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BLDC motor control for respirators

Ivan Lovas, Freescale Semiconductor - February 12, 2013

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Using high-speed sensorless BLDC motor control in medical devices

Introduction  

Brushless DC (BLDC) motors are widely used in medical devices due to their high reliability, high efficiency, low maintenance and many other advantages. One of the most popular applications is a sleep apnea machine. 

For people who suffer from sleep apnea, the most common treatment is the use of a continuous positive airway pressure (CPAP) device, which “splints” the patient’s airway open during sleep by means of a flow of pressurized air into the throat. The patient usually wears a plastic facial mask, which is connected by a flexible tube to a small CPAP machine.

 

 

 

 

Advanced models may warm or humidify the air and monitor the patient’s breathing to ensure proper treatment. For pressuring the air, a small and usually very-high-speed BLDC or PMSM motor is used.

 

 

Application Requirements 

The motor controls in the sleep apnea machine have very complex requirements. A first important requirement is MCU power. The dedicated MCU performs many other algorithms, so the motor control part should not consume more than 20 percent of total controller power. In addition to motor control, the controller should perform:

 

• Display control

• Communication

• Air humidity control

• Air temperature control

• Recording data to allow doctors to verify adequacy of treatment or adjust required pressure

• Safety algorithms

If we look more closely at this application, we can see that the motor control requirements are also very strict.

• Reliability: As sleep apnea machines are medical devices, reliability is first placed on the motor to start and operate with 100 percent reliability in any condition. That means the motor is already spinning in a forward or backward direction just before startup.

• Pressure control: The difference with standard BLDC applications is that a fan is typically controlled according to required pressure not by required speed. The machine must generate the pressure according to pre-defined ramps and patient requirements.

• High-speed operation and wide speed range: For small dimensions to allow enough airflow, the speed must be increased. On the other hand, when the high-speed operation is used it is easier to achieve the required air pressure.

The maximal speed typically ranges from 30,000 to 60,000 RPM. In minimal speed mode, the motor should operate from 1500 RPM. That equals roughly 2.5 to 5 percent of maximum speed. This is very complicated from the PI controller setting point of view due to the very wide range of speed. In the high-speed region, it is also very difficult to determine commutation instance, as only a few samples of BEMF are measured between commutations. For high-speed operation, a very fast and precise ADC converter is required.

• Low noise: A quiet motor makes sleep more comfortable for patients and their sleeping partners. Noise can be generated by the mechanical part of the fan, but it can also be caused by motor torque ripple.

• High accelerations and braking: The motor usually has small diameter lightweight impellers which provide lower inertia. This allows the system to operate with very high dynamics, such as acceleration and braking near 200,000 RPM per second. This dynamic is required to achieve the requested pressure in a very short time.

• Current control as an inner loop: To achieve such a high dynamic, the application must be equipped with a current controller. Otherwise, the motor can be overloaded by exceeding maximal motor current. The current control also has additional noise reduction. Small, high-speed BLDC motors usually have very low inductance compared to a conventional BLDC motor. In this case, the bipolar PWM cannot be used. A solution can be achieved with higher frequency, unipolar PWM strategy, or a power stage with input DC-DC converter to change motor supply voltage according to the motor speed.

• Memory footprint and MCU performance: All previous requirements must be met to achieve 20 percent power consumption.

 

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