Using a conductive sphere as a probe to characterize the sensitivity of soft piezoresistive films. Issue 18 (7th December 2020) - Green, Christopher
Abstract: Flexible piezoresistive films, such as, carbon black/polydimethylsiloxane (C‐PDMS) composites, are often used as skin analogs and integrated into complex array sensors for tactile sensing. The uniformity of the sensor characteristics heavily depends on the homogeneity of the composite. Therefore, the ability to locally characterize a film that will be integrated into a complex force sensor could be critical. Here, a method to characterize the local sensitivity of flexible piezoresistive films is presented. Using a conductive sphere, which was chosen over a flat probe to eliminate misalignment issues, the surface of a thin film composite is indented to characterize the change in resistivity in terms of average strain. Experiments were performed with 15 and 18 wt% carbon black C‐PDMS films of varying thickness. The contact radius of the probe with the piezoresistive film was estimated using the Johnson‐Roberts‐Kendall contact theory. Theoretical contact area estimates were found to agree with contact radius measurements carried out using optically transparent PDMS films observed through an optical microscope. Results show that C‐PDMS with 15 wt% carbon black exhibit a higher rate if change of resistivity and gauge factor than films of same thickness with 18 wt% carbon black. On the other hand, thicker films exhibit higher gauge factors for the two tested carbon black contents. Tests carried out at multiple locations yielded consistent sensitivity values, making these types of composites suitable for array type force sensors. Abstract : Soft piezoresistive films are widely used as force/tactile sensors, specifically as skin analogues in robotic applications, owing to their flexibility and ease of fabrication. Despite their favorable properties, they can exhibit variations in sensitivity and resistivity over the film area due to uneven dispersion of the conductive particles in the polymer binder. This work describes a conductive spherical probe‐based method to characterize sensitivity locally and non‐destructively. Additionally, the spherical geometry of the probe eliminates the need for an alignment system otherwise necessary for flat probes.