Christopher R. Wagner

A tactile shape display using RC servomotors

Tactile displays are used to convey small-scale force and shape information to the tip of the finger. In this paper, we present a 6/spl times/6 tactile shape display that uses commercial radio-controlled (RC) servomotors to actuate an array of mechanical pins. The display has a maximum pin deflection of 2 mm along with a resolution of 4 bits. Pin spacing is 2 mm with a pin diameter of 1 mm. The display can accurately represent frequencies up to 25 Hz for small amplitudes and the slew rate is limited at 38 mm/sec for larger amplitudes.

The Benefit of Force Feedback in Surgery: Examination of Blunt Dissection

Force feedback is widely assumed to enhance performance in robotic surgery, but its benefits have not yet been systematically assessed. In this study we examine the effects of force feedback on a blunt dissection task. Twenty subjects used a telerobotic system to expose an artery in a synthetic model while viewing the operative site with a video laparoscope. Subjects were drawn from a range of surgical backgrounds, from inexperienced to attending surgeons. Performance was compared between three force feedback gains: 0 (no force feedback), 37, and 75. The absence of force feedback increased the average force magnitude applied to the tissue by at least 50, and increased the peak force magnitude by at least 100. The number of errors that damage tissue increased by over a factor of 3. The rate and precision of dissection were not significantly enhanced with force feedback. These results hold across all levels of previous surgical experience. We hypothesize that force feedback is helpful in this blunt dissection task because the artery is stiffer than the surrounding tissue. This mechanical contrast serves to constrain the subjects hand from commanding inappropriate motions that generate large forces.

Force Feedback Benefit Depends on Experience in Multiple Degree of Freedom Robotic Surgery Task

Force feedback has been suggested to provide a number of benefits to surgery. Few studies, however, have addressed the benefit of force feedback in the context of the complexities of true surgical tasks. When information is limited (such as depth information in endoscopically guided tasks), force feedback may provide additional information that improves performance. We investigate a two-handed, six degree of freedom, endoscopically guided, minimally invasive cannulation task (inserting one tube into another tube) to test this hypothesis. We used twelve subjects, six of whom were experienced minimally invasive surgeons. Results suggest that force feedback reduces applied forces for both subject groups, but only the surgically trained group can take advantage of this benefit without a significant increase in trial time. We hypothesize that this training difference is due to the interaction between visual-spatial motor abilities and the information contained in the mechanical interaction forces.

Mechanisms of performance enhancement with force feedback

The addition of force feedback to virtual environments and teleoperators provides many benefits to the user. However, the mechanisms by which force feedback improves performance remain unknown. We present an experiment demonstrating that force feedback can provide a physical constraint to an operators motion, passively restraining the hand and reducing error even before the operator can voluntarily respond to the force stimulus. Because of the presence of force feedback, the magnitude of unwanted incursions into a virtual wall were reduced by up to 80%, as compared to the case with no force feedback. We also propose that a second order mechanical model of the operators hand can be used to quantify the benefits of force feedback. Using our model, we are able to account on average for over 95% of the variance in the force on the hand.

Force Feedback in a Three-Dimensional Ultrasound-Guided Surgical Task

Three-dimensional ultrasound (3D US) is a novel imaging modality that allows real-time visualization of internal body structures such as the heart, even through visually opaque blood and tissue. The real-time nature of 3D US allows minimally invasive manipulations to be carried out without an endoscopic camera. The quality of the images is not ideal, however, other senses might be used to augment a surgeon’s performance in a 3D US-guided procedure. We investigated the combination of haptics under 3D US in a force control task. Results suggest that stiffness of the tissue plays a significant role as to the relative importance of vision versus haptics in this type of task.

Embedded Sensors for Biomimetic Robotics via Shape Deposition Manufacturing

One of the greatest successes of biologically-inspired design has been the development of mechanically robust robots. One promising biomimetic fabrication technique is shape deposition manufacturing (SDM), which alternates material deposition and machining to produce robot structures with compliant joints and embedded actuation elements. In this paper, we add to the tools available to robot designers by describing the development of a range of sensing modalities. These include Hall-effect sensors for joint angle sensing, embedded strain gauges for 3 axis force measurements, optical reflectance sensors for tactile sensing, and piezoelectric polymers for contact detection. We demonstrate these sensors in the context of a simple robotic grasper using the biomimetic SDM process to simplify fabrication and produce robust sensing mechanisms

Integrating Tactile and Force Feedback with Finite Element Models

Integration of the correct tactile and kinesthetic force feedback response with an accurate computational model of a compliant environment is a formidable challenge. We examine several design issues that arise in the construction of a compliance renderer, specifically the interaction between impedances of tactile displays, impedances of robot arms, and the computational model. We also describe an implementation of a compliance rendering system combining a low-impedance robot arm for large workspace kinesthetic force feedback, a high-impedance shape display for distributed tactile feedback to the finger pad, and a real-time finite element modeler. To determine the efficacy of the integration of tactile and kinesthetic force feedback components, we conducted a study examining the user’s ability to discriminate stiffness. Subjects were able to reliably detect a 20% difference in rendered material stiffness using our compliance rendering system.

Spatial low pass filters for pin actuated tactile displays

A common tactile display design uses an array of mechanical pins covered by a rubber layer which acts as a spatial low pass filter. To characterize the perceptual relationship between this rubber layer and shape rendering, we conducted psychophysical experiments to examine the perception of a vertical line stimulus felt using rubber covers of varying thickness and stiffness. We found no significant change in perception for rubber thicknesses ranging from 1.5-3.0 mm and for stiffnesses ranging from 45-200 kN/m/sup 2/.

Intrinsic Embedded Sensors for Polymeric Mechatronics: Flexure and Force Sensing

While polymeric fabrication processes, including recent advances in additive manufacturing, have revolutionized manufacturing, little work has been done on effective sensing elements compatible with and embedded within polymeric structures. In this paper, we describe the development and evaluation of two important sensing modalities for embedding in polymeric mechatronic and robotic mechanisms: multi-axis flexure joint angle sensing utilizing IR phototransistors, and a small (12 mm), three-axis force sensing via embedded silicon strain gages with similar performance characteristics as an equally sized metal element based sensor.

Compliant background subtraction algorithms for tactile rendering

Relaying spatially distributed forces promises to enhance the performance of telemanipulation systems. However, the proper way to render tactile information from the sensor to the display is not clear for current displays, where the user maintains constant contact. We present a simple approach to rendering tactile information to improve performance in a lump localization task in a compliant object. The algorithms presented subtract uninformative background information from the tactile signal. Subtracting a fixed background pressure frame reduced lump localization error by up to 20% while decreasing the time required to find the lump by up to 44%. Subtracting a background frame that depends on applied force further reduced lump localization error by another 17%.