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Thread: XMC2Go Design Challenge: Fetal heart rate monitoring

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    mui
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    Fetal heart rate monitoring with XMC2GO

    XMC2GO Design Challenge 2014 Italy – Results Report


    The detection of fetal heartbeat is essential during the pregnancy both from the medical point of view and for ensuring maternal serenity. Unfortunately, the most common and effective devices are based on the Doppler Effect, which may arise some issues potentially harmful to the unborn. The project aimed to the realization of a device that can perform the basic function of Fetal Heartbeat Monitor and have some innovative features. Moreover, this device will be absolutely safe both for the mother and her baby.

    The project
    The idea is therefore to provide a device able to filter and amplify the signal coming from the fetus. Via a microphone and an appropriate conditioning, the signal will reach an analog to digital converter. Its microcontroller will then digitize the detected audio signal and send it via Bluetooth to a smartphone where a special APP will transmit the data on a cloud platform for possible future analysis. The system shall also be equipped with a headphone socket to ensure a local listening, and with a memory for storing intervals of audio detection. A research has been conducted among the ARM core-based microcontrollers, therefore performing and scalable, with ADC and other internal devices (obviously), but above all with a user-friendly development environment and integrated debugging. In fact, both the time-to-market and the low startup costs were defined as essential requirements.

    All these requirements lead us to choose the XMC2GO board by Infineon, which mounts an ARM Cortex M0 (XMC1100) MCU with several peripherals, and a controller specifically designed for professional debugging. All this for just over 5 Euros!

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    The possibility of having a memory and then transmit the collected data to a doctor who can correctly interpret them, make the device open to medical applications that go beyond the admittedly important tranquility resulting from listening to the fetal heartbeat. In addition, thanks to the ability to save audio samples, we can enable Bluetooth only when we are away from the paunch of the expectant mother. We are working to avoid exposure to ultrasound, and we do not want to irradiate the fetus with the Bluetooth signal! To better understand the project, we can see a block diagram that describes the system in its entirety.

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    Audio signal amplification
    The main idea is to amplify the signal received through a cone, such as the one used in the initial part of the stethoscope. The cone and especially its plastic body, possibly covered with Velcro or with another sound-absorbing material, are an effective solution for an initial amplification and for performing ambient noise reduction. The chosen solution amplifies the received signal using the stethoscope principle, thus improving its characteristics and its employment; at the same time, it ensures its safety: the fetus is never irradiated with any type of signal.

    Schematics

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    The first schematics version has been created to carry out the main provided features having a margin of adjustment through the addition of appropriate testing sections. The fateful wired prototype, today out of fashion, has not been prepared, and a PCB has been directly created. The project will be released Open Source and will include some optional parts for future implementations and for developers needs.
    Let's now see together the various sections of the schematics.

    Power supply
    As usual, power supply is a mandatory section. The device will be battery powered and therefore we have foreseen two button batteries, connected in parallel in order to increase the capacity. U6 (MCP1640) is the DC-DC converter in charge of managing the power supply. In fact, thanks to this IC, the supply voltage will maintain a 3.3V stable value during the battery discharge. It will be just the 3.3V voltage to power all the components on the board. In the power supply section you can also note U2 (TPS62730), an additional DC-DC converter (step-down in this case), ideal for managing the BLE113-A module. Actually, as suggested by its datasheet, the TPS optimizes the power of the Bluetooth module by saving energy through the bypass function. The latter is optional, so in the future we will decide whether to use it or not.

    Audio
    The audio section is obviously composed by an omnidirectional, high sensitivity microphone (electret). As an optional component, we have also included the possibility to mount a MEMS SMM310 microphone. The microphone signal is then pre-amplified by the NE5534 operational amplifier, single version of the famous NE5532 audio op-amp. However, this stage is optional and dedicated to the MEMS microphone. For use with a condenser microphone, signal arrives directly to a double active filter implemented with a Sallen-Key that in addition to amplify in two different steps, will also implement a low-pass filter tuned on 200Hz (maximum frequency provided by a fetal heart rate. The signal at the end of conditioning will be an input to the AD converter, a device integrated inside the XMC2GO module.The same signal comes also to U3 (LM4881), a specific headphone amplifier that will just make the signal audible from the headphones, connected through the X1 header.

    Microcontroller and external memory
    The processing task is entrusted to the XMC2GO module that integrates an ARM Cortex M0 microcontroller, which in turn integrates some peripherals, essential for this project: an A/D converter to acquire and digitize the audio signal, an UART to communicate with the BLE module, an SPI to manage the external EEPROM memory and an I2C to manage the accelerometer (optional). The external memory is entrusted to a SST25PF020B 2Mbit SPI EEPROM, more than enough to store audio samples of the fetal heart rate.

    Bluetooth
    The BLE module is a Bluegiga BLE113 that communicates with the microcontroller through the UART. Later we will evaluate whether to enable the DC-DC dedicated regulator in order to optimize power consumption.

    Accelerometer
    The digital accelerometer, added for future purposes, is a 3-axis (MMA8451) type. Among the card features there is a button that could be used for many functionalities and two LEDs (in addition to those already present on the XMC2GO board).

    Part list (preliminary)

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    The assembled board

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    Conclusions
    The project will be released as Open Source, both hardware and firmware, both the schematics and the assembly plan to be published in high resolution, the part list, the gerber files, and all the created documents.
    Last edited by mui; Feb 15th, 2015 at 07:07 PM.

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