System for controlling robot

A system for controlling a robot includes a control portion (7) for supplying a control signal to and receiving a control signal from a servo unit (6) for carrying out a signal supply and a signal feedback to an electric motor (5) for driving a shaft executing a Z-axis linear motion, and threshold value supply portion (8) for supplying a threshold value to the control portion. Based on a comparison between a motor torque instruction value and a threshold value supplied from the threshold value supplying portion, an alarm is delivered and the process subsequently proceeds to a step of dealing with an abnormal condition when said motor torque instruction value becomes greater than a predetermined threshold value.

TECHNICAL FIELD 
The present invention relates to a system for controlling a robot in which 
a control is carried out based on a detection of a force along the Z-axis 
of the robot. The device according to the present invention may be used in 
an industrial robot for carrying out, for example, an assembling job. 
BACKGROUND ART 
In general, when an industrial robot having a linked structure composed of 
a rotatable arm portion, a vertically movable shaft portion, and related 
devices is used to fix a worked article (workpiece) to an object article, 
it is possible that a related motion of a workpiece will deviate from a 
normal condition, for some reason, and an abnormal situation such as a 
collision of the workpiece with the object article occurs. Such an 
abnormality causes a failure of the robot and hinders the progress of the 
robot operation, and accordingly, effective measures are sought to 
minimize such problems and thus facilitate a smooth progress of the robot 
operation, although such appropriate measures have not yet been obtained. 
DISCLOSURE OF THE INVENTION 
It is an object of the present invention to provide an improved system for 
controlling a robot in which a control is carried out based on a detection 
of a force along the Z-axis of the robot. 
According to the present invention, there is provided a system for 
controlling a robot, characterized in that the system includes an electric 
motor for driving a shaft executing a Z-axis linear motion of the robot, a 
servo unit for carrying out a signal supply and a signal feedback to the 
electric motor, a control portion for supplying a control signal to and 
receiving a control signal from the stated servo unit, and threshold value 
supply portion for supplying a threshold value to the above control 
portion, a comparison between a motor torque instruction value and a 
threshold value supplied from the threshold value supply portion being 
carried out in the control portion, and an alarm being delivered and the 
process subsequently proceeding to a step of dealing with an abnormal 
condition, when the motor torque instruction value becomes greater than a 
predetermined threshold value, whereby a control is carried out based on a 
detection of a force along the Z-axis of the robot.

BEST MODE FOR CARRYING OUT THE INVENTION 
A schematic diagram of a system for controlling a robot according to an 
embodiment of the present invention is shown in FIG. 1, and an industrial 
robot for an assembling job to which the system of FIG. 1 is applied is 
shown in FIG. 2. 
The third axis mechanism i.e., the Z-axis of the robot 1 is shown in the 
left portion of FIG. 1. At the tip of a shaft 14, as the third or Z-axis, 
a band head 15 consisting of a band base portion 151 and grip fingers 152 
and 153 is mounted. The grip fingers grip a workpiece 2 and insert the 
workpiece into a hole 31 in an object article. 
The shaft 14 is driven by a motor 5 through a transmission system including 
a pulley 41, a pulley belt, and a ball-and-screw nut 17, providing a 
vertical movement in linear motion. 
A rotary motion is transmitted from the pulley transmission system to the 
ball-and-screw nut 17, and consequently, a rotary driving force is applied 
to the shaft 14, but due to an action of a ball-and-spline nut located 
beneath the ball-and-screw nut 17, the motion of the shaft 14, which is 
apt to rotate, becomes an up-and-down motion of the shaft 14 in a linear 
movement. In the ball-and-spline nut 16, a line of balls 161 lie between 
the internal surface of the nut and the shaft 14, and by the circulation 
of such a line of balls, the motion of the shaft 14, which is apt to 
rotate, appears as a vertical motion. (With regard to the use of a power 
transmission mechanism equipped with a ball-and-spline nut to drive the 
arm shaft of an industrial robot, reference can be made to, for example, 
Japanese Patent Application No. 61-222186, filed by the present 
applicant.) 
In the robot 1 shown in FIG. 1, a column 112 stands on a base stand 111, 
and the column 112 supports the first arm 121 which may rotate around the 
first axis 101. The top end portion of the first arm 121 is incorporated 
with the second arm 122 which may rotate around the second axis 102, and 
the top end portion of the second arm 122 is incorporated with the shaft 
14 which is accommodated in a shaft-receiving cylindrical enclosure 13 and 
is vertically movable along the third axis 103. The first arm 121, the 
second arm 122, and the shaft 14 are driven by a motor (not shown). 
In the robot 1, while the normal operation is carried out, the workpiece 2 
held in the hand 15 is properly inserted in the hole 31 of the object 
article 3, but when an abnormal condition occurs such that the position of 
the workpiece 2 does not match the position of hole 31 for some reason, so 
that the workpiece 2 strikes against the surface of the object article 3, 
and in particular, if an appropriate countermeasure is not provided, due 
to the continuous action of the drive force from the motor, the workpiece 
2 is pressed strongly against the object article 3. This may cause damage 
to the object article 3, the workpiece 2, the hand 15 and/or other 
portions of the robot 1, and further, may cause an undesirable emergency 
stoppage of the robot 1. The device of FIG. 1 has an advantage of 
eliminating such an undesirable problem. 
The action principle of the device shown in FIG. 1 will be described 
hereafter. When the shaft 14 comes down, and if the workpiece 2 does not 
coincide with the predetermined position of the hole 31 of the object 
article 3, but instead collides against the surface of the object article 
3, the reaction force F.omega. from the object article 3 acts on the 
workpiece 2 as the Z-axis force. 
When a pitch (lead) of a thread on the shift 14 is considered as lmm/rev, a 
reduction ratio of the pulley timing belt as 1/i, and an efficiency of the 
transmission system from the pulley 41 to the ball-and screw nut 17 as 
.eta., the following relative expression is effected for the torque Tm of 
the motor: 
##EQU1## 
Since each of l/2, 1/i, and 1/.eta. above is a constant, the formula (1) 
shows that the motor torque Tm is in proportion to the reaction force Fw 
applied to the workpiece. 
Accordingly, in the device of FIG. 1, the reaction force (Fw) acting on the 
workpiece can be monitored by monitoring the motor torque (Tm) 
instruction. 
The motor 5 is driven by the output from the drive portion 62 of servo unit 
6. Feedback of an output current from the drive portion 62 is carried out 
by a current feedback signal S(62-FB) to the drive portion 62. Feedback of 
a position feedback signal S(POS-FB) and a position feedback signal 
S(SP-FB) from the motor 5 is carried out to the torque instruction 
determination portion 61 of the servo unit 6. 
The output of the torque instruction determination portion 61 is delivered 
to the drive portion 62 as a torque instruction INS(TQ), which is also 
transferred to the control portion 7. The control portion 7 includes a CPU 
71, ROM 72, RAM 73, and an input/output device 74. A position deviation 
signal S(POS-DEV) is delivered from the servo unit 6 to the control 
portion 7, and a position instruction signal S(POS-INC) is delivered from 
the control portion 7 to the servo unit 6. 
A torque threshold value THR(TQ) is supplied from the threshold value 
supply portion 8 to the control portion 7. This supply is executed when an 
operator stores a threshold value prepared as a program to the RAM 73 of 
the control portion 7, using a key board or other appropriate means. A 
comparison between a torque instruction INS(TQ) and a torque threshold 
value THR(TQ) is carried out in the control portion 7. 
During normal operation, the motor 5 is driven according to the operation 
of the servo unit 6, but in a defective state wherein the workpiece 2 
operated by the shaft 14 does not coincide with the position of the hole 
31 of the object article 3 and a failure to insert the workpiece 2 into 
the hole 31 occurs, the rotation speed of the motor is decreased and a 
phenomenon of a non-following of the position occurs in the control of 
motor operation. Based on this, the torque instruction INS(TQ) value 
increases. This increase proceeds and when the result of a comparison 
between a torque instruction INS(TQ) value and a threshold value THR(TQ) 
in the control portion 7 indicates that the torque instruction value has 
become greater than a predetermined threshold value, the defective control 
state i.e., a failure to insert the workpiece, is determined, and based on 
this determination, an alarm is delivered at the alarm generation portion 
75, and the process subsequently proceeds to a step of dealing with an 
abnormal condition. As described above, in the device of FIG. 1, a 
threshold value THR(TQ) is supplied to the control portion 7, a comparison 
between a motor torque instruction value and a threshold value is carried 
out in the control portion 7, and in the servo unit 6, the torque 
instruction determination and the motor drive signal delivery are carried 
out based on the signal received from the delivered to the control portion 
7 and the motor 5, and consequently, the shaft motion as the Z-axis linear 
motion in the robot is carried out by being deriven by the motor 5. When 
the motor torque instruction value INS(TQ) becomes greater than the 
predetermined threshold value THR(TQ), an alarm is delivered at the alarm 
generation portion 75, and the process consequently proceeds to a step of 
dealing with an abnormal condition. The torque threshold values THR(TQ) 
are set in the robot operation programs, and when the robot performs a 
plurality of operations, it may select an individual threshold value 
corresponding to respective operations. FIG. 5 is an example showing a 
movement of the top end of the robot. In FIG. 5, each of P1 to P8 indicate 
a teaching point, P9 is a waiting position of the top end of robot, and 
each arrow indicate a locus of the top end of robot. 
An example of a program for a robot operation shown in FIG. 5 is shown in 
the chart below. 
______________________________________ 
ADDRESS INSTRUCTION 
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1 Move to P1 
2 Move to P2 
3 Move to P3 
4 Move to P4 
5 Move to P5 
6 If determination of INS(TQ) &gt; THR(TQ) is YES, 
jump to address 12, and if NO, proceed to next 
address. 
7 Move to P6 
8 Move to P7 
9 If determination of INS(TQ) &gt; THR(TQ) is YES, 
jump to address 12, and if NO, proceed to next 
address. 
10 Move to P8 
11 Jump to address 1 
12 Move to P9 
13 Deliver an alarm signal. 
14 Wait 
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In the above chart, the torque instruction value INS(TQ) may be converted 
to FW (kg) using the previously described formula (1 ). But in this case, 
the torque threshold value THR(TQ) must be a kg value in the 
FIG. 6 is a flow chart showing an example of the operation of the device 
shown in FIG. 1. In step S1, an operation of inserting the workpiece 2 is 
carried out by the motion of shaft 14, and in step S2, it is determined 
whether the torque instruction value INS(TQ) exceeds the threshold value 
THR(TQ). If the torque instruction value INS(TQ) is less than the 
threshold value THR(TQ), the process goes to step S3 and the normal 
operation proceeds, but if it is greater than the threshold value THR(TQ), 
the process goes to step S4 where a "failure to insert the workpiece" is 
determined, and in step S5, an alarm is delivered at the alarm generation 
portion 75, and subsequently, an abnormal condition is dealt with. 
A characteristic diagram for indicating the characteristic of time vs. 
torque instruction is shown in FIG. 7, to describe the motion of the 
device of FIG. 1. In the idle operation state (1), the torque instruction 
value INS(TQ) is at the lower level, and in a normal insertion operation 
(2), the torque instruction value INS(TQ) moves up to a level higher than 
(1) but still does not reach the threshold value THR(TQ). Then, if the 
torque instruction value INS(TQ) sharply increases to become an abnormal 
running state (3), the torque instruction value INS(TQ) exceeds the 
predetermined threshold value THR(TQ). Based on the detection of the 
abnormal running state (3), a "deliver alarm" and "dealing with an 
abnormal state" are carried out.