Soft Triboelectric Pressure Sensor with Spiking Calibration Signal

Soft triboelectric pressure sensors are growing interest due to increasing demand for various sensors for human-machine interaction and growing global energy concerns. However, signals of triboelectric sensors are significantly affected by miscellaneous factors such as environment and user motion. It is because the surface charge density of triboelectric layers depends on those factors. Although there have been a few attempts to compensate for the change in surface charge density, they were either limited to the change by a specific factor or had considerably limited pressure resolution. None of them could directly calculate current surface charge density to eliminate diverse factors including humidity, time decay and warm-up time while providing continuous pressure information. Here, we propose a triboelectric self-powered pressure sensor with a unique spiking calibration system to directly calculate and compensate for the change in surface charge density.<br/>The sensor generates two output signals: open-circuit voltage and spiking calibration voltage. While open-circuit voltage provides continuous pressure information, it is affected by the change in surface charge density just as in conventional triboelectric sensors. Contrarily, the spiking calibration voltage conveys discrete pressure information without the influence from the change in surface charge density. The spiking calibration voltage is generated based on geometrically switchable operation. A mechanical beam and a parallel-line patterned electrode were designed and fabricated to work as an electrical switch such that the connection depends only on the geometrical state of the sensor. Since closing the switch results in abrupt current spike to flow, a series of voltage spikes are observed with respect to applied pressure. The magnitudes of spikes were not affected by the speed of motion from 0.1mm/s to 5mm/s, and the direction of spikes indicates the direction of pressure change. For triboelectric layers, porous polyurethane and porous ethylene vinyl acetate were utilized due to their softness, high triboelectric performance and cheap price.<br/>Simultaneous analysis of the two signals gives an estimation of current surface charge density in real-time. By comparing the change in the open-circuit voltages between two consecutive spiked moments, current surface charge density can be calculated, which can be further utilized to compensate its change. Triboelectric sensor signals varied by a factor of over two for same pressure due to change in humidity (from relative humidity of 20% to 80% at 22°C) or applying successively higher pressure (up to 13kPa). Nevertheless, our sensor system could successfully eliminate the effect of the change in surface charge density by directly calculating and calibrating it.<br/>Furthermore, the sensor was integrated to a robotic arm to harvest crops in their own optimal growing environmental conditions to demonstrate its possible usage in smart farming robots. As demonstration, a gripper on the robotic arm was feedback controlled to harvest, transport and release a cherry tomato and lettuce with optimal relative humidity of 50% and 75% at 24°C, respectively. While both crops were grabbed with a same target pressure, the open-circuit voltages at the target pressure differed due to different optimal growing humidity conditions. Although conventional triboelectric sensors would have failed to pick them up due to either overestimation or underestimation of pressure, our sensor system could successfully harvest both crops due to its accurate pressure measurements based on the spiking calibration system.