Piezo VS Strain

Differences between the two sensors

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The Main Difference between Piezoelectric and Strain Gauge Force sensor

Auloma piezo force monitoring system, a quasi static behaviour

Auloma use the Low Impedance Voltage Mode (LIVM) technology in the piezoelectric force sensor to control our press fit modules until 444 kN. LIVM technology consist in a miniature IC metal oxide silicon field effect transistor (MOSFET) amplifier built into the housing of the sensor. The amplifier converts the high impedance voltage signal from the quartz element to a much lower output impedance level, so the readout instrument and long cable have a neglect effect on the signal quality. Because the high impedance input to the IC amplifier is totally enclosed and thus shielded by the metal housing. The LIVM sensor is relatively impervious to external electrostatic interference and other disturbances. The sensor amplifier is a common drain, unity gain “source follower” circuit with the source terminal brought out through a coaxial connector on the sensor body Low output impedance (less than 100 ohms) makes the sensitivity of the LIVM sensor independent of cable length within the frequency response limits outlined in the chart. Basic system sensitivity does not change when cables are replaced or changed. The sensitivity of the LIVM sensor is fixed at time of manufacture by varying the total capacitance across the quartz crystal element. The highest possible voltage sensitivity is obtained with no added capacitance across the element. The discharge time constant (TC) of the low Impedance Voltage Mode (LIVM) is a very important factor when considering the low frequency and the quasi-static response capabilities of an LIVM system. The TC value related on the data sheet means the time (in seconds) required for a sensor output voltage signal to discharge 63% of its initial value immediately following the application of a long term, steady state input change, this effect provide on quasi-static behaviour of the force sensor. In all applications, where is necessary monitoring processes with a constant value of reaction force, a quasi-static behaviour is fundamental to have a control, although if the piezoelectric sensor don't own a quasi-static behaviour, in all situation where the force continue in constant value, the value of force goes immediately to zero value.

The diagram below compare a the same servo press equipped with different force sensor. The blue trend correspond at the servo press with strain gauge force sensor. The red trend correspond at the servo press with a cheaper piezoelectric force sensor. The diagram shows the behaviour of the two sensor when the press is in stall for a constant force of 100 kN for a time of 15 seconds. The long dwell time shows how the crystals of the piezoelectric sensor loose the charge and output in volts correspond to a wrong force value. To have a reliable force measure with a piezoelectric force sensor is necessary use a sensor with the capability of loose slowly the charge associated to an amplifier capable of manage promptly the sensor discharge.

The graph above displays in red the quasi static behaviour of piezoelectric force sensor. The force value descend constantly and depends by the discharge time constant of the quartz

Why the piezoelectric force sensor

At first glance the piezoelectric force sensor wise the incorrect transducer for press applications, but this isn't true. In industrial applications where the quasi-static behaviour is accepted, the piezoelectric force sensor became relevant for its high resistance against the overload and the low maintenance. Our piezoelectric force sensor resist without problem to strong overload until 2 or 5 time the value of full scale. This feature mean a good safety guard to the overload risk during an automated process. Sometime automated device makes some failure, under the press could being misaligned parts, parts bad oriented, parts out of tolerance, or parts ruined. All this examples, in case coming under the press could produce an overload. The nature of piezoelectric sensor allow setting of the machine capable to working inside the overload limit. In mean time if the value of working overload remain inside the force sensor overload limit it don't lose the calibration. In eventually this could happen is possible replace the damaged sensor. All our sensors are calibrated and ready to use, the maintainer have to remove the damaged force sensor and plug in the new one with the right pre-load and restart. The damaged force sensor could be shipped to Auloma to have the recalibration needed. The nature of the piezoelectric crystal give to the sensor an elevate stiffness, the displacement measure hasn't some negative influences by sensor deformation. This elevate stiffens avoid the frequently recalibration by fatigue stress such as happen in strain sensor, this further advance is another source of cost saving.

Auloma strain gauge force monitoring system, a constant control

Auloma use the strain gauge force sensor to control the force in servo presses until 500 kN. In strain gauge force sensor, through a mechanical arrangement, the force being sensed deforms a strain gauge. The strain gauge measures the deformation (strain) as an electrical signal, because the strain changes the effective electrical resistance of the wire. The Strain gauge force sensor consists of four strain gauges in a Wheatstone bridge configuration. The electrical signal output is in the order of a few millivolts and requires amplification by an instrumentation amplifier. The output of the transducer is plugged into an algorithm to calculate the force applied to the transducer. A strain gauge force sensor takes advantage of the physical property of electrical conductance and its dependence on the conductor's geometry. When an electrical conductor is stretched within the limits of its elasticity such that it does not break or permanently deform, it will become narrower and longer, changes that increase its electrical resistance endtoend. Conversely, when a conductor is compressed such that it does not buckle, it will broaden and shorten, changes that decrease its electrical resistance endtoend. From the measured electrical resistance of the strain gauge, the amount of applied stress may be inferred.The strain gauge don't own the some performances of a piezoelectric force sensor in high dynamic applications. Measurement of dynamic force and impacts makes some strong stress in strain gauge foil and this fatigues solicitation after a period of time produce a loose of calibration. To avoid the loose of calibration or to increase the period of correct calibration is better don't exceed the 75% of strain gauge sensor full scale during the applications. When the strain gauge became opportune? In all applications where the force values don't increase constantly but own a progress which go up and down. The main difference within piezoelectric and strain gauge force sensor consist in the principle of working. The piezoelectric force sensor is a capacitor charged by the force impact and measure dynamic force that increase constantly or remain constantly in a period of time. In case the force decrease during the working cycle the piezoelectric force sensor don't show the real force values but show a negative values. This behaviour don't means that the piezoelectric force sensor is enable to monitoring process with progress which go up and down, if the waveform obtained is constant and repeatable is possible compare all waveform obtained in processes, but became impossible have a correct force signature after the point of force down. The strain instead read constantly the force value, the variance of force is measured by the variance of dimension of conductive strip and the feedback is constant. This behaviour in case of force progress which go up and down the strain gauge show in all point of force signature the real value and became the right device to control the force further the simple monitoring process.

Force with oscillation trend

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