Grinder robot

Abstract: A grinder robot having a grinder can be operated by both of a force control and a position control to grind a work surface quickly and accurately. When a grinding point by said grinder has not reached yet to a target position to finish grindings, force loop drive control means (C5) are selected to drive the grinder in order to grind a work by a force control. After a grinding point by said grinder has reached to said target position, then, position drive control means (C4) are selected to drive the grinder by a position control in order to execute precise grindings. A grinding amount in every grinding in a force control area is relatively large and a grinding speed in every grinding in a position control area is relatively fast. As a result, a work can be grinded quickly and accurately by the grinder robot.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention is related to a grinder robot to grind a work by using a grinder equipped at the tip of robot arm and by moving the grinder towards the work. Especially, this invention is related to a grinder robot by which a work can be grinded quickly and accurately. DESCRIPTION OF THE PRIOR ART In general, a grinding machine controls a grinder to move relatively to and from a work by a position feedback loop. In other words, a grinding machine in prior arts controls grinding points of a grinder to be target positions for grindings by a position loop, and makes a finishing surface to be a target surface. However, the grinding machine, in which a grinder is controlled to move by a position loop as mentioned above, detects only positions by the loop. It may, therefore, damage a grindstone by forcing it to grind a rugged surface of a work if the grinder is set directly to grind towards a target position. This is because the grinder cuts into the rugged surface. To avoid this, the grinder can be moved slowly in every shuttle move so as to reach the target position gradually. In this case, however, a problem to increase processing time arises. In order to avoid disadvantages arised from the use of this kind of grinder robot controlled by a position loop, it can be thought to detect force given to the grinder and to control robot arm's move so as to keep the force constant. In this method, however, it is difficult to grind a work surface completely smooth, because it cannot control the position of grinding point. From EP-A- 331 265 a position/force controlling apparatus for a working machine is known. According to this document a computing device executes position/force computation for the control of the position and of the force of the working tool. From US 4 817 848 an automatic ultrasonic wire bonder is known which employs a motion servo to maintain a known, constant and steady force by the bonding tool to the wire. EP-A-107 147 discloses a numerically controlled process for a machine tool. When the working path of the tool passes through an area where maching takes place the tool is made to move at a relatively high speed to the next actual maching location. SUMMARY OF THE INVENTION This invention is made to overcome the problems mentioned above. The object of this invention is, therefore, to provide a grinder robot, in which a grinder is able to grind surfaces quickly and accurately without cutting into the surfaces. According to the present invention this object is solved by a grinder robot for grinding a work placed on a working table by moving a grinder across a surface of said work, the grinder robot comprising a position detector for detecting a position of said grinder in a direction towards said work; a force detector for detecting a force on the grinder from the work; control means for controlling a movement of said grinder in said direction towards said work by feedback loops performing force control and position control; characterized by a loop selecting means for selecting a feedback loop performing force control until a grinding point of said grinder reaches to a target position for finishing, and for selecting a feedback loop performing position control after said grinding point of said grinder has reached to said target position. There is provided a grinder robot to grind a work by holding a grinder in the tip of a robot arm which is provided to move freely towards a working table; and then by moving the grinder towards a work placed on said working table, comprising; means for detecting force given to said robot arm from said work through said grinder; means for detecting positions of said grinder; and a controller for controlling the move of said grinder. The controller is also comprised of means for indicating target positions to finish grindings of said work; position loop drive control means; force loop drive control means; and means for selecting either one of said drive control means. After obtaining feedback values of position detecting signals detected by said position detecting means, said position control means control the move of said robot arm in a direction towards said work, by a position and velocity feedback loop according to said target positions, which are indicated by said indicating means. Said force loop drive control means control the move of said robot arm within said direction by a force feedback loop, after obtaining force feedback values detected by said force detecting means. Also, said selection means select either one of said drive control means, according to positions of said grinding points of said grinder and/or a force given to said grinder. According to the first aspect of this invention, position loop drive control means and force loop drive control means are provided in the controller. One of these drive control means may be selected according to grinding positions of the grinder and/or said force given to said grinder. For example, said force loop drive control means should be selected until the grinding point of the grinder has reached to the target, and said position loop drive control means should be selected after the grinding point of the grinder has reached to the target, under a condition that grinding force given to the work is in set values. As a result, a grinding may be carried out quickly and accurately. In the second aspect of this invention, there is provided a grinder robot which has a grinder in the tip of a robot arm and grinds work surfaces into any shape. The grindings are carried out with controlling the grinding force given to the work by the grinder, until a grinding point has reached to a target position for finishing. The grindings are also carried out with controlling a grinder position, after a grinding point has reached to the target position for finishing, under the condition that the grinding force is in set values. In this case, the feeding speed of the grinder drived by a position control, should be set faster than that of the grinder drived by a force control. According to the second aspect of this invention, a grinding amount of a surface in every grinding becomes large by setting the feeding speed of a grinder relatively slow, until a grinding point has reached to the target position for finishing and so far as a force control is selected. After a grinding point has reached to the target position for finishing and so far as a position control is selected, the feeding speed of a grinder is selected faster than that of the grinder drived by a force control. As a result, the total amount of processing time becomes short, and a surface of a work can be grinded into any shape correctly and efficiently by using a grinder robot of this invention. These and other objects, features and advantages of the present invention will be more apparent from the following description of a preferred embodiment, taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram showing a structure of a grinder robot according to this invention; Fig. 2 is a block diagram showing a control part of the grinder robot illustrated in Fig. 1; Fig. 3 is a block diagram showing a position loop drive control part; Fig. 4 is a block diagram showing a force loop drive control part; Fig. 5 is a flowchart showing one example of a control process of the grinder robot illustrated in Fig. 1; Fig. 6 and Fig. 7 illustrate outlines of various operations of the grinder robot shown in Fig. 1; Fig. 8 is a schematic diagram showing the whole structure of a grinder robot according to the second embodiment of this invention; Fig. 9 illustrates an outline of a grinding operation; and Fig. 10 and Fig. 11 are graphs showing the relations between grinding positions and feeding speeds of a grinder according to the second embodiment of this invention. DETAILED DESCRIPTION OF THE EMBODIMENTS Fig. 1 is a front view showing an outline structure of a grinder robot according to one embodiment of this invention. As shown in the figure, a working table 2 is placed on the bottom surface of a frame 1, which forms the main body of an apparatus. A work W to be grinded can be fixed on working table 2. A robot arm 3 of the 3 degrees of freedom Cartesian coordinate type is mounted on the top surface of said frame 1 to move free above said working table 2. On the top end of robot arm 3, a potentiometer (a position detecting means) 4 is mounted to detect vertical movements of the arm. The main body of a grinder 5 is also mounted in the tip of robot arm 3. A grindstone 6 is attached to rotate free to the main body of the grinder 5. A force sensor (force detecting means) 7, such as a strain gauge, is provided between the main body of grinder 5 and arm 3 to detect a force given to grindstone 6 from work W. A z axis motor 11 is also provided on arm 3 so as to move robot arm 3 vertically (z direction). Furthermore, a controller 9 is provided beside a side surface of said frame 1, which has a joy stick 8 to indicate feeding speeds and feeding directions of said grinder 5. Control members corresponding to control means C7 shown in Fig. 2 are built inside of this controller 9. In the grinder robot having said structure, position (Zo) of the finishing target is input by means of a keyboard (indicating means) 18. By operating joy stick 8, arm 3 can, then, be moved in ail directions in a horizontal plane (X Y directions) and with any speed, with pushing a rotating grindstone 6 of grinder to the upper surface of work W. Now, control means C7 will be explained below with referring to Fig. 2. Said control means C7 include position loop drive control means C4, force loop drive control means C5, and loop selecting means C6. After obtaining feedback values of position detecting signals, which are detected by potentiometer 4 (position detecting means), means C4 control the drive of said robot arm by a feedback loop concerning position and velocity in the direction towards said work according to said target positions input by keyboard 18 (indicating means). As described above, keyboard 18 indicates target positions for finishing up grindings of said work W. On the other hand, said means C5 control the drive of said robot arm in the same direction mentioned above, by a force loop according to force feedback values, which are detected by force sensor 7 (force detecting means). According to positions of grinding points by said grinder and/or forces given to said grinder, said loop selecting means C6 select either one of said drive control means C4 or C5. As explained above, control means C7 of this invention include position loop drive control means C4 and force loop drive control means C5, and select either one of these means C4 or C5 by loop selecting means C6 according to positions of grinding points and/or force given to said grinder. For example, said means C5 may be selected until the grinding point of the grinder has reached to a target position for finishing and said means C4 may be selected after the grinding point of the grinder has reached to said target position under the condition that the force given to the grinder is in set values. Fig. 3 shows a block diagram of a position loop drive control part, which corresponds to said position loop drive control means C4 shown in Fig. 1. Position loop drive control part 10 in this embodiment drives said arm 3 vertically (Z-direction). This part 10 includes a driver (servo amplifier) 12 connected to said Z-axis motor 11, a PID (proportional, integral, derivative) controller 13, a position comparator 14, and a speed comparator 15. These comparators 14 and 15 form a position feedback loop and a velocity feedback loop for instruction, according to position signals from said potentiometer 4 and velocity signals from said motor 11. Fig. 4 illustrates a block diagram of a force loop drive control part 16, which corresponds to said force loop drive control means C5 shown in Fig. 1. This part 16 in this embodiment drives said arm 3 vertically (Z direction) according to force feedback signals. This part 16 includes, also, a force comparator 17 to compare force signals detected by said force sensor 7 with a predetermined constant value. Driver 12 is driven by deviation signals from said comparator 17. A drive control procedure of Z axis motor 11 by position and force controls, is illustrated in Fig. 5. At first, whether a grinding point reaches to a target surface for finishing or not should be detected in step 501. A force control in step 502 may, then, be selected until the point reaches to the target surface. This force control is carried out by force loop drive control part 16 shown in Fig. 14. Thus, grindstone 6 of grinder 5 is pushed to work W with a constant force during the force control. On the other hand, step 504 may be selected to execute if the grinding point is judged to be on the target surface in step 501. In that step 504, whether the grinding force is more than a set value or not should be judged. In the case that the grinding force is more than the set value, step 502 may be selected to keep the grinding force within constant values by a force control executed in force loop drive control part 16 shown in Fig. 4. In step 504, if the grinding force is judged to be under the set value, then, step 505 may be selected to execute a position control which will be carried out by position loop drive control part 10 shown in Fig. 3. In step 503, a completion of grindings should be judged. If a completion of grindings is not seen in step 503, step 501 may then be chosen to execute again until a completion is found in step 503. As described above, work W may be grinded by rotating grindstone 6 attached to grinder 5 with a position control operation or a force control operation. In this grinding procedure, a force control may be selected to execute until the grinding point of the grinder reaches to a target surface and a position control may, then, be selected to execute after it has reached to the target surface. Therefore, grindstone 6 may not be damaged by cutting into surfaces even if these are rugged as shown in Fig. 6. Also, grindstone 5 may not apart from a work surface and may not grind surfaces over the target surface for finishing. In place A shown in Fig. 6, grinder 5 is force controlled in the pushing direction, because the grinding point does not reach to the target surface of work W for finishing. Grindstone 6, therefore, does not cut into the working surface and does not apart from it. The grinder, then, moves to place B where the grinding point has reached to the target surface of work W for finishing. In this place, the grinder is position controlled in the pushing direction. The grinder, therefore, does not grind the working surface exceeding the target surface for finishing. As a result, the work surface may be grinded into a perfect plane quickly. As shown in Fig. 7, a target surface for finishing may be set on position Z₁ which is just this side of the last target position Z₀ for finishing. In this case, a target plane for finishing may be formed only by the position control at the last stage of grindings. As a result, what is called a rough grinding can be processed quickly by the control shown in Fig. 5, and then a precise grindings can be processed only by the position control in order to increase grinding accuracy. In this embodiment illustrated above, grinder 5 may be moved manually on a surface by using joy stick 8 shown in Fig. 2. It is, however, possible to move a grinder automatically on a plane as described in the second embodiment later. In the embodiment illustrated above, arm 3 was moved against work W. It is, however, possible to move work W against arm 3 along all axes (X Y Z) or X and Y axes, because arm 3 and work W can be moved for each other. As mentioned above, the grinder robot in this embodiment may be operated by both of force control and position control in the direction towards a work. It is, therefore, possible to grind surfaces of a work quickly and accurately, without fears that a grinder might cut into a rugged work surface to cause a damage of the grinder, and a grinder might apart from a work surface. Now, a grinder robot according to the second embodiment of this invention will be explained below. The second embodiment is intended to further overcome a problem arised from a grinder robot in the first embodiment, in the case to move the grinder in a plane automatically. This problem is as follows. When grindings are executed with feeding a grinder in a constant speed automatically, a relatively slow speed may be selected as a grinder speed throughout the grindings. This is a speed set in conditions when grinding points are far from a target position for finishing and grindings of a work are executed with relatively large grinding force by a force control. If this speed for feeding a grinder is used throughout the entire process of grindings, the process time will become longer unnecessarily. This is because the slow speed should be used even in a position control area where most part of the work surface has been grinded into a target surface for finishing. If a high speed is selected throughout the entire grinding process, .hen, a grinding amount of every grinding in the force control area will become small. As a result, the total process time will become longer in both cases. In this second embodiment, then, the relation between the feeding speed V f in a position control area and the feeding speed V s in a force control area may be determined to be a fixed relation, that is, V f > V s . Fig. 8 shows a schematic structure of a grinder robot according to the second embodiment of this invention. This grinder robot is comprised of a robot body 20 and a controller 21. Robot body 20 is a 6 degrees of freedom cylindrical coordinates type robot, and it has, basic 3 axes ϑ, Z, R and 3 axes α, β, γ to control end effector's orientation. Grinder 5 fixed to the tip of robot arm can be moved to any position in space with any orientation using said 6 axes so as to grind work W with pushing a rotating grindstone 6 to the work. Grinding force of grinder 5 during said grindings, is always detected by a 6 axes force/torque sensor 22 placed between the robot arm 3 and grinder 5. The detected signals from the sensor 22 are, then, sent to controller 21. There are provided control means in controller 21. Grinding forces from grinder 5 and positions of the grinder tip are always observed by force detecting means and position detecting means. At grindings, loop selecting means may select force loop drive control means until a grinding point of the grinder reaches to a target position for finishing, which is indicated by indicating means, and select position loop drive control means after a grinding point has reached to said target position, under the condition that the grinding force is within set values, by the same way described in the first embodiment. Fig. 9 shows how a grinding is processed by said control means. At place A in the figure, the grinding point at the tip of the grinder has not reached vet to the target position Z₀ of work W. Grindstone 6 is, then, pushed to work W in order to grind the work surface by a force control. At place B in the figure, the grinding position at the tip of the grinder has already reached to target position Z₀ of work W. A position control may, then, be chosen in order not to grind into a work surface any more, and in order to finish the work surface into a predetermined shape accurately. In Figs. 10 and 11, the relation between grinding points and feeding speeds of a grinder is shown in the case that grindings are carried out automatically in order to grind a work surface into any shape by the grinder robot of this embodiment. The horizontal axes in the figure show positions Z of a grinder tip and the vertical axes show a feeding speed V of a grinder. From Fig. 10, it is understood that the feeding speed V of a grinder may be set to a relatively slow speed V s , when a grinding position Z of a grinder tip has not reached yet to a target position Z₀ and a force control is selected. It is also understood that the feeding speed V of a grinder may be changed to a relatively high speed V f , after grinding position Z of a grinder tip has already reached to target position Z₀ and a position control is selected. If a feeding speed of a grinder is set according to said speed relation, that is, V f > V s , a grinding amount in every grinding becomes relatively large in places where a force control is selected, because the feeding speed selected in these places is relatively slow. On the other hand, in places where a position control is selected, a relatively high speed is set to feed the grinder. As a result, the processing time for the entire grindings may be further reduced. As shown in Fig. 11, the feeding speed V f of a grinder may be changed to the feeding speed V s gradually between target position Z₀ and position Z₁, which is set to be close to Z₀. In this case, feeding speed of a grinder does not change abruptly throughout the entire grinding process. Smooth grindings can, therefore, be executed by said speed control. As mentioned above, in a grinder robot according to the second embodiment of this invention, a feeding speed of a grinder may be set relatively slow in places where a grinding point of a grinder has not reached yet to a target position and so far as a force control is selected. During the grindings, then, a relatively large grinding amount in every grinding of a work surface can be carried out. On the other hand, the feeding speed in places where a grinding point has already reached to a target and a position control is selected, may be set faster than that in places where a force control is selected. As a result, the processing time of the entire grindings may be further reduced.

Claims: A grinder robot adapted for grinding a work placed on a working table (2) by moving a grinder (5) across a surface of said work, the grinder robot comprising a position detector (4) for detecting a position of said grinder (5) in a direction towards said work; a force detector (7) for detecting a force on the grinder (5) from the work; control means (C4, C5) for controlling a movement of said grinder (5) in said direction towards said work by feedback loops performing force control and position control; characterized by - a loop selecting means (C6) for selecting a feedback loop (7, C5, 3) performing force control until a grinding point of said grinder (5) reaches to a target position for finishing, and for selecting a feedback loop (4, C4, 3) performing position control after said grinding point of said grinder (5) has reached to said target position. A grinder robot according to claim 1, characterized in that said control means selects said feedback loop (4, C4, 3) performing position control if a force on said grinder (6) from said work is within set values, and selects force control otherwise. A grinder robot according to claim 1 or 2, characterized in that said loop selecting means (C6) selects said feedback loop (4, C4, 3) performing position control or said feedback loop (7, C5, 3) performing force control in accordance with signals received from said position detector (4) and said force detector (7). A grinder robot according to any of the preceding claims, characterized in that a feeding speed across the surface of said work is set to a relatively slow speed V s when force control is selected, and the feeding speed is changed to a relatively high speed V f when a position control is selected. A grinder robot according to claim 4, characterized in that near the switching point between force control and position control the feeding speed of the grinder is changed gradually.

Inventors: JINNO MAKOTO, YOSHIMI TAKASHI, JINNO, MAKOTO, YOSHIMI, TAKASHI
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1990
Main classification: G05B19
IPC codes: B24B27, G05B19, B25J13, B25J9