Concept and Specification

The conceptual idea of the Shear-Force Display is based on one or more rigid pins, like the ones used for normal-force displays. A major difference is that the Shear-Force Display must be able to move the pin(s) lateral to the surface of the human skin. This augments the movement space to a two-dimensional space, with the consequence that two actuators are needed to drive a single pin.

The next points to consider are the shape and the size of the pin. In order to cause skin stretch, the pin must be able to remain in contact with the skin over the entire range of motion it covers. On the other side, the pin must not perceived as punctual (needle- like) tactile sensation. Both demands could be satisfied by maximizing the contact area of the pin at the skin. However, arguments against a large contact area are movement constraints, especially if there is more than one pin present. Resulting from this trade-off, a round pin with a plain top surface and a diameter of 1 mm was chosen. The desired pin movement was determined to be 2 mm in each direction. This value results mainly from the comb experiments described earlier. Previous pilot experiments with a similar setup showed that even a amplitude of lateral pin movement below 1 mm beneath the skin causes a clear perceptual sensation of skin stretch. Therefore, the chosen dimensions seem to be a good compromise concerning pin size and motion, and they allow to install four lateral movable pins within the area of the fingertip of the human index finger.

The skin reacts to stretching with a resistive force. To push against this resistance, the pin needs a certain force. In addition, there must be sufficient performance left to ensure an appropriate velocity of the pins, to provide the intended stimuli. The skin can physically be described as viscoelastic material, where force reactions in normal direction are nearly independent of tangential force reaction. Investigations on these viscoelastic properties of the skin show that the force response clearly depends on the strain rate. In a steady state, the force response solely depends on skin stretch [23].

However, more quantitative knowledge on the exact physical properties of the human skin at the tip of the index finger is needed to specify the required force and power of the actuator driving the pins. To this aim, an initial experiment was performed.

The experiment on skin stretch was performed by providing sinusoidal pin excitations of up to 150 µm perpendicular to the skin at several frequencies in the range of 2−200 Hz. The pin size was 0.5 mm in diameter and the stimulation time was 10 s per trial. The results, based on the viscoelastic model, show a steady-state shear modulus of 90 kN/m2 ∗_{. In the}

range of the stimulation frequency of 2 − 20 Hz, the shear modulus is about 70 kN/m2_.

At stimulation frequencies from 30 Hz up to 200 Hz, the shear modulus increases from 454 kN/m2 _{to 8.09 MN/m}2_.

According to the demands of the envisaged experiments, the maximum kinetic frequency of the pins was specified to 20 Hz. For this range, a maximum shear modulus of 90 kN/m2

can be expected. The maximum bulk modulus in this frequency range is 55 kN/m2_{. With}

the specified dimensions of the pin and a pin movement of 2 mm in each direction, a maximum driving-force of 0.355 N is required, according to the equations for purely elastic behavior of the hand tissue [23]. Tab. 3.1 lists the key specifications that are demanded

Table 3.1: Specifications for the Shear-Force Display.

Property Specification

Number of pins 4

Pin shape circular

Pin tip area 0.7854 mm2 _{(∅ 1 mm)}

Pin movement two axes, workspace 2 × 2 mm

Max. lateral force 0.355 N

Max. frequency 20 Hz

from the Shear-Force Display. Besides these quantitative specifications, it must also be considered that the display should be as compact and lightweight as possible.

Mechanical Concept

The main objective is to find a mechanical solution that transfers the rotatory motion of two servo motors to one pin, generating two-dimensional lateral pin motion. Fig. 3.1 shows the design concept of the tactile Shear-Force Display. The side view illustrates the

Figure 3.1: Functional principle of the Shear-Force Display.

mechanism to operate one pin in one axis of motion. All four pins are connected to the base plate by universal-joint shafts. This locks the rotatory motion of the pins and thus allows only pitch motion. The pin must be long enough to prevent undesired motion normal to the skin during pitch motion. Each pin is actuated by two servo motors. The figure shows one of the two actuator chains: The rotatory motion of the servo motor is transformed by the reduction rocker arm, which in turn moves the pin. These components are connected via lever linkages with ball joints on both ends. This ensures explicit transmission of linear motion with nearly zero backlash. The top view in Fig. 3.1 shows the arrangement of the four pins. The dotted cycle represents the hole in the cover plate, which enables the connection between the pins and the skin of the resting index finger.

According to this concept, an experimental prototype for one pin was built as a proof of principle [53]. The following section describes the final hardware setup of the tactile Shear-Force Display, as presented previously in [54–56].