Edited by Stephen J. Mraz
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. 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.
For medical applications, space constraints are often critical, so the smaller the motor, the better. Smaller motors also need less power, so they save energy. But there are drawbacks to smaller motors, the major one being less torque. And although you might think smaller means less expensive, building small and precise motors costs more. So users must weigh the trade-offs between size and price.
Some stepper-motor manufacturers are coming up with innovative methods to shrink motor size while simultaneously increasing torque. Lin Engineering, Morgan Hill, Calif., for example, has recently introduced the 4418 Series of Xtreme Torque motors. Compared to other stepper motors with similar mechanical traits, the 4418 Series provides up to 35% more torque without the need to increase power input. This was done by shortening the motor’s end caps and lengthening the stator and rotor, so the overall length is unchanged. And because torque is directly proportional to the number of stator laminations and rotor length, increasing both increases torque.
When it comes to motor size, length is often critical for fitting the motor into medical devices. NEMA standards do not account for length and length can vary among motor manufacturers and change with redesigns and upgrades. For example, the thinnest NEMA-14 stepping motor used to be 1.02-in. long. Today, the thinnest, the Lin 3809, ranges from 0.5 to 0.79-in. long and generates from 7.5 to 20 oz-in. of torque.
Generally, stepmotors come with a standard square flange and mount to a bracket. But for some motors, especially modular ones, demand custom housings. In one application, for example, a handheld liposuction device needed a stepmotor to create suction. So the company used a Lin 3809 modular motor molded into a plastic housing that did not require screws to mount the motor. This saved room and kept the device small enough to be handheld.
Another application, an instrument used to cut into human eyes to create an air pocket during surgery, used the same motor. But a plastic-injection-molded housing with a twist-and-lock design, rather than screws, held the motor in place.
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.
All stepmotors have some step error. Therefore, engineers generally choose motors with the least amount of error. Step error is measured in increments of arc-minutes, where 1 arc-min equals 1/60°, or 0.0167°. The industry average error for a 0.9° stepmotor at 64× microstepping is 4.5 arc-min, or 0.075°. But some motors, including Lin Engineering’s 0.9° stepmotors, have average step errors of 1.5 arc-min, or 0.025°. Naturally, though, 0.9° steppers tend to have slightly less torque than 1.8° motors.
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.
Two major factors determine a stepmotor’s resonant speed: torque stiffness and inertia. Changing a motor’s torque properties can shift the resonant point to a lower or higher speed. This can be done by changing the driver’s power-supply voltage or the current delivered by the driver. Increasing either of these shifts the resonant frequency to a higher speed.
Changing the inertia of the load on the motor also shifts the resonant frequency. The most common method is to increase inertial load by simply adding a damper to the rear side of the stepmotor. This increases the overall inertial load and shifts the resonant frequency to a lower speed. If the resonant “sweet spot” is high enough, the motor will never see it and, therefore, generate less noise.
But motor noise can stem from other causes as well. Three other culprits are: mechanical issues, electrical issues, or using the wrong motor.
The major components that rotate inside a stepper motor are the front and rear bearings. They fit inside the motor’s end caps and are installed on the motor’s shaft. Most noisy motors have poorly fitted bearings in the rear cap. The fit could be cogged, which puts too much stress on the bearings, or be too tight or too loose. These defects are determined by the motor-manufacturer’s assembly processes as well as by the motor-component’s dimensional tolerances. If quiet operation is vital, ask your motor manufacturer to hold certain tolerances within the bearing bore of the end cap and question how it handles critical dimensions.
Motors can also emit electrical noise. It is not audible but can be seen on oscilloscopes or other electrical devices within the application. Electrical noise can be caused by an imbalance in the stepmotor’s resistance and inductance.
Bipolar stepmotors have two phases. Each phase is wound with the same number of turns and, in theory, each phase should use the same amount of copper wire. Therefore, the resistance and inductance of these two wires should be the same. But if a motor is poorly made, these two values will not be equal.
Nowadays though, most manufacturers use automated winders and you will seldom see motors fail final inspections due to resistance or inductance imbalances. However, some manufacturers use low-quality magnet wire with inferior insulation or coatings, or an automatic winder might nick the wires and strip off some of the protective coating. If two sections of the same wire are exposed to each other, electrical leakage can create electrical noise in the motor and affect the driver’s electronics. To avoid this, select a motor from a manufacturer that uses quality wire. Or, request that the motor be built with a heavy-build wire. Heavy-build wire is double coated with protective material. Even if it gets nicked during winding, there is little risk of cutting through both coatings.
Finally, noise can also be due to a simple misapplication. In many cases, users choose motors that create more torque than needed, which causes vibrations and noise. So simply choosing smaller motors can lead to quieter medical devices. In general, engineers should chooses motors with 20% more torque than their application needs. Anything more is overkill. Another approach is to microstep the motor through the driver electronics. Forcing the motor to take smaller steps significantly reduces oscillations at each step are and noise goes down.
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.