Miniature Motion in Medical Devices

Ventilators use slotless, brushless motors
"For the ventilator business, the trends in motion control are driven by manufacturer requirements to increase the pressure and flow levels to cure a broader spectrum of pulmonary diseases," says Loic Lachenal, global medical segment manager for Portescap "New devices are also capable of supporting respiratory conditions for smaller people and children with higher breath per minute count, so the increase in therapy frequency is placing greater demands on the motor."

One challenge is to manage the heat rise inside the motor, which directly relates to the life of the motor. Temperature has a direct impact on the life of the ball bearings. Since children have a faster breathing pattern than adults, the extra number of breaths per minute influences the pressure and flow in the system.

"Reliability is utmost important for ventilation, and our motors are used to rotate an impeller in a blower, which creates a flow generation function," says Lachenal. "We provide the air flow through an OEM's impeller and blower design and either sell motors to the OEM or integrate the blower and impeller, and ship a complete solution."

With a goal of closely following the breathing pattern of a patient, the motor typically accelerates from 10,000 to 15,000RPM up to 50,000 to 60,000RPM in a very brief amount of time (usually 100 to 200 milliseconds). The shorter the acceleration time, the more comfortable it is for the patient because the time for accelerating affects the air flow. Low inertia motors help to create a very responsive device.

"The more frequently the system operates (50 to 60 times per minute for children compared to adults who breathe 35 to 40 times per minute), the more demanding the performance becomes, and the more temperature the motion control system is dissipating because faster operation requires more current," notes Lachenal.

One way Portescap is helping to manage the temperature inside the motor is with a new slotless, brushless motor design. As in typical slotless motors, this design has no iron in the air gap between the magnet and the copper wire of the stator, which helps to provide smooth operation. It also has no detent torque, and very low vibration levels, which is important for high-speed applications. Portescap's newest slotless design with optimized magnet and winding maximizes torque output without increasing power consumption.

"As a benefit, OEMs can use the same amount of current that they use with existing motors, and get more out of the new motor," says Lachenal. "Or if they want to lower temperature inside their device, they can reduce the amount of current and still maintain the same performance level."

Modular-style controllers
"The basic trend we see in medical devices is the miniaturization of designs, and new modular style controllers we have developed to help with those requirements,” says Biren Patel, applications engineer with Maxon Motors. "With our positioning controllers, for example, we now have an EPOS2 module, which is only 1.5 inch by 0.5 inch with a PCI Express-style connection layout, so the customer can design a motherboard and plug the module into it."

Modular style controllers such as Maxon Motors' EPOS2 modules offer design flexibility and reduce time to market by integrating the motion control more tightly into the device rather than using a black box mounted in a panel.

Modular controllers provide flexibility because the motherboard can trace out the connections, rather than using long cables connecting to a black box. The design approach helps reduce weight, and there are fewer cables to rout through the device.

With a module, a key advantage is that the customer can design the motion control more tightly into the device rather than using a black box mounted in a panel. Basically, the printed circuit board (PCB) module can be plugged into the motherboard to achieve greater design flexibility. In applications with limited available space, the motherboard can implement multiple axes in a small volume and can be mounted either vertically or horizontally. The approach offers tight integration with existing electronics, and there is no need to run extra communication cables, since power and communications is traced out on the PCB.

Modules are available to provide position, velocity, or torque control for brush-type or brushless motors. Patel says motor selection is generally application dependent, but there is a move toward brushless solutions in medical applications where the motors often need to last longer. As the cost of brushless is coming down, more and more applications are moving in that direction.

"A key advantage of modular style controllers is their ability to help companies reduce the time to market," says Patel. "With the motion controllers becoming smaller, engineers don't need to design their own drives and controllers. This approach offers a solution that fits more easily into their existing design rather than redesigning the control scheme."

By using a module, all the customer needs to develop is the logic to feed into the controller, which takes care of the trajectory generation; moving the motor; and checking the encoder feedback to verify motor positioning. Options for standard modules include motor types, different feedback types (encoder or absolute encoder), plus the ability to provide dual-loop operation.

For positioning a linear screw, where there is a need to know the position of the screw versus motor position because of backlash issues, separate encoders can be mounted on both the lead screw and the back of the motor. The encoder on the lead screw is used for positioning to compensate for backlash within the gearbox.

Custom firmware can also be used to create a simple routine of point-to-point moves, for example, and the customer can use digital I/O to initiate moves rather than writing a program and communicating over the serial or CANopen bus.

As far as software is concerned, the approach Maxon has taken is to use industry standards (IEC 61131-3), which are intended for industrial robotics rather than creating their own protocol. "As more engineers are moving between medical, robotics, and automation, they often don't need to relearn the software but use what they already know and reduce the learning curve," says Patel.