Human Tactile Spatial Sensitivity for Tactile Feedback

In this paper, we quantify several spatial capabilities of the hmnan tactile system needed for tactile feedback, or teletaction. Psychophysics experiments measure the amplitude resolution of the human tactile system, the effects of shear stress on grating orientation discrimination, and the effects of viscoelasticity {creep and relaxation) on tactile perception for static touch. The results are used to determine teletaction system design parameters. We find that 10% amplitude resolution is sufficient for a teletaction system with a 2 mm elastic layer and 2 mm tactor spacing. 1 I n t r o d u c t i o n Information about texture, local compliance, and local shape is important in applications such as telesurgery or handling of fragile objects in telerobotics. Figure 1 shows a general configuration of a tactile feedback (or teletaction) system. One possible application is on a robotic laparoscopic telesurgery system. The tactile sensor is mounted on the end effector (the laparoscopic instrument), and the tactile display is mounted on the master manipulator (the user interface). The tactile display presents information recorded by the tactile sensor to the user. Ideally, the patterns felt by the user would be indistinguishable from direct contact with the environment. The tactile display needs to generate surface stresses that realistically represent data collected by the tactile sensor. To fully control surface stress, the ideal tactile display system would be an infinite density array of 3 DOF actuators. Teletaction systems are composed of a tactile sensor, a tactile filter, and a tactile display. We discuss fingertip teletaction systems with high spatial detail but low temporal resolution. Most teletaction work has focused on tactile sensors and displays [Shimoga 1992; Howe et al 1995]. Tactile sensors are comparatively well understood [Howe 1994], but are generally designed for shape recognition and manipulation tasks, not teletaction. Typical tactile sensors range in size from a 1 mm square with 8 × 8 elements [Gray and Fearing 1996] to a 16 mm square with 8 x 8 elements *This pape r is a synopsis of " H u m a n Psychophysics for Teletact ion Sys tem Design" which will a p p e a r in haptics-e, The Electronic Journal of Haptics Research. 0-7803-5886-4/00/$ 10.00© 2000 IEEE 77 Environment _ ~ Measured Strain ~ Stress Pattern i s ..... I =lF''r I , , . u.. Figure 1: A block diagram representation of a general teletaction system. [Howe et al 1995] to a 25 mm diameter cylinder with 3 × 16 elements [Nicolson and Fearing 1995]. Tactile sensors typically respond to the normal component of strain though there are tactile sensors tha t measure normal and shear stress [Domeniei and DeRossi 1992]. A tactile filter converts tactile sensor da ta to tactile display data. Some concerns of tactile filter design include spatial and temporal sampling differences between the sensor and display, anomaly and noise filtering of the sensor data, and conversion of strain profiles from the sensor to normal and shear displacement profiles or normal and shear force profiles for the display. The filters are typically computers with A /D and D/A boards. Tactile displays originated with tactile reading aids for the blind using piezoelectric-driven pins and direct pneumatic actuation [Bliss 1969]. Progress in tactile displays has been slow, due to the demanding mechanical requirements. An ideal display requires 50 N /cm 2 peak pressure, 4 mm stroke, and 50 Hz bandwidth; that is, a power density of 10 W / c m 2 with an actuator density of 1 per mm 2. The performance requirements are a result of the 70 SA I mechanoreceptors per cm 2 [Johansson and Vallbo 1979] and force and displacement for compression of the finger [Serina et al 1997]. Tactile display designs have used solenoids [Fischer et al 1995], shape memory alloy [Howe et al 1995; Hasser and Daniels 1996], pneumatics [Cohn et al 1992; Caldwell et al 1999], and MEMS [Ghodssi et al 1996]. Voice coil actuators have also been used [Paw[uk et al 1998], but result in a large apparatus. Electrocutaneous stimulation [Kaczmarek et al 1991] is mechanically quite simple; however, the perceptual effects are hard to analyze. Typically, tactile displays control either displacements or forces. In a displacement display, an array of pins is shaped into a contour. In a force display, the pin array will produce a surface stress distribution representing the data. The tactile display's spatial density is limited by actuator size. Currently, the spacing between the centers of the pins is around 2 mm [Cohn et al 1992; Howe et al 1995].