# Patent

## Title
Power assist device and its control method

## Abstract

It is possible to provide a power assist device which can maintain a stable contact state without causing an oscillation phenomenon even if a robot is brought into contact with an environment. A method for controlling the power assist device is also provided. The power assist device includes: an inner force sensor which detects an operation force applied by an operator; an operation handle having the inner force sensor; a robot arm which supports the operation handle; an actuator which drives the robot arm; the actuator and a control device which measure or estimate a force applied when the robot arm is brought into contact with an environment; and the actuator and the control device which detect or estimate a motion speed of the operation handle. The control device acquires a corrected external force according to the operation force detected by the inner force sensor and an external force detected by the actuator and the control device as external force derivation means and controls the actuator so that the corrected external force acts on the operation handle.

## Background
This is a 371 national phase application of PCT/JP2008/066212 filed 9 Sep. 2008, which claims priority to Japanese Patent Application No. 2007-237034 filed 12 Sep. 2007, the contents of which are incorporated herein by reference.
FIELD OF THE INVENTION
The present invention relates to a power assist device and its control method.
BACKGROUND OF THE INVENTION
Conventionally, at the scene of manufacture, a robot referred to as a power assist device is used so as to reduce labor and improve workability. As a control method of the power assist device, mechanical impedance control based on a position control (called an admittance control) is widely adopted.
In the admittance control, normally, position control with a high gain is performed and its target position is determined based on a force sensor. Then, force applied on the power assist device not through the force sensor is not considered at the control action, and a manipulator is hardly actuated. Namely, when the power assist device is brought into contact with an environment (the external) at the position except the detection part of the force sensor, the power assist device is continued to be actuated without consideration of the contact.
Hereafter, situations where humans and robots share a space will increase more and more so that the admittance control will consider the connection of a robot with a person or an environment
The Non-patent Literature 1 shown below discloses a method of the admittance control with consideration of connection with an environment by using estimated value of external force acting on the power assist device not via any force sensor.
In the control method with consideration of the connection with the environment disclosed in the Non-patent Literature 1, compliant action following unknown external force can be realized. However, in this control method, there is known that, when an operator brings the robot into contact with the environment and then presses the robot to the environment further, the robot vibrates along the pressing direction and the anti-pressing direction (so-called oscillation phenomenon occurs). Essentially, the oscillation phenomenon should not occur in the state that the operation force is perfectly balanced with the external force. However, even slight difference between the two causes the oscillation phenomenon. The oscillation phenomenon becomes a problem in the case that the connection of the robot with the environment is premised such as pressing work with a power assist device.
For solving the problem, hitherto, such a method that an operator adjusts pressing condition of the power assist device is only obtained. With regard to parameters concerning the action of the power assist device, viscosity resistance is set to be increased so as to suppress the oscillation phenomenon. However, in this case, another problem occurs that the operation force required for the operation of the power assist device is increased so as to worsen the operability, whereby it is undesirable in practical use.
Namely, with regard to the conventional art, there is no method effective to suppress the oscillation phenomenon caused in the pressing work with the power assist device.
[Non-patent Literature 1]: Masahito Iso, Hirokazu Seki, and Yoichi Hori, Sensorless power assist method using impedance control considering load characteristics, Technical Meeting on Industrial Instrumentation and Control, IEE Japan, Vol. IIC-02-40 (2002)
SUMMARY OF INVENTION
Problems to be Solved by the Invention
The purpose of the present invention is to provide a power assist device which can maintain a stable contact state without causing an oscillation phenomenon even if a robot is brought into contact with an environment, and to provide a method for controlling the power assist device.
Means for Solving the Problems
The first aspect of the present invention is a power assist device which includes an operation part operated by an operator; an operation force detection means for detecting operation force applied on the operation part; a robot arm for supporting the operation part; a drive means for driving the robot arm; an external force measurement means for measuring force applied when the robot arm is brought into contact with an environment, or an external force estimation means for estimating force applied when the robot arm is brought into contact with the environment; and a speed detection means for detecting movement speed of the operation part, or a speed estimation means for estimating the movement speed of the operation part, wherein a control device is provided that obtains corrected external force based on the external force obtained by the external force measurement means or the external force estimation means and a component along the external force of the operation force detected by the operation force detection means, and controls the drive means so as to maintain the state that the operation force and the corrected external force act on the operation part and the component along the external force of the operation force and the corrected external force acting on the robot arm are always balanced with each other.
In an embodiment of the power assist device according to the present invention, preferably, the control device compares the component along the external force of the operation force detected by the operation force detection means with the external force obtained by the external force measurement means or the external force estimation means, regards the speed of the operation part calculated from the external force as speed of the operation part along the external force calculated from the component along the external force of the operation force so as to obtain the corrected external force when the component along the external force of the operation force is opposite to the external force and the external force is larger than the component along the external force of the operation force, and adopts the external force without any operation as the corrected external force when the component along the external force of the operation force has the same direction as the external force or the external force is smaller than the component along the external force of the operation force.
In another embodiment of the power assist device of the present invention, preferably, the control device compares the component along the external force of the operation force detected by the operation force detection means with the external force obtained by the external force measurement means or the external force estimation means, regards the external force as a component of the operation force along the external force so as to obtain the corrected external force when the component along the external force of the operation force is opposite to the external force and the external force is larger than the component along the external force of the operation force, and adopts the external force without any operation as the corrected external force when the component along the external force of the operation force has the same direction as the external force or the external force is smaller than the component along the external force of the operation force.
The second aspect of the present invention is a control method of a power assist device which includes an operation part operated by an operator; an operation force detection means for detecting operation force applied on the operation part; a robot arm for supporting the operation part; a drive means for driving the robot arm; an external force measurement means for measuring force applied when the robot arm is brought into contact with an environment, or an external force estimation means for estimating force applied when the robot arm is brought into contact with the environment; or an external force estimation means for estimating force applied when the robot arm is brought into contact with the environment; and a speed detection means for detecting movement speed of the operation part, or a speed estimation means for estimating the movement speed of the operation part, wherein corrected external force is obtained based on the external force obtained by the external force measurement means or the external force estimation means and a component along the external force of the operation force detected by the operation force detection means, and the drive means is controlled so as to maintain the state that the operation force and the corrected external force act on the operation part and the component along the external force of the operation force and the corrected external force acting on the robot arm are always balanced with each other.
In an embodiment of the control method of the power assist device according to the present invention, preferably, the speed along the external force of the operation part calculated from the component along the external force of the operation force detected by the operation force detection means is compared with the speed of the operation part calculated from the external force obtained by the external force measurement means or the external force estimation means, the speed of the operation part calculated from the external force is regarded as the speed along the external force of the operation part calculated from the component along the external force of the operation force so as to obtained the corrected external force when the speed along the external force of the operation part calculated from the component along the external force of the operation force is opposite to the speed of the operation part calculated from the external force and the speed of the operation part calculated from the external force is larger than the speed along the external force of the operation part calculated from the component along the external force of the operation force, and the external force without any operation is adopted as the corrected external force when the speed along the external force of the operation part calculated from the component along the external force of the operation force has the same direction as the speed of the operation part calculated from the external force or the speed of the operation part calculated from the external force is smaller than the speed along the external force of the operation part calculated from the component along the external force of the operation force.
In another embodiment of the control method of the power assist device according to the present invention, preferably, the component along external force of the operation force detected by the operation force detection means is compared with the external force obtained by the external force measurement means or the external force estimation means, the external force is regarded as a component of the operation force along the external force so as to obtain the corrected external force when the component along the external force of the operation force is opposite to the external force and the external force is larger than the component along the external force of the operation force, and the external force without any operation is adopted as the corrected external force when the component along the external force of the operation force has the same direction as the external force or the external force is smaller than the component along the external force of the operation force.
Effect of the Invention
According to the power assist device of the present invention, when the power assist device is brought into contact with the environment, the oscillation phenomenon is prevented and the stable connection state is maintained.
According to the power assist device of the present invention, instead of correcting the operation force, the external force is corrected to be limited so as to suppress the oscillation phenomenon on the power assist device without spoiling the operability.
According to the power assist device of the present invention, the oscillation phenomenon on the power assist device is suppressed more certainly without spoiling the operability.
According to the control method of the power assist device of the present invention, when the power assist device is brought into contact with the environment, the oscillation phenomenon is prevented and the stable connection state is maintained.
According to the control method of the power assist device of the present invention, instead of correcting the operation force, the external force is corrected to be limited so as to suppress the oscillation phenomenon on the power assist device without spoiling the operability.
According to the control method of the power assist device of the present invention, the oscillation phenomenon on the power assist device is suppressed more certainly without spoiling the operability.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 It is a schematic drawing of state that a power assist device according to an embodiment of the present invention is brought into contact with an environment.
FIG. 2 It is a flow chart of admittance control according to the embodiment of the present invention.
FIG. 3 It is a speed vector diagram for explaining application of the present invention to a multidimensional system. (a) is a first speed vector diagram, (b) is a second speed vector diagram, (c) is a third speed vector diagram, and (d) is a fourth speed vector diagram.
FIG. 4 It is a schematic drawing of an experimental device for confirming effect of application of the present invention. (a) is a plan view and (b) is a side view.
FIG. 5 It is a graph of variation with time of tip position, operation force and estimated external force at the admittance control. (a) shows an experimental result of the normal admittance control (with only the information of operation force), (b) shows an experimental result of the conventional admittance control (with the information of operation force and estimated external force), and (c) shows an experimental result of the admittance control according to the embodiment of the present invention.



DETAILED DESCRIPTION
Explanation will be given on the mode for carrying out the invention referring appended drawings.
Firstly, explanation will be given on a power assist device 1 and control method thereof according to an embodiment of the present invention referring work shown in FIG. 1 as an example.
As shown in FIG. 1, the power assist device 1 includes a robot arm 2, an inner force sensor 3, an operation handle 4, an actuator 5, a control device 6 and the like.
The robot arm 2 is supported by the actuator 5 which is a drive means, and the robot device can assume various postures by the actuation of the actuator 5 following commands from the control device 6.
The operation handle 4 which is an operation part is disposed at the tip of the robot arm 2 through the inner force sensor 3 which is an operation force detection means, and an operator can moves the robot arm 2 to a desired position by holding the operation handle 4. The operation force applied on the operation handle 4 by the operator at the time of the movement is detected by the inner force sensor 3.
The actuator 5 can not only support and drive the robot arm 2 but also detect the posture of the robot arm 2 and detect the operation speed of the operation handle 4 as an operation speed detection means.
Based on the information of posture of the robot arm 2 and the information of operation speed of the operation handle 4 detected by the actuator 5, the control device 6 which acts as an external force estimation means and an operation speed estimation means estimates external force (contact force) applied on the robot arm 2 by an environment.
Though estimated value estimated by the control device 6 is adopted as the external force applied by the environment, it may alternatively be constructed that a force sensor which is an external force measurement means for connection detection is provided separately and actual measurement value is adopted as the external force applied by the environment.
When the robot arm 2 is brought into contact with the environment (for example, an obstacle 7), operation force fh by an operator and external force fe by the obstacle 7 act on the robot arm 2. In admittance control, the operation force fh and the external force fe are detected or estimated, and based on the detected values or estimated values, target tip position xdm at which desired impedance is realized is calculated by the control device 6. Then, the control device 6 controls the actuator 5 for driving the robot arm 2 so as to move tip position xd of the robot arm 2 to the target tip position xdm.
In the state shown in FIG. 1, based on the information obtained by estimating (or directly measuring) the operation force fh and the external force fe, a formula 1 shown below is materialized.

αfh+fe=M{umlaut over (x)}d+D{dot over (x)}d  [Formula 1]

In the above formula, a indicates assist ratio, M indicates inertial force, and D indicates viscous force. In below explanation, for simplifying the explanation, it is assumed that α=1.
In the case of calculating the target tip position xdm with the information of a plurality of force (that is, the operation force fh and the external force fe) as mentioned above, the resultant of the plurality of force must be “zero” and balanced so as to set the target tip position xdm to a fixed value.
Delay of force detection by the inner force sensor 3, delay caused by time for calculation must be considered so as to obtain the target tip position xdm, set to a fixed value, by the control device 6.
In an embodiment of the control method of the power assist device 1, a control method that the external force fe used in the calculation with the formula 1 is corrected corresponding to the state of the operation force fh and the external force fe so as to prevent an oscillation phenomenon from being caused on the power assist device 1. Explanation will be given on the concrete control method.
In an one-dimensional model, in the state that the tip position xd moves at speed Vd, when the operation force fh acts, a formula 2 shown below is materialized and speed Va is obtained.




  
    
      
        
          
            
              
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In the above formula, T indicates sampling time.
When the speed Va and the external force fe have the same direction (that is, Va*fe>0), the external force fe is adopted as corrected external force fes without any operation. When the speed Va and the external force fe have opposite directions (that is, Va*fe<0), the corrected external force fes is obtained with a formula 3 shown below.




  
    
      
        
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Accordingly, the next target tip position xdm of the operation part is obtained from the operation force fh and the corrected external force fes, and the operation force fh and the corrected external force fes are maintained to be balanced with each other, whereby the calculation result of the target tip position xdm is not oscillated and the stable connection state is maintained.
Next, explanation will be given on a control flow of admittance control according to the present invention referring FIG. 2.
As shown in FIG. 2, in the control method of the power assist device according to the embodiment of the present invention, when control action is started, firstly, the external force fe from the environment is measured or estimated (S01) and the operation force fh from an operator is measured (S02).
Next, based on the external force fe and the operation force fh obtained at the steps (S01) and (S02), the control device 6 judges conditions (S034 and S03-2), and the control flow advances to a step (S04) in the case that both the steps (S03-1) and (S03-2) are satisfied and the control flow advances to a step (S05) in the other cases.
At the step (S04), the external force fe is corrected to the corrected external force fes so as to make the next target speed “zero”. Since the speed Vd at the time that the robot arm 2 is brought into contact with the environment is about “zero”, the above control can be regarded that the external force fe is limited to the corrected external force fes so as to prevent vibration of the robot arm 2.
At the step (S05), the external force fe is adopted as the corrected external force fes without any operation.
Based on the corrected external force fes obtained at the step (S04) or (S05), the control device 6 calculates target acceleration Ad (S06), and the obtained target acceleration Ad is temporally integrated so as to calculate the target speed Vd and the target tip position (S07), and then the control device 6 controls the drive of the actuator 5, which supports the robot arm 2, so as to materialize the target speed Vd and the target tip position xdm. Each of calculating processes shown in FIG. 2 is performed by the control device 6.
The target acceleration Ad is obtained with below formula 4.

Ad=(fh+fes−D·Vd)/M  [Formula 4]

The power assist device 1 includes: the inner force sensor 3 which detects the operation force applied by an operator; the operation handle 4 having the inner force sensor 3; the robot arm 2 which supports the operation handle 4; the actuator 5 which drives the robot arm 2; the actuator 5 and the control device 6 which measure or estimate the force applied when the robot arm 2 is brought into contact with an environment; and the actuator 5 and the control device 6 which detect or estimate the movement speed of the operation handle 4. In the control method of the power assist device 1, the corrected external force fes is obtained based on the operation force fh detected by the inner force sensor 3 and the external force fe obtained by the actuator 5 and the control device 6, and the actuator 5 is controlled so that the corrected external force fes acts on the operation handle 4.
According to the above construction, when the robot arm 2 is brought into contact with the environment (the obstacle 7), the oscillation phenomenon on the power assist device 1 is prevented and the stable connection state is maintained.
The operation force fh detected by the inner force sensor 3 is compared with the external force fe obtained by the actuator 5 and the control device 6. When the external force fe is larger than the operation force fh, the external force fe is limited to be not larger than the operation force fh so as to obtain the corrected external force fes, and when the operation force fh is larger than the external force fe, the external force fe is adopted without any operation so as to obtain the corrected external force fes.
Accordingly, instead of correcting the operation force fh, the external force fe is corrected to be limited so as to suppress the oscillation phenomenon on the power assist device 1 without spoiling the operability.
Furthermore, when the operation force fh detected by the actuator 5 and the control device 6 has opposite direction to the external force fe obtained by the actuator 5 and the control device 6 and the external force fe is larger than a predetermined threshold, the external force fe is limited to make the speed calculated from the operation force fh and the external force fe “zero” so as to obtain the corrected external force fes, and when the external force fe is smaller than the predetermined threshold or has the same direction as the operation force fh, the external force fe is adopted without any operation as the corrected external force fes.
Accordingly, by setting the threshold so as to provide a dead band in the control action, the oscillation phenomenon on the power assist device 1 is suppressed more certainly without spoiling the operability.
The control method of the power assist device according to the embodiment of the present invention is not limited to be adopted to a one-dimensional model and is adoptable to a multidimensional model.
Explanation will be given on the case that the control method of the power assist device according to the embodiment of the present invention is adopted to a multidimensional model referring FIG. 3. Herein, for simplifying the explanation, it is assumed that viscous force D=0.
As shown in FIG. 3(a), in the state that a speed vector Va by the operation force and a speed vector Tfe/M obtained from the external force exist as an example, the speed vector Va by the operation force obtained with the formula 2 can be indicated as FIG. 3(b) by dividing it into components (V1, V2) of the direction of the speed vector Tfe/M obtained from the external force and the direction perpendicular thereto.
As shown in FIG. 3(c), when the speed vector Tfe/M has opposite direction to V1 and is larger than V1, the speed vector Tfe/M is corrected and the corrected speed vector Tfe/M is referred to as −V1.
Then, as shown in FIG. 3(d), the movement direction finally obtained from the operation force and the corrected external force is Vk. Namely, any force does not act along the direction of the external force from the environment so that the oscillation is prevented.
The operation force applied along a tangent of the environment (that is, the operation of moving the power assist device along the tangent of the outer edge of the environment) is not prevented.
Next, explanation will be given on experimental results for confirming effect of adoption of the control method of the power assist device according to the embodiment of the present invention referring FIGS. 4 and 5.
As shown in FIGS. 4(a) and (b), in the power assist device 1, the inner force sensor 3 is disposed at the tip position of the robot arm 2, and the operation handle 4 is disposed at the tip position of the robot arm 2 through the inner force sensor 3.
An experimental device is constructed that a hook 8a of a spring balance 8 is hanged on the operation handle 4 and an operator pulls the operation handle 4 via the spring balance 8 so as to displace the robot arm 2.
When the operator pulls the operation handle 4 via the spring balance 8, the robot arm 2 is brought into contact with the environment (the obstacle 7), and the inner force sensor 3 detects the operation force fh at the time of the contact of the robot arm 2 with the obstacle 7. The estimated external force fe by the contact and the tip position xd are obtained by calculation based on information of position obtained from the actuator 5 driving the robot arm 2 and the like.
The experimental results with the above experimental device are shown in FIGS. 5(a), (b) and (c). In each of FIGS. 5(a), (b) and (c), the axis of abscissas indicates time (s), and a dotted line shown at the position just after the time 9 (s) in the graph indicates the timing of the contact of the robot arm 2 with the environment (the obstacle 7).
Firstly, explanation will be given on the experimental result of normal admittance control (namely, the robot arm 2 is assist-controlled with only the information of the operation force fh).
As shown in FIG. 5(a), in this case, after the contact of the robot arm 2 with the environment (the obstacle 7), the tip position xd and the operation force fh have been changing while the contact scarcely affects them.
On the other hand, the estimated external force fe by the contact which is not used for the control changes widely so that it is obtained that the robot arm 2 may break the environment in this control method.
Next, explanation will be given on the experimental result of the admittance control further considering the information of the estimated external force fe by the contact (namely, the robot arm 2 is assist-controlled with the information of the operation force fh and the estimated external force fe by the contact).
As shown in FIG. 5(b), in this case, after the contact of the robot arm 2 with the environment (the obstacle 7), the tip position xd does not make an inroad into the environment further, and it is obtained that the oscillation phenomenon occurs according to the change of the estimated external force fe by the contact.
In this case, though the oscillation phenomenon can be prevented by weakening the pressing force of the robot arm 2 to the environment (the obstacle 7) by the skill of the operator, this operation requires delicate adjustment of the force.
Finally, explanation will be given on the experimental result of the admittance control to which the control method of the power assist device according to the embodiment of the present invention is adopted so that the estimated external force fe by the contact is corrected and the information of the corrected external force fes is considered.
FIG. 5(c) shows the experimental result of the admittance control according to the present invention (namely, the robot arm 2 is assist-controlled with the information of the external force fe corrected corresponding to the operation force fh (the corrected external force fes)).
In this case, the tip position xd does not oscillate and is stable at a fixed position from the time at which the robot arm 2 is brought into contact with the environment (the obstacle 7) to the time at which the robot arm 2 reaches the pressing state, whereby it is confirmed that the stable contact state is maintained.
Accordingly, by adopting the control method of the power assist device according to the embodiment of the present invention, even if the robot is in contact with the environment, the oscillation phenomenon does not occur on the power assist device, whereby the power assist device can be provided that the stable contact state is maintained at the pressing work.
INDUSTRIAL APPLICABILITY
The power assist device and the control method thereof according to the present invention can be used not only for a power assist device provided in an assembly line of a motorcar or the like but also for various uses widely in which a power assist device is used.

## Claims

1. A power assist system comprising
an operation part operated by an operator;
an operation force detection device that detects operation force applied on the operation part;
a robot arm that supports the operation part;
a drive mechanism that drives the robot arm;
an external force measurement device that measures force applied when the robot arm is brought into contact with an environment, or an external force estimation device that estimates force applied when the supporting part is brought into contact with the environment; and
a speed detection device that detects movement speed of the operation part, or a speed estimation device that estimates the movement speed of the operation part,
wherein a control device is provided that obtains a corrected external force based on the external force obtained by the external force measurement device or the external force estimation device and a component along the external force of the operation force detected by the operation force detection device, controls the drive mechanism so as to maintain the state that the operation force and the corrected external force act on the operation part and the component along the external force of the operation force and the corrected external force acting on the robot arm are always balanced with each other, compares the component along the external force of the operation force detected by the operation force detection device with the external force obtained by the external force measurement device or the external force estimation device, regards the speed of the operation part calculated from the external force as speed of the operation part along the external force calculated from the component along the external force of the operation force so as to obtain the corrected external force when the component along the external force of the operation force is opposite to the external force and the external force is larger than the component along the external force of the operation force, and adopts the external force without any operation as the corrected external force when the component along the external force of the operation force has the same direction as the external force or the external force is smaller than the component along the external force of the operation force.

2. A power assist system comprising comprising
an operation part operated by an operator;
an operation force detection device that detects operation force applied on the operation part;
a robot arm that supports the operation part;
a drive mechanism that drives the robot arm;
an external force measurement device that measures force applied when the robot arm is brought into contact with an environment, or an external force estimation device that estimates force applied when the robot arm is brought into contact with the environment; and
a speed detection device that detects movement speed of the operation part, or a speed estimation device that estimat6es the movement speed of the operation part,
wherein a control device is provided that obtains a corrected external force based on the external force obtained by the external force measurement device or the external force estimation device and a component along the external force of the operation force detected by the operation force detection device, controls the drive mechanism so as to maintain the state that the operation force and the corrected external force act on the operation part and the component along the external force of the operation force and the corrected external force acting on the robot arm are always balanced with each other, compares the component along the external force of the operation force detected by the operation force detection device with the external force obtained by the external force measurement device or the external force estimation device, regards the external force as a component of the operation force along the external force so as to obtain the corrected external force when the component along the external force of the operation force is opposite to the external force and the external force is larger than the component along the external force of the operation force, and adopts the external force without any operation as the corrected external force when the component along the external force of the operation force has the same direction as the external force or the external force is smaller than the component along the external force of the operation force.

3. A control method of a power assist device comprising:
an operation part operated by an operator;
an operation force detection device that detects operation force applied on the operation part;
a robot arm that supports the operation part;
a drive mechanism that drives the robot arm;
an external force measurement device that measures force applied when the robot arm is brought into contact with an environment, or an external force estimation device that estimates force applied when the robot arm is brought into contact with the environment; and
a speed detection device that detects movement speed of the operation part, or a speed estimation device that estimates the movement speed of the operation part,
wherein a corrected external force is obtained based on the external obtained by the external force measurement device or the external force estimation device and a component along the external force of the operation force detected by the operation force detection device,
the drive mechanism is controlled so as to maintain the state that the operation force and the corrected external force act on the operation part and the component along the external force of the operation force and the corrected external force acting on the robot arm are always balanced with each other,
the speed along the external force of the operation part calculated from the component along the external force of the operation force detected by the operation force detection device is compared with the speed of the operation part calculated from the external force obtained by the external force measurement device or the external force estimation device,
the speed of the operation part calculated from the external force is regarded as the speed along the external force of the operation part calculated from the component along the external force of the operation force so as to obtain the corrected external force when the speed along the external force of the operation part calculated from the component along the external force of the operation force is opposite to the speed of the operation part calculated from the external force and the speed of the operation part calculated from the external force is larger than the speed along the external force of the operation part calculated from the component along the external force of the operation force, and
the external force without any operation is adopted as the corrected external force when the speed along the external force of the operation part calculated from the component along the external force of the operation force has the same direction as the speed of the operation part calculated from the external force or the speed of the operation part calculated from the external force is smaller than the speed along the external force of the operation part calculated from the component along the external force of the operation force.

4. A control method of a power assist device comprising:
an operation part operated by an operator;
an operation force detection device that detects operation force applied on the operation part;
a robot arm that supports the operation part;
a drive mechanism that drives the robot arm;
an external force measurement device that measures force applied when the robot arm is brought into contact with an environment, or an external force estimation device that estimates force applied when the supporting part is brought into contact with the environment; and
a speed detection device that detects movement speed of the operation part, or a speed estimation device that estimates the movement speed of the operation part,
wherein a corrected external force is obtained based on the external force obtained by the external force measurement device or the external force estimation device and a component along the external force of the operation force detected by the operation force detection device,
the drive means is controlled so as to maintain the state that the operation force and the corrected external force act on the operation part and the component along the external force of the operation force and the corrected external force acting on the robot arm are always balanced with each other,
the component along the external force of the operation force detected by the operation force detection device is compared with the external force obtained by the external force measurement device or the external force estimation device,
the external force is regarded as a component of the operation force along the external force so as to obtain the corrected external force when the component along the external force of the operation force is opposite to the external force and the external force is larger than the component along the external force of the operation force, and
the external force without any operation is adopted as the corrected external force when the component along the external force of the operation force has the same direction as the external force or the external force is smaller than the component along the external force of the operation force.