JP7172248B2 - Calibration method of force control robot and tool coordinate system - Google Patents

Description

The present invention relates to a force-controlled robot and a tool coordinate system calibration method.

Patent Literature 1 below discloses a force-controlled robot calibration apparatus and method capable of calibrating necessary parameters in consideration of installation errors even when the installation accuracy of the robot is low. This background art relates to a force control robot in which a tool is attached to the tip of a robot arm through a force sensor, and obtains the measurement values of the force sensor when the robot arm is moved in a plurality of postures, and the measurement values. A plurality of parameters including the tool weight, the gravity direction vector, and the tool center-of-gravity position vector are calculated by obtaining the posture data of the force sensor when the tool is pressed.

JP 2012-040634 A

By the way, in the force-controlled robot, calibration of the reference point of the tool, that is, the tool center point (TCP) is also performed. As a calibration method of this TCP, the reaction force (contact force) detected by the force sensor when the tip of the tool is pressed against a predetermined calibration jig is compared with a predetermined threshold value, and the tip of the tool and the calibration jig are compared. A method (touch sensing method) for obtaining a contact position with a tool is known.

However, in the above touch sensing method, it is difficult to set a threshold value. Accuracy is a problem because the amount of movement of the tip of the tool is an error.

SUMMARY OF THE INVENTION The present invention has been made in view of the circumstances described above, and an object of the present invention is to improve the accuracy of calibrating the reference point of a tool in a force-controlled robot.

In order to achieve the above object, in the present invention, as a first solution for a force-controlled robot, a predetermined tool is attached to the tip of a robot arm via a force sensor, and based on the detected value of the force sensor, A force-controlled robot in which the robot arm is controlled, wherein the detected value before the tool contacts the calibration jig by controlling the robot arm to press the tool against a predetermined calibration jig and the amount of movement of the tool, and a second function indicating the relationship between the detected value after the tool contacts the calibration jig and the amount of movement of the tool. a reference point obtaining unit for obtaining a tool center point of the tool as an intersection of the first function and the second function; and tool coordinates unique to the tool based on the tool center point and a calibration unit for calibrating the system.

In the present invention, as a second solution for a force control robot, in the first solution, the function acquisition unit sequentially acquires time-series data regarding the detection value and the movement amount from the start of pressing of the tool. and obtaining the first function and the second function based on the time-series data.

According to the present invention, as a third solution to the force control robot, in the first or second solution, the tool includes a float folder provided with an elastic element that can be stretched in one axial direction of the tool coordinate system. The force sensor is attached to the force sensor through the

In the present invention, as a fourth solving means related to the force control robot, in the third solving means, the function acquisition unit is configured to generate a Acquiring a third function indicating the relationship between the detected value and the movement amount of the tool, and the reference point acquisition unit acquires the elastic characteristic of the elastic element in addition to the tool center point. adopt.

In the present invention, as a fifth solution related to a force control robot, in any one of the first to fourth solutions, the tool is a tool that contacts a predetermined workpiece and performs predetermined processing. adopt the means.

According to the present invention, as a sixth solution to the force control robot, in any one of the first to fourth solutions, the tool contacts the object to be measured in order to detect the position of the object. A means of being a contact is adopted.

Further, in the present invention, as a solution to the tool coordinate system calibration method, there is provided a tool coordinate system calibration method unique to a predetermined tool mounted on the tip of a robot arm, wherein the tool is a predetermined calibration jig. a first function indicating the relationship between the detected value of the contact force acting on the tool before contacting the tool and the amount of movement of the tool; and the detected value and the tool after the tool contacts the calibration jig. and a reference point obtaining step of obtaining a tool center point of the tool as an intersection of the first function and the second function. and a calibration step of calibrating a tool coordinate system specific to said tool based on said tool center point.

According to the present invention, it is possible to improve the calibration accuracy of the reference point of the tool in the force-controlled robot as compared with the conventional art.

1 is a block diagram showing the overall configuration of a force-controlled robot according to one embodiment of the present invention; FIG. FIG. 4 is a cross-sectional view showing the structure of the distal end portion of the force-controlled robot according to one embodiment of the present invention; 4 is a flow chart showing calibration operation of the force-controlled robot according to one embodiment of the present invention. 4 is a graph showing contact force characteristics of a force-controlled robot according to one embodiment of the present invention;

An embodiment of the present invention will be described below with reference to the drawings.
The force-controlled robot according to this embodiment includes a robot arm 1 and a robot controller 2, as shown in FIG. The robot arm 1 is a multi-joint robot as shown, and is a robot main body that performs desired work under the control of a robot controller 2 . This robot arm 1 is fixedly installed on a horizontal floor surface.

More specifically, the robot arm 1 includes a first swivel portion 1a, a base portion 1b, a first joint 1c, a first arm 1d, a second joint 1e, a second arm 1f, a third joint 1g, and a tip portion 1h. ing. The first turning portion 1a is provided between the floor surface and the base portion 1b, and sets the turning angle of the base portion 1b with respect to the floor surface. That is, the first turning portion 1a is a device for turning the base portion 1b at a predetermined turning angle in the horizontal plane. The base portion 1b has a lower portion rotatably supported on the floor via the first rotating portion 1a, and an upper end to which a first arm 1d is connected via a first joint 1c. The base portion 1b is a portion of the robot arm 1 that is relatively heavy.

The first joint 1c includes a rotating shaft in a horizontal posture and a driving unit for setting a rotation angle (first joint angle) of the rotating shaft. Adjust the elevation angle of 1d. The first arm 1d is a rod-shaped member having a predetermined length, the proximal end of which is supported by the base 1b via the first joint 1c, and the distal end of which is connected to the second joint 1e via the second joint 1e. .

The second joint 1e includes a rotating shaft in a horizontal posture and a driving unit for setting the rotation angle (second joint angle) of the rotating shaft. 2 Adjust the elevation angle of arm 1f. The second arm 1f is a rod-shaped member having a predetermined length, the base end of which is supported by the first arm 1d via the second joint 1e, and the distal end portion 1h is connected to the distal end via the third joint 1g. there is The third joint 1g includes a rotating shaft in a horizontal posture and a driving unit for setting the rotation angle (third joint angle) of the rotating shaft. Adjust the elevation angle of the portion 1h.

Here, the first turning section 1a described above outputs the detected value of the first turning angle to the robot controller 2 as the first turning angle detected value T1. Also, the first joint 1c outputs the detected value of the first joint angle to the robot controller 2 as the first joint angle detected value E1. The second joint 1e also outputs the detected value of the second joint angle to the robot controller 2 as the second joint angle detected value E2. Further, the third joint 1g outputs the detected value of the third joint angle to the robot controller 2 as the third joint angle detected value E3.

The tip portion 1h includes a force sensor 1i, a second turning portion 1j, a float holder 1k and a tool M as shown in FIG. The force sensor 1i is provided on the most proximal side of the distal end portion 1h as shown in the figure, and detects the reaction force (external force) acting on the tool M when the tool M comes into contact with the calibration jig J or the like. It is a force sensor that detects force. The force sensor 1i outputs to the robot controller 2 a detection value (contact force detection value Ep) indicating the contact force. Incidentally, the portion 1i in FIG. 2 is, more precisely, the portion in which the force sensor is accommodated.

The second turning portion 1j is provided next to the force sensor 1i at the distal end portion 1h and on the proximal end side, and has one end connected to the force sensor 1i and the other end connected to the float folder 1k. The second swivel portion 1j has a swivel shaft whose posture is set in the longitudinal direction (vertical direction in FIG. 2) of the tip portion 1h. (Second turning angle).

That is, the second turning portion 1j is a portion that turns the float folder 1k around the turning axis at the second turning angle with respect to the force sensor 1i. Such a second turning section 1j outputs the detected value of the second turning angle to the robot controller 2 as the second turning angle detected value T2.

The float folder 1k is a holding member that holds the tool M detachably and pushably in one axis direction (x-axis direction) of the tool coordinate system (x-axis, y-axis and z-axis) shown in FIG. , a shaft 1m, a tubular member 1n and a coil spring 1p (elastic element). The shaft 1m is a rod-shaped member having a predetermined length, one end (rear end) of which is fixed to the other end of the second turning section 1j, and the other end (front end) of which is fixedly attached to a tubular member 1n. For example, the diameter of the shaft 1m is set slightly larger than the inner diameter of the tubular member 1n, and the tubular member 1n is fixedly attached to the tip of the shaft 1m by press-fitting the tip of the shaft 1m into the tubular member 1n.

The tubular member 1n is a bottomless cylindrical member having a predetermined inner diameter, and openings are formed at both ends. Of the pair of openings in this tubular member 1n, one end (rear end) side opening is fixedly attached to the tip of the shaft 1m described above, and the rear end portion of the tool M can be pushed out into the other end (tip) side opening. is inserted in the

As illustrated, the coil spring 1p is accommodated in the inner space (cylindrical inner space) of the tubular member 1n so that one end faces the tip end face of the shaft 1m and the other end faces the rear end face of the tool M. there is The coil spring 1p functions as a compression spring that can expand and contract in the longitudinal direction of the tip portion 1h, that is, in the uniaxial direction (x-axis direction) of the tool coordinate system shown in FIG.

That is, when the tool M is pressed from the front end side to the rear end side, one end of the coil spring 1p contacts the front end surface of the shaft 1m and the other end of the coil spring 1p contacts the rear end surface of the tool M, further When M is pressed, the coil spring 1p contracts in the longitudinal direction of the tip portion 1h.

The tool M is attached to the tip of the tip portion 1h, and is a tool for machining a work, for example. This tool M is detachably and pushably mounted via the float folder 1k. When this tool M is used to machine a work, it is important to adjust the contact force of the tool M with respect to the work, that is, to control the position of the tool M by the robot arm 1 . Calibration processing of the reference position of the tool M, ie, the tool center point, which will be described later, is indispensable for accurate position control of the tool M.

For such a tool M, a unique coordinate system (tool coordinate system) of the tool M is set as shown in FIG. 2, for example. This tool coordinate system is a three-dimensional coordinate system for control of the force-controlled robot, and is therefore also a unique coordinate system for the robot arm 1 . In FIG. 2, of the three orthogonal axes (x-axis, y-axis, and z-axis), the x-axis is set in the direction of the central axis of the bar-shaped tool M, and the y-axis is set in one of two directions perpendicular to the central axis. The z-axis is set in the other of the two directions perpendicular to the central axis.

On the other hand, the robot controller 2 is a software control device that optimally controls the position of the tool M by controlling the robot arm 1 based on a prestored control program. That is, the robot controller 2 issues first and second turning commands based on the first to third joint angle detection values E1 to E3, the first and second turning angle detection values T1 and T2, and the contact force detection value Ep. Values Q1, Q2 and first to third joint angle command values R1 to R3 are calculated, and these first and second turning command values Q1, Q2 and first to third joint angle command values R1 to R3 are applied to the robot arm 1. to control the first and second turning parts 1a and 1j and the first to third joints 1c, 1e and 1g.

In the force control robot configured as described above, each movable part of the robot arm 1, that is, the first turning part 1a, the first joint 1c, the second joint 1e, the third joint 1g, and the second turning part 1j is controlled by the robot controller 2. A workpiece (not shown) placed on the workbench D is machined by being appropriately controlled. That is, the robot controller 2 controls the relative position of the tool M with respect to the work, and the work is machined by bringing the tool M into contact with the work with a desired contact force.

Here, the present invention relates to calibration of the position of the tool M, that is, calibration of the tool coordinate system, performed before the robot arm 1 processes a workpiece in such a force-controlled robot. The robot controller 2 has an operation mode (calibration mode) for calibrating the position of the tool M in addition to a normal operation mode for machining the workpiece on the worktable D.

For this reason, FIG. 1 shows a calibration jig J that is used to calibrate the tool coordinate system, omitting the workpiece. This calibration jig J has a reference plane j1 whose mounting position with respect to the workbench D is strictly controlled. This reference plane j1 is an x-axis reference plane provided for calibrating the x-axis among the three orthogonal axes (x-axis, y-axis, and z-axis) of the tool coordinate system. The calibration jig J is provided with a y-axis reference surface and a z-axis reference surface (not shown) in addition to the reference surface j1.

Although the details will be described later, the robot controller 2 controls the robot arm so as to press the tool M against the calibration jig J in the calibration mode, thereby adjusting the contact force before the tool M contacts the calibration jig J. A first function representing the relationship between the detected value Ep and the movement amount of the tool M, and a second function representing the relationship between the contact force detection value Ep after the tool M contacts the calibration jig J and the movement amount of the tool M. to get the functions of

More specifically, the robot controller 2 sequentially acquires time-series data representing the relationship between the contact force detection value Ep and the amount of movement of the tool M from the start of pressing of the tool M against the calibration jig J as calibration data. , a first function and a second function are obtained based on this calibration data. That is, the robot controller 2 sequentially stores calibration data that is sequentially acquired in the internal memory, reads the calibration data from the internal memory, and performs predetermined information processing to obtain the first function and the second function. to get

Also, the robot controller 2 acquires the tool center point of the tool M as the intersection of the first function and the second function. Furthermore, this robot controller 2 calibrates the tool coordinate system based on the tool center point. In other words, such a robot controller 2 corresponds to the function acquiring section, the reference point acquiring section, and the calibrating section in the present invention.

Next, the calibration process, that is, the calibration method of the force-controlled robot according to this embodiment will be described in detail along the flowchart shown in FIG.

The calibration process in the force control robot is for accurately associating the above-described tool coordinate system (specific coordinate system of the tool M) with the specific coordinate system of the workpiece (work coordinate system). Through this calibration process, the positional relationship between the tool M and the workpiece in the three-dimensional space becomes clear, so that the workpiece can be machined with high accuracy using the tool M.

This calibration process is performed for the above-described orthogonal three axes (x-axis, y-axis, and z-axis). Of these three axes, calibration of the x-axis corresponding to the axial direction of the tool M will be described first. The robot controller 2 controls each movable part of the robot arm 1 to position the tool M at a position (initial position x ₀ ) where it does not come into contact with the reference plane j1 (step S1).

Then, the robot controller 2 starts moving the tool M from the initial position x0 toward the _x -axis direction, that is, toward the reference plane j1, and starts acquiring the contact force detection value Ep in the x-axis direction. That is, the robot controller 2 sequentially moves the tool M in the x-axis direction to obtain time-series data (calibration data) indicating the relationship between the movement amount of the tool M and the contact force detection value Ep (step S2). ). When the robot controller 2 moves the tool M by a predetermined displacement amount ΔL (step S3), the robot controller 2 stops the movement of the tool M and ends acquisition of time-series data (calibration data) (step S4).

That is, the robot controller 2 obtains a plurality of contact force detection values Ep corresponding to the displacement of the tool M over a predetermined displacement amount _ΔL from the initial position x0, that is, the movement of the tool M, through the series of processes of steps S1 to S4 described above. A contact force characteristic indicating the relationship between the quantity and the contact force is obtained as data for calibration. The initial position _x0 and the predetermined displacement amount ΔL are movement amounts estimated in advance so that the tool M, which is not in contact with the reference surface _j1 at the initial position x0, sufficiently contacts the reference surface j1.

FIG. 4A is a graph showing an example of the contact force characteristics (calibration data). This contact force characteristic is a line graph consisting of four linear portions f1 to f4 as shown. Of the four linear portions f1 to f4, the first linear portion f1 has a contact force characteristic (for calibration) corresponding to a region (movement region) in which the tip of the tool M does not contact the reference surface j1 during movement of the tool M. data), and although there may be some noise, the contact force is almost zero.

The second linear portion f2 is a contact force characteristic (calibration data ), and the rate of change (slope) of the contact force is relatively steep. The third linear portion f3 is the contact force characteristic (calibration data) corresponding to the region where the coil spring 1p is contracted, and the rate of change (slope) of the contact force compared to the second linear portion f2 is moderate.

Finally, the fourth linear portion f4 is the contact force characteristic (calibration data) corresponding to the region where the coil spring 1p has completely contracted, and the contact force is lower than that of the third linear portion f3. The rate of change (slope) is steep. That is, such first to fourth linear portions f1 to f4 are provided by the structure for holding the tool M at the tip portion 1h of the robot arm 1. As shown in FIG.

Since the tip portion 1h shown in FIG. 2 has a holding structure for holding the tool M via the float folder 1k, it exhibits a contact force characteristic consisting of the first to fourth linear portions f1 to f4. When 1k is deleted and the tool M is held by a rigid holding structure, the contact force characteristic becomes as shown in FIG. 4(b). That is, the retention characteristic in this case consists of two linear portions, an initial linear portion fa and a late linear portion fb.

The initial straight portion fa corresponds to a movement area where the tip of the tool M is not in contact with the reference plane j1, similarly to the first straight portion f1 described above, and the contact force is approximately zero. Like the second straight portion f2, the latter straight portion fb is a region after the tip of the tool M contacts the reference plane j1, and the rate of change (inclination) of the contact force is relatively steep. That is, in this case, since the float folder 1k does not intervene, that is, the connection between the robot arm 1 and the tool M in the x-axis direction is not a flexible connection but a rigid connection. There is no area to

Now, the robot controller 2 identifies the four linear portions f1 to f4 as individual regions in the contact force characteristics obtained by the processing of steps S1 to S4 (step S5). For example, among the calibration data, a data set of a plurality of adjacent contact force detection values Ep that give approximately the same rate of change, that is, four data sets respectively corresponding to the four linear portions f1 to f4 are generated, and each data Sets are set to individual regions corresponding to the four straight line portions f1 to f4.

Then, the robot controller 2 applies the method of least squares or the like to a plurality of calibration data (time-series data) belonging to each data set to obtain functions of the four linear portions f1 to f4 (first to fourth functions ) is calculated (step S6). A series of steps S1 to S6 described above correspond to the function acquisition step of the present invention.

Then, the robot controller 2 calculates the tool center point as the intersection point P1 between the function of the first linear portion f1 (first function) and the function of the second linear portion f2 (second function) (step S7 ). The processing of step S7 corresponds to the reference point acquisition step of the present invention.

Here, the y-axis and z-axis other than the X-axis are directions in which the float folder 1k does not function. That is, the float folder 1k holds the tool M in a flexible state only in the x-axis direction. Therefore, the contact force characteristics obtained for the y-axis and z-axis are as shown in FIG. A point of intersection Pa is calculated as the tool center point.

Further, the robot controller 2 calculates the spring constant (elastic characteristic) of the coil spring 1p as the slope of the function (third function) of the third linear portion f3 (step S8). Then, the robot controller 2 uses the tool center point acquired in step S7 to correct the origin of the tool coordinate system, that is, calibrate the tool coordinate system (step S9). The process of step S9 corresponds to the calibration process of the present invention.

According to this embodiment, the tool center point is calculated as the points of intersection P1 and Pa between the first function and the second function with respect to the three orthogonal axes (x-axis, y-axis, and z-axis). It is possible to calibrate the tool coordinate system in the robot with higher accuracy than before. That is, according to the present embodiment, the tool coordinate system is set with higher precision than before with respect to all of the three orthogonal axes, ie, the x-axis to which the float folder 1k imparts elasticity, and the y-axis and z-axis to which elasticity is not imparted. It is possible to calibrate to

Further, according to this embodiment, the spring constant (elastic characteristic) of the coil spring 1p (elastic element) is obtained based on the third function. can be calibrated at the same time. Therefore, according to this embodiment, it is possible to realize machining of a workpiece with higher precision than in the past.

It should be noted that the present invention is not limited to the above-described embodiments, and for example, the following modifications are conceivable.
(1) In the above embodiment, the case where the present invention is applied to the force-controlled robot having the robot arm 1 has been described, but the present invention is not limited to this. That is, the configuration of the robot arm in the present invention is not limited to the robot arm 1. FIG.

(2) In the above embodiment, the case where the present invention is applied to a force-controlled robot that can move along three orthogonal axes (x-axis, y-axis, and z-axis) has been described, but the present invention is not limited to this. For example, the present invention may be applied to a force-controlled robot that can move only on one axis or only on two axes. In this case, one or two axes are to be calibrated.

(3) In the above embodiment, the spring constant of the coil spring 1p was obtained, but the present invention is not limited to this. The spring constant of the coil spring 1p may change as the float folder 1k is used. By obtaining the actual spring constant in the calibration process described above, the position of the tool M with respect to the workpiece, that is, the contact force with respect to the workpiece, can be controlled more accurately in the actual operation (actual machining of the workpiece) of the force-controlled robot after configuration. becomes possible. Therefore, machining with higher accuracy can be achieved.

(4) In the above embodiment, the case where the tool M is a tool for machining a workpiece has been described, but the present invention is not limited to this. The tool M may be, for example, a contact that contacts the object to be measured for position detection of the object. Such a contactor is used, for example, for three-dimensional measurement of the shape of an object to be measured.

In the case of three-dimensional measurement, that is, when the tool M is a contactor, the object to be measured is placed on the workbench D, and the robot arm 1 is operated to bring the tool M (contactor) into contact with each part of the object to be measured. The three-dimensional shape of the object to be measured is measured. In such three-dimensional measurement of an object to be measured, the position of the object to be measured may be confirmed by bringing the tool M (contactor) into contact with a specific portion of the object to be measured at the start of measurement. In such three-dimensional measurement, for example, when the position of the object to be measured is not the normal position, the reference point of the tool M can be calibrated with high accuracy according to the present invention, so the position of the object to be measured can be corrected. It is possible to reset to the normal position with high precision.

Reference Signs List 1 robot arm 1a first turning section 1b base 1c first joint 1d first arm 1e second joint 1f second arm 1g third joint 1h tip section 1i force sensor 1j second turning section 1k float folder 1m shaft 1n tubular member 1p Coil spring (elastic element)
2 robot controller D worktable E1 1st joint angle detected value E2 2nd joint angle detected value E2 3rd joint angle detected value Ep Contact force detected value J Calibration jig j1 Reference plane M Tool Q1 First rotation angle command value Q2 2 Turning angle command value R1 1st joint angle command value R2 2nd joint angle command value R3 3rd joint angle command value T1 1st turning angle detected value T2 2nd turning angle detected value

Claims (6)

A force control robot in which a predetermined tool is attached to the tip of a robot arm via a force sensor, and the robot arm is controlled based on a detection value of the force sensor,
A first method for showing the relationship between the detected value before the tool comes into contact with the calibration jig and the movement amount of the tool by controlling the robot arm so as to press the tool against a predetermined calibration jig. a function acquisition unit that acquires a function and a second function that indicates the relationship between the detected value after the tool contacts the calibration jig and the amount of movement of the tool;
a reference point acquisition unit that acquires a tool center point of the tool as an intersection of the first function and the second function;
a calibration unit that calibrates a tool coordinate system specific to the tool based on the tool center point;
A force control robot, wherein the tool is attached to the force sensor via a float folder having an elastic element that is elastic in one axial direction of the tool coordinate system. The function acquisition unit sequentially acquires time-series data relating to the detection value and the movement amount from the start of pressing of the tool, and acquires the first function and the second function based on the time-series data. The force-controlled robot according to claim 1, characterized in that: The function acquisition unit acquires a third function indicating the relationship between the detected value and the amount of movement of the tool when the elastic element contracts after the tool contacts the calibration jig,
3. The force control robot according to claim 1, wherein the reference point acquisition unit acquires elastic characteristics of the elastic element in addition to the tool center point. 4. The force control robot according to any one of claims 1 to 3, wherein the tool is a tool that contacts a predetermined workpiece and performs predetermined processing. The force control robot according to any one of claims 1 to 3, wherein the tool is a contactor that contacts the object to be measured for detecting the position of the object. A method for calibrating a tool coordinate system specific to a predetermined tool attached to the tip of a robot arm, comprising:
a first function representing a relationship between a detected value of contact force acting on the tool before the tool contacts a predetermined calibration jig and an amount of movement of the tool; a function obtaining step of obtaining a second function indicating the relationship between the later detected value and the amount of movement of the tool;
a reference point obtaining step of obtaining a tool center point of the tool as an intersection of the first function and the second function;
calibrating a tool coordinate system specific to the tool based on the tool center point;
A method for calibrating a tool coordinate system, wherein the tool is attached to a force sensor via a float folder having an elastic element that is elastic in one axial direction of the tool coordinate system.

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