Strain Sensors for monitoring press forces

Monitoring the process forces on presses is not only important for ensuring plant availability, but also has a decisive influence on the quality of the press elements

Indirect force measurement with Strain Sensors

The measurement can be measured directly in the force flow via load cells or indirectly in the force bypass via strain transducers. Strain transducers for force measurement have advantages in terms of their compact dimensions and ease of retrofitting outside the workspace. In addition, the press forces that occur can be measured in an overload-proof manner and the dynamic properties of a system are not impaired. By means of force measurement via strain transducers, strains induced by the press force in the rigid structures of the machine are measured. The strains are tiny deformations in the frame, which are proportional to the applied press force. To measure the strains, the strain transducers are force-fitted to the frame of the press in the so-called force bypass where the deformations occur.

Force measurement in forming technology fulfills the following functions:

  • Limit value monitoring to avoid overloads
  • Tool protection in case of jammed parts
  • Process monitoring (e.g. joining, holding and pressing forces)
  • Tool monitoring (monitoring of wear)

The use of strain sensors for force measurement has the following advantages:

  • No impairment of the height of the working area
  • Overload resistant
  • Protected mounting outside the work area
  • Easy to retrofit, as the screw connection can be easily made to the existing frame.
  • Cost-effective installation. This applies in particular to large nominal forces, for which force sensors must be dimensioned relatively large

However, the fast cycles in metalworking vouch for high demands on the measurement technology used. For example, drifting of the zero point or the measurement signal due to harsh environmental conditions such as vibration and temperature changes must be avoided. In response to this challenge, X-SENSORS has developed two functions that can be used to easily tare away drift of the measurement signal in cyclic applications.

The teach-in mechanisms for performing a zero adjustment are named after their mode of operation as “internal and external” zero adjustment. They are designed to be used in fast, cyclic applications and to ensure a stable measurement signal there.

For very fast load changes within 220 ms, such as can be found in eccentric presses, the internal, AC-coupled zero adjustment is available. By means of an integrated integral function, only the fast load changes are made visible in the output signal. Zero drift, on the other hand, is automatically masked out. The following graphic describes the behavior of the compensated output signal during very fast load cycles.

For cycle times of 0.15…150s there is alternatively a control input to trigger the zero point adjustment from external. The strain transducers of X-Sensors can be adjusted to zero via the controller using this external zero point adjustment. This input for the zero point adjustment is internally pulled to positive with a resistor. If this reset input is open, the sensor is in measurement mode (Operate). If, on the other hand, the reset input is pulled to 0V, the signal is set to zero (Reset) and the zero point adjustment has been carried out.

The following diagram describes the behavior of the strain transducers depending on the reset input:

Our strain sensors feature high accuracy and an analog signal path , allowing effective measurement and control of process parameters. Furthermore, the common industrial input signals 0-10 V and 4-20 mA are available as output signals . The respective product families differ in terms of accuracy, contact area, mounting type and measuring range.

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