The Autonomous Multi Sensor Drifter (AMuSeD) project aims to facilitate insitu environmental monitoring for maritime research. Drifter buoys are used to study ocean currents by drifting on the water surface, following the ocean current, and transmitting their position. By measuring additional valuable environmental parameters, the drifter's usability is greatly enhanced. However, the additional use of multiple sensors significantly increases the requirements and complexity of the platform in terms of electronics, data processing, power consumption, and the sensor technologies. Currently existing solutions for multi-sensor drifters are expensive and usually limited to a specific type of sensor. Therefore, many research projects are developing their own monitoring solutions to meet their specific measurement needs.

A general environmental monitoring solution is needed that meets the requirements of maritime measurements, is cost effective, and allows modular extensions with custom sensors. With AMuSeD, the Institute of Mechatronics in Mechanics seeks to address this specific problem.

The project focuses on three aspects:

  1. The development of a cost-effective modular hardware and software for data processing, storage and transmission.
  2. The research on wave energy harvesting to meet the increased energy demand of the additional sensors.
  3. The development of new sensor technologies that have a form factor suitable for maritime applications and are low cost yet reliable.

The result is an interdisciplinary project that requires innovation in several Domains to achieve its set goals.

Low-Cost Sensor Drifter
Autonomous Multi Sensor Drifter (AMuSeD) and CARTHE
Testing of the low-cost multi sensor drifter and CARTHE drifter

AMuSeD PCB with GPS, IMU, Temp.-Sensor, SD-Card, Power-Management and MCU
AMuSeD PCB with GPS, IMU, Temp.-Sensor, SD-Card, Power-Management and MCU

The overall hardware design consists of:

a. The fixed Main board that integrates the basic requirements of a maritime monitoring platform

b. The optional external sensor board that enables connection of custom sensors

c. The optional external satellite communications board that enables use of various satellite networks from large established Providers to novel nano-satellite IOT companies.

The main board integrates GPS tracking and data processing of all selected sensor devices. It implements power management that enables very low power consumption by controlling not only the main board, but also the power supply to the connected external communication and sensor boards. Sensors for onboard temperature and a MEMS IMU for measuring the sea state are already integrated which allows to use the main board as a stand-alone hardware to record position and onboard sensor data on the integrated SD card. The boards functionallty can be extended by a sensor board to integrate additional sensors and/or a communication board to transmit the data over a prefered communications network.

No programming skills are required to use the main board, as the user configuration is implemented by a simple configuration file. This configuration file sets up the required measurement parameters, which then allows the main controller to schedule measurement tasks and log and transmit data using an event-driven state machine.

To integrate additional sensors, the external sensor board - e.g., an Arduino or similar - simply needs to be programmed to read sensor values from the connected physical sensors. The collected measurement data can then be transmitted from the sensor board (Arduino) to the main board via a simple communication protocol. The main board takes care of managing and transmitting the received data, scheduling the next tasks, and setting the low-power modes.

As instrumentation and processing power increase, so does the power consumption of the platform. Since drifters powered only by batteries have a limited lifetime, long-term applications rely on means of energy harvesting. One common solution is to use solar power, but this can only be used on buoys with sufficient space and may be completely useless in polar regions. In this case, the use of wave energy with its high availability becomes particularly interesting. Within the scope of this project, investigations are being carried out on the implementation of a wave energy converter for ocean drifters. The working principle is based on a non moored self-reacting point absorber (SRPA), which uses relative motion between the buoy and the drogue. Since SRPAs are usually studied for large power plants, IMEK has focused on optimizing this operating principle for small-scale energy harvesting. The power take-off is realized by an electrical generator in the buoy. Since the motion of a SRPA is linear, a direct-drive linear generator has the great advantage of simple design and implementation.
In a cooperation between the Institute for Mechatronics in Mechanics (IMEK) and the Institute of Mechanics and Ocean Engineering, a small scale wave energy converter is developed and optimized, which can be integrated into a small, free-floating sensor drifter. The development of the mechanical part is based on mechanical simulations and experiments in the TUHH Wave-Flume. The generator design uses electromagnetic FEM simulations and a generator testbench implementation to optimize the power output for the low wave frequencies. The developed prototype currently achieves an output power of up to 400mW and we still have promising ideas for improvement.

Small-Scale Kinetic Wave Energy Harvester for Drifter
The prototype of the kinetic wave energy harvester is tested in the TUHH wave flume
Small Scale Wave Energy Harvester Linear Generator
Linear generator used for kinetic wave energy harvesting

Inductive Conductivity Sensor
Inductive conductivity sensors on testbench

To equip a compact low-cost drifter with sensors, sensors are needed that have a compact design, are inexpensive, and still have high accuracy. Especially when large-scale drifter deployment is planned, the cost of sensors quickly exceeds the funds available for the project. For this reason, the development of compact and low-cost sensors for small measurement applications is another goal of the AMuSeD project. In particular, the parameters water temperature and conductivity (salinity) are in the foreground. Inductive conductivity sensors can be used to measure salinity in a reliable and stable manner over long periods of time. Inductive conductivity sensors have the advantage of having a protective housing that prevents biofouling and corrosion. In contrast, conductivity cells require more maintenance but are much less expensive to purchase. Conductivity cells are often used in low-cost applications, although inductive sensors would offer clear advantages in the long run. In addition, most conductivity sensors on the market are designed for industrial applications and have large transducers. Especially in IoT applications, there is often a need for compact and low-cost sensors that offer simple data interfaces. IMEK is therefore working on the development of a compact integrated inductive conductivity sensor to make salinity measurements in the ocean more available.

Compact integrated inductive conductivity sensor for low-cost salinity measurement in maritime monitoring applications
Compact integrated inductive conductivity sensor for low-cost salinity measurement in maritime monitoring applications

Contact: Julius Harms