Tear Down: EnOcean STM 550 Energy Harvesting Multi-Sensor

August 19, 2022

Blog

Energy harvesting devices capture energy from the environment, convert that energy to electricity, and put it to work. The energy could come from the sun or other (man-made) light sources, or from vibrations, wind, or water. While this technology is not new, the ability to harness that power and use it as the sole source of energy for small embedded devices is.

The real key to energy harvesting is having a system whose power consumption is low enough that it can be operated using one of these alternative sources. That statement seems obvious, but it’s a lot harder than it sounds, assuming you want that system to perform a meaningful action. In most cases, we’re talking about milliwatts of power or less, which could be enough to drive one of the latest ultra-low-power MCUs.

One vendor that’s been able to accomplish this feat is EnOcean, with its STM 550 self-powered sensor. It extracts energy from ambient light using small solar cells, though the STM 550 still contains a backup battery if the solar light source is removed.

 

From Switches to Sensors

EnOcean is fairly well known for producing switches powered by energy harvesting. During an internal review of those switches, the question was asked whether that relatively simple switch could be used as the basis for a more complex multi-sensor product. Hence, the STM 550 was conceived, a device that’s small enough to unobtrusively hang on a wall. From spec to prototype took about 18 months.

The multi-sensor can collect five different forms of data:

• temperature
• humidity
• light
• acceleration
• magnetic contact (to determine whether a door or window is open or closed)

The sensor’s largest component, taking up most of the surface area on one side, is the solar cell, taking up approximately one square inch. Enough energy is realized with this sensor to power the entire device under indoor lighting conditions — that includes sensing, processing the information, and transmitting it to an external source.

 

That said, the average power consumption of the system is less than 3 μA. During normal operation, the solar cells charge the tiny lithium battery, which powers the system. The battery, which appears on the backside of the PCB, can hold enough charge to run the sensor for about four days, which should be far more than necessary. EnOcean has left the door open to insert yet another battery for even longer run times with no light source.

The magnetic contact (the glass-like element on the side) reads a magnet that would be attached to the door or window. If the magnet is present, the door/window is closed and vice-versa. Building administrators like to connect this sensor to an HVAC system. If the door or window is open, the HVAC shuts down to conserve energy.

“One feature that’s really special is the tiny acceleration/vibration sensor that can detect many different conditions,” says Matthias Kassner, a Vice President at EnOcean. “A primary industry for this device is building automation, and many of these buildings operate with older equipment. The sensor would know when a system is not running in its typical manner, like when a large air-conditioning unit needs maintenance. The sensor is so sensitive it can detect whether a desk is occupied based on a vibration from the user.”

Two Radio Options

Two different radio transmitters are available, both optimized for low power. They’re designed into the device using a modular configuration, allowing EnOcean to swap in one or the other based on customers’ needs, without altering the device's form factor.

One is the company’s own EnOcean radio, based on an ST device that operates in the sub-gigahertz range. It conforms to an international ISO standard. This medium is suited for long distances and sees less interference. The second option is a Nordic Bluetooth radio, which can connect directly to Bluetooth-based lighting systems or other end points.

The STM 550 is designed with two separate antennas. One is for the sub-gigahertz or Bluetooth radio and the second is an NFC antenna, which is required because the device can be parameterized via NFC. In other words, the NFC function lets the user configure things such as which sensors are reporting, how often updates are sent, security settings and device IDs, and so on. Using NFC, the user can make these adjustments with an NFC-enabled smartphone and then do a simple download.

Most savvy engineers know how difficult antenna design can be, and the EnOcean engineering team tackled that portion completely in-house. In a tiny form factor, they had to squeeze two antennas and avoid interference from a metal solar cell on the top side and a metal plate on the back.

According to Kassner, “This is an area of expertise for us. We have a big RF department. We have our own small test chambers where we can automatically rotate the device on all axes. But it’s the tiny form factor that really makes it complicated.”

Another space saver is the small amount of memory that’s needed, as hardly anything is stored on the device. It simply senses and transmits. Not having to store anything or perform any Edge processing helps keep the power consumption down. If processing is required, that would be handled by a downstream device like a gateway that receives data from the STM 550.

While a larger form factor would have allowed for a larger solar array, that wasn’t seriously considered by the EnOcean engineers, because the output power is regulated to a maximum of 1 dBm at the antenna base in the U.S., although it goes up to 10 dBm in Europe. That larger form factor would have simplified the antenna design as well, but keeping to the smaller package makes the STM 550 quite appealing.

Other design criteria considered (and discarded) include the use of Wi-Fi and LoRaWAN. Wi-Fi was ruled out because of its high power consumption. And LoRaWAN didn’t make the cut simply because it’s not the use case the team was targeting. LoRaWAN is not an “always on” type of configuration; rather it excels in applications that need to be “pinged” at a much lower frequency.

At the end of the day, the STM 550 represents an impressive design. Squeezing so much functionality and operating through energy harvesting is quite an engineering feat.