Remote Palpation Instruments for Minimally Invasive Surgery
We are developing remote palpation systems to convey tactile information from inside a patient’s body to the surgeon’s fingertips during minimally invasive procedures. These new instruments will contain tactile sensors that measure pressure distribution on the instruments as tissue is manipulated. The signals from these sensors will be sampled by a dedicated computer system, which will apply appropriate signal processing algorithms. Finally, the tactile information will be conveyed to the surgeon through tactile “display” devices that recreate the remote pressure distribution on the surgeon’s fingertips. Creation of remote palpation technology will increase safety and reliability in present minimally invasive procedures, and bring the advantages of minimally invasive techniques to other, more complex procedures, which are not possible today.
The target application for our remote palpation technology is lump localization in minimally invasive thoracic surgery. In this procedure, hard nodules in the lung must be localized for excision. This is trivial using traditional palpation with the fingers because the nodule is much stiffer than the surrounding lung tissue. Using minimally invasive techniques; however, the process can be frustrating and time-consuming. The development of a remote palpation instrument that maps the surgeon’s finger motions to the instrument tip while providing tactile feedback will decrease localization times. A prototype palpation instrument (shown below) was constructed. It consisted of the tactile array sensor on the instrument tip, the shape display against the surgeon’s finger, and a cable drive mechanism to couple the motions of the surgeon’s finger to the instrument tip. An early version of this device was tested in a surgical setting using a pig. Many important design requirements were learned, which will be incorporated into the next-generation instrument currently under construction.
Researchers: Bill Peine and Jae Son
A Tactile Shape Display Using RC Servomotors
Tactile displays attempt to realistically simulate skin deformations that occur when interacting with real objects by transmitting small-scale shape information to the fingertip. Teletaction (experiencing the sensation of touching a remote object) and virtual environments are both domains in which an effective tactile display is crucial to establishing a realistic sense of presence. Lederman and Klatzky have shown that when spatially distributed contact forces to the fingertip are removed during contact, spatial acuity, pressure sensitivity, orientation detection and detection of a lump by palpation are all markedly impaired. Roughness perception is also impaired, but only moderately so. These data argue for the potential importance of displaying spatially distributed forces to the skin in domains such as remote medicine, surgical tools for minimally invasive surgery, and virtual training applications.
The dominant tactile display design uses an array of stimulators that contact the skin to achieve a force distribution on the fingertip. Other previous designs have used shape memory alloy, pneumatics, voice coil actuators, and solenoids. However, due to tradeoffs between display bandwidth and actuator density, no universally satisfactory solution has emerged. Moy and Fearing showed that the optimal tactile display had to have an actuator density of 1 per square mm, have a 2 N per tactor force capacity or have a 2 mm indentation, have at least 3 bits of height resolution, and have at least 50 Hz bandwidth [9]. No previous display has been able to meet these criterion, however, these parameters motivate many of the design decisions made.
We propose that a tactile display using RC servomotors in can achieve a high bandwidth, high actuator density, large vertical displacement, and firm static response for a relatively low cost and simple construction. To validate this design hypothesis, we first constructed a five pin, uniaxial tactile display. Seeing that RC servos can provide a successful actuation mechanism for a pin based tactile display, we enlarged the design and incorporated higher performance servomotors. Our final display vertically actuates a 6×6 array of 36 mechanical pins at a 2 mm spacing to a height range of 2 mm with a resolution of 4 bits. For a vertical displacement of 2 mm, the 10% to 90% rise time is 41ms.
Researchers: Chris Wagner
Towards Realistic Vibrotactile Display in Virtual Environments
This paper investigates the use of vibrotactile feedback to add a further dimension of reality to virtual environments. We describe a system that incorporates low-frequency force feedback and an inexpensive vibrotactile display system composed of modified audio speakers, a PC sound card, and a 2-channel current amplifier. We examine the initial problem of stiffness perception based on this vibrotactile feedback. To determine the appropriate waveforms to use in virtual environments we examine the waveforms that are produced when subjects tap on surfaces of varying stiffness with an aluminum stylus. A simple exponentially damped sinusoidal model is presented for these vibrations. This model is then used to produce waveforms for an experiment that assessed the validity of this characterization and the effectiveness of vibrotactile stimulation in conveying stiffness information in a virtual environment. The results are compared with a similar experiment in which subjects tapped directly on different real surfaces using a stylus. Using these simple waveforms for vibrotactile display in our test system gives a relatively accurate description of the phenomenon and allows subjects to accurately distinguish surface stiffness.
We used only one stylus for all trials. With these restrictions, there are two variables that must be considered. First, what difference in waveforms distinguishes one surface from another? Answering this question will allow us to determine the proper shape of the waveforms to be replayed. And second, what is the difference in the waveforms produced when a person taps on the surface with differing velocity? Finding the answer to this second question will allow us to appropriately scale the vibrations with differing impact speeds.
In order to answer these questions, it is necessary to obtain data relating to contact vibrations with different surface stiffness and impact velocities. For this purpose, we used an aluminum stylus fitted with a spherical steel tip 10 mm in diameter. This stylus was instrumented with an accelerometer in the tip, a force sensor mounted coaxially with the tip, and a tracking lever arm mounted on a potentiometer to record its position during a tapping operation. We obtained an estimate of the velocity at the instant of impact through backward difference. Figure 1 shows a schematic view of the stylus.
Researchers: Parris Wellman
Tactile Shape Displays for Small-Scale Shape Feedback
The tactile display in our prototype system consists of a line of 10 individually actuated pins that are raised against the fingerpad. Shown below is a drawing of our design. A line configuration was chosen since the palpation instrument will be scanned across the tissue allowing motion to provide the other dimension. This also simplifies the design. SMA wires are used to drive the pins. As electric current heats the wire, it goes through a phase transformation and shortens, thus pushing the pin up. With this design, each pin can move 3 mm and produce over 1 N of force. A primary problem with SMA is the slow response times. We overcame this by using water cooling and position feedback for each pin from optical sensors. Figure 5 shows the response of the pin as a function of desired position frequency. The output displacement drops by 30% (-3 db point) at 40 Hz. This satisfies the design specification set by the finger speed experiments.
Researchers: Parris Wellman and Bill Peine
