6.5  Summary

Novel charge- and flux-based memristive sensing schemes are presented based on the unique property of memristors to memorize the charge (time integral of current) and the flux (time integral of voltage). The proposed methods can be used for capacitance, inductance, and power measurements. Although inductance and capacitance sensing are far from being new problems, the use of a memristor reduces the measurement to a straightforward resistance measurement which can be performed fast. We have shown that the TiO                   2   memristor can be used for charge-based measurements. Spintronic memristors are proposed for both charge- and flux-based capacitance and inductance measurement. The effect of the device geometry on the memristive behavior of a spintronic device is studied to determine proper geometries for memristive charge- and flux-based sensing. In the presence of the non-adiabatic spin-torque effect, the spintronic memristor shows memristive behaviors at low current/voltage regimes and within the desired geometries the device has a constant modulation of the memristance (memductance) with respect to the charge (flux) applied, which can be used for capacitance (inductance) measurement or both. Spintronic memristors show 3-6 orders of magnitude higher sensitivities compared to the TiO2   memristor devices.

Since the memristor holds the information, it is possible to use the memristor in a readout circuit which measures the memristance and also resets the memristor for the next measurement. Thus, unlike other time domain methods, memristive sensing does not need extra hardware for time/frequency measurement. In fact, a memristive sensor can be simply implemented by using a switch. Furthermore, memristor devices are fabricated in nano-scale dimension and this makes memristors a candidate for low power micro-system applications. The memristive sensing method is suitable for measuring time-varying inductances and capacitances and it has the potential to be used in novel inductive and capacitive sensors. For example, the simplest form of a capacitor consists of two metal plates, separated by an insulator and the capacitance is expressed as C  = ε0 εrA ∕d  where ε0   denotes the permittivity of free space, εr  is the dielectric constant of the insulating material between the plates, A  express the area of the plates, and             d  is the distance between the plates. The capacitance changes, if at least one of the parameters εr  , A  , or d  is changed. By measuring the capacitance periodically, any movement of the plates or changing in the dielectric constant (e.g. due to a finger-touch on the dielectric) can be measured or detected. Due to the non-volatility, zero-leakage, high endurance, and small cell size of the memristors, memristive sensing is promising for ultra-low power capacitive (e.g. touch) sensors.