Bluetooth LE inertial measurement unit delivers orientation data for wearable applications


Nordic nRF52832 SoC-powered SFM2 by Sensor Maestros enables orientation/motion end products with Bluetooth LE link to third-party apps

Nordic Semiconductor today announces that Denver, CO-based technology company, Sensor Maestros, has selected Nordic’s nRF52832 Bluetooth® Low Energy (Bluetooth LE) multiprotocol System-on-Chip (SoC) to provide the core processing and wireless connectivity for its ‘SFM2’ (Sensor Fusion Module) wearable inertial measurement unit (IMU). 

The ultra-low power SFM2 integrates a nine-degrees-of-freedom (9DoF) IMU—including a 3-axis gyroscope, 3-axis accelerometer, and 3-axis magnetometer—to deliver accurate absolute orientation data, as well as pressure/altitude, temperature, and humidity sensors. The SFM2 provides up to 833Hz sensor fusion output data rate (ODR) and up to 1667Hz ODR for raw accelerometer and gyroscope output. Five or more SFM2 devices can be used and synchronized by Sensor Maestros’ Android and iOS ‘smSENSR-LOGR’ smartphone app. 

Bluetooth LE throughput capability

The SFM2 utilizes the nRF52832 SoC’s Bluetooth LE throughput capability to provide data rates approaching 50kBps, which are demonstrated to be feasible on newer mobile devices, claims Sensor Maestros. The company says SFM2 is the smallest (40.9 by 28 by 14mm) and lowest power wearable IMU in the market that provides both Bluetooth LE and USB connectivity. Other features include Fusion ODR oversampling, time-stamped sensor data, and coordinate system function ‘tare’. When attached to any object, SFM2 can be used to determine the object’s orientation and motion in 3D space. It can be attached via a small Velcro strap that slides through the bottom of the enclosure. 

The Nordic DevZone forum, direct ticket support, and excellent SDK have also been very helpful throughout the development process
Franks Roberts II, Sensor Maestros

The device is designed to enable a wide array of end applications such as indoor positioning and navigation, gaming, sports and fitness, balance and rehabilitation equipment, vibration monitoring, and pattern recognition. For example, SFM2 could be employed for a sophisticated sports watch, drone control, or in human motion applications such as gait analysis, sports motion, and impact analysis, and aiding in the diagnosis and treatment of peripheral and central vestibular disorders (for example, dizziness and vertigo). It can be supplied as an assembled PCB with a 250mAh or 130mAh battery for integration into third party devices, or as a complete product in an enclosure for badging by OEMs. 

The SFM2 uses the Nordic nRF52832 SoC’s 64MHz, 32-bit Arm® Cortex® M4 processor with floating point unit (FPU) to provide the processing capability needed to run the solution’s complex sensor fusion algorithms. The processor is designed to support the FP and Digital Signal Processing (DSP) computations typical of high-end wireless applications. The nRF52832 SoC also provides Bluetooth 5 support including 2x on-air raw data bandwidth (2Mbps); 8x broadcasting ability with advertising extensions that increase the advertising packet payload size to 251 bytes, and an improved channel coexistence algorithm. 

In addition, the nRF52832 enables Bluetooth LE connectivity to third-party smartphone or tablet apps (which the company can develop for the OEM)—for example allowing an end user to steer their drone or review their smartwatch sensor data—as well as connectivity to the smSENSR-LOGR companion app allowing complete device configuration for developers via a Bluetooth 4.0 (and later) smartphone. 

Low power consumption

When powered by a 130mAh Li-Poly battery, SFM2 provides approximately 50 hours of battery life (at 104Hz ODR) between charges, thanks in part to the ultra-low power characteristics of the Nordic SoC. The nRF52832 has been engineered to minimize power consumption, with features such as the 2.4GHz radio’s 5.5mA peak RX/TX currents and a fully-automatic power management system that reduces power consumption by up to 80 per cent compared with Nordic’s nRF51 Series SoCs.

Nordic’s nRF52832 multiprotocol SoC combines the Arm Cortex processor with a 2.4GHz multiprotocol radio (supporting Bluetooth 5.2, ANT™, and proprietary 2.4GHz RF protocol software) featuring -96dB RX sensitivity, with 512kB Flash memory and 64kB RAM. The nRF Connect SDK (Software Development Kit) is recommended for nRF52832 software development. The nRF Connect SDK integrates the Zephyr RTOS, supports applications using Bluetooth LE, Bluetooth mesh and Thread, and includes examples, Bluetooth LE profiles and driver support for all peripherals. The SDK also includes a migration of Nordic’s Bluetooth SoftDevice Controller from the company’s proven SoftDevices used with the nRF52 Series. The nRF5 SDK can still be used for development with Nordic’s S132 SoftDevice, a Bluetooth 5-certifed RF software protocol stack for building advanced Bluetooth LE applications. 

“We selected Nordic’s nRF52832 for SFM2 because of key features like the SoC’s low power consumption, Arm Cortex processor with FP capability, the 2Mb PHY optimizing throughput capability, and generous Flash and RAM memory allocation,” says Franks Roberts II, Owner/Founder of Sensor Maestros. 

“The Nordic DevZone forum, direct ticket support, and excellent SDK have also been very helpful throughout the development process.”