Self-calibrating Level Measuring Module (2012)
It took me almost a year to design a reliable capacitive level measuring sensor which is not only accurate but also serves an innovative self-calibration approach. In order to measure the capacitance, I used a RLC circuit where C is the target capacitor, R is the total resistance including leakage resistance and electrode resistance, and a very large inductor (L = 1H) which is big enough to decrease the resonance frequency in the presence of a very small capacitance. The self-resonant frequency of the RLC circuit can be accurately measured by a FPGA operating at 270MHz. The combination of the high resolution pulse width measurement capability of the FPGA and the comparatively low resonance frequency of RLC leads the sensor to be so sensitive.
Although a sensitive capacitive measuring sensor is necessary in measuring the level of the pouring material in a vessel, it is not enough to be reliable. The dielectric of the target material, which plays a vital role in measuring the level of the material, can vary by temperature, humidity and impurities. Thus, the first calibration of this device (a common required action for all types of sensors) cannot be enough for long-term usage. As such, these sensors need to be calibrated more often. I’ve designed a new electrode which is really helpful for self-calibration, and is installed next to the main electrode. There are some aluminum cylinders with predefined heights on the calibration electrode which are connected to the GND of the circuit. Grounding these aluminum cylinders creates a E field isolation so that the total capacitance formed by this electrode and the vessel remains constant while the pouring material reaches these cylinders. As such, the effect of these aluminum cylinders on the capacitance-height curve derived from the calibration electrode can be used to self-calibrate the sensor.
The capacitance-height curve resulting from the main electrode and the calibration electrode are shown below. The points at which the slope is changed demonstrate the position of aluminum cylinders.
