Thermal Sensing
Because the conductivity of the fluid or gel increases with temperature, a thermistor is incorporated for thermal compensation. Thermal energy from the embedded electronics is used to heat the finger above ambient temperature, similar to the biological finger. This enables the material properties of contacted objects to be inferred from thermal transients measured by the thermistor on the surface of the core. Upon contact with a test object, the derivative of temperature (dT/dt) has several reproducible features (Fig). The initial negative peak (cooling) is similar for all materials; it occurs when the cooler skin is pushed against the warmer core. After the inflection point, the curves diverge and are dependent on heat capacity and conductivity of the object.
Temperature (top) and its derivative (dT/dt, bottom) following contact (vertical arrows) with large plastic and copper test pucks.