Abstract
In this paper a new low-cost stretchable coplanar capacitive sensor for liquid level sensing is presented. It has been 3D-printed by employing commercial thermoplastic polyurethane (TPU) and conductive materials and using a fused filament fabrication (FFF) process for monolithic fabrication. The sensor presents high linearity and good repeatability when measuring sunflower oil level. Experiments were performed to analyse the behaviour of the developed sensor when applying bending stimuli, in order to verify its flexibility, and a thermal characterization was performed in the temperature range from 10 °C to 40 °C to evaluate its effect on sunflower oil level measurement. The experimental results showed negligible sensitivity of the sensor to bending stimuli, whereas the thermal characterization produced a model describing the relationship between capacitance, temperature, and oil level, allowing temperature compensation in oil level measurement. The different temperature cycles allowed to quantify the main sources of uncertainty, and their effect on level measurement was evaluated.
Highlights
Additive manufacturing (AM) technologies appear to be very appealing for the fabrication of sensors: in particular, the two main classes of AM-based sensors can be summarized as follows: sensors for engineering applications and sensors for medical applications [1]
Fused filament fabrication (FFF) technology is widely employed in this field and different studies were developed to correlate process parameters with sensor performance, in particular to improve the conductivity of 3D-printed tracks [3,4,5,6]
As a matter of fact, the extruded conductive materials employed for these purposes are plastic-based materials doped with conductive elements and the principle underlying the relation between temperature and resistance is complex and not fully understood
Summary
Additive manufacturing (AM) technologies appear to be very appealing for the fabrication of sensors: in particular, the two main classes of AM-based sensors can be summarized as follows: sensors for engineering applications and sensors for medical applications [1]. FFF is extensively employed to manufacture piezoresistive-based sensors, ranging from accelerometers [7] to force and motion sensors [8,9,10], allowing the fabrication of systems with integrated sensors and actuators [1,2], enabling technology development in forefront fields such as the Internet of Things and the Internet of Robotic Things [11]. FFF technology has been used to fabricate bendable coplanar sensors based on the capacitive principle, which have been characterized in terms of no sensitivity to bending stimuli, and from a thermal point of view, by using an industrial climatic chamber. Other applications may involve the detection of liquid or the measurement of liquid level in flexible bags and tubing for medical use, or the detection of liquid leakage
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