## 4 Results and Discussion
### Sensor Robot Design as Access Point
#### 4.1.2 Image Projector (Safety Lighting) for Alerting Signals to Vehicles  
The sensor robot device also includes an image projector (220 in Figure 7a) in the projection unit (220 in Figure 7b). It projects an image or text with a laser and lighting onto a road surface when the vehicle approaches. Here, the projected image can include a video or image message that notifies the driver of a road condition or a detour route. Also, the image as safety lighting can include a warning message indicating the state of the road. For example, a warning video can consist of a message indicating the state of the road, such as \"under construction\" and \"incident section.\" The projected image can also indicate a detour route. For example, the 'detour route image' can include a message indicating a direction in which the vehicle should bypass, such as '->' and '>>>'. In this case, the image to be projected can be set by the image manager of the message library unit (240 in Figure 7a).  
The image projector can project an image onto a road surface when the approaching vehicle is within a preset range set by the detector. For example, when the ultrasonic detector detects a distance that is fewer 10 m and RADAR detects a distance of less than 200 m from the approaching vehicle, the image is projected on the road surface to warn the driver. For efficient battery usage, it can stop projecting an image while no vehicle is approaching the robot device.  
The image projector includes a light source unit inside the package. We adopted a light source unit including red, green, and blue laser diodes and connected it to Arduino, as shown in Figure 7b. In this case, the plurality of light-emitting devices can emit color by combining at least one white beam. The light source unit can emit light generated in each of the laser diodes by a driving current applied from the power supply unit (260 in Figure 7a) according to a predetermined driving signal. It can also control the projection angle to project the image within the distance measured by the detection unit.  
Here, for the purpose of setting the accurate angle of the laser lighting, the projection manipulator, as shown in Figure 8a, controls the image projector to project an image on a road surface within a distance from the vehicle, measured by the detector, based on a vertical axis relative to the road surface. A portion of the robot device can also be horizontally rotated to control the projection angle.  
This is an example of an kinematics of the end effector (projection light) on the transformation matrix:  
\[\begin{array}{c}\mathbf{cos}\theta_{\Pi}\quad-\mathbf{sin}\theta_{\Pi}\quad \quad\mathbf{0}\quad\mathbf{0}\\ \quad\mathbf{sin}\theta_{\Pi}\quad\quad\mathbf{cos}\theta_{\Pi}\quad\quad \mathbf{0}\quad\mathbf{0}\\ \quad\mathbf{0}\quad\quad\mathbf{1}\quad L_{\Pi}\\ \quad\quad\mathbf{0}\quad\quad\mathbf{0}\quad\quad\mathbf{0}\quad\mathbf{1} \end{array} \tag{1}\]  
When the image projecting unit projects an image in a direction opposite to an approaching vehicle, part of the robot device is rotated horizontally by 180 degrees to the left on the vertical axis of the road surface, as shown in Figure 8b. The projection angle can be controlled to project an image in  
Figure 8: (**a**) Projection; (**b**) simple R-R manipulator and the kinematics.  
the vehicle direction. The lower end B, in Figure 7a, has a hemispherical shape, at least a portion of which is convex downward. In addition, it can include a bottom surface B1, at least a portion of which is in contact with the road surface as the first revolving joint, as shown in Figure 8b.