Electromechanical servo presses
FORCE MEASUREMENT CHAIN
The force is measured by a piezoelectric quartz force transducer directly at the tool holder, at the point where it also arises and is to be regulated.
Threaded spindle and pressure pipe friction are thus not included in the measurement.
Due to the very high stiffness of the quartz discs, one can practically speak of a displacementless force measurement.
Strain gauges must deform to measure forces and therefore have low stiffness.
The piezoelectric quartz force transducer is characterised by its perfect linearity over the entire measuring range.
This means that even small forces can be measured precisely and then regulated.
By using a charge amplifier with two measuring ranges, the measuring range can be switched automatically and thus the resolution can be changed.
It is possible, for example, to make initial contact with the component in a small force range with high resolution (“latching on”) and to press it in with high force by automatically switching to the second measuring range.
Another advantage arises when the same press-fit station or assembly cell is used for several components. It is possible to define a small measuring range for components with lower force requirements and a high measuring range for components with higher force requirements.
This increases the accuracy and the tolerances can be better monitored.
This is not possible with a measurement chain made of strain gauges.
Due to the position of the piezoelectric force transducer directly on the tool holder, the measurement result is not affected by the heat emitted by the motor or the mechanical screw drive.
There are systems on the market in which the force measurement is integrated in the housing of the servo press.
With these systems, one can notice an impairment of the measuring system especially at high clock rates.
The force measurement chain should be checked regularly (recommendation 1x per year).
For this purpose, reference measurements are carried out with a calibrated force measuring chain.
To enable regular force balancing, a counter stop is required which can be moved to with Fmax. and corresponding installation space to bring a reference sensor between the counter stop and the tool holder.