Symbol

Just because it came back to mind recently, here's a symbol I propose to represent piezo film as a circuit element. It's based on the existing symbol for a crystal, which is fine because it reflects the capacitive nature of any piezo sensor. But here, we want to show that the sensor is flexible...

Meanwhile, it is worth mentioning that a piezo sensor can be modelled (for example, in a circuit emulation package) as a voltage source that exactly follow the applied mechanical property (stress or strain), connected in series with a capacitor (whose value is the same as the piezo film sensor, which can be measured or calculated from its dimensions). When this arrangement is connected to a fixed resistive load (such as the input of an oscilloscope), a voltage divider is formed. At low frequencies, the impedance of the sensor is much higher than that of the fixed resistor, and so the voltage across the resistor (the input to the scope) is very small. At higher frequencies, the impedance of the capacitor gets much less, and eventually the full "open circuit" voltage developed by the film appears across the resistive input. Unfortunately, it just happens that many real-life applications involve frequencies that are moderate or slow in electrical terms, especially those resulting from human-initiated events. This makes it more challenging to capture the full "potential" electrical signal from the film - we need to use rather high input resistance (often 10 or 100 M ohm, even into the 1-10-100 G ohm ranges for very slow events or small sensors). Charge amplifiers work differently, actually presenting very low resistance to the sensor (so that all the charge flows into the feedback network of the amplifier), but high resistances are still required internally to maintain the desired low frequency response.

Detecting Force

I'd like to start with a discussion about the trace shown here. This was obtained using a short metal rod pressing on a loadcell, in turn compressing a piezo film sensor fixed to a very rigid block (i.e. the loadcell is mechanically in series with the piezo film, which can't bend or stretch).

The upper (blue) trace is the output of a 100 lb compression load cell, with effective sensitivity of around 7.5 mV per Newton.

The lower (red) trace is the output of a piezo film sensor type NDT1-220K (MEAS p/n 1005935-1), connected to a charge amplifier with 100 mV/nC sensitivity, and a lower frequency limit of 0.1 Hz.

Note that the loadcell trace stays always above the initial baseline that exists at left-hand edge of screen.

With the piezo trace, it "follows" the force on the initial push, then "undershoots" on release of that force, and starts rising again from a "false" baseline, with this behaviour repeating and getting "worse". As I keep cycling the force, the piezo signal eventually spends as much time below the baseline as above – in other words, it becomes perfectly AC-coupled with zero net DC result.

The implication here is that the piezo signal cannot tell you much about the true force condition at any instant in time. It can give an indication of the magnitude of the AC (dynamic) part, but even here, the dynamic portion is not necessarily "accurate" compared with the true piezo sensitivity. The magnitude of the dynamic component is attenuated slightly, and the extent of this attenuation is dependent upon the exact frequency of excitation, relative to the cut-off frequency of the electronic filter. Note that the excitation here was approx 2 Hz while the filter frequency was 0.1 Hz, more than one decade away, but there is still amplitude error. On top of this, there may also be a phase error introduced by the filter.

This set-up was special, in that the piezo film sensor was pretty well clamped between two rigid blocks (the loadcell above, and a thick aluminum block below). In many other practical cases, the film may be fixed to a substrate that can undergo some bending, in which case the high "d31" piezo sensitivity to strain along the machine direction of the film would certainly generate additional contribution to the final output signal. It is not really possible to separate out the contributions from thickness-mode and stretch-mode excitation. In fact, even on the trace shown here, it is possible that the initial high peak from the piezo sensor (which seems stronger relative to later compression events in the trace) could have some additional contribution from machine-direction stretching.

Conclusion: it's really tough to measure "force" directly, using piezo film.

In many cases, it may be better to design a system that allows the film to stretch as a result of applied force.

Setting up the blog

The intent of this blog will be to offer insight and commentary to applications of piezo film, in a more conversational way than we offer in our published technical manual and articles. Although the author works for Measurement Specialties, Inc (www.meas-spec.com), the content here is not necessarily endorsed by the company.