Slotted DC motors
In terms of brushless designs, another important category for portable medical device applications is slotted motors. With these motors, Portescap typically winds the core by hand to increase power density compared to slotless motors where the core is automatically wound. Some benefits with a slotted design are more flexibility in controlling the trade-off with I2R losses and the ability for designers to maximize efficiency compared to torque.
Typically, a slotted motor allows for better thermal dissipation compared to a slotless design. “With the slotted design, we have had success developing autoclavable designs which create an excellent application match for surgical hand tools: bone shavers, reamers, bone saws, bone drills, surgical staplers and any tools that need to be autoclaved,” said Martha. “Typically these applications operate at a high RPM and need to stay relatively cool while the surgeon is using them in their hand.”
Since most of these slotted motors are custom motors, Portescap engineers work with customers to optimize the exact working point for the application. Considerations might include optimizing peak torque or no load speed and torque, or optimizing the stator and magnet to achieve the desired result.
A primary application for stepper motors is handheld pipettes used in medical labs. The big factor in the OEM’s design is ergonomics, matching the motor precision to the metering requirements of the pipette. The motor drives the ergonomics as the performance requirements dictate the motor diameter and length.
In the past, these applications have used a motor with a leadscrew coupled to the shaft, which would translate the rotary motion into linear motion. But this solution wasn’t efficient because the design introduced other mechanical components in the system. “As linear actuators enabled direct coupling into the mechanism, better magnet design and materials improved the linear performance that the actuators could produce as well as offer a size reduction,” Beckstoffer said. “The linear actuators fit tightly into the pipette housing, with a larger section at the top that the user grips, and the linear motor which moves directly down the length of the pipette.”
Linear actuators are ideal for the pipette designers because steppers provide precision movements and inexpensive solutions for electronic control of move distances. The actuators are limited in size because the leadscrew is an integral part of the rotor, and has to travel through the rotor itself. There is a forwarding looking trend to look at disc magnet technology.
Using the disc magnet technology is in some ways a step back because it can’t be used with the linear actuator. But the technology potentially allows motor diameters to go down as small as 10 mm, which would reduce the size and weight of the pipette itself.