Ventilator design solution from Renesas ElectronicsFollow article
A lack of ventilators available to hospitals instigated by the coronavirus pandemic has inspired many companies to design and share their “ready-to-assemble” solutions featuring readily available components. One open-source ventilator system design was created and published by Renesas Electronics. The design is based around building a portable ventilator for non-ICU patients.
Ventilators provide vital support to the patient by delivering air when breathing is restricted. Ventilators do have an operating mode that allows for invasive breathing in the most severe clinical cases. While understanding the different modes of ventilation and ventilator settings requires years of professional training, this article discusses the most common mode of ventilation, which is the assist control mode. We’ll then take an inside look and examine Renesas’ open-source ventilator design solution.
The assist control mode delivers air to the patient based on volume or by pressure. The exact values of volume and pressure are estimated by healthcare professionals based on various parameters, including the patient’s condition, age and weight.
The system block diagram for Renesas’ ventilator design solution highlights the main functional blocks and provides recommendations for the most suitable products. Design components are highlighted with each recommended product discussed in more detail following the functional description.
Renesas Electronics Ventilator – Block diagram
The sensors in the medical ventilation devices play a crucial role in ensuring accurate and fast measurement of air flowing in and out of the patient’s lungs. The placement of each sensor is dependent on its functional purpose. Inspiration sensors are equipped in the device and they monitor the gas and flow inhaled by the patient. The gas is dry and is mixed with a specific concentration of oxygen that is patient dependent. Similarly, expiration sensors are also placed inside the ventilator for measuring the air exhaled by the patient. The exhaled air is moist because it has been humidified artificially and by the patient’s lungs.
Lastly, proximal sensors are very close in proximity to the patient, typically connected to the mask. Proximal sensing of airflow, volume and pressure are needed to guarantee the health of the system by way of leakage measurements. The proximal sensor also allows the detection of small respiratory signals that can be suppressed by the noise present in the system. The ventilator system reference design from Renesas supports all three of these sensor types. In the block diagram, the inhale and exhale paths are demonstrated in blue and pink, whereas proximal sensing is shown in grey. The overall purpose of the sensor is to guarantee the health of the system, the safety of the patient, and to ensure the system reacts appropriately in dynamic situations.
The air from the blower is controlled through a sensor feedback loop by monitoring flow and pressure sensors. In assist control mode of ventilation, the gas flow rate readings can be taken using the FS1023-DG gas flow sensor module, which consists of a flow sensor, amplification circuit and housing. The sensor module can measure the gas flow at a rate of 0 to 35 litres/min and output both in digital I2C and analog formats. The response time of the FS1023-DG is less than 5ms ensuring that the ventilator detects breathing without any delays. During pressure-controlled ventilation, the pressure sensors are utilized to monitor the amount of pressure being delivered to the patient during each inhale. Both flow rate and pressure data are sent to the RX23W MCU using the I2C communication protocol. The flow rate data is converted to tidal volume information by the same MCU.
The two proximal sensors that are present in this design are proximal air pressure sensor and oxygen sensor. The former monitors the pressure of air inhaled by the patient, whereas the latter measures the concentration of oxygen in the inhaled air. Since the output of the oxygen sensor has a limited voltage range, an operational amplifier is used to improve the resolution of the sensor readings prior to inputting the signal to the RX23W MCU’s built-in analog-to-digital converter (ADC). The ISL28148FHZ-T7Ais a great low-power opamp option for an oxygen sensor signal conditioning application and features rail-to-rail input and output and low supply current (900µA). The RX23W MCU controls the ratio of oxygen by opening and closing valves that lead to the oxygen tank.
The recommended flow sensor in this design is optimized for operation with a regulated 5V supply voltage. Buck regulator ICs are a great choice for converting a high voltage from battery source (24V) to a low voltage value required for the flow sensor. The ISL8541 synchronous buck regulator offers a compact, yet highly efficient solution for this application. It supports an input range of 3V to 40V and does not require external compensation. In fact, only a minimum number of external components are needed when using this buck converter, which reduces the BOM count of the overall system. A variety of buck converter application designs featuring the ISL854x family can be found in "Doing More with Buck Regulator ICs" white paper from Renesas.
Processing and User Interface
The 32-bit RX23W MCU is the main “brain” behind the processing of the sensory data from flow, pressure, and oxygen sensors. It opens and closes the valves that control oxygen delivery and it controls pressure during the expiration stage.
The RX23W MCU also drives external peripherals, including LCD display, fan, buzzer and touch key through existing GPIO, SPI and PWM ports. It also supports Bluetooth® 5 Low Energy (BLE) technology allowing the designers to implement wireless communication functionality to the end device. RS Components stock a target boardfor the RX23W MCU with an embedded emulator that can be utilized in the initial evaluation and prototyping stages.
The RX23T functions as the MCU for the motor driver circuit of the blower. This 32-bit MCU features an integrated FPU (floating point unit) that allows the implementation of complex algorithms for motor torque and speed control. To test the performance of the RX23T MCU, consider the YROTATE-IT-RX23T motor control kit that is designed to drive 3-phase permanent magnet motors up to 48Vdc with a peak current of 5A. If you are new to the RX23T family of microcontrollers and need help to get started with programming, the Renesas Starter Kit is currently on offer! The kit includes the core board with RX23T MCU, LCD display module, on-chip debugging emulator, and associated software on DVD-ROM.
Both MCUs operate on 3.3V supply voltage regulated by a series combination of ISL85410 (RS:121-7276) buck converter and high voltage ISL80410 LDO with adjustable VOUT and low quiescent current. The MCUs monitor and reset each other increasing the safety of the ventilator system.
The motor control system for the blower unit brings its own unique set of design challenges. Apart from demonstrating a very fast and precise reaction, the motor for the blower unit must operate smoothly with low audible noise.
In the ventilator design solution from Renesas, the motor is controlled by implementing a 3-phase bridge configuration consisting of bridge MOSFETs and HIP2103 half-bridge drivers. Three ISL28191 operational amplifiers are used (one for each phase) for current monitoring and current limiting purposes and their output is connected back to the RX23T. The HIP2103-4DEMO1Z Demonstration Board , which is designed to drive a 3-phase BLDC motor, along with its documentation can be a good place to start if you want to quickly familiarize yourself with a 3-phase bridge application for the HIP2103.
Power and Battery Management
This ventilator design is intended to be a portable ventilator used in hallways, homes or for non-ICU patients. Hence, the battery life is a major concern for this application. A way to ensure extended battery life is to optimize the system components’ power consumption by switching to low power or sleep modes when the ventilator is not in use. The brightness of the LCD display, for example, is adjusted depending on the ambient light intensity by utilizing the ISL29034 digital light sensor.
The ISL94202battery pack monitor IC controls power to the system as well as monitors the status of the pack. The RX23T working with the ISL94202 improves the efficiency and maximizes on time and battery life. The ISL94202 supports 3 to 8 battery cells connected in series and works with most lithium batteries, including Li-ion CoO2, Li-ion Mn2O4, and Li-ion FePO4.
A battery backup system using the ISL81601bidirectional controller (Source: renesas.com)
The ISL81601 is a 60V buck-boost controller used to charge the internal batteries of the charger. The device features internal and external current sense allowing for more involved charging algorithms. The controller enables the ventilator to be powered up by an external battery pack or a plug-in power supply. The corresponding evaluation board ISL81601EVAL1Zis currently available at RS Components’ online store.
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