Power optimization for battery-less BLE beacons
Devices like smartphones have brought major changes to our day-to-day life. They are our gateway to information affecting our lives directly in real time, relating to our health, environment, and even how we make purchases. However, most of this information has to be ‘pulled’ out, which means that either it is obtained through a connection with another device or by searching the web. These methods require that users have to initiate an action when they want data. Sometimes users may not even know where or what to look for, such as when looking for offers on a product in a store.
The solution is to have a system that can ‘push’ messages to users in real time. As smartphones are the best gateway through which information can be pushed to users, this system should effectively send data to it without any hassle. There is where beacons come into the picture.
In wireless terminology, a beacon is a system that broadcasts messages so that they can be received by user devices in the vicinity. Beacons allow hassle-free data transfers to a user’s device without requiring their intervention. Current devices such as smartphones support various options that can be used to enable beacon functionality. To ensure large scale adoption of beacons -- including support by a major section of devices, interoperability, low installation cost and low power mode operation -- Bluetooth Low Energy (BLE) has become a ubiquitous choice for beacon communication.
BLE is widely used for low power wireless communication in applications requiring the transfer of data within a relatively small radius. A Wireless Sensor Node (WSN) is an example. Data is taken from a sensor and is usually sent to a smartphone. A typical application flow in these sensor nodes is as shown below:
Figure 1. Typical flow diagram of BLE sensor devices (Source: Cypress)
These beacons/sensors need to be powered from a source that allows them to function continuously while still maintaining the size-factor of the overall device. Powering these types of beacons from a wired source is rarely feasible as they are either on a human body or placed remotely; therefore, use cases that require wires for power do not make sense. Battery powered sensors introduce issues such as finite operating life, the need to recharge them frequently, and environmental impact due to disposal.
If we truly want beacons that do not require any kind of maintenance, then we need to utilize unharnessed energy from the surrounding environment such as light, motion, pressure or heat. This will enable an “install-and-forget” approach where the beacons will remain powered through the lifetime of the device.
This is where energy harvesting comes into play. Energy harvesting is a method of collecting minute amounts of unharnessed energy from the surrounding environment and storing it. This stored energy is used for powering the WSN device, collecting sensor data, and transmitting data over BLE.
Figure 2. Energy Harvesting WSN device block diagram (Source: Cypress)
The energy harvesting system (EHS) is a circuit that includes an energy harvesting device (EHD), an energy harvesting power management IC (PMIC), and a storage device. The energy harvesting PMIC ‘trickle’ charges the storage device (typically a capacitor) with energy provided by and EHD such as a solar cell, vibration sensors or piezoelectric devices. The EHS then uses this stored charge to provide energy to another embedded device. Depending on the state of the WSN, the EHS output power varies. When active, energy is consumed, and the voltage from the EHS begins to drop. When in a low power state, the voltage from the EHS rises since the storage device is being charged. The following image shows an example of an EHS output voltage varying with embedded device activity over a period of time.
Figure 3. EH output variation due to Device activity over a period of time (Source: Cypress)
For devices powered by an EHS, the energy consumed during an active state should not exceed the available energy in the EHS. Figure 4 shows an EHS powered system where the energy consumption of during an active state is greater than the amount the EHS can provide. The output voltage of the EHS slowly drops due to consumption, eventually shutting down the output completely.
Figure 4. WSN shutdown due to insufficient power (Source: Cypress)