mechanical resonance issues of stepper motors, some manufacturers employ high-microstepping drivers to reduce mechanical resonance; others use specialized stepper motor vibration-damping pads to lower noise; and some even drill holes in the motor shaft or add rubber sleeves to reduce vibration transmission from the shaft.

[Advantages of Using Brushless DC Motors in Infusion Pumps, Syringe Pumps, and Enteral Feeding Pumps]

Brushless DC motors exhibit no significant mechanical resonance across their entire operating speed range. Especially within the speed range of infusion pumps and feeding pumps, they can be considered virtually silent.

Stepper motor motion is discrete. With each pulse, the stator's magnetic field instantly "jumps" to a new position, exerting a step-like pulling force on the rotor. This abrupt "pulling" action contains rich harmonics that can easily coincide with the natural frequency of the mechanical system, triggering resonance. In contrast, the stator magnetic field of a brushless DC motor rotates continuously and smoothly, generating a similarly smooth and continuous electromagnetic torque, making it difficult to excite significant oscillations.

The service life of brushless DC motors is essentially equivalent to that of stepper motors.

Brushless DC motors are highly efficient and energy-saving, offering a distinct advantage, particularly when powered by batteries. The author has observed in tests with many medical infusion pump and feeding pump clients that, with the same 2000mAh battery, stepper motors typically last about 4 hours, while brushless DC motors often exceed 16 hours. Many users might be puzzled by this: brushless DC motor efficiency is around 85%, while stepper motor efficiency is about 60%. Why is there such a significant difference in usage time?

The explanation is quite simple. According to the formula "Motor Efficiency = Output Power / Input Power," stepper motors typically use constant current drive. With a fixed load, the input power of a stepper motor changes little across different speeds, while its output power is directly proportional to speed. This is equivalent to the numerator in the formula changing while the denominator remains largely constant. Therefore, in low-speed applications (infusion pumps, syringe pumps, and feeding pumps all operate at relatively low speeds), the efficiency of stepper motors is extremely low.

Brushless DC motors are different. Their input power also varies due to PWM speed control, allowing the motor to maintain high efficiency across different speeds. At ultra-low speeds, the input current of a brushless DC motor can drop to around 40mA.

Some users might ask: Can't the input power of a stepper motor be reduced? The answer is yes, but for ordinary users, the control is relatively complex and difficult.

Brushless DC motors generate less electromagnetic interference (EMI). The leads of a stepper motor act as efficient "antennas," effectively radiating and conducting the inherent switching noise from the drive circuit. In contrast, brushless DC motors with built-in drive circuits minimize interference sources and propagation paths at the system level through extremely short power loops.

Brushless DC motors have a wide speed regulation range. Stepper motors are suitable for medium to low-speed applications, while brushless DC motors can still guarantee relatively high torque output at 3000 rpm. This enables brushless DC motors to meet a wide range of flow requirements and easily fulfill the need for fast priming and bubble removal in infusion pumps at a flow rate of 2000 ml/h.

Brushless DC motors employ closed-loop control, eliminating the possibility of missed steps inherent in open-loop stepper motor control.

[Potential Issues with Using Brushless DC Motors]

Implementing closed-loop control for brushless DC motors requires knowledge of servo control technology related to motors, which can be challenging for ordinary motor users.

For the same physical size, the output torque of a brushless DC motor is smaller than that of a stepper motor, necessitating a certain gear reduction ratio to increase torque.

In terms of overall cost, the usage cost of brushless DC motors is comparable to that of stepper motors.

[Recommendation]

Nidec offers a low-cost brushless DC servo motor series, the 24H, specifically developed for high-speed printers from Canon and Ricoh. It achieves precise angular positioning control with an accuracy of 0.9 degrees, features a built-in drive circuit, and is highly integrated. Control is very simple, equivalent to the effect of a stepper motor plus driver, accepting pulse and direction control signals, with each pulse moving the motor 0.9 degrees. Currently, several domestic infusion pump clients have been using it for over 10 years.

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