TODO: update electrical circuit diagram to reflect current design
The electronics system provides power to the ventilator, real-time control of its sensors and actuators, safety protections and alarms, and a user interface for delivering information and controlling the parameters of its operation.
As with the pneumatics system, the electrical system is designed to be amenable to global supply chains, be low cost, and yet be robust and reliable.
A dual-computer architecture was selected. There is a Cycle Controller based on STM32 that handles the real-time control of the ventilator’s sensors and actuators. Connected to this is a UI Computer based on a Touchscreen, Raspberry Pi 4, and Linux to provide the user interface. This design allows us to delegate the most critical functions to a simple design that can be reasonably reviewed and tested in its entirety, while also allowing enough complexity in the UI Computer to provide a comprehensive user interface.
The division of responsibility between the two computing elements is divided roughly by the immediacy of a potential failure. For the class of operations in which a potential failure could result in a hazard too quickly for reasonable human intervention, the Cycle Controller is used. For the class of operations in which a potential failure can reasonably be addressed by human interaction in a reasonable amount of time given a timely alarm, the UI Computer is used. This design also allows for self-checking between the two computing elements to mitigate single-point failures.
The STM32 microcontroller was selected because it is more powerful than Arduinos or similar popular microcontrollers, but it is still cheap, widely available, well-documented, and has extensive debugging tools. The STM32 is proven to be a reliable medical electrical component and has a diverse I/O set for pneumatic and mechanical control. The operation of the Cycle Controller is kept as simple as possible to facilitate comprehensive review, test, and verification of its function.
The Raspberry Pi was selected as the UI controller for its wide compatibility with existing human-machine interfaces and robustness to supply chain shock. The Raspberry Pi also enables rapid software and UI development by supporting a Linux-based operating system and the common HDMI interface. It supports remote monitoring, as well as automated testing, through its wired and wireless network interfaces. While powerful and flexible, the complexity of its operating system means that real-time control cannot be sufficiently guaranteed or verified. Hence all time-critical operations, such as ventilator pneumatic control, are handled by the STM32.
A serial communication link using checksums to ensure data integrity provides reliable communication of settings and data between the two computing elements.
Each computing element has its own alarm system as well as a watchdog timer. If either computing element becomes non-responsive, the watchdogs will put it into a safe state and restart it. The UI Computer monitors the Cycle Controller and vice versa, and will issue an alarm if the other stops responding. Each controller can trigger an audible alarm system independently, and the UI controller can display alarms visually as well. The motherboard features bright LEDs in 3 colors to provide visual alarms independently of the touchscreen interface. The Cycle Controller and UI Computer each have independent control of their own set of bright red, yellow, and green LEDs. Due to an error in the motherboard design, the Cycle Controller only has command of the red LED in its set. This will be fixed in a future revision.
A custom motherboard has been designed to host both the UI Computer and Cycle Controller, as well as all the required communication, filtering, safety protections, sensors, motor drivers, and power supplies. This improves both the manufacturing assembly and the electromagnetic immunity of the electronics.
All mainboard design files are maintained in this repository in the ../../pcb directory.
Wiring materials and assembly guide on this page.
All key elements of the pneumatic system are controlled by electronics. The WS7040 blower features its own driver, which is powered by 12VDC controlled by the motherboard with a PWM signal. The RespiraWorks-designed proportional pinch valves are driven by stepper motors, which are controlled by powerSTEP01 stepper driver ICs. The powerSTEP01 is available in a daughterboard compatible with the STM32 developer module - this synergy enables rapid hardware and software development around controlling airflow, and provides a proven electronic design for integration into the next motherboard revision.
The differential pressure (dP) sensors that monitor inhale/exhale flow rates are board mounted as well. These are automotive differential pressure sensors which are cheap and widely available in large quantities worldwide. They monitor the pressure difference across the two venturi-based flow sensors as well as the delivered patient pressure. They output an analog electrical signal which is filtered and conditioned on the motherboard and delivered to the STM32 Cycle Controller. They are pneumatically connected to the venturis by 2.5mm flex tubing. In the case of the single-ended pressure measurements only one of the two pressure ports is connected by 2.5mm tubing, and the second port is referenced to atmospheric pressure.
The oxygen supply is controlled by a proportional solenoid. The motherboard features two high-current MOSFET switches fused with resettable thermal fuses and protected by flyback diodes for operating inductive loads. These switches are capable of fixed on/off control or proportional PWM control of a 12V load. One of these switch channels is used for the proportional solenoid. The other channel is uncommitted and can be used for a future feature, such as humidifier heating control.
For future expansion of features, the motherboard also provides 4 powered I2C ports that can be configured for 3.3V or 5V operation. These can be used to command actuators, read sensors, or interface with other supporting devices such as an external humidifier or uninterruptible power supply (UPS).
An early decision during the electronics design was to standardize the input voltage to 12 VDC. This enabled rapid prototyping around the WS7040 blower and driver. Using 12VDC enables the use of widely-available sealed lead-acid security system batteries or automotive batteries as a backup or primary power source in field hospitals. 12VDC is also a common battery standard and there exist 60601-1 compatible wall adapters for operation in a more conventional medical facility.
The custom motherboard features DC/DC converters to supply and regulate the logic voltages for the on-board electronics. In the next iteration of the motherboard, backup batteries shall be present for supplying emergency shutdown alarms (when both mains and backup battery are absent or depleted) as well as providing real-time clock support for monitoring, diagnostics and logs.
A 7-inch capacitive touchscreen is selected as the primary user interface. This is a color display capable of receiving user input and refreshing data with 50 ms latency. Both the display and the integrated speakers are operated through the HDMI interface of the UI controller.
The cycle controller has access to its own dedicated audio system. This is a simpler piezo buzzer, driven by a square wave signal and capable of the required output volume. The buzzer and LEDs provide a secondary emergency interface in the event of a touchscreen failure.