An open-source emergency ventilator design with battery backup – EDN.com

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As COVID-19 spread in early 2020, the global shortage of emergency ventilators was one of the biggest concerns. The increasing demand for ventilators meant they would soon be out of supply, so a team of design engineers at Monolithic Power Systems (MPS) sought to help create a solution to this crisis.

While MPS is not a medical device manufacturer, its engineers and designers are highly experienced with power electronics and motor control. Given the technical architecture of ventilators, respirators and ventilator-type machines, the design team leveraged its expertise to aid the fight against the global pandemic.

This article will summarize the development process of an open-source “emergency ventilator” while demonstrating how power solutions can enhance the development of such medical system designs.

Benchmarking design

When considering the specific needs that could address the impending ventilator shortage, the team turned toward the maker/DIY community. This search led to the Emergency Ventilator Project (E-Vent) being developed by a technical and medical team at MIT. MIT’s E-Vent project was an open-source endeavor that provided clinical and design information for teams around the world to build their own emergency-use ventilators by automating a manual bag-valve mask (BVM) as shown in Figure 1.

Figure 1 MIT’s E-Vent prototype provides reference information for ventilator designs. Source: MIT

“Ventilator” is widely used to describe devices that provide ventilation, but it is important to note the differences between an emergency-use automated BVM machine compared to a full-featured ventilator.

Full-fledged ventilators are complicated machines. Ventilators don’t simply force air in and out of a patient’s lungs—they also control the air volume, flow rate, oxygen content, and even the air temperature and humidity. Additionally, any conditions that could endanger a patient must be monitored to trigger appropriate alarms or corrective actions.

By contrast, an automated BVM, such as the E-Vent system, is not a full-featured ventilator. While automated BVMs provide ventilation and include basic levels of safety monitoring, they are emergency-use bridge devices that automate an otherwise manual process.

Automated BVMs are intended to sustain a patient until a ventilator is available while allowing medical personnel to support multiple patients at once. It’s worth mentioning here that BVM is a bridge device that could potentially pose medical risks, and should only be used when necessary.

Figure 2 The comparison highlights the difference between ventilator and BVM. Source: Drager and Ambu

Ventilator’s improved design

The design team followed the main architecture of MIT’s E-Vent system, which uses an Arduino-based microcontroller to process user inputs, monitor key parameters, and drive a DC motor to squeeze the BVM. However, it employed MPS solutions to enhance the power management by including seamless battery backup and improved the motor control by utilizing a brushless DC (BLDC) smart motor. The design also integrated the commercially available drivetrain components to create a compact device (Figure 3).

Figure 3 The prototype highlights the open-source emergency ventilator design. Source: MPS

The design team integrated the following components into the open-source emergency ventilator design:

  • MP2759, a switching charger with power path management, integrated as part of the EV2759-Q-01A evaluation board.
  • MPM3510A, a synchronous step-down converter module, integrated as part of the EVM3510A-QV-00A evaluation board.
  • MP3910A, a boost PWM controller, integrated as part of the EV3910A-S-00A evaluation board.

While a custom-printed PCB can integrate these components on a single board, the MPS solution utilizes premade, readily stocked evaluation boards of each product to reduce development time while creating a compact solution.

Figure 4 The diagram highlights the basic building blocks of the emergency ventilator system. Source: MPS

For this solution, the EV2759-Q-01A evaluation board takes the primary input power from a standard 19 V AC power adapter—such as those used on laptops—and maintains charge on the integrated backup battery while the battery supplies power to the ventilator system. The evaluation board has a maximum input voltage of 36 V and can handle a battery with 1 to 6 cells in series with varying battery regulation voltages and a maximum 3 A charging current (Figure 5).

Figure 5 The EV2759-Q-01A evaluation board facilitates a backup battery for the ventilator design. Source: MPS

The battery used in this solution is a 14.8 V, 4S, 6,000 mAh lithium pack that is widely used in remote control (RC) cars and small robots. If the AC power is removed, the system power and operation are undisturbed, allowing for continuous operation of the emergency ventilator through a power outage or while transporting a patient. Moreover, the EV2759-Q-01A evaluation board can be configured to trigger an alarm during an AC power loss event. This alarm is cleared once the AC supply power is restored.

Next, the MPM3510A converter module steps down the main system voltage to supply the Arduino microcontroller with continuous, stable power. A reliable and accurate power supply is a necessity since the Arduino controls the system. The converter module accommodates a wide 4.5 V to 36 V input voltage range, and has an adjustable output down to 0.8 V with 1.2 A of continuous load current capability.

Finally, the MP3910A controller boosts the main system voltage for supply to the 24-V BLDC smart motor. Like the Arduino, the motor driver requires a reliable power source to deliver consistent performance. With emergency ventilator systems, inconsistent motor performance can lead to inaccurate air delivery. The PWM boost controller has a standard 9 V to 14 V input voltage range, and it tightly regulates the 24-V output to provide a sufficient current for peak power motor demands.

The motor that squeezes the BVM design is EVKT-MSM942077-24, a 77-W BLDC smart motor with a 42-mm frame size and an integrated smart motor module, the MMP742077-24-C. The motor is connected to the Arduino via the RS485 interface, and is controlled utilizing commands from the MPS-developed “MSMMotor.h” Arduino library.

The EVKT-MSM942077-24 smart motor module makes BLDC motor control more accessible. Within the smart motor module, there is an integrated magnetic angular position sensor, a field-oriented controller (FoC), and power drivers. Furthermore, it has RS485 and PULSE/DIR input interface options, 77 W of continuous power output, and 0.3° position resolution.

The motor reference board allows designers to use current measurement to implement a homing routine that detects when the mechanical squeeze arms contact a built-in mechanical hard stop in the “opening” direction. This hard stop sets the motor zero-position during start-up to ensure consistent performance, which eliminates the need for a separate homing switch and simplifies system design.

Ventilator’s mechanical design

The mechanical design of the emergency ventilator is simple and utilizes standard commercial parts wherever possible. Where commercial parts are not used, this solution incorporates simple, custom mechanical parts. For example, the squeeze paddles are 3D-printed blocks that conform to the shape of the BVM machine.

The technical requirements for the mechanical design demand a specific range of breaths-per-minute (BPM) at various inhale-to-exhale (I:E) ratios. Based on the ranges for both the BPM and I:E ratios provided by the MIT project, along with their recommendations for force requirements to squeeze a BVM, the design team calculated the necessary gear reduction to use with the EVKT-MSM942077-24 motor control module.

The team installed a standard 100:1 gearbox and gear set with a final reduction of 200:1 to achieve the required speed and torque levels. Other components—such as pressure sensor, alarm buzzer, display panel, buttons, knobs, and switches—are all standard components that can be replaced with any equivalent parts that are available to the system designer.

Figure 6 Main gears of drivetrain have input from the motor and 100:1 planetary gearbox. Source: MPS

After the design team at MPS created its first functional prototype, word was spread using videos, a project website, and network outreach. Multiple contacts and groups were informed of the development process, and assistance was provided to integrate electrical features unique to the design. However, MPS did not manufacture this design.

Once this solution was created, many ventilator manufacturers had caught up with the supply shortages, and COVID-19 treatment options were less focused on ventilation, particularly emergency-use ventilators like the BVM machine. For future use, MPS designed a second version of the prototype that will be available if needed (Figure 7).

Figure 7 Version 1 (left) and version 2 (right) of the emergency ventilator prototype are shown side by side. Source: MPS

The design team at MPS would like to thank the MIT E-Vent Project and countless others in the maker community that were also part of this global effort. The ideas in the community provided vital reference material during the development of this emergency-use ventilator design. The team also deeply appreciates the time and effort from the medical team at Santa Clara Valley Medical Center (SCVMC) for assisting with testing the device on a lung simulator to finetune system design details, such as the BVM squeeze paddle shape and torque requirements.

To learn more about the design or components used in the automated BVM machine, including the full BOM, 3D design files, and the Arduino control code, check out the “Open-Source Ventilator Project” page on the MPS website.

Aaron Quitugua-Flores is principal engineer at Monolithic Power Systems (MPS).

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