Detection system of autonomous mobile robot based on ultrasonic sensor

An extension of the ultrasonic sensor is a good supplement to the existing detection system of mobile robots. It has been fully demonstrated in experimental applications, and it has certain practicality in obstacle detection and robot pose adjustment. However, this method needs to be further improved in real-time and accuracy.

One of the most important ultrasonic level sensor for mobile robots to acquire autonomous behavior is to acquire knowledge about the environment. This is achieved by using different ultrasonic sensor measurements and extracting information from those measurements. Sensors such as vision, infrared, laser, and ultrasonic have all been used in mobile robots. Ultrasonic sensors have been widely used in mobile robot sensing systems due to their high cost performance and simple hardware implementation. However, ultrasonic sensors also have certain limitations, mainly due to the large beam angle, poor directivity, and instability of distance measurement (under non-vertical reflection). Therefore, multiple ultrasonic sensors or other sensors are often used to compensate. In order to make up for the deficiencies of the ultrasonic sensor itself and improve its ability to obtain environmental information, this paper designs a detection system composed of an integrated ultrasonic sensor and a stepping motor.

1 Analysis of the detection principle and method of ultrasonic sensors

The basic principle of an ultrasonic sensor is to send (ultrasonic) pressure wave packets and measure the time taken for the wave packets to be transmitted and returned to the receiver.

Among them, it is the distance between the target and the ultrasonic sensor; c is the ultrasonic wave speed (in order to simplify the description, the influence of temperature on the wave speed is not considered when it is measuring the distance discussed below; t is the time interval from emission to reception.

Because measuring distance with ultrasound is not a point measurement. Ultrasonic sensors have certain diffusion characteristics. The emitted ultrasonic energy is mainly concentrated on the main lobe, and attenuates in a wave-like shape on both sides of the main wave axis, with a diffusion angle of about 30° left and right. In fact, the calculation method of formula over time is based on the successful, vertical reflection of ultrasonic waves. However, it is difficult for a mobile robot to ensure the stability of its own motion posture. The detection method which an ultrasonic sensor is fixed on the body of the mobile robot is used. When the mobile robot deviates from a parallel wall, the detection system is often difficult to obtain the actual distance. In addition, when the divergence characteristic of ultrasound is used to measure obstacles, it can only provide the distance information of the target obstacle, but not the direction and boundary information of the target. These defects greatly limit the practical application and promotion of ultrasonic sensors.

Based on theoretical analysis and continuous testing, this paper uses a four-phase stepper motor to drive a single integrated ultrasonic sensor to rotate to form a dynamic sensing system.

2 Detection system is composed of integrated ultrasonic sensor and stepper motor

2.1 Structural design

The ultrasonic sensor is welded on the PCB board, the board is built up by a steel pipe, and the other end of the steel pipe is connected to the shaft of the stepping motor, and the stepping motor is fixed under the robot chassis. The ultrasonic sensor control signal and output signal which are connected to the control board on the vehicle body through the signal line. In addition, a cone-shaped sleeve made of foam material is added in front of the probe of the ultrasonic sensor, the diameter of the upper mouth is 22 mm, the diameter of the lower mouth is 16 mm, and the height is 20 mm. In this way, the beam angle of the transmitted wave and the angle at which the reflected wave is received are greatly restricted. In order for the robot to adjust its posture, it needs to determine its own rotation direction and reference position. Therefore, a simple photoelectric encoder composed of a direct infrared photoelectric sensor and a turntable is made by ourselves. The distribution of 2 direct infrared photoelectric sensors is shown and they are horizontally arranged on the midpoint connecting line on both sides of the robot car body at 180° intervals. The turntable and the rotating arm are connected on a concentric circle, as shown by the outer circle in the figure, the 1, 3 scale lines are separated by 27°; the 2, 1 scale lines are separated by 180°, and the 1 scale line and the center of the ultrasonic sensor are kept on the same horizontal line. I alone conduction is used as the reference coordinate, I and II are simultaneously guided to determine the direction of rotation, and Ⅱ single pass is used as the navigation reference when the robot returns along the wall.

The integrated ultrasonic sensor is driven to rotate by a stepping motor, and the direction of the ultrasonic sensor's central axis perpendicular to the robot body is used as the coordinate reference for its own posture adjustment. The stepping motor adopts a 4-phase 4-beat step angle of 1.8°, and 1 step per revolution,The ultrasonic sensor detects once, and sends the measuring value to the upper computer through the serial port.

2.2 Detection system hardware design

The detection system hardware is mainly composed of ultrasonic generating circuit, ultrasonic receiving circuit, stepper motor speed control module, etc. The core of the system is the single-chip , which mainly completes the signal transmission and reception, controls the stepping motor, and transmits data to the robot host computer for processing.

The ultrasonic transmitter circuit uses the P11 port of the single-chip to output the transmitter pulse, and is driven by the 74HC04 to connect the ultrasonic sensor. They  enhance its output current capability and increase the transmission distance of the ultrasonic sensor.

The ultrasonic receiving and processing circuit adopts integrated circuit. it is a dedicated integrated circuit for infrared receivers. Here CX20106 is used as an amplifying and detecting device for receiving signals from ultrasonic sensors, and good results have also been achieved. After the pre-amplifier receives the reflected signal from the ultrasonic receiving probe, it amplifies the signal with a voltage gain of about 80 dB. Then the signal is sent to the limiting amplifier to make it into a rectangular pulse, and then the frequency is selected by the filter to filter out the interference signal, the carrier frequency is filtered out by the detector to detect the command signal, and after shaping, it is output by pin 7 low level. The falling edge of the pulse output from pin 7 is input through the INT0 port of the microcontroller.

The transmitter circuit and the receiver circuit of the integrated ultrasonic sensor use the same sensor pin input/output. If the input/output is not isolated, the receiver circuit and the transmitter circuit will be greatly affected. The CMOS bidirectional analog switch is used to realize the transmission and reception isolation. The stepping motor control module adopts the control mode of ring pulse distributor L297 + double H-bridge power integrated circuit L298. P1.6, P1.7, and P2.3 of the single-chip microcomputer are respectively connected to the CW, clock, and enable control terminals of L297 to control the forward and reverse rotation, clock signal, start and stop of the motor.

2.3 Detection system software design

The software of the detection system is mainly composed of a main program module, an interrupt service program module, and a ultrasonic sensor has transmitting and receiving module. The main program module of the detection system is mainly explained here.

Ultrasonic sensor and stepper motor measurement and control modules are controlled by different single-chip microcomputers, so the sensing system and the upper computer of the mobile robot must rely on the I/O port line and serial asynchronous communication between the single-chip microcomputers. The flag T is used to switch actions. When T=0 and OFF=0 are satisfied at the same time, it is an ordinary detection process of ultrasonic sensors; when T=1, OFF=0, it is used to adjust the azimuth before each cycle measurement; OFF=1 Is waiting for the next action. The timer T0 is used to calculate the time of the echo, so the distance value d=0.334×(TH0×256+TL0)/2. one trigger pulse is given to the stepper motor. Then determine whether the next action is to do sensor detection or to adjust the azimuth angle of the robot itself, which enters a new cycle.

3 Experiment and application of detection system on mobile robot

3.1 Find the closest point to the wall

In this paper, the design idea of finding the closest point to the wall is based on ultrasonic ranging. Selecting the time-level distance measurement method, and limit the receiving range of the ultrasonic sensor by setting the receiving echo threshold and adding a sound-absorbing sleeve before the probe. The measured beam angle is about ±20° at a

distance of 75 cm, and the effective angle that can receive reflected waves is about ±40°.

The approximate conical beam of the ultrasonic sensor determines the reflection distance of the closest point every time it measures the distance. Even if the beam angle deviates to the dotted line, the actual distance is still the value measuring along the beam centerline. Theoretically, the distance measured within the transmitting beam angle should be the same, but the shock time of the ultrasonic sensor and the setting of the receiving threshold, including the reflection of the wall, will have a certain impact on the distance measurement. Measured by experiments, within a certain angle (approximately ±20°), the measuring distance value does not change significantly, and its neighboring values are relatively close (no more than 2 mm). When the deflection angle continues to increase, the changes in adjacent measurement values also increase significantly. Therefore, one method is to use these two critical points to find the angle between the beam and the wall (that is, the closest point to the wall), and the stepper motor drives the ultrasonic to rotate to find these two critical points. When two adjacent values are continuously detected below 2 mm, it is considered that it has entered the stable zone, and the point where the change occurs before and after is set as the critical point. All points within this critical point are recorded, and then the midpoint is calculated. The midpoint is the closest point between the wall and the ultrasonic sensor. it shows a set of measured data. Within 72°～108°, it is the stable area of distance measurement. Outside of this, the adjacent deviation of the measured distance exceeds 8 mm, and with the angle It will be further enlarged when turned to both sides. Experiments were performed by changing the distance between the integrated ultrasonic sensor and the wall within 50 cm and 200 cm. As a result, the measured error of the vertical angle to the wall was limited to 2 step angles.

3.2 The detection system is applied to the robot to navigate along the wall

Autonomous mobile robots detect information about the current environment during movement. The distance information detected each time is measured on the premise of the current robot motion posture. While is walking in a straight line along the wall, the robot guarantees the accuracy of its trajectory through the joint perception of distance measurement and its own posture. Ultrasonic distance measurement has been widely used. After testing the relationship between ultrasonic detection angle and distance measurement, ultrasonic sensors can be used to measure the azimuth angle of the vehicle body (to determine its own posture) according to the method of calculating the closest point. The measured closest point is the actual distance between the robot and the wall. The reference coordinates of the robot are determined by the direct infrared sensor 1 on the simple encoder, and the closest point is calculated according to the information stored during each step of the stepping motor. Between the reference coordinates and the closest point, the angle traversed by the stepper motor is used to determine the deflection angle between the robot and the wall, and then the deflection angle is transmitted to the wheel drive control system to adjust the azimuth angle.

3.3 Search for obstacles

The use of a stepping motor to drive the industrial ultrasonic sensor

 to rotate is functionally similar to multi-sensor detection. Mobile robots usually use multiple ultrasonic sensors around the body to obtain more information, thereby increasing the range of obstacles and determining target direction and boundary information. In contrast, one advantage of the rotation method is that the detection density can be automatically adjusted according to the tightness of the obstacle. The number of additional sensors is limited by its own conditions, and the tightness of the rotation method is only related to the step angle of the stepper motor. The increase of the detection density can greatly improve the resolution of the angle, thereby strengthening the determination of the target direction and boundary information.

This system is an extension of the function of the ultrasonic proximity sensor and a good supplement to the existing detection system of mobile robots. It has been fully demonstrated in experimental applications, and it has certain practicality in obstacle detection and robot pose adjustment. However, this method needs to be further improved in real-time and accuracy.