Pressure and power transducer

CNT based force sensors: Process Monitoring in forming tools#

© Fraunhofer ENAS
Sensor principle of the CNT based force sensor.
© Fraunhofer ENAS
In cooperation with Fraunhofer IWU developed load cell with four sensor cells for process monitoring + SEM cross section image of the CNT network embedded in parylene.

For precise process monitoring of cutting and forming processes a vicinity of the sensor system to the working point is necessary to determine the real forces in machine tools. This vicinity of the working point requires a high sensitivity of the sensors to obtain a high stiffness of the sensor environment. A to large stiffness loss due to the integrated sensors would lead to an accuracy decrease of the components and to a degradation of dynamic properties of tools and machines.

To satisfy these requirements a novel force sensor was developed. The sensor principle is based on a network of vertically aligned carbon nanotubes (CNTs) which electrical resistance can be altered by compression.

For realizing a load cell, CVD-grown CNT structures were arranged as resistors in a Wheatstone bridge configuration and integrated in a deformation element. For process monitoring a sensor cell with four testing points in a circular arrangement was developed. Following results could be obtained from testing the load cell in a press test stand:

  • For normalized load until nominal load, a sensitivity up to 75 mV/V could be achieved. This is 35 times higher as it is the case for standard metallic strain gauges with 2 mV/V sensitivity.
  • The deformation element has a stiffness of 8 kN/µm. Comparable commercial available force sensors with nominal load between 1 and 5 kN have a stiffness from 0.05 to 0.2 kN/µm. Therefore, the stiffness of the CNT sensors is one to two orders of magnitude higher.