Exo Sense Pi, an Environmental Sensor, with a Twist

Author:
Ulderico Arcidiaco, Sfera Labs

Date
03/31/2022

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A look "behind the scenes" at why Sfera Labs developed its indoor environmental sensor, Exo Sense Pi, in a crowded market with many competitive alternatives

Sfera Labs recently introduced a new indoor environmental sensor, Exo Sense Pi. While there is a peak of interest in indoor sensors that can monitor air quality and people occupancy, this is a very crowded space. For a company specializing in industrial solutions, wouldn't it have been better to stay clear of the environmental sensors market? 

Defining Exo Sense Pi

All Sfera Labs products are platforms that can easily be used as the hardware foundation for applications. As such, an environmental sensor by the company should not be just like any other closed hardware, implementing a pre-defined set of functions around the sensors built in the device. Instead, it should be a sensor platform, leaving lots of space for developers. It should also be small and have a non-descriptive look. The problem is that environmental sensors based on microcontroller architectures limit the freedom and creativity of developers.

Complex logic must reside elsewhere, maybe in the "cloud" or an external local controller.

Sfera Labs’ expertise lies in developing Raspberry Pi- based hardware, and therefore its goal was to create a sensor platform based on a powerful and flexible Raspberry Pi. Unfortunately, no Raspberry Pi model could fit the target size of a circuit board for the chosen 80 mm X 80 mm enclosure. 

The most appropriate version of Raspberry Pi was the Compute Module, but the size and constraints of the DDR2 SODIMM connector made it impossible to squeeze it into the available space. WiFi and BTLE wireless connectivity were also desired, but the prospect of developing an off-Pi wireless solution was not appealing. However, in October 2020, Raspberry Pi released the Compute Module 4. The CM4 marks a total departure from the original versions, with a different, more compact form factor and integrated wireless connectivity. Simultaneously, Sensirion, a leading manufacturer of IC sensors, shipped two advanced digital sensors, one for temperature and humidity, the other for air quality (VOC - volatile organic compounds). Those releases brought all of the main pieces of the puzzle together.

A True Sensor Platform for Developers

Thanks to the embedded CM4, Exo Sense Pi marks a radical departure from other environmental multi-sensor products. Instead of a traditional microcontroller, it embeds a quad-core ARM Cortex-A72 (ARM v8) 64-bit processor at up to 1.5 GHz, up to 8GB of LPDDR4 ECC RAM, up to 32GB embedded eMMC Flash memory, or an external μSD card. Those specifications bring the power of a high-performance server from only a few years ago, and shrink it into an 80 mm X 80 mm wall-mount plastic enclosure with a total weight of 90 grams. The Exo Sense Pi also leverages the tremendous amount of software resources of the Raspberry Pi ecosystem. Application development time and costs can be dramatically reduced compared to a functionally similar development on a microcontroller. More importantly, this platform allows the development of applications that would not be possible on the constrained resources of even a 32-bit microcontroller. The CM4 board also offers a wireless stack with 2.4 GHz and 5.0 GHz IEEE 802.11 b/g/n/ac WiFi, as well as Bluetooth 5.0 and BLE.

No Free Lunch

Unfortunately, having such a powerful CPU on a small environmental sensor also creates issues. A fast CPU architecture and the high-performance wireless module require a considerable amount of power. This is then dissipated, causing a substantial self-heating of the device. The power supply stage must deliver a substantial amount of current and gracefully handle peaks as the CM4 draws power based on the processor's load. Exo Sense Pi draws about 2W when the CPU is lightly loaded and over 5W with 100% CPU load. A typical low-power MCU-based environmental sensor may draw just 100mW.

While a small regulator could be based on a simple LDO design, Exo Sense Pi requires an efficient switching regulator, adding cost, complexity, internal heat, and potential switching noise issues. Exo Sense Pi uses an Analog Devices LT8640S capable of delivering up to 6A at 5V output and has an internal architecture specifically designed to minimize EMI emissions while delivering high efficiency at high switching frequencies.

Internal Heat Compensation

In most environmental sensors, the internally generated heat may be small and constant, so the temperature, humidity, and air quality sensors output are relatively easy to compensate with a fixed correction factor or a simple formula. Exo Sense Pi self-heat is significant and subject to broad changes based on CPU load and other operating conditions. The air temperature inside the enclosure is typically around 5°C higher than external air temperature when lightly loaded but could go up to 10°C higher under heavy continuous load. There is no way to compensate for this with a static correction factor, so two LM75A temperature sensors, one underneath the Compute Module and one close to the raised PCB where the temperature, humidity, and air quality sensors are installed. The temperature gradient between them is used to estimate the amount of self-generated heat and compensate all affected sensors.

 

Click image to enlarge

Figure 1: Temperature compensation sensors

 

Sensors and Interfaces

The temperature and humidity sensor is a Sensirion SHT40, connected to the Compute Module I2C bus. The VOC air quality sensor is a Sensirion SGP40. Both sensors sit on a raised PCB close to the lower-left ventilation grille. The upper grille, thanks to the air convection movement caused by the temperature gradient between the inside of the case and the environment air temperature, creates a natural airflow to improve the performance of the sensors. 

Exo Sense Pi also has a Texas Instruments OPT3001 Ambient Light Sensor, a Panasonic EKMC1604111 PIR motion sensor, and a TDK ICS-43432 digital I2S microphone for audio recording and environment noise detection. A Linux utility running on the CM4 processes the audio stream for noise detection. It computes the dB(A) noise level, as required by the IEC-61672 international standard for sound level meters. dB(C) weighting is also available.

Besides the internal sensors, Exo Sense Pi provides external interfaces, like a RS-485 serial interface, connected to the CM4's UART. It has two 5V-level I/O or generic digital inputs to connect 1-Wire sensors or a Wiegand keypad/badge reader. Finally, an open-collector output controls a small load or an external power relay.

As shown in Figure 2, Exo Sense Pi also has a Microchip MCP79410 real-time-clock with backup battery, an ATECC608 crypto authentication chip, an internal buzzer that can be driven by a PWM line or as an on-off buzzer, and a factory-installed optional D7S earthquake detection vibration sensor from OMRON.

Click image to enlarge

Figure 2: Exo Sense Pi block diagram

 

Typical Applications

Exo Sense Pi was envisioned to be a sensors platform that allows the development of non-typical sensor-based applications. It can be used to create rich and innovative applications running onboard the sensor and not on external processors. For example, Exo Sense Pi could be used as a controller for remote technical shelters, monitoring all essential environmental data, using the motion sensor for intrusion detection, and the microphone to sense issues with cooling fans or other mechanical equipment. Connected to Modbus RTU energy meters and actuators, it could analyze and report electrical load and energy quality data and turn on lights when motion is detected if the ambient light is low. It could also serve as an access control system when connected to a Wiegand keypad and badge reader. Finally, it could use the BTLE radio on the CM4 for assets and personnel tracking.

Summary

Although the sensors market is crowded with traditional sensors, it is void of powerful and easily programmable devices like Exo Sense Pi. This is an entirely new development for smart sensors, and marks a departure from all other existing solutions.

 

Sfera Labs

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