Abstract:
A mobile robot includes: a main body; a drive unit that moves the main body; a contact force detecting unit that detects a contact force acting against an obstacle; and a controller that controls the drive unit to move the main body toward a target object under a condition that the contact force detected by the contact force detecting unit is in a predetermined range.

Description:
RELATED APPLICATION(S) 
     The present disclosure relates to the subject matters contained in Japanese Patent Application No. 2007-187241 filed on Jul. 18, 2007, which are incorporated herein by reference in their entirety. 
     FIELD 
     The present invention relates to a mobile robot and a method for controlling the mobile robot, for safely moving toward a target in a situation, such as in a crowd of people, where being bumped with surrounding people and obstacles. 
     BACKGROUND 
     In a conventional mobile robot, there is used a control method to basically stop moving or to largely avoid a course of movement when people are around the robot while the robot is attempting to follow a target object, thereby securing a safety of the people. 
     An example of such technique is disclosed in JP-A-2006-146491. 
     However, conventional control method is nearly store, which follows a person through a crowd of people. 
     SUMMARY 
     One of objects of the present invention is to provide a mobile robot and a controller for the mobile robot, for safely moving toward a target in a situation where being bumped with surrounding obstacles including people, by controlling contact force between the mobile robot and the obstacles. 
     According to an aspect of the invention, there is provided a mobile robot including: a main body; a drive unit that moves the main body; a contact force detecting unit that detects a contact force acting against an obstacle; and a controller that controls the drive unit to move the main body toward a target object under a condition that the contact force detected by the contact force detecting unit is in a predetermined range. 
     According to another aspect of the invention, there is provided a method for controlling a mobile robot, wherein the mobile robot includes: a main body; a drive unit that moves the main body; and a contact force detecting unit that detects a contact force acting against an obstacle, and wherein the method includes: receiving the contact force from the contact force detecting unit; and controlling the drive unit to move the main body toward a target object under a condition that the contact force is in a predetermined range. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a block diagram illustrating a controller of a mobile robot according to an embodiment of the present invention; 
         FIG. 2  is a schematic view illustrating an example of a contact force detecting mechanism according to the embodiment; 
         FIG. 3  is a schematic plan sectional view illustrating a configuration of the contact force detecting mechanism shown in  FIG. 2 ; 
         FIG. 4  is a concept diagram illustrating a target person following action and a contact force compliant motion according to the embodiment; 
         FIG. 5  is a diagram illustrating an impedance model defining characteristics in action of the mobile robot at the time of bumping into surrounding people; and 
         FIG. 6  is a flowchart illustrating a composite action of a following action and a contact force compliant motion in the mobile robot. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, a mobile robot and a controller for the mobile robot according to an embodiment of the present invention will be described with reference to the drawings. 
     In the following description, it is assumed that the mobile robot is used in a situation where the mobile robot follows a person leading at a predetermined distance in a retail store. The mobile robot is configured as a shopping cart robot that is used by a customer who walks and places a picked-up merchandise into the shopping cart robot. 
     As shown in  FIG. 2 , a mobile robot  1  includes a robot body  2  and wheels  3  attached to the robot body  2 , and the mobile robot  1  is movable in any direction on a floor. Contact force detecting mechanisms  4  are disposed around the robot body  2 . The contact force detecting mechanisms  4  are preferably attached to a plurality of upper and lower positions of the robot body  2 , to cope with high and low of surrounding people. A controller  30  is provided in the robot body  2 . The controller  30  controls the wheels  3  to be driven based on an output of the contact force detecting mechanism  4 . 
     For example, the wheel  3  may be used as a omni directional wheel like a spherical wheel. And the wheel  3  may be used as a single directional wheel. 
       FIG. 3  shows a configuration of the contact force detecting mechanism  4 . A three-axis force sensor  5  fixed to a center shaft of the robot body  2  is disposed at the center of the contact force detecting mechanism  4 , and four radial members  6  extending from the three-axis force sensor  5  in a radial direction are disposed. A ring member  7  is disposed to connect leading ends of the radial members  6  in a circumferential direction. The ring member  7  is disposed to protrude from an outer periphery of the robot body  2 . The radial members  6  are disposed to penetrate the robot body  2  at an opening formed on the robot body  2 , and transfer a contact force applied from the ring member  7  to the three-axis force sensor  5  without interference from the robot body  2 . 
     The three-axis force sensor  5  detects two-dimensional forces FX and FY on the plane and a moment MZ, for example. A “6 DoF force/torque sensor system (Product Name)” that is available on the market from Nitta Corporation may be used as the three-axis force sensor  5 . 
     A touch force sensor (now shown) that detects contact forces f 1  to f 6  with a person is surrounded around the whole ring member  7 . The three-axis force sensor  5  detects a composite force applied to the mobile robot  1 , while the touch force sensor detects which portion of the mobile robot  1  is contacting with a person with how much force. The composite force applied to the mobile robot  1  may be calculated using information of a torque sensor provided at a driving shaft of the wheel  3 . 
       FIG. 4  shows a situation in which the mobile robot  1  follows a target person  20  who leads the mobile robot  1 . Generally, the mobile robot  1  is controlled to move to keep a constant distance from the target person (leading person)  20 . However, when the mobile robot  1  cannot find a movable space for the mobile robot  1  toward the target in the crowd of people, the mobile robot  1  detects a contact force with surrounding people  21  and advances to move forward with an appropriate contact force. 
       FIG. 1  shows a control block diagram of the controller  30  of the mobile robot  1 . First, a distance from a leading target person  20  is detected by a main feedback loop, and a velocity of the mobile robot  1  is controlled to keep the distance in a constant range. When the mobile robot  1  is in contact with the surrounding people  21 , the controller  30  generates a velocity vector for realizing a compliant motion (impedance model control) to reduce the contact force and combines the generated velocity vector with a velocity control system that keeps a constant distance with the target person. 
     The controller  30  sets a target value of a predetermined contact force in accordance with a physical size of the surrounding people  21 , and the mobile robot  1  advances while safely being in contact with the surrounding people  21  based on the compliant motion of the impedance (spring, mass, and damper) model defining characteristics in action at the time of contact. 
       FIG. 5  shows an impedance model defining characteristics in action at the time of contact. Specifically, a filter G(s) represented by a quadratic transfer function represented by the following equation (1) may be realized.
 
 G ( s )=1/( Ms   2   +Ds+K )  (1)
 
     In the equation (1), M denotes a mass, D denotes a damper, and K denotes a spring constant. 
     An output at the time of inputting an external force Δf represented by the following equation (2) is composed to a constant distance keeping control system and a velocity instruction to the mobile robot may be generated.
 
Δ f =TARGET CONTACT FORCE  fr −CONTACT FORCE  f   (2)
 
     When a contact force or an acceleration exceeds an (predetermined) upper limit, the spring constant of the impedance model is changed to zero (or small value) and thus a safe control to put a body in the crowd of people is performed. The upper limit of the contact force and the mass constant (upper limit of acceleration) of the impedance model is changed with a weight of a contact person. In case of bumping into a plurality of people, the upper limit of the contact force and the mass constant of the impedance model is changed with the weight of the lightest person. 
     The position of the target person  20  may be obtained by use of a radio tag, an image captured by a camera, a distance sensor, or the like (not shown). The physical size (weight or hardness of body surface) of the surrounding people  21  may be obtained by use of a radio tag, on-line data detected by a robot, or the like. 
     The controller  30  may control the movement of the mobile robot  1  as follows. When the contact force exceeds a first critical value, the mobile robot  1  is stopped. When the contact force is larger than a second critical value (first critical value&lt;second critical value), an avoiding action of moving the mobile robot  1  in a direction of an external force that generates the contact force is performed. Accordingly, it is possible to improve the safety of the mobile robot  1 . 
     The controller  30  may control the movement of the mobile robot  1  as follows. An acceleration of the mobile robot  1  is detected or calculated. When the acceleration exceeds the first critical value, the moving robot  1  is stopped. When the acceleration is larger than the second critical value (first critical value&lt;second critical value), the moving robot  1  is driven in a direction of the external force. Accordingly, reckless driving of the mobile robot  1  is suppressed, and the external force applied to the mobile robot  1  is attenuated, thereby preventing the mobile robot  1  from being destroyed. 
       FIG. 6  shows a flowchart for generating a velocity instruction to the mobile robot  1  by composing the aforementioned following action and the contact force compliant motion. That is, a position of the target person is detected (Step S 1 ), and then a constant distance keeping velocity control operation is carried out (Step S 2 ). In case where the velocity composition is not necessary, the velocity instruction is commanded (Step S 3 ). In case where the velocity composition is necessary, the contact force is detected (Step S 4 ), constants of the spring, mass, and damper are input (Step S 5 ), a contact force limitation value is input (Step S 6 ), and then an impedance model (filter) operation is carried out (Step S 7 ). A velocity vector operation is carried out on the basis of the result (Step S 8 ), and then a velocity composition with the result of the constant distance keeping velocity control operation is carried out (Step S 9 ). A velocity instruction is commanded to the mobile robot  1  (Step  10 ). 
     Basically, a translational two-dimensional force detected by the three-axis force sensor  5  may be superposed upon a translational velocity instruction, and a linear rotational moment may be superposed upon a rotational velocity instruction. However, a force that can be detected by the three-axis force sensor is a composite force. Accordingly, actually, the velocity instruction is limited to the force of the mobile robot  1  and the surrounding person  21  detected by the surrounding touch sensor. 
     The above-described embodiment is described as only an example, and the present invention is not limited to the specific configuration described above. For example, in a case where the mobile robot  1  having a map of the store leads a person while passing through the crowd of people toward a target, the mobile robot  1  may be configured similarly as described above. In addition, in a case where the mobile robot  1  follows a person in the crowd of people outdoor or leads the person, similar configuration is applicable. Moreover, the target or the surrounding obstacle is not limited to a person, and may be a non-mobile object or a mobile object, for example, another mobile robot. 
     As described above, according to the present invention, it is possible to control a mobile robot to move toward the target while the mobile robot safely contacts with the surrounding obstacles by controlling the contact force between the mobile robot and the obstacles such as surrounding people. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.