Using Piezo Film Sensors as Accelerometers

Accelerometers are used in instruments to measure acceleration (rate of change of velocity with time) and to measure inclination (the angle of inclination between the longitudinal axis of an object and a line perpendicular to a plane tangent to the earth's surface). Inclination measurements can be considered "DC" or steady-state measurements. In theory, acceleration can be steady-state, but in practice, acceleration is usually a short-term, transient phenomenon.

In non-tilt applications (short-term acceleration), piezoelectric geophones or piezoelectric film sensors can be used as sensors. Any type of piezoelectric sensor has an equivalent circuit of an AC voltage source in series with a capacitor (plus other reactive elements that produce second-order effects, which are not analyzed here). Typical capacitance values ​​are hundreds of picofarads to a few nanofarads. Capacitive coupling of the voltage source is why the device cannot provide a steady-state tilt measurement.

The equivalent capacitance mentioned above, plus the shunt resistance of the input or subsequent amplification or buffer circuit, forms a single-pole high-pass filter (HPF). In the best case, the larger the shunt resistance, the longer the time constant of the pole in the high-pass filter. This means that the acceleration can be measured for a longer time before the time constant effect degrades the measurement.

From a practical point of view (considering the availability of components), a value of 1GΩ can be chosen. Because this resistor value is large, the amplifier used must have very low bias or leakage current, preferably in the order of 1pA.

Figure 1 is a schematic diagram of a practical circuit. The piezoelectric film sensor is device X1. In the prototype design, the measurement-specific LDTM-028K device was used. A small gravity is already applied to one end of this sensor. Adding a large gravity to this end can increase the sensitivity. The sensor is connected to the non-inverting input of the op amp U1 through R1. R1 prevents overvoltage from damaging the op amp input. This is likely to happen if the sensor is subjected to very high acceleration (such as a heavy blow). R1 ​​also serves to reduce the amplitude of the signal from X1. R1 in this circuit is 1GΩ. R2 is the input shunt resistor. A leakage current of 1pA will flow through R2, which is also 1GΩ, generating a 1mV bias voltage (added to the actual bias voltage of the op amp). R2 is connected to a 2.5V reference voltage to set the quiescent output voltage of the op amp. The op amp is the ISL28158 (or any other device with ultra-low input bias/leakage current). The operational amplifier is powered by a +5V DC supply. The DC gain is set by R3 and R4, which is +2V/V in this circuit. The 1μF capacitor (C3) forms a low-pass filter, reducing the circuit's response to higher-frequency vibrations. Film capacitors are preferred here, as ceramic capacitors may introduce additional unpleasant piezoelectric effects (also known as microphonics). If additional low-pass filtering is required, an additional pole can be added at the output of the op amp, shown with R5 and C4.

Mount X1 on the circuit board so that gravity is applied to the underside of the board and the sensor bends in the direction of the acceleration you want to measure.

Reducing R1 can increase sensitivity. Adjusting C3 and C4 can change the time constant of the low-pass filter.