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Page 425 Chen et al. Intell Robot 2023;3:420-35 https://dx.doi.org/10.20517/ir.2023.24

The normalized square difference, which is calculated using equation (1), is used for matching. The stereo
measurement method using parallax is shown in Figure 5.

2.5. Face detection by skin color detection at a distance
Two cameras are separately connected to the Raspberry Pi, and the distance measurement is executed by
using the parallax. Considering the calculation capacity, when the camera resolution was set to 320 × 240,
the detection of objects far away was difficult. In this study, face detection using skin color is introduced.

Skin color detection is conducted by setting the HSK color space [10-12] and extracting only colors that fall into
that range. The procedure of face detection is presented as follows, and an example of the face detection
result is shown in Figure 6:

1. The y-coordinate with the most consecutive pixels is searched for based on its length, which should be
equivalent to the face width.

2. By multiplying its width by the appropriate face ratio, the result is expected to be the length of the face.

3. The face ratio is determined empirically; the ratio of 1.2 works best in this study.

2.6. Vibration of SMA
The tactile actuator consists of a metal pin and an SMA wire with a diameter of 0.075 mm and a length of 5
mm. The structure is shown in Figure 7.

An SMA is a metal composed of Ti-Ni-Cu, which changes its shape when the temperature changes. When
the temperature rises to the transformation temperature, the length of the SMA wire shrinks to 95% of its
original length. The contraction of the SMA actuator is controlled by the PWM current, which consists of
two states in one cycle: ON and OFF. The ON current generates heat inside the wire, by which an SMA wire
shrinks. When the pulse current stops in an OFF state, the wire releases heat into the air and returns to its
original length . By adjusting the frequency and the duty ratio of the pulse current, different vibration
[13]
patterns can be created that can be applied to human skin. In our previous studies, we discovered that the
SMA actuator is able to generate vibrations with a frequency of up to 500 Hz. We also invented a structure
to amplify the micro-contractions of the SMA wire, leading to greater vibrations, as shown in Figure 7B [14,15] .
When an electric current is applied, the wire contracts, thereby lifting the pin upward. Then, when the
current stops, the wire instantly returns to its original length and the pin also returns to its original position.
The contraction and the return to the initial length cause the vibration of the pin to be sufficiently
recognized by a user as a tactile sensation.

2.7. Signal amplifier circuit
The Raspberry Pi only provides a voltage of 3.3 V with a maximum current of 80 mA. The SMA wire
requires approximately 100 mA to generate the maximum vibration, and thus, a current amplifier is
required. The control circuit shown in Figure 8 was originally designed to control the vibration motors and