July 21, 2020
Since automatic ventilators must be equipped with appropriate sensor technology for monitoring changes and reacting to these such as pressure flow, volume, and respiration rate, the module comes with a HALL sensor interface and connectors for an optional pressure sensor add-on board. This important data can directly be visualized on a Raspberry Pi with touch screen using the free, open-source firmware developed for the reference design module by Trinamic.
“Our goal with this reference design is to show engineers how medical ventilators can be build using readily-available components, bypassing increased lead-times of traditional components during the COVID-19 crisis,” explains Michael Randt, Founder of Trinamic. “Right from the start, we decided to use a high-RPM turbine motor based on a BLDC motor. Together with pressure and volume flow sensors, the dynamically controlled motor enables both pressure-controlled and flow-controlled ventilation modes. In line with the Trinamic Open-Source Ventilator (TOSV) project, which led to this reference design, both hardware and software are open-source.”
Drawing on previous experience with the control of CPAP device turbines, one thing was clear: providing fast and dynamic control of low-induction BLDC motors can be exceptionally challenging. It requires careful consideration of the trade-off between high switching frequency and current ripples and their consequent switching and stator losses, respectively. Thanks to the embedded TMC4671 servo controller IC, the module generates a PWM frequency and current controller clock of 100 kHz independent of the microcontroller. With this, the current consumption of the system is reduced, with no performance penalty, by up to 15 percent when compared to a frequency of 25 kHz.
As mentioned, an add-on board for I2C and analog sensors allows support for pressure sensors from various manufacturers. The TMC4671+TMC6100-TOSV-REF can be plugged into a single-board computer to access the user interface and high-level control functions. In case of the TOSV project, a Raspberry Pi with touch screen display was used. All hardware, firmware, and software is open-source under the MIT license.
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