A piezoelectric element generates an electric field (and a measurable voltage from the field) in response to applied pressure. However, a piezoelectric element generates such electric fields only in response to changes in the applied pressure. In other words, a piezoelectric element conventionally acts as a dynamic pressure sensor. This by itself may be useful for measuring constantly pulsing pressures, such as vibrations in a car or other mechanical machine, or for example in seismometers. This dynamic response may also be utilized in some computer user interfaces where the mere presence of a short press is sufficient for input recognition without the need for absolute pressure determination.
Because of this dynamic nature of the electrical field response, conventional devices with piezoelectric sensors have not been typically used for sensing a constant pressure or for determining the absolute value of a dynamic or constant pressure.
Piezoelectric elements also change shape in response to a dynamic electrical field. Thus, in addition to dynamic pressure sensing, some devices also provide haptic feedback (e.g., active vibration) using the same piezoelectric element used for pressure sensing. However, relatively high driving voltages (e.g., about one hundred volts) are typically used for haptic feedback, as compared with the smaller voltages (e.g., one or two volts) typically generated in response to pressure changes. Because the piezoelectric element acts as a capacitor, the use of two very different voltage ranges can pose the practical problem of preventing high voltages stored in the piezoelectric element (due to haptic feedback operations) from being discharged into sensitive low-voltage amplifier electronics used for dynamic pressure sensing.