Patent Description:
The document <CIT> discloses a robot controller, the document <CIT> discloses a robot arm control method and a control device and the document <CIT> discloses a motor controller. In order to reduce takt time and improve the accuracy of finished workpieces, there is a growing demand in recent years for laser processing in, for example, welding or cutting. The speed and accuracy of the laser processing are usually improved by attaching a laser output device to the arm tip of articulated robots.

The arm of a typical articulated robot is operated by driving servo motors to rotate the joint shafts of the robot. The driving force of the servo motors is transmitted to the joint shafts through the reduction gears connected to the servo motors. However, such an articulated robot can have a discrepancy between the position of the robot arm tip specified to the servo motors and the actual position of the robot arm tip. The discrepancy is due to the bending of the spring components of the reduction gears. Such bending can occur, for example, when the robot arm is subjected to a gravitational force (hereinafter, gravitational torque) or when one of the joint shafts of the robot arm is subjected to an interference force (hereinafter, interference torque) from other joint shafts.

To overcome the bending problem, there has been a suggestion to calculate a correction value based on the amount of bending and to add the correction value to the position command (see Patent Literature <NUM>, for example). To be more specific, the amount of bending is found by calculating the external torque applied to the reduction gear of each joint shaft and dividing the external torque by the spring constant of the reduction gear. The amount of bending corresponds to the amount of position error of the robot arm tip due to the bending or to the amount of position error of the joint shaft angle. The correction value based on the amount of bending is added to the position command in the opposite direction.

In an articulated robot, the behavior of bending in the reduction gear differs between components due to gravitational torque and components due to interference torque. Therefore, bending correction is not always performed properly when the correction is based directly on the external torque, which is the sum of the gravitational and interference torques, as in the above-described well-known art.

In view of these circumstances, an objective of the present disclosure is to provide a robot control device that is less influenced by the bending of the reduction gears so as to correct the position error of the robot arm with high accuracy.

To achieve the objective, the robot control device of a first aspect of the present disclosure separately finds the gravitational and interference torques to be applied to the reduction gears connected to the joint shafts. The gravitational and interference torques thus obtained are separately used to find bending correction values, and these values are used to correct the position command to be given to the motors. The invention is as set out in the independent claims, further aspects of the invention are outlined in the dependent claims. Embodiments that do not fall within the scope of the claims do not describe part of the invention.

To be more specific, the robot control device of the first aspect of the present disclosure is a robot control device for controlling the movement of a robot arm having a joint shaft driven through a reduction gear connected to a servo motor, the robot control device including:.

This structure separates the influence of the gravitational torque and the influence of the interference torque on the bending and finds the position-command correction values separately for these torques, thereby correcting the position error of the robot arm with high accuracy.

The second position-command correction value is preferably obtained by compensating the amplitude and phase of the bending correction value obtained based on the interference torque.

This structure ensures the correction of the influence of the bending due to the interference torque.

The robot control device of a second aspect of the present disclosure is a robot control device for controlling the movement of a robot arm having a joint shaft driven through a reduction gear connected to a servo motor, the robot control device including:.

This structure separates the influence of the gravitational torque and the influence of the interference torque on the bending and finds the compensation value for the motor current with respect to the interference torque. This enables the correction of the position error of the robot arm with high responsivity.

The current compensation value is preferably obtained by compensating both the amplitude and phase of the bending correction value obtained based on the interference torque and multiplying the bending correction value by a predetermined coefficient.

The servo motor may be one of n servo motors, the n being an integer not less than <NUM>,.

This structure performs the bending correction by using the position command of the servo motors connected to the respective joint shafts. This eliminates the need for a dedicated component such as a torque sensor, thereby reducing the cost.

It is preferable that the external and gravitational torques should be obtained by dynamic calculation and that the interference torque should be obtained by subtracting the gravitational torque from the external torque.

This structure can easily separate the gravitational and interference torques from each other, thereby correcting the position error of the robot arm caused by the bending.

As described above, the present disclosure separates the bending due to the gravitational torque and the bending due to the interference torque from other joint shafts, thereby correcting the position error of the robot arm with high accuracy.

The present disclosure will now be described in detail by means of exemplary embodiments with reference to the drawings. It should be understood that the exemplary embodiments are merely exemplary of the present disclosure and are not intended to limit the present disclosure, its application, or use.

As described above, the bending of the spring components of the reduction gears causes a position error of the robot arm. To correct the bending, it is preferable to take the external torque applied to the reduction gears into consideration. The external torque contains the gravitational torque and the interference torque, which is applied from other joint shafts. The inventors of the present application have found that the gravitational and interference torques have different influences from each other on the position command after the bending correction. To be more specific, they have found that these torques have different response frequencies from each other. This finding will be described as follows by taking the operation of a vertical articulated robot as an example.

<FIG> shows the structure of a well-known vertical articulated robot used for laser welding and cutting. This six-axis vertical articulated robot <NUM> includes robot arm <NUM> and joint shafts J1 to J6. Robot arm <NUM> consists of a plurality of parts. These parts are connected to each other through joint shafts J1 to J6. Joint shafts J1 to J6 are connected to servo motors (not shown) through respective reduction gears (not shown). Robot control device <NUM> drives the servo motors according to the position command θc (see <FIG>) so as to rotate joint shafts J1 to J6 by desired amounts, thereby controlling the operation and attitude of robot arm <NUM>. Robot arm <NUM> has laser output device <NUM> mounted at its tip. Laser output device <NUM> is connected to a laser light source (not shown) for generating laser light <NUM> and an optical fiber (not shown) for guiding laser light <NUM>.

In the following description, robot <NUM> includes a total of six joint shafts: main three shafts J1 to J3, which determine the attitude of robot <NUM> as a whole, and distal three shafts J4 to J6, which determine the orientation of the arm tip. Note that the directions X, Y, and Z shown in <FIG> may be used to describe the position and movement of robot <NUM>.

Assume that laser output device <NUM> emits laser light <NUM> to cut workpiece <NUM> and that laser output device <NUM> has a mass of about <NUM>. Once the position and direction of radiation of laser light <NUM> are determined, the attitude of robot arm <NUM>, or the angle of each joint shaft of robot <NUM> is uniquely determined. Thus, the joint shafts have no control redundancy, so that when workpiece <NUM> is being cut into a circle, all the joint shafts operate.

Assume that workpiece <NUM> is cut into a circle with a diameter of <NUM> at a rate of <NUM>/min. In this case, the operation time is about <NUM> seconds, and the operating frequency is about <NUM>. Each joint shaft operates upon receiving a sinusoidal position command θc corresponding to the operating frequency. The actual frequency at which each joint shaft can operate in response to the position command θc is defined as the position response frequency, and the actual position of each joint shaft (hereinafter, actual position) is referred to as θL. The position command θc is an angle command indicating the amount of rotation angle of each joint shaft. The actual position θL indicates the actual amount of rotation angle of each joint shaft. The position response frequency is determined by the natural oscillation frequency of each joint shaft of robot <NUM> and the characteristics of the control device (see <FIG>) for driving each joint shaft. The following Formula <NUM> defines a transfer function WcL until each joint shaft reaches the actual position θL after receiving the position command θc.

In the following description, the position response frequency is the lower one of the following: the frequency at which the amplitude of the transfer function WcL is half (-<NUM> dB); and the frequency at which the phase of the transfer function WcL is <NUM> degrees behind.

Main three shafts J1 to J3 are so physically large and heavy that it is difficult to improve the position response frequency. For example, when a robot has an arm length of <NUM> and a maximum load capacity of about <NUM>, the position response frequency is at its minimum of about <NUM> when the arm is extended (the inertia is at its maximum).

<FIG> show the response characteristics of the transfer function with respect to the operating frequency of the main shafts. More specifically, <FIG> shows the frequency response characteristics of the amplitude of the transfer function WcL, and <FIG> shows the frequency response characteristics of the phase of the transfer function WcL. <FIG> shows a time waveform of the position command θc and the actual position θL when a sine wave of <NUM> is applied as the position command θc to the main shafts having the frequency characteristics shown in <FIG>.

As shown in <FIG>, when the position response frequency is <NUM>, the amplitude is half (-<NUM> dB) and the phase is <NUM> degrees behind. Furthermore, as shown in <FIG>, the amplitude of the actual position θL becomes half of the position command θc, and the phase is <NUM> degrees behind.

Thus, when workpiece <NUM> is laser-cut into a circle with a diameter of <NUM> at a rate of <NUM>/min by operating the main shafts at a position response frequency of <NUM>, the diameter of the circular trajectory traced by the tip of robot arm <NUM> is halved, resulting in an inappropriate trajectory.

Meanwhile, distal three shafts J4 to J6 are smaller and lighter in weight than main three shafts J1 to J3. Therefore, these shafts J4 to J6 have a position response frequency of at least <NUM> even when laser output device <NUM> of about <NUM> is mounted at the tip of robot arm <NUM>. Hence, robot arm <NUM> can have additional joint shaft J7 at its tip. Main shafts J1 to J3 can be in the stopped state whereas distal shafts J4 to J6 and the additional joint shaft J7 together can perform positioning or track following. This operation enables robot arm <NUM> to have sufficient position responsivity at an operating frequency of at least <NUM>. Thus, joint shafts J1 to J5 are brought to a complete standstill so as to keep rotation center position <NUM> of joint shaft J6 still.

<FIG> shows the structure of the seven-axis vertical articulated robot in which joint shaft J7 has been added in parallel to the endmost joint shaft J6. This seven-axis vertical articulated robot <NUM> can move the endmost joint shafts J6 and J7 alone so as to make the tip of robot arm <NUM> trace a desired trajectory such as a circle or an oval.

<FIG> shows the trajectory of the arm tip of robot <NUM> shown in <FIG>.

At time t1, joint shafts J1 to J5 of robot arm <NUM> are driven to move laser radiation position <NUM> to the center of a desired circular trajectory, and then laser radiation is started. At time t2, laser radiation position <NUM> is moved in the +Y direction and placed on the circular trajectory by rotating joint shafts J6 and J7 in rotation directions <NUM> and <NUM>, respectively. At times t3 to t5, joint shafts J6 and J7 are rotated so that laser radiation position <NUM> travels along the circular trajectory. At time t6, laser radiation is ended.

<FIG> show the response characteristics of the transfer function with respect to the operating frequency of the robot arm tip. More specifically, <FIG> shows the frequency response characteristics of the amplitude of the transfer function WcL, and <FIG> shows the frequency response characteristics of the phase of the transfer function WcL. In <FIG>, the robot arm tip indicates joint shafts J6 and J7, respectively. <FIG> shows a time waveform of the position command θc and the actual position θL when a sine wave of <NUM> is applied as the position command θc to the robot arm having the frequency characteristics shown in <FIG>. Since joint shafts J6 and J7 are smaller and lighter in weight than the main shafts as mentioned earlier, the position response frequency of these shafts J6 and J7 is, for example, about <NUM>, which is higher than that of the main shafts.

Therefore, as shown in <FIG>, when the position response frequency is <NUM>, the amplitude is half (-<NUM> dB), and the phase is <NUM> degrees behind. As shown in <FIG>, when the frequency of the position command θc is <NUM>, the actual position θL does not have a substantial amplitude attenuation or phase delay. Thus, the actual position θL follows the position command θc better than in the case shown in <FIG>.

In the case shown in <FIG>, when workpiece <NUM> is laser-cut into a circle with a diameter of <NUM> at a rate of <NUM>/min, the tip of robot arm <NUM> can draw a circular trajectory approximately as commanded as shown in <FIG>.

However, the movement of joint shafts J6 and J7 causes an interference torque on joint shafts J1 to J5, thereby bending the reduction gears connected to joint shafts J1 to J5. The rotational trajectories of main three shafts J1 to J3 are particularly far from the tip of robot arm <NUM>. The inventors of the present application have found that a slight bending of the reduction gears connected to these joint shafts can cause rotation center position <NUM> of joint shaft J6 to greatly fluctuate, thereby greatly affecting the trajectory drawn by the tip of robot arm <NUM>.

The inventors of the present application have actually investigated the fluctuation of rotation center position <NUM> of joint shaft J6 when workpiece <NUM> is laser-cut into a circle with a diameter of <NUM> at a rate of <NUM>/min. This investigation has revealed that joint shaft J6 has a fluctuation of about <NUM>, causing the trajectory drawn by the tip of robot arm <NUM> to have an error of <NUM>% or so. In seven-axis vertical articulated robot <NUM> shown in <FIG>, the characteristics of the reduction gears are different from one joint shaft to another. Therefore, the amount of bending differs from one reduction gear to another, thereby causing rotation center position <NUM> of joint shaft J6 to fluctuate irregularly.

<FIG> shows the trajectory of the arm tip of seven-axis vertical articulated robot <NUM> when the reduction gears are bent.

At time t1, joint shafts J1 to J5 of robot arm <NUM> are driven to move laser radiation position <NUM> to the center of a desired circular trajectory, and then laser radiation is started. Similar to the case shown in <FIG>, at time t2, laser radiation position <NUM> is moved in the +Y direction so that the tip of robot arm <NUM> is placed on the circular trajectory by rotating joint shafts J6 and J7. However, the interference torque due to the movement of joint shafts J6 and J7 bends mainly the reduction gear of joint shaft J1, thereby causing rotation center position <NUM> of joint shaft J6 to deviate in the +Y direction. At time t3, laser radiation position <NUM> is traveled in the +X direction along the circular trajectory. The interference torque bends mainly the reduction gear of joint shaft J2, thereby causing rotation center position <NUM> of joint shaft J6 to deviate in the +X direction. However, the difference in the amount of bending between the reduction gears connected to joint shafts J1 and J2 roughly halves the amount of the position error in the +X direction of the rotation center position <NUM> of joint shaft J6, compared with the amount of the position error in the +Y direction. Also at each of times t3 to t5, the interference torque causes a position error in rotation center position <NUM> of joint shaft J6. As a result, while the position command θc has a circular trajectory, the actual position θL has an oval trajectory.

When main three shafts J1 to J3 are moved in a six-axis articulated robot, the diameter of the circular trajectory may be reduced about <NUM>% by an error. The trajectory error of the actual position θL is smaller than this error, but it exceeds the maximum error allowable when processing is performed with a robot. Therefore, some measures should be taken for this.

<FIG> shows a diagram of the position error of the robot arm tip due to the external torque. Main three shafts J1 to J3 are subjected to external torques τddyn1, τddyn2, and τddyn3, respectively, which are the sum of the gravitational and interference torques: (τg1+τa1), (τg2+τa2), and (τg3+τa3), respectively. For convenience of explanation, <FIG> only shows the bending amount θs2 of joint shaft J2.

In this case, the bending correction value θsc (hereinafter, position-command correction value) to be added to the position command θc is expressed by the following Formula <NUM> where Ks represents the spring constant of the reduction gear. <MAT> where θs represents the amount of bending caused between the later-described primary and secondary sides of the reduction gear.

The bending correction value θsgc (hereinafter, first position-command correction value) to reduce a gravitational torque τg and the bending correction value θsgc to reduce interference torque τa are expressed by the following Formulas <NUM> and <NUM>, respectively. <MAT> <MAT>.

Consequently, the entire position-command correction value θsc is as shown in Formula <NUM>.

When the position-command correction value θsc expressed by Formula <NUM> is added to the position command θc to perform bending correction, the first position-command correction value θsgc to reduce the gravitational torque τg is fully corrected for the following reason.

While main three shafts J1 to J3 are in the stopped state, the gravitational torque τg has a frequency of <NUM>. The robot has an arm length of <NUM> and a maximum load capacity of about <NUM>, whereas main three shafts J1 to J3 have an operational angular velocity of about <NUM>/sec. and rotate only half of the circle per second. Therefore, even when main three shafts J1 to J3 are in action, the gravitational torque τg changes less than <NUM>. Consequently, the oscillation frequency of robot arm <NUM> twisted by the gravitational torque τg is also less than <NUM>. Even when the position response frequency of main three shafts J1 to J3 is <NUM> or so, the actual position θL can fully follow the position command θc, achieving bending correction.

Meanwhile, the bending due to the interference torque τa cannot be fully corrected even when the bending correction value θsac to reduce the interference torque τa is added to the position command θc for the following reason.

In the above-described cutting operation (workpiece <NUM> is cut into a circle with a diameter of <NUM> at a rate of <NUM>/min), the center frequency of the bending of robot arm <NUM> due to the interference torque τa is <NUM>. As a result, as shown in Formula <NUM>, the frequency of the bending correction value θsac to reduce the interference torque τa is also <NUM>.

<FIG> show the response characteristics of the transfer function with respect to the operating frequency of the main shafts when the reduction gears are bent. More specifically, <FIG> shows the frequency response characteristics of the amplitude of the transfer function WcL, and <FIG> shows the frequency response characteristics of the phase of the transfer function WcL. <FIG> shows a time waveform of the position command θc and the actual position θL when a sine wave of <NUM> is applied as the position command θc to the main shafts having the frequency characteristics shown in <FIG>. <FIG> shows a time waveform of the position command θc and the actual position θL when a sine wave of <NUM> is applied as the position command θc to the main shafts.

As shown in <FIG>, the actual position θL fully follows the first position-command correction value θsgc, which has a frequency of <NUM>. Meanwhile, as shown in <FIG>, the amplitude of the actual position θL is half as high as that of the bending correction value θsac to reduce the interference torque τa, and the phase of the actual position θL is <NUM> degrees behind.

In the above-described well-known approach, the position-command correction value θsc is found based on the external torque τddyn as a whole without dividing it into the gravitational torque and the interference torque. Therefore, for example, if the position-command correction value θsc is increased with a decrease in the actual corrected amplitude of the bending due to the interference torque τa, the first position-command correction value θsgc to reduce the gravitational torque τg is also increased. As a result, the arm is raised more than it is pulled down by gravitation.

The inventors of the present application have found that the external torque can be separated into the gravitational torque and the interference torque so as to correct the position error of robot arm <NUM> based on these separate torques, thereby enabling robot arm <NUM> to provide a desired trajectory. This finding will be described in detail as follows.

<FIG> shows a functional block diagram of the position control of seven-axis vertical articulated robot <NUM> shown in <FIG>. This diagram includes a schematic internal structure of the robot mechanism and the robot control device. Robot mechanism <NUM> is a mechanical drive unit of robot <NUM> and includes servo motors <NUM> (hereinafter, motors), reduction gears <NUM>, and encoders <NUM>. Although not illustrated, robot mechanism <NUM> includes robot arm <NUM>. Motors <NUM> are connected to joint shafts J1 to J7 of robot <NUM> shown in <FIG> through respective reduction gears <NUM>. Motors <NUM> drive joint shafts J1 to J7 to control the operation and attitude of robot arm <NUM> according to the control signal from servo control units <NUM> of robot control device <NUM>. Encoders <NUM>, which are connected to motors <NUM>, detect their amount and speed of rotation and send the detection signals as feedback signals to respective servo control units <NUM>.

In the following description, motor <NUM>, reduction gear <NUM>, and encoder <NUM> connected to joint shaft J1 may hereinafter be referred to as the first motor, the first reduction gear, and the first encoder, respectively. Similarly, the motors connected to joint shafts J2 to J7 may hereinafter be referred to as second-to-seventh motors. Servo control unit <NUM> and bending correction block <NUM> connected to the first motor may hereinafter be referred to as the first servo control unit and the first bending correction block, respectively. Similarly, servo control units <NUM> connected to the second-to-seventh motors may hereinafter be referred to as the second-to-seventh servo control units. Bending correction blocks <NUM> connected to the second-to-seventh servo control units may hereinafter be referred to as second to fifth bending correction blocks. The position commands and the position-command correction values sent to the joint shafts may hereinafter be referred to as the position commands θc to θ7c, and the position-command correction values θsc to θ7sc, respectively.

Robot control device <NUM> includes operation-teaching unit <NUM>, main control unit <NUM>, servo control units <NUM>, and bending correction blocks <NUM> (bending correction means). Operation-teaching unit <NUM> stores the trajectory of robot arm <NUM> acquired in the teaching process and the rotation of motors <NUM> to draw the trajectory.

Main control unit <NUM> receives an instruction from operation-teaching unit <NUM>, and outputs the position commands θc to θ7c to joint shafts J1 to J7, respectively, of robot <NUM> according to the trajectory of robot arm <NUM> of robot mechanism <NUM> stored in operation-teaching unit <NUM>.

First-to-seventh servo control units <NUM> control the rotation of first-to-seventh motors <NUM> of robot mechanism <NUM> such that the actual position θL follows the position commands θc to θ7c sent from main control unit <NUM>.

Bending correction blocks <NUM> are located between main control unit <NUM> and the respective servo control units <NUM> in such a manner as to correspond to joint shafts J1 to J7. Bending correction blocks <NUM> generate the position-command correction values θsc to θ7sc based on the position commands θc to θ7c received from main control unit <NUM>. The position-command correction values θsc to θ7sc thus generated are added to the position commands θc to θ7c, respectively, and are sent to first-to-seventh servo control units <NUM>, respectively.

Each functional block in robot control device <NUM> may be composed of an independent circuit, or of a single integrated circuit. Alternatively, a combination of some functional blocks may be composed of a single integrated circuit. The functions of main control unit <NUM>, servo control units <NUM> and bending correction blocks <NUM> are achieved mostly by executing a software program on an integrated circuit such as a CPU.

<FIG> is a diagram showing the bending of the reduction gears in the robot mechanism. As shown in <FIG>, motor <NUM>, reduction gear <NUM>, and part of robot arm <NUM> coupled with them are extracted as load <NUM> from robot mechanism <NUM>. Load <NUM> includes the following: first arm <NUM> as a motor mounting base; motor <NUM> connected to arm <NUM>; reduction gear <NUM> having: primary side <NUM> connected to motor <NUM>, and secondary side <NUM> including bearing <NUM>; and second arm <NUM> rotatably connected to secondary side <NUM>.

Primary side <NUM> of the reduction gear is connected to rotor <NUM> of motor <NUM> through the shaft of motor <NUM>. Primary side <NUM> is rotated by the angle corresponding to a motor rotation position θM sent from servo control unit <NUM>. Reduction gear <NUM> converts the motor rotation position θM into the arm rotation position (actual position) θL at a reduction ratio Rg shown in Formula <NUM>.

However, as shown in <FIG>, reduction gear <NUM> includes spring component <NUM> between primary side <NUM> and secondary side <NUM>, so that Formula <NUM> is satisfied only when the stretch of the spring, or the bending, is constant.

<FIG> is a block diagram of the load shown in <FIG>. The diagram includes the following: a motor current command IM to drive motor <NUM>; a torque constant Kt of motor <NUM>; the reciprocal <NUM>/Rg of the reduction ratio shown in Formula <NUM>; the spring constant Ks of reduction gear <NUM>; the amount of bending θs caused between primary side <NUM> and secondary side <NUM> of the reduction gear; and an external torque τddyn applied to robot arm <NUM>. Note that motor <NUM> is supplied with a current evoked by motor current command IM.

Motor transfer function <NUM> has: a moment of inertia JM around the shaft including rotor <NUM> of motor <NUM> and primary side <NUM> of the reduction gear; and a viscous friction coefficient DM. Load transfer function <NUM> has: a moment of inertia JL around the shaft including second arm <NUM> and secondary side <NUM> of the reduction gear; and a viscous friction coefficient DL.

As shown in <FIG>, the motor rotation position θM is obtained based on the motor current command IM. The motor rotation position θM is multiplied by the reciprocal <NUM>/Rg of the reduction ratio so as to obtain a first value. Meanwhile, the arm rotation position θL is obtained based on the external torque τddyn. The arm rotation position θL is subtracted from the first value so as to calculate the bending amount θs generated between primary side <NUM> and secondary side <NUM> of the reduction gear. The bending amount θs thus obtained is multiplied by the spring constant Ks so as to obtain a second value. The second value is added to the external torque τddyn. The second value is also multiplied by the reciprocal <NUM>/Rg of the reduction ratio so as to obtain a third value. The third value is subtracted from the product of the motor current command IM and the torque constant Kt of motor <NUM>.

Since <FIG> shows a typical control block diagram of a motor connected to a load and a reduction gear, the remaining functions will not be described in detail.

<FIG> is a block diagram of the first servo control unit for comparison with <FIG>. In servo control unit <NUM>, position control block <NUM> receives a fourth value, which is obtained by adding a position command θc and a bending correction value θsco (also referred to as position-command correction value θsco) sent from bending correction block <NUM>. The motor rotation position θM is subtracted from the fourth value so as to obtain a fifth value. The fifth value is multiplied by a position proportional gain Kpp so as to generate a speed command ωc. The motor rotation position θM is obtained from a detection signal of first encoder <NUM>, which is a position detector. The following description will be focused on the structure of first servo control unit <NUM>, but the description holds true for the structure of the second-to-seventh servo control units <NUM>.

In speed control block <NUM>, the motor rotation position θM is differentiated to obtain a motor speed ωM. The motor speed ωM is subtracted from the speed command ωc so as to obtain a sixth value. The sixth value is multiplied by a velocity proportional gain Kps so as to obtain a seventh value. The sixth value is also integrated and multiplied by a velocity integral gain Ki so as to obtain an eighth value. The seventh and eighth values are added together to calculate the current to be supplied to first motor <NUM>, thereby obtaining the motor current command IM.

<FIG> shows a detailed structure of the bending correction block shown in <FIG>. Bending correction block <NUM> includes dynamic calculation block <NUM>. As shown in Formula <NUM>, finding the position-command correction value θsco requires finding the external torque τddyn in advance. However, attaching a torque sensor to each joint shaft to find the external torque τddyn is not preferable because torque sensors are generally expensive. Moreover, attaching a torque sensor to each joint shaft causes robot arm <NUM> to bend more.

To avoid this happening, dynamic calculation block <NUM> finds an external torque value τdyn by using the position commands θc to θ7c sent from main control unit <NUM> for all the joint shafts. In dynamic calculation block <NUM>, dynamic calculation is performed using the position commands θc to θ7c for all the shafts, the speed components, which are the differential values of the position commands θc to θ7c, and the acceleration components, which are second-order differential values. This calculation finds the external torque applied to each joint shaft.

The external torque value τdyn is multiplied by the reciprocal of the spring constant Ks (obtained by sign inversion) so as to find the position-command correction value θsco.

As described earlier, the external torque value τdyn contains both the gravitational torque τg and the interference torque τa. Therefore, the actual corrected amplitude of the bending due to the interference torque τa having a high frequency is small. If the position-command correction value θsco is increased accordingly, the position-command correction value to reduce the gravitational torque τg is also increased. The arm is raised more than it is pulled down by gravitation as described earlier.

<FIG> is a block diagram of the first servo control unit in the present exemplary embodiment. In first servo control unit <NUM>, the functional blocks except bending correction block <NUM> are the same as those shown in <FIG>, so that their description is not be repeated here. <FIG> shows the detailed structure of bending correction block <NUM> shown in <FIG>. Bending correction block <NUM> (bending correction means) includes dynamic calculation block <NUM> and gravity calculation block <NUM>. Block <NUM> finds the external torque value τdyn using the position commands θc to θ7c sent from main control unit <NUM> to all the joint shafts. The external torque applied to each joint shaft in dynamic calculation block <NUM> is calculated in the same manner as that shown for comparison. To be more specific, when the number of the joint shafts is n (n is an integer not less than <NUM>), one bending correction block <NUM> receives n position commands θc to θnc corresponding to the n servo motors <NUM> sent from main control unit <NUM> and correct the position error of robot arm <NUM>, which is caused by the bending of reduction gears <NUM>, based on these position commands.

Meanwhile, in the present exemplary embodiment, each bending correction block <NUM> separates the gravitational torque τg and interference torque τa from each other to calculate the respective position-command correction values. Gravity calculation block <NUM> receives the position commands θc to θ7c in the same manner as dynamic calculation block <NUM>. Block <NUM> finds the gravitational torque τg through dynamic calculation with the above-mentioned speed and acceleration components set at zero.

The gravitational torque τg is subtracted from the external torque value τdyn so as to obtain the interference torque τa as shown in Formula <NUM>.

As described above, in the present exemplary embodiment, each of the external torque value τdyn, the gravitational torque τg, and the interference torque τa is calculated in bending correction block <NUM>.

Furthermore, the gravitational torque τg thus obtained is used to obtain the first position-command correction value θsgc from Formula <NUM>.

Meanwhile, the bending correction value θsac to reduce the interference torque τa is calculated by Formula <NUM> shown below.

As mentioned earlier, the amplitude and phase of the bending correction value θsac should be compensated to correspond to the actual position θL. For example, they should be subjected to PD compensation including proportion comprehension and phase lead compensation. The value θskc (hereinafter, second position-command correction value) obtained by the compensation is shown in Formula <NUM>. <MAT> where Kpa represents a PD compensation proportional gain and a coefficient for amplitude compensation; Kda represents a PD compensation differential gain and a coefficient for phase compensation; and (s - θ sac) represents the first derivative of θsac with respect to time.

The position-command correction value θsc is obtained from the values calculated by Formula <NUM> and Formula <NUM>.

Gravity calculation block <NUM> and the calculate function of bending correction block <NUM>, which is shown in Formula <NUM>, will be together referred to as first position-command-correction-value calculation means <NUM>. Dynamic calculation block <NUM>, gravity calculation block <NUM>, and the calculate function of bending correction block <NUM>, which is shown in Formulas <NUM>, <NUM> and <NUM>, will be together referred to as second position-command-correction-value calculation means <NUM>.

As shown in <FIG>, first servo control unit <NUM> receives a new position command (θc + θsc) obtained by adding the position-command correction value θsc shown in Formula <NUM> to the position command θc sent from main control unit <NUM> to first servo control unit <NUM>. First servo control unit <NUM> uses the new position command to control the driving of first motor <NUM>.

<FIG> show the response characteristics of the transfer function with respect to the operating frequency of the main shafts in the present exemplary embodiment. More specifically, <FIG> shows the frequency response characteristics of the amplitude of the transfer function WcL, and <FIG> shows the frequency response characteristics of the phase of the transfer function WcL. <FIG> shows a time waveform of the first position-command correction value θsgc and the actual position θL when a sine wave of <NUM> is applied as the first position-command correction value θsgc to the main shafts having the frequency characteristics shown in <FIG>. <FIG> shows a time waveform of the second position-command correction value θskc and the actual position θL when a sine wave of <NUM> is applied as the second position-command correction value θskc to the main shafts.

As shown in <FIG>, the actual position θL fully follows the first position-command correction value θsgc. This value θsgc has a frequency of <NUM>, which is the maximum oscillation frequency of robot arm <NUM> twisted by the gravitational torque τg. Furthermore, as shown in <FIG>, the actual position θL fully follows the second position-command correction value θskc having a frequency of <NUM> in both amplitude and phase.

As described hereinbefore, in the present exemplary embodiment, the gravitational torque τg and the interference torque τa, which are different in operation response frequency and are contained in the external torque τddyn applied to robot arm <NUM>, are separately calculated. These torques are used to find the first and second position-command correction values θsgc and θskc. This enables correcting the position command without overestimating the influence of bending due to the gravitational torque τg. These position-command correction values θsgc and θskc are added to the original position command θc so as to correct the position error of robot arm <NUM> with high accuracy. Bending correction blocks <NUM> receive seven position commands θc to θ7c sent from main control unit <NUM> to seven servo control units <NUM>. Bending correction blocks <NUM> find the external torque value τdyn and the gravitational torque τg through the dynamic calculation based on the position commands θc to θ7c. This eliminates the need to provide a dedicated component such as a torque sensor for measuring the torque, thereby reducing the cost of robot <NUM> and robot control device <NUM>. The interference torque τa is obtained by subtracting the gravitational torque τg from the external torque value τdyn. This allows easy separation between the gravitational torque τg and the interference torque τa, thereby correcting the position error of robot arm <NUM> caused by bending.

These days, the position teaching of robots is more and more performed not by the teaching playback method but offline using a GUI on a PC. According to the teaching playback method, it does not matter if there is a discrepancy between the tip position of robot arm <NUM> specified to motors <NUM> and the actual tip position as long as repeatability is high. Meanwhile, offline teaching needs absolute position accuracy, so that if bending correction is inappropriate, the robot cannot be controlled properly.

In the present exemplary embodiment, the position error of robot arm <NUM> due to bending can be corrected with high accuracy, so that the offline teaching can be performed smoothly.

<FIG> is a block diagram of the first servo control unit in the present exemplary embodiment. The present exemplary embodiment differs from the first exemplary embodiment in the following three aspects. Firstly, bending correction block <NUM> finds an interference torque current compensation value Isc (hereinafter, current compensation value) based on the interference torque τa. Secondly, the current compensation value Isc is added to the motor current command IM. Thirdly, the first position-command correction value θsgc alone is added to the position command θc.

<FIG> shows a detailed structure of the bending correction block shown in <FIG>.

In bending correction block <NUM>, gravity calculation block <NUM> calculates the gravitational torque τg, and the interference torque τa is calculated by Formula <NUM> similar to the first exemplary embodiment. The position-command correction value θsc shown in Formula <NUM> is changed as shown in Formula <NUM>.

Meanwhile, the interference torque τa is used not to find the bending correction value θsac but to find the current corresponding to the bending correction value θsac. In this case, similar to the second position-command correction value θskc in the first exemplary embodiment, the current also needs to be compensated in amplitude and phase. Therefore, the current compensation value Isc is expressed by Formula <NUM> below. <MAT> where Kpb represents an interference-torque-current-compensation proportional gain, which is a coefficient for amplitude compensation. Kdb represents an interference-torque-current-compensation differential gain, which is a coefficient for phase compensation. (s-τa) represents the first derivative of τ a with respect to time. The other calculation components are the same as mentioned above.

The motor current command IMb sent from speed control block <NUM> is expressed by Formula <NUM> below based on the value calculated by Formula <NUM>.

Dynamic calculation block <NUM>, gravity calculation block <NUM>, and the calculate function of bending correction block <NUM>, which is shown in Formulas <NUM> and <NUM>, are together referred to as current compensation value calculation means <NUM>.

As shown in <FIG>, the position-command correction value θsc (= θsgc) expressed by Formula <NUM> is added to the position command θc sent from main control unit <NUM> to first servo control unit <NUM>, so that a new position command (θc + θsgc) is obtained. The new position command is sent to the position control block <NUM> of first servo control unit <NUM>. Furthermore, the current compensation value Isc is added to the motor current command IM generated in position control block <NUM> and speed control block <NUM> so that a new motor current command IMb is obtained. The new position command (θc + θsgc) and the new motor current command IMb are used to control the driving of first motor <NUM>.

As described above, in the present exemplary embodiment, the current compensation value Isc to reduce the interference torque τa is added to the motor current command IM so as to apply bending correction based on the interference torque τa directly to the motor current. This improves the response of load <NUM>, or in other words, the effect of correcting bending caused by the interference torque τa.

In the present exemplary embodiment, the bending due to the interference torque τa is corrected not by adding the position-command correction values θsc to θ7sc to the position commands θc to θ7c sent from outside block <NUM> or <NUM>, but by adding the current compensation value Isc to the motor current command IM generated in blocks <NUM> and <NUM>. Thus, the bending is compensated indirectly, so that the current compensation value Isc added to the motor current command IM does not necessarily entirely contribute to the bending reduction. Therefore, in some cases, determining the exact current compensation value Isc needs to perform operation verification using an actual articulated robot and gain adjustment.

The first and second exemplary embodiments have described seven-axis vertical articulated robot <NUM>; alternatively, however, the number of joint shafts is not limited to seven and can be properly changed according to the specification of the robot. Robot control device <NUM> is configured so that n servo control units <NUM> (n is an integer not less than <NUM>) drive the n servo motors <NUM> so as to drive n joint shafts connected to the n servo motors <NUM> through reduction gears <NUM>. Furthermore, bending correction blocks <NUM> and <NUM> receive the n position commands θc to θnc sent from main control unit <NUM> to the n servo control units <NUM>, and correct the position error of robot arm <NUM>, which is caused by bending, based on these position commands. Thus, the robot control device of the present disclosure is applicable to articulated robots with two or more shafts.

Claim 1:
A robot control device (<NUM>) for controlling movement of a robot arm (<NUM>) having a joint shaft (J1 - J7) driven through a reduction gear (<NUM>) connected to a servo motor (<NUM>), the robot control device (<NUM>) comprising:
a main control unit (<NUM>) configured to transmit a position command to the joint shaft (J1-J7);
a bending correction means (<NUM>-<NUM>) configured to correct a position error of the robot arm (<NUM>), the position error being due to bending of the reduction gear (<NUM>), the bending correction means (<NUM>) including:
a first position-command-correction-value calculation means (<NUM>) configured to calculate, based on the position command, a gravitational torque as part of an external torque causing the bending of the reduction gear (<NUM>) and to find a first position-command correction value based on the gravitational torque; and
a second position-command-correction-value calculation means (<NUM>) configured to calculate, based on the position command and the gravitational torque, an interference torque as part of the external torque, the interference torque being due to interference to which the joint shaft (J1-J7) is subjected and to find a second position-command correction value by multiplying an amplitude and a phase of a bending correction value obtained based on the interference torque by coefficients for amplitude compensation and phase compensation, respectively; and
a servo control unit (<NUM>) configured to drive the servo motor (<NUM>) based on a new position command obtained by adding the first position-command correction value and the second position-command correction value to the position command,
wherein the first position-command correction value θ sgc, the bending correction value θsac, and the second position-command correction value θ skc are obtained by the formulae <MAT> <MAT> <MAT>
where τ g represents the gravitational torque, τ a represents the interference torque, Ks represents the spring constant of the reduction gear, Kpa represents a PD compensation proportional gain and a coefficient for the amplitude compensation, Kda represents a PD compensation differential gain and a coefficient for the phase compensation, and(s·θ sac) represents the first derivative of θsac with respect to time.