To date several methods proposed to estimate the position of a mobile robot using ultrasonic sensors, one of them is using two artificial landmarks to determine the mobile robot‟s current position and orientation. In (Feng-ji et al., 1997), sixteen sonar sensors mounted around the mobile robot are used to detect obstacles. The purpose was to provide an estimation of the robot‟s absolute position and orientation in its working space. Using map matching from sonar sensors and matches it to the real working place map. Error in position and orientation from this system was 10cm and 10 deg.
Ghidary (1999)developed home robot localization system; the localization method utilizes ultrasonic and infrared signals simultaneously. The transmitter, which is mounted on the mobile robot, transmits both ultrasonic and infrared signals at the same time. The receivers are located at fix points in the ceiling of the room use the received infrared signal as a trigger to measure The Time Of Flight (TOF) of ultrasonic waves. The location of robot is computed by measuring its distance from three receivers. The heading of the robot is computed by measuring its position in two successive points while the robot moves from one point to the other. The performance and validity of this system are evaluated using one transmitter and six receivers located in a 6m x 4m room. Error in positioning was less than 5 cm in X and Y position.
Shoval (1999) used ultrasonic sensors for the measurement of angular position of a mobile robot relative to a known ultrasonic source. In this work proposed using of phase difference measurement to determine a mobile robot‟s angular position relative to a known ultrasonic source. Two ultrasonic sensors attached to the sides of the robot, and serve as receivers. A third sensor is positioned
18 at an off-board location. This sensor only transmits signals at a rate controlled by the mobile robot (control and synchronization can be achieved by radio link but in our bench top set-up a wired link was used). When the transmitting sonar fires an ultrasonic wave, the two receivers are set for “listening mode” ready to receive the signals. A fast measurement system calculates the phase difference between the received signals in both sonar, where the magnitude of the phase difference is
proportional to the difference of the Euclidean distance between the receivers and the transmitter.
Based on ultrasonic sensory information, an approach is proposed for localization of autonomous mobile robot (AMRs). In the proposed method, it is proven that the combination of three ultrasonic transmitters and two receivers can determine both the position and the orientation of an AMR with respect to a
reference frame uniquely. In this manner, since only ultrasonic sensors are used, the proposed method will be highly cost-effective and easy to implement (Wu & Tsai, 2001). Such localization system not possible to be used in real applications because; the transmitters installed very near to each other only 20 cm apart which mean small coverage area. The experiment done in static mode the robot was not moving and that is not the case in mobile robot applications.
Shoval (2001) presented a method for measuring the relative position and orientation between two mobile robots using a dual binaural ultrasonic sensor system. Each robot is equipped with a sonar transmitter that sends signals to two receivers mounted on the other robot. It is assumed (but not implemented in this system) that the two robots can synchronize events with an infrared or radio link. The receivers measure the distance to the transmitter on the other robot, and a geometric model determines the relative position and orientation of the two robots based on a combination of the data from all four receivers.
In Shoval (2001), experimental results show the accuracy and operational limitations of the technique. The described method can assist multi-robot systems where cooperation is required in terms of relative position and orientation. In addition, the system can be implemented in tasks where a single robot is required to position itself accurately against other point. For example, a stationary
transmitter/receiver setup can be installed in docking stations, doorways or narrow passageways to guide the robot to the desired position and orientation regardless of the accuracy of its other positioning systems. The simplicity of operation and the low
19 cost for the obtained accuracy make the system advantageous for implementation is multi-robot system or in industrial environments.
In Hwang et al (2006) indoor GPS system using ultrasonic sensor has been built, this system consists of one transmitter having ultrasonic and RF and two receivers. The transmitter irradiates RF and ultrasonic, and the receivers calculate corresponding distance with reference to the RF signal. The two distance values are used for determining the location of the transmitter by trigonometrical functions. Due to the characteristics of ultrasonic sensors, noise occurs in sensor values by surrounding temperature or obstacle. Location error is minimized by prediction and correction of the noise with Linear Kalman Filter. In order to prove the effectiveness of this system, experiments were carried out in a room dimensioned by 3.5m*2.2m, wherein the location error showed 2cm max in x and y position.
Ultrasonic GPS has high positioning accuracy for indoor applications with the fact that, reflection of ultrasonic wave in case of dynamic environment leads to error in positioning. Moreover, coverage area for most ultrasonic transducers is only 2-meter; as a result, ultrasonic GPS to be used in factories or large warehouses need to spread large number of ultrasonic transmitter modules as well. Using large number of ultrasonic transmitters has health drawback on labors in the place and increasing the positioning system cost.