Design

Biomimetic inspiration

Figure 1: Intermediate ridges in the human skin (left), and the corresponding pins in the TacTip (right)

The sense of touch is one of the key human modalities. Tactile sensory input should also be provided to robots to allow them to perform complex manipulation, grasping and exploration tasks. The TacTip sensor is a biomimetic, dome-shaped device based on the human fingertip.

Chorley et al. (2009) were inspired to consider the behaviour of the human glabrous (hairless) skin, as can be found on the palms of our hands and the soles of our feet. They built on previous research showing that Meissner’s Corpuscles work in tandem with the intermediate ridges (Fig. 1), to provide edge encoding of a contacted surface. When the human finger makes contact with an object or surface, deformation occurs in the epidermal layers of the skin and the change is detected and relayed by its mechanoreceptors. The TacTip device seeks to replicate this response by substituting intermediate ridges with internal pins on the inside of its skin. The pins deflect when the TacTip contacts an object and the resulting deformation is optically tracked by a camera.

Technical Design

The four main characteristics of the TacTip's design are listed below:

1. Modular philosophy The latest generation of the TacTip sensor is designed to allow its tip to be quickly replaced. This allows different designs of the dome-shaped part of the sensor, which is in contact with the external environment. Different versions of the tips can be manufactured and tested without changing or re-fabricating the overall structure. The bayonet mount permits the tip and the base structure of TacTip to be connected and disconnected in a fast and user-friendly manner.

Sectional view of tactip

Figure 2: Tip of the TacTip sensor. Left: view from the top, Right: side view

2. Materials The tip of the TacTip is created in two stages: the rigid white plastic (VeroWhite) is 3D printed in a Stratasys Objet series printer, and the black rubber skin (Smooth-on Vytaflex 60) is created through a moulding process. The mould could be adapted to investigate the effect of morphological changes of the skin (shape, layout and number of pins) on performance and sensitivity. The physical properties of the two materials used in the latest version of TacTip are reported in the following table:

 

VeroWhite

Vytaflex 60

Tensile Strength

58 MPa

6 MPa

Elongation at break

10% – 25%

480%

Hardness Shore A

85

60

 

The soft material used to fill the tip of the TacTip is Techsil RTV27905, which has a penetration (mm) [19.5g cone] of 3 to 7 and is completely clear, in order to allow accurate images to be collected by the camera.

3. Pin Layout The hexagonal layout of the pins in the TacTip’s soft tip is designed so that so that pins are equidistant from each other in the camera image. Since there are 6 LEDs in the TacTIp, this arrangement also keeps the pins as far as possible from the LEDs that illuminate the tip. This avoids the white LED lights being erroneously identified as pins by image processing algorithms.

Figure 3 view of the internal part of the TacTip

4. Sensor safety The TacTip has been purposely designed so that there is no physical contact between the fragile optical elements (camera, LED circuit) and the TacTip's external surface. This significantly contributes to the robustness of the sensor. In case of a strong collision between the sensor and an object, the soft tip dilutes the effect of the impact, and the crucial components of the sensor (LED circuit, camera) are well protected.

In summary, The TacTip possesses enough similitudes with its biological counterpart to make it an ideal platform for biomimetic research and, at the same time, is a low-cost, open-source, robust and easily customable structure. The STL and CAD files for the TacTip parts described above can be found in the Downloads section.