MPS’s Open-Source Emergency Ventilator with eMotionTM and Battery Backup
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As COVID-19 spread in March of 2020, the global shortage of emergency ventilators was one of the biggest concerns. The increasing demand in ventilators meant they would soon be out of supply, so a team 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, MPS leveraged their expertise to aid the fight against the global pandemic.
This article will summarize the development process of MPS’s open-source “emergency ventilator” while demonstrating how MPS power solutions can enhance the design and development of any electronic system.
When considering the specific needs that could address the impending ventilator shortage, MPS 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 reference clinical and design information for teams around the world to build their own emergency-use ventilators by automating a manual bag-valve mask (BVM) (see Figure 1).
Figure 1: MIT E-Vent Version 3 Prototype (1)
1) This image was obtained from MIT.
Full-fledged ventilators are complicated machines (see Figure 2). 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.
Figure 2: Ventilator and BVM Example (2) (3)
2) This image was obtained from Dräger.
3) This image was obtained from Ambu.
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.
“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. A BVM machine is a bridge device that could potentially pose medical risks, and should only be used when necessary.
MPS’s Emergency Ventilator Improved Design
MPS’s design follows 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. The MPS solution improves on the power management by including seamless battery backup, and improves the motor control by utilizing a brushless DC (BLDC) smart motor. This new design integrates commercially available drivetrain components to create a compact device (see Figure 3).
Figure 3: MPS Open Source “Emergency Ventilator” Version 2 Prototype
The main integrated components include the following devices:
- The MP2759, a switching charger with power path management (as part of the EV2759-Q-01A evaluation board).
- The MPM3510A, a synchronous step-down converter module (as part of the EVM3510A-QV-00A evaluation board).
- The MP3910A, a boost PWM controller (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: MPS Emergency Ventilator System Block Diagram
For this solution, the EV2759-Q-01A takes the primary input power from a standard 19V 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 EV2759-Q-01A has a maximum input voltage of 36V and can handle a battery with 1 to 6 cells in series, with varying battery regulation voltages and a maximum 3A charging current (see Figure 5).
The battery used in this solution is a 14.8V, 4S, 6000mAh lithium pack that is widely used in 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. The EV2759-Q-01A can be configured to trigger an alarm during an AC power loss event. This alarm is cleared once the AC supply power is restored.
Figure 5: MPS EV2759 Board and Backup Battery
The MPM3510A 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 MPM3510A accommodates a wide 4.5V to 36V input voltage range, and has an adjustable output down to 0.8V, with 1.2A of continuous load current capability.
The MP3910A boosts the main system voltage to supply the 24V 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 MP3910A has a standard 9V to 14V input voltage range, and tightly regulates the 24V output to provide a sufficient current for peak power motor demands.
Figure 6: MPS EV3910A (Left) and EVM3510A (Right)
The motor that squeezes the BVM is MPS’s EVKT-MSM942077-24, a 77W BLDC smart motor with a 42mm frame size (NEMA-17) and an integrated MPS smart motor module (the MMP742077-24-C). These are both part of the MPS eMotionTM system line of servo drive modules and kits. The motor is connected to the Arduino via the RS485, and is controlled utilizing commands from the MPS-developed “MSMMotor.h” Arduino library.
The EVKT-MSM942077-24 provides a powerful, compact, and simple solution to make 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 (see Table 1). This device has RS485 and PULSE/DIR input interface options, 77W of continuous power output, and 0.3° position resolution. The EVKT-MSM942077-24 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 (see Figure 7).
Table 1: Smart Motor Module Components
|MP6570||Three-phase BLDC controller with a high-accuracy angle sensor||1|
|MP6710||eMotion SystemTM servo motor controller||1|
|MPM3510A||36V/1.2A modules, synchronous step-down converter with integrated inductor||1|
|MP1907A||100V, 2.5A, high-frequency half-bridge gate driver||3|
|MPQ2013A||40V, 150mA, low quiescent current linear regulator||2|
Figure 7: Ventilator System Performing Homing Operation at Start-Up
The mechanical design of MPS’s 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.
The technical requirements for the mechanical design provide the 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, MPS calculated the necessary gear reduction to use with the EVKT-MSM942077-24. MPS 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 the 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 8: Main Gears of Drivetrain with Input from Motor and 100:1 Planetary Gearbox
After 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 version 2 prototype that will be available if needed (see Figure 9).
Figure 9: MPS’s Open-Source “Emergency Ventilator” (Version 1 on the Left, Version 2 on the Right)
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 this community provided vital reference material during the development of this emergency-use ventilator design. MPS 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 fine-tune system design details, such as the BVM squeeze paddle shape and torque requirements.
To learn more about the design or components used in our automated BVM machine (including the full BOM, 3D design files, and the Arduino control code), check out the “Open-Source Ventilator Project” page.
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