2.2.1. System Design
We equipped the Limpet with nine exteroceptive sensing modalities. The sensors incorporated in the Limpet are temperature, pressure, humidity, optical, distance, sound, magnetic field, accelerometer, and gyroscope. We conducted a literature survey to understand what parameters are relevant to offshore environments and monitoring of offshore structures and chose the sensors accordingly. We also got input from the industrial partners involved in the ORCA Hub project on the parameters that are interesting to monitor for the offshore platforms. These discussions influenced the choice of sensing modalities on the Limpet system. The main purpose of the Limpet is to monitor the offshore assets and the environmental conditions surrounding them. The Limpet can be modelled as an instrument, where each of its sensors converts a physical measurement variable into a corresponding signal variable as shown by Figure 2
B. The signal variables are fed to the on-board microcontroller, which can then transmit the signal variable using one of several communication systems. In this work, we give one example of the multi-functional capabilities of the Limpet. Table 1
describes how the physical measurement from each sensor on the Limpet can be related to different measurands on offshore platforms. The table is based on the instrument model described in Figure 2
A. It gives an example of a few potential measurands, and not all the measurands possible with each sensor, and it gives an overview of the multi-functionality that can be achieved with each sensor on the Limpet.
In this work, we demonstrate fault detection in equipment, specifically in wind turbines. We achieve the fault detection using a distance sensor, which is a Time-of-Flight (ToF) sensor. Not only does the distance sensor achieve condition monitoring and fault detection remotely, it gives an extra parameter, distance, which can be extremely useful in different applications (e.g., blade deflection monitoring), or when integrating it with other robots. The approach we used requires a sensor that is capable of remotely detecting the mechanical rotation of the wind turbine. For example, this approach can be performed using a magnetic sensor to measure magnetic reluctance. However, using a magnetic sensor would require the blades to be metallic. An accelerometer could also be used, by attaching it to the blade to monitor the speed and vibration, but the number of faults that can be detected are limited. A combination of a photodiode and phototransistor is another way to achieve fault detection in wind turbines, but it requires a complex setup and would only give indication of one parameter, which is absence of blade.
C indicates the instrument model for the Limpet as considered in this work. The measurands normal operation and fault can be related to the physical measurement variable light. We use the distance sensor on the Limpet, which converts the physical measurement variable light to a signal variable distance, which is fed into the microcontroller’s I2
C bus. We use the microcontroller to process the sensor data on-board and check them against a classifier. We use the distance measurements to classify if the machine is operating normally, or if there is a fault in the machine hindering its performance. The Limpet can send the data to a PC using one of several different communication systems. We can then use spectral analysis on the data to further reduce the information density and classify the type of fault detected.
The primary communication method of the Limpet is Wi-Fi. The Limpet uses Wi-Fi to transmit its data to a PC. We incorporated an ESP8266, which is a System-on-a-Chip (SOC) Wi-Fi module with integrated TCP/IP protocol stack capable of giving any microcontroller access to the Wi-Fi network, on the Limpet. We use Mosquitto [37
], which implements a messaging protocol known as Message Queuing Telemetry Transport (MQTT), to send the data wirelessly from the Limpet to the PC. MQTT is a light-weight publish/subscribe messaging protocol used for remote communication. The received data can then be plotted in a real-time basis using MATLAB or saved on the PC and processed later.
We designed the Limpet to have robust communication. We can use multiple communication methods with the Limpet, including serial, Wi-Fi, LoRa and optical communication. LoRa is a digital wireless data communication technology that enables long-range transmission with low power consumption [38
]. LoRa technology is provided by the LoRa Alliance, which is a non-profit association of more than 500 member companies that are developing and promoting LoRaWAN open standard for IoT. We achieve optical communication with a combination of the RGB LED and the optical sensor on the Limpet. Wi-Fi and serial communication do not allow agent-to-agent communication, unlike LoRa and optical communication. We incorporated a communication fail-over mechanism on the Limpet. If the primary communication method (Wi-Fi) fails for any reason, the functions of the Wi-Fi are assumed by a secondary communication module, which makes the system more fault-tolerant and robust in communication and data transmission.
The Limpet has an on-board programming port (JTAG). We programmed the Limpet using a SEGGER J-Link programmer together with a JTAG Adapter (Olimex ARM-JTAG-20-10). We programmed it in C/C++ using Atmel Studio 7. We used the standard Universal Asynchronous Receiver-Transmitter (UART) protocol for communication. The UART protocol uses a high idle line, which is pulled low at the start of a message.
Cost and functionality were the most important factors that we considered when designing the Limpets. Our rationale behind the system design was to keep the costs as low as possible without sacrificing functionality. The Limpet has a diameter of 50 mm, a height of 7 mm and weighs 17 g with, and 10 g without, the battery. We designed the Limpets to have a size and weight ideal for ease of fabrication, manufacturing, and assembly. The total cost of electronic components used in the design of the Limpet is about £22.
2.2.2. Electrical Design
The Limpet consists of a single two-layer Printed Circuit Board (PCB) and a detachable Li-Ion coin cell battery. We designed a fully integrated PCB incorporating a low–power microcontroller (ATSAMD21G18A), RGB LED (LTST-N683EGBW), battery holder (BK-877) for a rechargeable Li-ion battery (LIR2477), charging IC (MCP73812T), charger connector, programming port [JTAG] (Molex 532610571), and a communication connector as shown by Figure 1
. The PCB includes several exteroceptive sensors, which are: Temperature and Humidity Sensor (Si7006), IMU [Accelerometer and Gyroscope] Sensor (LSM6DS3), Optical Sensor (VEML6040), Sound Sensor (SPU0414HR5H-SB), 3-Axis Magnetic Sensor (MLX90393), Pressure Sensor (BMP280), Distance Sensor (VL53L0). Figure S1
shows the PCB schematic. The communication connector is connected to UART of the microcontroller. Therefore, we can use this connector to connect the Limpet to different communication systems. We designed the Limpet to use a single PCB for control, communication, and sensing.
The sensors on the Limpet, except for the microphone, are controlled by the microcontroller through the I2C bus. The microphone is an omnidirectional Micro-Electro-Mechanical System (MEMS) sensor, with an analogue output and a frequency range of 100 Hz to 10 KHz. In this work, we use the distance sensor for fault detection. The distance sensor on the Limpet is the smallest range sensor on the market today. It is a ToF laser-ranging module that can measure sub-mm distances for a range between 0 and 2.2 m. We also demonstrate the use of the optical sensor for local communication between two Limpets.
The Limpet could be used in a wide variety of areas, which leads to a wide range of temperatures that the components must be tolerant too. The datasheets of the sensors used show that they can be used in a wide range of temperatures. We designed the Limpet for the North Atlantic region which has a water temperature range of between 6 °C and 17 °C. This temperature range was within the allowed range of all the components used on the Limpet. The sensors used on the Limpet also have a wide humidity range. Some of the sensors used are splash resistant such as the IMU, hall-effect, and optical sensors. If we were to deploy the Limpet for field tests, it would require encapsulation to ensure resistance to extreme temperature and humidity conditions and saltwater spray. As noted in Section 1.4
, the casing would have to comply with the regulations used in the specific area it would be deployed in.
We power the Limpet by a rechargeable 3.6 V 160 mAh lithium-ion coin cell battery. We can recharge the battery using a 6Vdc plug-in power supply. We included header pins on the PCB for connecting the power supply to recharge the battery. The Limpet has a battery life @160 mAh of 0.87 to 1600 h. We calculated the minimum battery life by assuming the Limpet has all the sensors, RGB LED, microcontroller and Wi-Fi communication continuously on. The Limpet will consume approximately 182.9 mA (ESP8266 consumes 135 mA, RGB LED consumes 20 mA, sensors consume 20.9 mA, microcontroller consumes 7 mA), which allows for a battery life of about 0.87 h or 52.2 min. We calculated the maximum battery life by assuming the Limpet is in sleep mode, where it consumes an average current of 0.1 mA. In this mode, the battery life of the Limpet can reach about 1600 h or 67 days. In the application presented in this work, the Limpet battery life is expected to last an average of 20 h if it is used for continuous monitoring. If we program the Limpet to sample once every hour, the battery life can go up to an average of 120 h.
2.2.3. Mechanical Design
We designed the PCB of the Limpet using Eagle PCB Design Software and fabricated them on double-sided Cu-FR4-Cu 0.1 mm boards using an external company called Minnitron Ltd. (Kent, UK). We purchased all electronic components from RS Components and Digi-key Electronics. We soldered the components on the PCB using a reflow soldering process. In this process, we cut solder paste stencils from vinyl using a Laser Cutter (Epilog Laser Fusion 32).
We fabricated the protective housing from an optically clear, semi-rigid polyurethane resin. We developed the housing by casting the resin in a 3D printed mold. We did not cover the surfaces of the sensors by resin to keep them exposed to the external physical measurement variables. A picture of the encapsulated Limpet and the mold can be found in Figures S2 and S3
We designed the Limpet explicitly for manufacturability; it consists of a single PCB and therefore mass manufacture is a simple case of placing a batch order with a PCB foundry. The PCB consists of surface mount components, except for the communication connector, and can be autonomously populated with pick-and-place machines at the point of manufacture. Systems that are made of a single PCB with mostly surface components allow for scaling up to huge collectives of agents easily without sacrificing functionality [39
]. Assembly of one Limpet takes seconds as it is a matter of only connecting the coin cell battery. Once the battery is connected, there is no need to remove the battery from the Limpet again as it can be charged on-board. As a result, of the Limpet being highly manufacturable and easy to assemble, it is easy to mass-produce Limpet agents and deploy them in huge collectives.