A comparison

Differences between the two technologies, illustrated by typical values

When using strain gauges any elastic deformation of the measuring body is first
converted to a change in strain gauge resistance, so that a Wheatstone bridge circuit electrical output signal can then be generated.

By contrast the basis of the piezoelectric effect is that crystals under compressive loading generate an electric charge that is directly proportional to the force applied.

Fig. 1: Strain gauge transducer with bridge circuit

This charge is then converted to a proportional output voltage with the aid of an amplifier. The two technologies complement each other perfectly. On one hand, strain gauges achieve far better stability for long-term measurements, as well as higher linearity.

Fig. 2: Piezoelectric sensor crystal principles

Piezoelectric sensors are far smaller in construction and their high natural frequency makes them ideal for dynamic applications.

When measuring with piezoelectric sensors, there is virtually no displacement, as the quartz already forms the mechatronic component with an electrical output signal. The sensitivity of a piezoelectric sensor does not usually depend on its size or on the volume of quartz, but on the material being used and its geometry.

Q = q11 · n · F

Q = generated electrical charge
q11 = material constant, e.g. 4.3 pC/N
n = number of quartz elements
connected in series
F = mechanical loading or force

With strain gauge sensors, the elastic deformation is used in accordance with Hooke’s Law, to achieve the change in strain gauge resistance. The following correlation applies to the change in resistance:

F = E · A · ε

ε = measuring body strain
E = measuring body modulus of elasticity
A = measuring body cross-section
F = mechanical loading or force

∆R / R = k · ε

∆R = change in resistance under load
R = resistance when not under load
k = k (gauge) factor of the SG

Comparing the systems of piezoelectric and strain gauge technology reveals a
significant difference in electrical sensitivity:

Case 1:
Piezo sensor 5 kN => characteristic -4.3 pC/N
SG sensor 5 kN => characteristic 2 mV/V=> 0.4 µV/N

Case 2:
Piezo sensor 140 kN => characteristic -4.3 pC/N
SG sensor 140 kN => characteristic 2 mV/V=> 0.014 µV/N

With piezoelectric measurement technology with a sensor variable, it is possible to implement an additional measuring range without loss of accuracy or resolution.

Strain gauge technology has advantages with regard to long-term stability, as with piezoelectric measurement technology, it is practically impossible to implement a test setup with infinite insulation resistance.

In practice, there is often a drift of about 1 N/min, which is why measurement has to be restricted to a few minutes, subject to the requirements of the measurement task. For a measuring range of 50 kN with 215000 pC, charge signal -4.3 pC / N and a drift requirement of <0.05 %, the maximum measurement time is about 25 minutes.

With strain gauge based sensors, the full bridge circuit can achieve excellent linearity. This avoids having to compensate for additional interference effects such as temperature variations. It also makes strain gauge sensors more suitable for high-precision measurement tasks in partial load areas,
such as for reference transducers. When space is tight and installation restricted, the piezoelectric sensor comes into its own. With the same measuring range and similar performance characteristics, the piezoelectric transducer is up to thirty times smaller in construction than a comparable
strain gauge transducer.

Fig. 3: Piezo measuring chain with MP85A-FASTpress process controller

There is room in measurement technology for both transducer technologies, strain gauge and piezoelectric, for mechanical quantities. The two are complementary and where strain gage performance ends, piezoelectric performance begins. Piezoelectric measurement technology has been added
to HBM’s product line enabling us to always provide the optimum metrological solution.

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