A Tethering Device for Mobile Robot Guidance

Abstract

A new human and robot interface tool, so-called Navi-Guider, which makes it easy to handle mobile robots, is presented in this paper. The Navi-Guider is easily mounted on a mobile robot and is able to detect a length and a direction of the tether pulled out by a user. Those detected factors are utilized for the robot guidance and control. This paper addresses detailed hardware and software architecture of the Navi-Guider and demonstrates the practical usability of the system through actual experimental tests. The new device, Navi-Guider, is an intuitive control tool for moving mobile robots from a place to another place just by pulling the tether.

Keywords

Navi-Guider Mobile Robot Intuitive Control System Tethering System

1. Introduction

Although robot technology has been advanced significantly in the past decade, up until now traditional manipulator robots only have been used in the industry for some specified purposes. Recently the concept of intelligent robots has appeared and popularly studied by many academic researchers as well as by appliance industrial engineers. It is highly expected that human-friendly intelligent robots would be essential in the near future for health-care, home monitoring, and various human supporting purposes. It has been anticipated by a lot of researches (Hashima, M. et al, 1997; Tanaka, T. et al., 1997; Bischoff, R., 1997; Sato, M. & Kosuge, K., 2000) that the robot would support human life incredibly in a near future. However, unfortunately, there are many challenging issues in mobile intelligent service robot such as indoor localization, indoor navigation, pattern recognition, intelligent control, mobility control, smart decision, etc. With regard to mobility control, even though there are many theoretical approaches, a dominant practical concept has not been reported yet. In this paper, we introduce a mobility-guiding device for maintaining a certain relationship between human and robot, which is called “Navi-Guider” (Ahn, H. S. et al., 2006; Nah, S. I. et al., 2006; Na, S. I. et al., 2007). Specifically, in this paper, we describe an easy control scheme for Navi-Guider, which can provide a convenient interface between a mobile robot and a user. The Navi-Guider system helps the user to move the robot easily from a place to a place. The mobile robot could be controlled by a tether which is pulled out by the user. That is, Navi-Guider system is able to detect the length and the direction of the tether. Then it converts those factors into control signals that are used to maneuver the mobile robot. These converted signals are again transmitted to the mobile robot through the RS232 serial communication cable. The wheels of the mobile robot are then controlled based on these received signals. Thus the mobility-guiding device introduced in this paper is a kind of tethering system for mobile robot control.

From a literature search, however we find that the concept of tether-based robot guidance is not new. In fact, the concept of tether-based robot control has been introduced in a number of publications. In particular, we can see that the idea of using tether for a robot control was given in (Sinden, F. W., 1990) for the first time. In (Sinden, F. W., 1990), a problem of not tangling multiple tethers was discussed. In (Xavier, P. G., 1999), a shortest path planning when a robot is connected by a tether was discussed, and in (Fukushima, E. F. et al., 2000; Hirose, S. & Fukushima, E. F., 2002), some ideas of using tether for robot guidance were introduced. However, a technical detail was not described in those publications. In (Rekleitis, I. et al., 2001), a virtual tether concept was introduced for the first time for a map construction. In (Hert, S. & Lumelsky, V., 1999), a similar tethering problem as (Sinden, F. W., 1990) was solved in three dimensional space, and in (Krishna, M. et al., 1997; Choi, S. K. et. al., 1995), applications of tether for a ground robot and an under-water robot were presented. Specifically, a concept of robot cooperation using tether was introduced in (Choi, S. K. et. al., 1995), which is very similar to the cooperation of cliff-robots. However, all publications given above do not provide technical details in terms of actual hardware development and actual experimental implementation. Furthermore, problems associated with the indoor intelligent robot navigation have not been addressed in existing publications. In this paper, we provide an overall concept of tethering robot systems, hardware components used in the Navi-Guider, and control algorithms used for guiding the tethered-robot. This paper consists of as follows. In Section II, the concept of Navi-Guider system is explained. In Section III, configuration of the Navi-Guider system is presented. In the Section IV, implementation of Navi-Guider is briefly described. Experimental test results will be given in Section V and conclusions are given in Section VI.

2. The Concept of The Navi-Guider System

The human and robot interaction is important in closely performing a certain amount of cooperative works. From a practical point of view, man in the control loop is reliable and desirable in some case, because as of now it is far away from present technology to design a fullyautonomous mobile intelligent system. The so-called Navi-Guider system was developed for easily handling the mobile robot when it cannot make own guidance command. The Navi-Guider is an innovative device of maintaining a certain relative relationship between a human and a robot. A mobile robot can work as an assistant for human in various situations. For instance, if the user requires the robot to move towards a particular workplace, then a fully intelligent robot can perform the task easily like a human. However, unfortunately this is not the case in current technology, because the robot lacks recognition and decision capabilities. Thus, man in the robot control loop is necessary in certain cases and for a reliable mobility control. To maintain this relationship (i.e., man in the control loop), the control of the mobile robot should be simple and easy. The Navi-Guider system is an easy control tool to move the robot. If the user wants to move the robot, he just pulls out the tether of the Navi-Guider system. A length and a direction of the tether pulled-out are measured and converted into available digital values, and then these data is transmitted to the mobile robot. Using these data, the wheels of the mobile robot are controlled. Thus, the user could easily move the robot to his/her intention. As shown in Fig. 1, the user pulls out the tether of Navi-Guider that is equipped on the mobile robot. The length of the tether pulled-out from Navi-Guider by a user is measured by the sensor. Let the length pulled-out be l and let the angle of the tether pulled-out between the orientation of the mobile robot and the direction of the tether pulled-out be θ. The measured data is an electric signal, and they are converted into available digital values. The converted values are then utilized for the control of the mobile robot. The moving and rotating speed of the mobile robot is controlled by these measured values. The bigger l and θ are, the faster the moving and the rotating speed of the mobile robot will be.

Fig. 1.

Navi-Guider, Which is kind of tethering device for mobility control of mobile robot

3. The System Configuration of Navi-Guider System

The Navi-Guider system provides the user with a convenience in moving the robot. The Navi-Guider is composed of two different systems: a sensing system and a control system. The purpose of the sensing system is to measure the length and orientation of the tether pulled-out, and the purpose of the control system is to provide commands to the robot wheels. There are two main design issues:

Interface between the user and the guiding-system: A user, who might not be familiar with robot and/or hardware operation, should handle the guiding-system easily.

Interface between the guiding-system and the robot: The system should have a plug-and-play function for interfacing with various robot platforms.

The control system is to solve these two design issues. That is, the controller should provide mechanical and electrical interfaces between the user and the guiding-system, and the guiding-system and the robot such that the robot follows well the user's trajectory. The sensing system is composed of SAM7SXX Mini Board, rotary encoder, and rotary potentiometer. The main processor of SAM7SXX Mini Board is ARM7TDMI ARM Thumb. Currently, the sensing system is connected to the pioneer robot through the RS232 serial port. The mechanical hardware components of the sensing system are reel and reed, which are used for winding the tether. The control system receives data measured from the sensing system and uses it for the speed control of wheels.

3.1. The Sensing System

The sensing system of the Navi-Guider consists of two components. The first one is a mechanical system and the other one is an electric system. The role of the mechanical system is to wind the tether using reel and reed, and to detect the length and the direction of the tether pulled-out using the encoder and potentiometer. The processor converts the physical values into the available digital values and transmits those to the mobile robot through RS232 serial port. In what follows, we will describe those components.

3.1.1 The Mechanical Configuration of Navi-Guider System

The Navi-Guider measures two attributes, which are the length and the direction of the tether pulled-out. As shown in Fig. 2, to implement the Navi-Guider system, we use two mechanical parts; reel and reed. The top picture of Fig. 2 shows the configuration of the reel and reed. The primary role of the reel is to wind and unwind the tether. Using the angle of the rotations of the wound reel, we can calculate the length of the tether pulled-out. The diameter of the reel is 5 cm and the maximum number of the rotations is fourteen. Thus, the tether is wound and re-wound by 210 cm smoothly. To measure the direction of the tether pulled-out, we use the rotating reed that is fixed on the shaft, and the tether is passed through the hole in the reed. Therefore, the reed rotates the direction of the tether pulled-out. In the Navi-Guider system produced, the maximum rotation angle of the reed is from −30 to 30 degrees. The bottom picture of Fig. 2 shows the configuration of the sensors. The rotation axis of the rotary encoder is firmly connected to the reel's rotation axis and the rotation axis of the potentiometer is firmly connected to the reed rotation axis. Thus, those sensors could measure the rotation angle of the reel and reed by the tether pulled-out.

Fig. 2.

The mechanical configuration of Navi-guider

3.1.2 The Electrical Configuration of Navi-Guider System

The Navi-Guider uses two sensors to measure the length and the direction. Such sensors convert the physical values into the electrical values. As shown in Fig. 3, the component of the length is converted into electrical pulse signal by the encoder. The potentiometer converts the direction of the tether pulled-out into the analog electric signal. To use those values in the control system of the mobile robot, the electric signals are reconverted into available digital values by a processor. The converted digital values are transmitted to drive the mobile robot through RS232 serial port in the processor.

Fig. 3.

The configuration of the sensing system

3.2. The Control System

As described in the previous subsection, there are two sensing components in the Navi-Guider. A rotary encoder is used for measuring the length of the tether pulled out, and a rotary potentiometer is used for measuring the orientation of the tether. The job of the Navi-Guider control system is to make a command for a wheel speed control such that the mobile robot can follow the user's trajectory smoothly. The turn of robot is achieved by making the left wheel and the right wheel rotate with different speeds. Fig. 4 shows the overall signal flow from the potentiometer/encoder to the motor control board. The rotary encoder provides, based on the rotation of solid shaft, a voltage output corresponding to the length of the tether. The rotary potentiometer used in the Navi-Guider is M22 series of Honeywell. The microprocessor mounted in the Navi-Guider calculates the length and the orientation of the tether pulled out. The measured signals are transmitted to the onboard computer through RS232. Then, based on these signals, the onboard computer generates wheel speed commands to turn the robot direction and to follow the user's trajectory as quickly and stably as possible.

Fig. 4.

The signal flow-chart in the Navi-Guider control system

Fig. 5 depicts the block diagram of the Navi-Guider control system. The Navi-Guider controller consists of one open-loop forward controller and one closed-loop feedback controller. The open loop controller is to provide an accurate wheel speed commands so that the mobile robot follows the desired trajectory with a less error. The feedback controller controls the motors so that the wheels speeds are feedback-controlled. Fig. 6 shows the mobile robot guided by the Navi-Guider. Denoting the left and right wheels' speeds by V_L and V_R, we calculate them by

V_{L} = V + Δ V_{L}

(1)

V_{R} = V + Δ V_{R}

(2)

where V is the nominal velocity, and ΔV_L and ΔV_R are additional velocities of the left and the right wheels. Now, it is necessary to decide V, ΔV_L and ΔV_R such that the robot follows the desired trajectory. Here, we know that the nominal velocity, V, of the mobile robot is determined by the length, l, of the tether, and additional velocities ΔV_L and ΔV_R are determined by orientation angle 9 Therefore, since the nominal velocity of the robot is proportional to the length of tether, it can be determined by V = k₁ · l. Similarly, the additional velocities can be determined, when θ ≥ 0, by the following formula:

Δ V_{L} = k_{2} θ

(3)

Δ V_{R} = 0,

(4)

and when θ ≤ 0, by the following:

Δ V_{L} = 0

(5)

Δ V_{R} = k_{2} | θ |

(6)

Fig. 5.

The Block diagram of the Navi-Guider control subsystem

Fig. 6.

Mobile robot guided by the Navi-Guider

However, a simple idea given above will be only valid when |θ| is small. That is, when |θ| ≫ 0, it is better to take the nominal velocity as zero (or very small), because it is not possible to rotate the robot when moving fast. Thus, we need to consider two different cases. When |θ| ≤ θ^*, the wheel speeds are determined by the above equations. However, when |θ| > θ^*, then the wheel speeds are determined by

V = k_{3} (l - k_{4} | θ |)

(7)

and the additional velocities are calculated as ΔV_L = v_c when θ ≥ 0, and as ΔV_R = v_c when θ < 0. The next problem is to determine k₁, k₂, k₃, k₄, v_c, and the upper boundary, θ^*, of the orientation angle. There is no particular way to determine k₁, k₂, k₃, k₄, v_c, and θ^*. However, k₄, can be determined by some rules given as follows:

k_{4} (l) = \frac{l}{θ_{max}}

(8)

where θ_max is the maximum angle the tether can deflect. From experimental tests, however, although the above algorithm has been effective for the obstacle avoidance and smooth movement, there have been some exceptions cases. As depicted in Fig. 7, the actual trajectory is usually deflected from the desired trajectory. In such cases, although the system is still acceptable from an operation point of view, when the system is deflected from the desired trajectory a lot, the robot would collide to obstacles. Thus, in the Navi-Guider system, we have considered two cases, which are differentiated based on the sonar measurement such as:

When there is no detection of obstacle from sonar observations, the robot is guided by the above algorithm.

However, when obstacles are detected, the robot stops at the turning point. Then, after turning the robot heading direction by amount of θ, the robot follows the tether pulled-out by the user. Thus, when detecting obstacles, the nominal velocity is enforced to zero.

Fig. 7.

Desired trajectory and actual trajectory

For computing the control signal, we are using a simple PID controller with a sampling time ΔT, given as

u (k) = K_{P} e (k) + K_{I} s a t (e (k)) + K_{D} \frac{e (k) - e (k - 1)}{Δ T}

(9)

where K_P, K_I, K_D are PID control gains, and sat(e(k)) = e(k) if |e(k)|< e^* and sat(e(k)) = sign(e(k))e^* if |e(k)| ≥ e^*. For determining control gains, an automatic tuning method such as Ziegler-Nochols tuning was used.

4. The Implementation of Navi-Guider System

The Navi-Guider system produced is shown in Fig. 8. The system is composed of a potentiometer, a rotary encoder and a processor. The potentiometer used in the Navi-Guider system is M22 series of Honeywell, and the rotary encoder used is S30 Series of the Metronix. The processor used is SAM7SXX Mini Board. The bottom picture of Fig. 8 shows the implementation of the guiding system on the mobile robot. The developed system could transmit signals to the control system that is implemented on the laptop computer 30 times per second using the RS232 serial communications. Using the transmitted signal from Navi-Guider, the control system controls the wheel velocity of the robot.

Fig. 8.

Implementation of the Navi-Guider system

5. The Experimental Result

We tested the guiding system produced using Pioneer robot of ActivMedia robotics. The experimental tests were performed using two robots to see whether a follower robot follows the trajectory of a leading robot. As shown in Fig. 9, Navi-Guider is set on robot 2 and the wire of the Navi-Guider is connected robot 1. To control and move robot 2, robot 1 moves around the experimental task platform with Navi-Guider. The robot 2 moves to the direction and with a velocity according to the direction and length of the tether pulled by the robot 1. In order to acquire the trajectory of two robots, we use the localization sensor that is called StarLITE (Chae, H. S. et al., 2006). Fig. 10 shows the experimental results comparing the trajectory of the robot 1 with robot 2. The blue line is the trajectory of the robot 1, and the red one is trajectory of the robot 2. As a result, we can see that the trajectory of the robot 2 is quite similar to the robot 2. Also we can see that trajectory of the robot 2 is smooth.

Fig. 9.

Navi-Guider test: trajectory following

Fig. 10.

Trajectory the robot using Navi-Guider

6. Conclusion

In this paper, we presented an easy and intuitive control tool that is called Navi-Guider. The Navi-Guider could guide and maneuver a mobile robot using the length and direction of the tether which is pulled-out by the user. This system is simply implemented on a chip which is composed of a rotary encoder, a potentiometer and a processor. The operation of the Navi-Guider is so simple that it is suitable for handling human friendly mobile robots. Also, since the Navi-Guider is a plug-and-play device, it can be attached to various mobile robot platforms. For the verification of the performance, we implemented the guiding system on Pioneer robot of ActivMedia robotics. From numerous experimental tests, it was revealed that the Navi-Guider works reliably and favorably.

Footnotes

7.

This research was supported by the Home Service Robots Project of the Ministry of Information and Communication and the National Research Laboratory Program of the Ministry of Science and Technology in the Republic of Korea

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