Prototypes

The current EnHANT prototypes showcase ultra low power wireless communications, energy harvesting, and energy-adaptive networking functionalities enabling the future EnHANTs.  All prototype components except the MICA2 motes are custom designed and developed for this project.

EnHANT Prototype

The prototypes communicate with each other wirelessly using ultra-low-power Ultra-Wideband Impulse-Radio Communication Modules (UWB-M). Ultra-Wideband Impulse Radio (UWB-IR) is fundamentally different from traditional narrow-band systems since the information is transmitted using very short pulses, and most of the transceiver circuitry can be shut down between the pulses to significantly reduce power consumption. The UWB-M is based on a custom 90nm CMOS receiver and transmitter chip that requires 1.65nJ/bit and 280pJ/bit, respectively. The UWB-M operates in a 500MHz band around a center frequency of 3.8GHz using a synchronized On-Off keying modulation which permits low power timing acquisition.

The UWB-M uses a dedicated printed circuit board which contains the custom UWB receiver and transmitter chips, a wideband RF switch to interface to the antenna, and a field programmable logic chip. The FPGA provides functionalities including preamble detection, CRC generation and verification, MAC address generation and detection, and buffering. Targeting low data rate applications, the UWB-M offers a flexible data-rate of up to 1Mbps, resulting in short packet durations. The UWB-M is further equipped with a 1.5cm×1.2cm antenna designed for omnidirectional UWB communication and provied by the Krishnaswamy Research Group.

A block diagram of the EnHANT prototype, and its interactions with the testbed

Each prototype is integrated with an energy harvesting module (EHM) that harvests and stores energy. The EHM consists of a solar cell, a small battery, and logic for near-real-time monitoring of the the energy harvested by the solar cell, and the battery charge level. The current prototypes harvest energy using custom organic solar cells fabricated on glass (shown below on the left) or plastic (shown below on the right) in the Columbia Laboratory for Unconventional Electronics. Organic solar cells efficiently harvest energy from indoor light, can be manufactured directly on flexible substrates using roll-to-roll fabrication techniques, and will enable future EnHANTs to be low cost, small, and flexible

The solar cells are charging the EHM’s small battery. When the prototype’s transceiver generates and sends out a wireless message, we flash the EHM’s LED, which reduces the energy level in the battery, emulating energy consumption by the radio.

The prototype Control Module (CM) is based on a legacy MICA2 mote that runs TinyOS with an added Fennec Fox software framework. The CM implements the MAC, forwarding, and routing protocols tailored for the UWB-IR transceivers. The UWB-M is integrated with the CM such that packets originating in the TinyOS application layer are sent wirelessly via the UWB-IR transceiver. The CM adapts the prototype’s networking and communication patterns, based on the energy states reported by the EHM. The CM implements simple energy-harvesting-adaptive link data rate adaptation, flow control, and collection tree adaptation policies.

Organic solar cell manufactured on glass

Flexible organic solar cell

Prototype Testbed

The testbed includes 6 EnHANT prototypes, a control and monitoring system, and a software-based light control system. For control and monitoring, the prototypes are placed on MIB600 programming boards and accessed from a PC via Ethernet. On the PC, a Java-based graphical monitoring system shows in real time the network topology, data rates, energy harvested, battery levels, and the individual packets transmitted.

To enable experimentation with various energy harvesting network adaptations, the EnHANTs testbed also includes a custom-designed software-based light control system as shown. The Arduino-based system uses Pulse Width Modulation (PWM) to precisely control the irradiance (light intensity) incident on each prototype’s solar cell. Dark box enclosures and 3D printed mounting fixtures were built to ensure repeatable placement of the solar cells, and full control over the light conditions. This ensures that experimental evaluations of energy harvesting adaptive policies are based on the same energy inputs. A Java script controls the intensity of the lights with remarkable granularity and precision.

A dark box enclosure used in the light control system

Monitoring and control system user interface

Demonstration

In our interactive demo we show several EnHANT prototypes communicating with each other. The prototypes determine how much power their solar cells are generating, and how much energy their EHM battery contains. Based on that, the prototypes jointly determine their communication parameters (data rates, sleep/wake cycles, network topology). Using the graphical monitoring system, demo participants can quickly observe the changes in the communication parameters. Using the oscilloscope, participants can observe the UWB-Impulse Radio (IR) synchronizedon-off keying (SOOK) modulation scheme while the nodes communicate.

Demo participants can observe the network adapt to varying light energy conditions using a user-operated module of the light system. We additionally demonstrate the effects on the network communication patterns when the light energy is based on different real world irradiance traces.

Demo Video

Video of the EnHANTs demo, filmed in December 2011, is available here. 

Previous Demos