Engineering Design of Haptic Interfaces

Thus far, a number of haptic feedback systems have been discussed in the context of user performance and haptic perception in surgery. This section of the review intends to describe the technical means by which artificial haptic feedback is made possible. In the broadest sense, a haptic interface is a form of configurable human-machine interface that actively engages the human haptic perception. Haptic interfaces can be designed to stimulate either the human kinaesthetic or cutaneous sensory subsystems or both. Artificial haptic interfaces have been developed over several decades but significant advancements were made in the 1990s when multi DoF haptic interfaces for the kinaesthetic haptic perception first became commercially-available [109]. These haptic interfaces were designed for the human user to actively explore and touch simulated or remote environments. As a surrogate for touching a real environment, the user will manipulate an interface that can, in turn, exert forces back on to the user so as to simulate contact by eliciting the user’s kinaesthetic haptic perception and proprioception. The mechatronic design of such haptic devices is challenging. One of the key challenges that has characterised the development of such interfaces is that of transparency [109]. An ideal, fully transparent haptic interface would be able to perfectly simulate real- world haptic interaction without the user being conscious that they are not interacting with the real-world but in fact, a simulated environment. One of the first designs to address the challenges of haptic transparency and allow kinaesthetic-based force-feedback was the PHANToM device, developed by Massie and Salisbury [110]. To put the notion of transparency in to more concrete terms, Massie and Salisbury describe two contrasting conditions that illustrate this point. Firstly, “free space must feel free” which means that the

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user should not perceive any force when moving in free space. This requirement has implications for the mechatronic design of haptic interfaces: quantities such as inertia and friction should be minimised and there should be no unbalanced weight which would break the illusion that the user is interacting with a virtual environment. Secondly, Massie and Salisbury state that “solid virtual objects must feel stiff,” for this to be true, sufficient forces and torques are required of the haptic interface in order to effectively resist the user as they contact a virtual rigid object. These transparency requirements necessitate conflicting engineering challenges. Firstly, in order to generate large forces, large (and consequently heavy) actuators are required which would increase inertia and compromise the first transparency requirement. Conventional robotic manipulators would make use of a gear train in conjunction with a lightweight actuator to resolve this problem, but such an implementation would introduce friction, inertia and backlash which all serve to degrade the users’ sensation of “free space feeling free”. The PHANToM design used a number of strategies to optimise for transparency. Pre-tensioned tendon-capstan assemblies provide friction-free, low inertia torque amplification of the motors. This allows the use of relatively low weight motors that can still exert high torques on to the counter-balanced linkage mechanism that the user manipulates. This PHANToM design was commercialised by Sensable Technologies (now Geomagic, MA, USA) and it has been optimised over subsequent decades and adapted in to other designs. Whilst the design has evolved, many of the original design features are utilised in commercially-available haptic interfaces today including the use of pre-tensioned tendon-driven capstan assemblies. The Quanser HD2

system (Quanser, Ontario, Canada) uses two coupled manipulators of a similar architecture to the PHANToM device in a parallel manipulator configuration. The W6D device (Entact Robotics, Ontario, Canada) also uses a parallel-linkage architecture. These configurations are capable of generating comparatively higher loads and recreating stiffer collisions but at the expense of workspace limitations and singularity problems. The Sigma, Omega and Delta systems (Force Dimension, Nyon, Switzerland) use a delta-based parallel kinematic architecture that can generate high forces without singularity issues. These devices can also incorporate active gravity compensation so as to improve transparency by making the electromechanical assembly feel weightless to the user.

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Figure 2.11 A commercial PHANToM device, originally developed by Massie and Salisbury [110].

Figure 2.12 The Omega.3 device that uses a Delta manipulator (image courtesy of Force Dimension, 2015).

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Another significant challenge in the design of transparent haptic interfaces is a computational one. Television can accurately simulate real-world human visual experience because of the temporal resolution limitations in the human visual sensory system. The human brain can interpolate between individual frames so that a series of discrete images received by the human visual sensory system at frequencies in the order of 30Hz will appear continuous and indistinguishable from human perception of the real world. Unfortunately for haptic interface designers, the human haptic perceptual system has a more acute temporal resolution which warrants haptic interfaces to have update rates in the region of 1kHz [111]. This has computational implications for the control systems of haptic interfaces which must resolve the forces being exerted on to the user within timescales in the order of magnitude of a millisecond. Whilst modern computers have gained significant computational power since the development of the original kinaesthetic haptic interfaces, computing the forces that a haptic interface must generate is still a significant challenge. It is especially challenging when considering haptic interactions with environments that require non-linear calculations, such as deformable soft tissue surfaces. For perfect transparency in a master-slave configuration, the slave manipulator must be capable of conducting perfect environment force sensing and dynamic modelling with minimal latency in the data transmission. It is for these reasons that force-reflecting master-slave systems can potentially become unstable, a potentially catastrophic event in robot-assisted surgery. This has motivated researchers to consider the sensory substitution techniques reviewed earlier as a haptic rendering method for robotic surgery since the approach is inherently robust to control instability.