Haptic sensing recreates the sense of touch when interacting with equipment, such as a medical device, by applying forces, vibrations, or motions. A force feedback loop is created such that the movement of the user is manipulated in a meaningful way, appropriate to the application. The haptic sensor, such as a fiber Bragg grating, can convert a mechanical force such as strain or pressure into an electrical signal which can then manipulate a motor to give a resistance, vibration or other tangible sensation. Some haptic techniques do not use touch for example ultrasound and concentrated air pockets can create sensation around the user.
All haptic sensors use a similar mixture of the principles (force, vibration and motion) to reconstruct the sense of touch. There are four main types of haptic sensor: fiber Bragg gratings (FBGs), eccentric rotating mass vibration (ERMV) motors, linear resonant actuators (LRAs) and piezo haptic sensors.
Fiber Bragg Gratings (FBGs)
To help surgeons, visualization technologies are used to give surgeons some level of feedback on the procedures. Laparoscopy (otherwise known as keyhole surgery) and robotically assisted surgery typically use fiber-optic endoscopes or small digital cameras to give a direct in situ view of the tips of the tools. This gives surgeons visual feedback but leaves them without the sense of touch, making it challenging for surgeons to differentiate between the harness and textures of various tissues. In addition, otherwise simple tasks, such as tying a knot in a surgical suture, are not as precise or consistent as can be achieved by human hands. To overcome this challenge, fiber Bragg gratings (FBGs) are finding their way into surgical tools to enable the forces experienced by the tools to be translated back to the surgeon via mechanical force feedback or visualizations.
FBGs inscribed into the core of an optical fiber are highly sensitive to longitudinal strain such that, as the strain increases along an optical fiber, the central wavelength reflection response of the FBG linearly shifts towards higher wavelengths. This shift can be monitored and fed back into motors attached to the handle of the surgeon’s tools.
FBGs are often written into Fibercore’s bend insensitive SM fibers to enable low bend loss sensors that can be mechanically routed through small bend radii without notably attenuating the optical signal.
Eccentric Rotating Mass Vibration (ERMV)
ERMV motors are setup to have a non-concentric rotation such that as a magnetic force is applied to the rotating mass, a forward or backward motion can be created due to the asymmetrical centripetal force. The same method can be used to create lateral vibrations.
Linear Resonant Actuators (LRAs)
LRAs create uniaxial oscillating forces through magnetic and electric currents. A voice coil is forced against a moving mass through current input, the mass in turn is attached to a spring which creates a magnetic field when it oscillates. The magnetic field then causes the actuator to vibrate with a sufficient magnitude that can be felt by touch.
Piezo haptic sensors can be used to generate a vibration as piezo materials generate electrical current when the materials are stressed. Subsequently, when the material is bent, deformed, compressed or elongated, the piezo haptic sensor can generate a vibration. The broader frequency range of piezos and multi-directional vibrations capable with piezos offer them advantages over LRAs and ERMVs.