Capacitive sensing

In electrical engineering, capacitive sensing (sometimes capacitance sensing) is a technology, based on capacitive coupling, that can detect and measure anything that is conductive or has a dielectric constant different from air. Many types of sensors use capacitive sensing, including sensors to detect and measure proximity, pressure, position and displacement, force, humidity, fluid level, and acceleration. Human interface devices based on capacitive sensing, such as touchpads, can replace the computer mouse. Digital audio players, mobile phones, and tablet computers will sometimes use capacitive sensing touchscreens as input devices. Capacitive sensors can also replace mechanical buttons. A capacitive touchscreen typically consists of a capacitive touch sensor along with at least two complementary metal–oxide–semiconductor (CMOS) integrated circuit (IC) chips, an application-specific integrated circuit (ASIC) controller and a digital signal processor (DSP). Capacitive sensing is co

Capacitive sensor to monitor the level of drug inside an injector pen

Sylvain Paul Fernand Joly

Diabete is a disease which is spreading faster than ever, impacting now a larger population. Patients suffering from this disease need the injection of insulin to control extracellular sugar levels. Pharmaceutical companies developed injector pens to allow patients to inject themself the right amount of drug. But the dosage depends on many parameters as the previous meal, current blood sugar level... and a wrong dosage can lead to health problem e.g., loss of conciousness at short terms and severe diseases like cardiac pathologies at long terms... To prevent these situations, a smart pen cap has been developed at Valtronic Technologies to follow and log the individual injections by measuring the level of drug in the pen, the temperature, and the relative humidity to allow the patient and the medical staff to have an history of the injections and the climatic conditions.Injector pen and smart pen cap have been firstly presented with a non-exhaustive list of different physics which can be used to measure a level of liquid. For need of precision, the capacitance technique is the best technology choice. Therefore, the smart pen cap is based on a capacitive sensor. Different capacitance converter topologies have been compared through a bibliographic study. It appears that the Lock-in converter gives the best sensitivity and Charge Sensitivity Amplifier has the lowest power consumption. A trade-of was to use a capacitance-to-digital-converter. A chip from Analog device has been selected due to its high resolution below the femto-farad level, its simple interface and its availability on the market which allows a fast development and industrialisation. Then, switch and electronic structure are presented. First tests with an empty device have been conducted to find the noise floor, which is below 0.1fF.Subsequently, a study of different electrodes configuration has been performed. Firstly, an analysis on two parallel flat electrodes has been conducted using theoretical and simulation approaches. Then, two semi-cylindrical electrodes have been analysed. A comparison has been made between two theoretical methods which are: considering an infinite number of parallel flat electrodes following the shape of the semi-cylindrical electrodes and the conformal mapping methods. This last method gives coherent results when comparing real measurements and simulation. The device is equipped with semi-cylindrical electrodes which allow for a small package and a good correlation between capacitance and drug level with a convertion slope of 0.5fF/µL. Tests have been performed which give an error of +-1 IU and +-4 IU for large injection (set point equal to 72 IU).Finally, the study of some parasitic effects is presented in the last section. An important effect is the climatic effect which affects the capacitance due to variation of electric permittivity linked to temperature change and water absorption of certain materials as plastics. To compensate for this effect, reference electrodes and a climatic sensor have been implemented. Another important effect comes from the cartridge position in the non-uniform electric field of the smart pen cap. A four electrodes device has been realized to allow a perpendicular rotation of the electric field to obtain a more constant averaged electric field. As a consequence, the position impact of the cartridge has been reduced. Other effects have been considered. The device can detect a minimum injection of 52uL.

EPFL2021

Passive UHF RFID Systems for Capacitive Sensing Applications

Kerem Kapucu

In this work, a complete passive UHF RFID system for capacitive sensing applications is developed to be used in food quality monitoring and supply chain management. All the components of the RFID system including the base station, the base station antenna, the tag chip, and the tag antenna is designed, implemented, and tested. The operation of all the components are shown by measurements and their performance is characterized. The remote powering subsystem consists of the tag antenna and the rectifier. Measurement results showed a maximum power conversion efficiency of 65% for the antenna-rectifier pair operated at 866 MHz. The rectifier is a three-stage, differential rectifier with threshold compensation, that offers high power conversion efficiency. The tag antenna is an inductively-coupled, meandered dipole that is impedance matched to the tag chip. The dimensions of the tag antenna are 83 x 20mm, and the measured far-field gain of the tag antenna is 1.2 dB. A supply generation subsystem is implemented in order to generate a stable, regulated supply voltage for high-precision sensor interfaces. The supply generation subsystem consists of the low drop-out (LDO) voltage regulator, a bandgap voltage reference, and a supply insensitive current source. The power supply rejection ratio (PSRR) of the LDO is greater than 55 dB. Data communication is realized by backscatter modulation because it offers lower circuit complexity and low power consumption on the tag side. The scattering properties of the tag antenna and the backscatter modulation is theoretically analyzed and an optimum antenna impedance for backscatter communication is calculated. The scattering properties of the tag antenna are characterized by measurements in an anechoic chamber. One of the innovations of this work is the novel pulse-width modulation based backscatter communication, in which the information is encoded in the duty-cycle of the signal. This method decreases the circuit complexity and power consumption on the tag side when used with phase-locked loop (PLL) based sensor interfaces. A monostatic base station with a homodyne receiver is implemented for the passive RFID system. A microstrip patch antenna is custom designed and fabricated for the base station. The input return loss of the base station antenna is measured to be -20 dB, whereas its measured far-field gain is 3.6 dB. An isolator that is based on a directional coupler with an intentional mismatch at the unused port is designed and fabricated for the base station. The measured directivity of the isolator is 50.5 dB. A PLL-based capacitive sensor interface is implemented for differential sensors. The integration of such a low-power circuit in an RFID tag is the main innovation of this work. The differential nature of this sensor interface makes it immune to supply voltage variations. The sensor interface can work with supply voltages as low as 0.3 V. The power consumption of the interface is 300 nW, for 0.3 V supply voltage. The resolution of the sensor interface is determined to be 6.4 bit by post-layout simulations. Two tag architectures for two different approaches for sensor interfaces are proposed: the generic passive UHF RFID platform for sensing applications and the SimpleTag - a novel, low-power UHF RFID tag for differential printed capacitive sensors. The circuits are implemented in UMC 0.18 um CMOS technology.

EPFL2015

Capacitive sensor to monitor the level of drug inside an injector pen

Sylvain Paul Fernand Joly

EPFL2021

Passive UHF RFID Systems for Capacitive Sensing Applications

Kerem Kapucu

EPFL2015

Integrated sigma-delta interface for capacitive sensors

Capacitive sensor to monitor the level of drug inside an injector pen

Passive UHF RFID Systems for Capacitive Sensing Applications

Capacitive sensor to monitor the level of drug inside an injector pen

Integrated sigma-delta interface for capacitive sensors

Passive UHF RFID Systems for Capacitive Sensing Applications