Development of Omnidirectional Collision-Detecting Bumper Using Noncontact Sensors

The omnidirectional mobile mechanism has high movement performance. We developed a prototype omnidirectional collision-detecting bumper using potentiometers for such a mobile mechanism. However, the previous prototype had low durability because it used contact-type sensors and a link mechanism to convert from the displacement of the bumper to the rotation of the potentiometers. In this paper, we propose a new prototype omnidirectional collision-detecting bumper using noncontact-type sensors. In an experiment, the new prototype showed higher accuracy than the previous one


Introduction
Omnidirectional mobile vehicles have been studied for a long time because of their high moving performance [1][2][3]. Omni wheels [4] and Mecanum wheels [5,6] are typically used because they are easy to implement, i.e., they can realize omnidirectional motion merely by replacing the wheels of existing vehicles. However, it is difficult for such wheels to negotiate steps, gaps, and on-slopes because their traveling ability depends on passive rollers. We have developed a ball wheel drive mechanism [7] that uses active-rolling balls as drive wheels, which showed higher climbing ability for steps and gaps and better control on slopes than the conventional omnidirectional wheel [8].
In the case of applying the omnidirectional mobile vehicle as a robot or an electric wheelchair, it is considered that it should have a system that detects and avoids obstacles in any direction to ensure the safety of the mobile vehicle. The conventional methods, for example, the omnidirectional vision system [9,10] and the method using a laser range finder (LRF) [11], are well known and provide accurate solutions but they require expensive sensors and time-consuming calculations. Other conventional methods using IR sensors and ultrasonic sensors [12,13] are less-expensive but they require many sensors and are have low robustness to distance noise.
In our previous study [14], an omnidirectional collision-detecting bumper (Omni Bumper) using three potentiometers was developed as a prototype low-cost collision-detecting device. Figure 1a shows an overview of the Omni Bumper with the potentiometers. The performance of collision detection by the bumper was shown in three degrees of freedom through a movement experiment. However, the bumper had a problem of low durability because it used a link mechanism to convert from the displacement of the bumper to the rotation of the potentiometers. Therefore, we developed a new Omni Bumper using non-contact sensor instead of the link mechanism, as shown in Figure 1b, to solve this problem. The new Omni Bumper uses PSD (Position Sensitive Detector) [15] sensors to measure the displacement of the bumper. In this paper, we present the details of the new bumper and experimental results.

Abstract
The omnidirectional mobile mechanism has high movement performance. We developed a prototype omnidirectional collision-detecting bumper using potentiometers for such a mobile mechanism. However, the previous prototype had low durability because it used contact-type sensors and a link mechanism to convert from the displacement of the bumper to the rotation of the potentiometers. In this paper, we propose a new prototype omnidirectional collision-detecting bumper using noncontact-type sensors. In an experiment, the new prototype showed higher accuracy than the previous one.

Noriyuki Shimpuku* and Hiroyuki Miyamoto
Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Japan The i x and i y obtained from Eq. (3) According to Figure 2b γ π γ π π γ π π = = + = + and . Thus, from Eq. (5), we have the following simultaneous linear equations containing three unknowns, Δx, Δy and Δθ. 1 1 These equations can be expressed as Where, 1 3 x A x y x y b=(ΔxΔyΔθ) T (11) If A -1 exists Δx, Δy and Δθ can be calculated as According to the positional relationship between the sensors shown in Figure 1b, det A=2u+v+w≠0.

Experiments Positioning device
It is important to evaluate the accuracy of the new Omni Bumper. In the experiment in our previous study [14], an external force was applied manually to the bumper. However, it is considered that conducting an experiment manually takes a long time and the result may contain human error. Therefore, we also developed an automatic positioning device to decrease human error and perform the experiment effectively. Figure  3 shows an overview of the positioning device and Table 1 shows the specifications of the positioning device.

Experiment to determine accuracy
Using the positioning device, the displacement detecting accuracy of the new Omni Bumper was evaluated. Figure 4 Table 2 shows the specifications of the omnidirectional mobile vehicle.

Experiment on collision avoidance
In this experiment, the omnidirectional mobile vehicle was moved forward by manipulating a joystick until the bumper collided with an obstacle. Figure 9 shows a flowchart of the collision avoidance algorithm and Figure 10 shows the result of the experiment. On the basis of the displacement of the Omni Bumper, the omnidirectional mobile vehicle stopped and then moved backward when there was no input via the joystick.

Discussion
It is considered that the new Omni Bumper produced better results than the previous bumper because the error due to the distortion of parts of the bumper was reduced. Although the new Omni Bumper, unlike the conventional methods, cannot detect multiple obstacles, this is not considered a problem because the result of collisions is manifested as a displacement of the bumper. Since it is intended to be used in indoor environments, the movable range of the bumper is considered sufficient. With the use of the positioning device, the total number of samples was increased tenfold and the total time taken to measure all axes changed from about 90 min to about 13 min.

Conclusion
In this study, we developed a new Omni Bumper with noncontact sensors as an Omni directional collision-detecting device. We also developed a positioning device to evaluate the accuracy of the new Omni Bumper. The experimental results show: • The new Omni-Bumper was accurate equal or more than the previous one.
• The omni-directional mobile vehicle with the new Omni Bumper could detect a collision with an obstacle and it stopped and went back according to the displacement of the bumper.
Our future work is to improve the accuracy and expand the measuring range of the Omni Bumper.  Actuator Robotis Dynamixel AX-12×5