Force sensors measure the applied force from the proportional deformation of a spring element: the larger the force, the more this element deforms. However, the frequency range of this type of sensor is limited by this element having to be sufficiently elastic to undergo large-amplitude deflections reflecting the load.

Force Sensor Construction and Operation

Usually force sensors designed on the piezoelectric principle exhibited by quartz. Under load, quartz crystals produce an electric charge proportional to the mechanical load applied: the higher the load, the higher the charge. Thus, in piezoelectric force sensors, quartz serves as both the spring element and the measurement transducer.

Unlike strain gages that can measure static forces, piezoelectric force sensors are mostly used for dynamic-force measurements such as oscillation, impact, or highspeed compression or tension. Any force applied to the piezoelectric sensing element produces a separation of charges within the atomic structure of the material, generating an electrostatic output voltage. The polarity of the voltage generated depends on the atomic structure of the material and the direction in which the force is applied.

However, any leakage path lets electrons redistribute across the material, dropping the voltage output back to zero. Internal leakage paths are formed by impurities within the crystal while external paths are created by the electronics used to measure the voltage generated. All leakages must be considered to determine the discharge time constant (DTC). The DTC typically follows an exponential curve similar to an RC time constant and is used to determine the sensor’s lowest frequency response.

Thanks to the high rigidity of the crystal, the measuring deflections are very small, usually just a few microns. Where slow, quasi-static processes are being measured, this virtual absence of displacement keeps measurement error to an absolute minimum . Yet quartz also offers unrivalled accuracy in the case of much faster processes.

The quartz crystal converts each physical quantity to be measured (force, pressure or acceleration) into an output signal that is precisely linear and hysteresis-free. A quartz force sensor consequently has an extremely wide measuring range while remaining compact, robust and highly sensitive.

Typical Force Sensor Applications

  • Quality assurance in manufacturing and assembly
  • Bench testing, e.g. force measurement to check material quality
  • Automation technology, e.g. for monitoring quality during reshaping or bonding operations
  • Measurement of cutting forces involved in machining
  • Biomechanics
  • Product testing
  • Materials testing
  • Dynamic weighing of moving vehicles
  • Wheel force measurement