How to Choose the Right Stepper motor for Medical Use

When selecting a stepmotor for a medical device, engineers need to consider many factors. For example, a systolic pump may require accuracy in a small package while lab devices, such as a blood sampler, may need to be exceptionally quiet. Although requirements for stepmotors vary from device to device, there are several factors that should always be considered.

Making it small
In terms of size, stepmotor manufacturers adhere to various NEMA frame sizes. NEMA is the industry standard governing motor dimensions, including the size of the front flanges used to mount motors to devices. NEMA sizes for stepmotors range from NEMA-8 to NEMA-42 Motor. For comparison, NEMA-8 motors with 0.8-in.2 front flanges generate about 2 to 3 oz-in. of torque, while NEMA-42 motors have 4.2-in.2 front flanges and output over 2,000 oz-in. of torque.

Pinpoint accuracy
Stepmotors rotate in terms of degrees. Each step can be in increments of 1.8°, 0.9°, or even 0.45°. This is the inherent and natural step the motor takes. Motors can also be microstepped or forced to take even finer increments. For example, a 0.9° motor can be made to step every 0.45°, which is called half-stepping the motor. And 64× microstepping a motor divides the 0.9° into 64 steps of 0.014°. Microstepping is usually handled by the driver electronics.

Smooth, quiet steps
Stepmotors can stop and hold position at any location a program tells it to. But as their name indicates, they take steps when moving. For example, to complete a full revolution, a 1.8° motor takes 200 steps. A stepmotor’s rotational speed is stated in terms of step pulses, or hertz, and is considered a frequency. At certain speeds, stepmotors resonate and vibrate loudly, and this vibration translates into jerky motion. The loud noise is due to the rotational frequency matching the motor’s inherent resonant frequency, which every stepmotor has. And the resonant frequency is generated with each step the motor takes. But there are ways to either eliminate or diminish the noise a motor makes.

Reliability and quality
When looking for reliable stepmotors, engineers often request MTBF (mean-time-between-failure) data to ensure the potential motor will last a certain number of cycles. Stepmotors typically have an MTBF of over 20,000 hr of continuous operation. When stepmotors operate at their bearings’ rated axial and radial loads or less with temperatures kept to less than 50°C, stepmotors usually last 20 years, assuming a 50% duty cycle.

So, whether your application involves making intricate cuts within a patient’s eyes, pumping critical bodily fluids, or any other type of medical application, taking a motor’s size, accuracy, smoothness of motion, noise level, quality, and reliability are a must.

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