During contact with an object, external forces deform the skin and fluid path around the electrodes, resulting in a non-linear distributed pattern of impedance changes containing information about force magnitude, direction, point of contact and object shape. Dynamic ranges of force sensing and resultant impedances span about a factor of 1000 as a result of asperities molded into the inner surface of the skin. Sensing range depends on fill volume, rubber durometer and asperity geometry; resolution (~0.01N) is limited by sampling electronics. Tangential forces result in sliding of the skin, causing a distributed pattern of impedance changes in electrodes along the sides of the finger. Non-linear regression techniques are required to interpret the data if explicit force vectors are to be extracted. We have used several machine learning techniques to explore data extraction including Kalman filters, neural networks, Gaussian processes and support vector machines.
Predicted (red) force vectors (Z = normal, X, Y = tangential) compared to actual forces (blue) for two of a set of manual contacts with the BioTAC. More systematic datasets for regression and validation are now being generated by contact with various probes controlled by a stepper-motor.
Publications
Arian, M.S., Blaine, C.A., Loeb, G.E., Fishel, J.A., Using the BioTac as a tumor localization tool, IEEE Intl. Conf. on Haptic Interfaces for Virtual Environment and Teleoperator Systems (Haptics), 443-448, 2014.Download PDF
Lin, C.H., Fishel, J.A., Loeb, G.E., Estimating point of contact, force and torque in a biomimetic tactile sensor with deformable skin, SynTouch LLC, 2013.Download PDF
Su, Z., Fishel, J.A., Yamamoto, T., Loeb, G.E., Use of tactile feedback to control exploratory movements to characterize object compliance, Frontiers in Neurorobotics, 6:7, 2012.Download PDF
Wettels, N. and Loeb, G.E., Haptic feature extraction from a biomimetic tactile sensor: force, contact location and curvature, IEEE International Conference on Robotics and Biomimetics (ROBIO), 2471-2478, 2011.Download PDF
Wettels, N., Smith, L.M., Santos, V.J. and Loeb, G.E., Deformable skin design to enhance response of a biomimetic tactile sensor, IEEE International Conference on Biomedical Robotics and Biomechatronics (BioRob), 132-137, 2008.Download PDF
Wettels, N., Santos, V.J., Johansson, R.S. and Loeb, G.E., Biomimetic tactile sensor array, Advanced Robotics, 22(8):829-849, 2008.Download PDF
Vibration Sensing
When the human finger slides over various surfaces vibrations are produce from the sliding friction. The human finger has specialized receptors to detect these vibrations which are essential for detection of slip and discrimination of surface textures as well as general tool usage. These same vibrations travel very efficiently through the BioTac fluid which is incompressible. They are then detected by the pressure transducer which has specially designed analog filters to optimize the sensitivity. The addition of fingerprints to the BioTac have been demonstrated to greatly enhance these vibrations.
As the ratio of tangential to normal force crosses the threshold of slip. Vibrations are produced and measured by the BioTacThe addition of fingerprints greatly enhances the sensed vibrations!
Publications
Fishel, J.A., Design and use of a biomimetic tactile microvibration sensor with human-like sensitivity and its application in texture discrimination using Bayesian exploration, University of Southern California, Doctoral Thesis, 2012.Download PDF
Fishel, J.A., Loeb, G.E, Bayesian exploration for intelligent identification of textures, Frontiers in Neurorobotics, 6:4, 2012.Article Link
Fishel, J.A., Loeb, G.E., Sensing tactile microvibrations with the BioTac - comparison with human sensitivity, IEEE International Conference on Biomedical Robotics and Biomechatronics (BioRob), 1122-1127, 2012.Download PDF
Loeb, G.E. and Fishel, J.A., The role of fingerprints in vibrotactile discrimination, Whitepaper for DoD Physics of Biology, 1-3, 2009.Download PDF
Fishel, J.A., Santos, V.J. and Loeb, G.E., A robust micro-vibration sensor for biomimetic fingertips, IEEE International Conference on Biomedical Robotics and Biomechatronics (BioRob), 659-663, 2008.Download PDF
Thermal Sensing
Because the conductivity of the fluid or gel increases with temperature, a thermistor is incorporated for thermal compensation. Thermal energy from the embedded electronics is used to heat the finger above ambient temperature, similar to the biological finger. This enables the material properties of contacted objects to be inferred from thermal transients measured by the thermistor on the surface of the core. Upon contact with a test object, the derivative of temperature (dT/dt) has several reproducible features (Fig). The initial negative peak (cooling) is similar for all materials; it occurs when the cooler skin is pushed against the warmer core. After the inflection point, the curves diverge and are dependent on heat capacity and conductivity of the object.
Temperature (top) and its derivative (dT/dt, bottom) following contact (vertical arrows) with large plastic and copper test pucks.
Publications
Lin, C.H., Erickson, T.W., Fishel, J.A., Wettels, N., and Loeb, G.E., Signal processing and fabrication of a biomimetic tactile sensor array with thermal, force and microvibration modalities, IEEE International Conference on Robotics and Biomimetics (ROBIO), 129-134, 2009.Download PDF
