ACCELERATION AND DECELERATION CONTROL SYSTEM FOR HORIZONTAL-JOINT ROBOTS

Abstract: In the acceleration and deceleration control system for horizontal-joint robots, the distance between the center of rotation of a horizontal-joint robot (A), which has a plurality of arms (3, 5) adapted to be turned in a horizontal plane, and a reference position, such as the position of a wrist portion (51) at the free end section of the arms (3, 5) is computed to deter­ mine appropriate acceleration of a movement of the free end section of the arms (3, 5) on the basis of the results of the computation. Thus the arms (3, 5) can be moved speedily and smoothly by effectively utilizing a servomotor for turning the same.

Description: The present invention relates to a system for controlling acceleration and deceleration of a horizontally articulated robot having a plurality of interconnected swing arms that are angularly movable in horizontal planes. In a system for controlling axis movement of a robot having a plurality of movable axes, acceleration and deceleration control is generally effected so as not to impose shocks and vibration on a mechanical system when starting and stopping the axis movement. There has been developed in recent years a horizontally articulated robot having a plurality of interconnected swing arms that are angularly movable in horizontal planes, for use in work where many jobs are carried out in horizontal planes, such as for assembling semiconductors or installing electronic parts on printed-circuit boards. The horizontally articulated robot is basically constructed of a first swing arm mounted on the distal end of a post erected on a bed fixed to a floor, the first swing arm being angularly movable horizontally, a second swing arm having one end swingably attached to the distal end of the first swing arm, the second swing arm also being angularly movable horizontally, and a wrist mounted on the distal end of the second swing arm. When an object to be moved is gripped by the wrist of the horizontally articulated robot and the swing arms are angularly moved, acceleration and deceleration of a servomotor are controlled in initial and final periods of angular movement of the interconnected swing arms so as to turn them smoothly. The conventional system for controlling acceleration and deceleration of such a robot has a drive source which can produce a torque great enough to move the object when at a position remotest from the center of rotation of the arms. In the conventional control system, even when the object is located more closely to the center of rotation of the arms than the remotest position, the object is moved at the same low acceleration as that when the object is located in the remotest position. Therefore, the time required to move the object is unnecessarily long and the ability of the drive source is not properly utilized. It is therefore desirable to provide a system for controlling acceleration and deceleration of a horizontally articulated robot, the system being capable of effectively utilizing a servomotor which drives arms and of quickly moving the arms. JP-A-60 57407 by Suwa Seikosha K.K. discloses a robot control system having the features of the preamble of accompanying claim 1. EP-A-0 076 331 by the present applicant discloses a robot controlling device for controlling the speed of rotation of a single telescopic arm in dependence on the current arm length. According to the present invention, there is provided a system for controlling acceleration and deceleration of a horizontally articulated robot having a plurality of interconnected swing arms that are angularly movable in horizontal planes, said system comprising:- processing means for computing the distance from the centre of rotation of the arms to a reference point at the distal end of the arms; and accelerating/decelerating means responsive to the computed distance for setting an acceleration for movement of the distal end of the arms; characterised in that:- said processing means is operable to compute a first distance between the centre of rotation and a present position of the reference point, and to compute a second distance between the centre of rotation and a target position of the reference point; and wherein said accelerating/decelerating means is operable to determine a first acceleration corresponding to the first distance and a second acceleration corresponding to the second distance; whereby the movement of the distal end of the arms from the present position to the target position includes an acceleration phase at the rate of said first acceleration, a constant speed phase, and a deceleration phase at the rate of said second acceleration. When a command is issued for moving an object gripped by a wrist, the distances from the center of rotation of the arms, about which the first swing arm (and consequently all the interconnected swing arms) rotate, to an initial position of the object and a target position thereof are immediately computed. Then accelerations to be effected when the distal arm end is accelerated and decelerated are computed, and points for starting acceleration and deceleration of the robot are varied dependent on the distance from the center of rotation of the arms. Reference is made, by way of example, to the accompanying drawings in which:- Fig. 1 illustrates a horizontally articulated robot to which the present invention can be applied; Fig. 2 is a schematic view showing the arrangement of arms; Fig. 3 is a diagram explaining acceleration; Fig. 4 is a diagram explanatory of an acceleration table; Fig. 5 is a schematic view explaining movement of the arms; Fig. 6 is a diagram showing acceleration/deceleration characteristics; and Fig. 7 is a flowchart of operation. Fig. 1 shows by way of example a horizontally articulated industrial robot including an angularly movable post on a bed installed on a floor. Designated at A is a robot mechanism including a post 1 mounted on a bed 2 and angularly movable in a prescribed angular range (ϑ) and a turning mechanism 21 disposed in the bed 2 for angularly moving the post 1. A first arm 3 is horizontally supported by a support shaft 4 inserted in the post 1, and a second arm 5 which is also horizontally angularly movable is mounted on the distal end of the first arm 3. A wrist 51 is attached to the distal end of the second arm 5. The bed 2 houses a drive mechanism 22 vertically movable within a prescribed range (Z) for controlling the height of the arm 3, the drive mechanism 22 being capable of vertically actuating the support shaft 4 to control the height of the first arm (operation arm). The first arm 3 houses a drive mechanism for turning the second arm, and the second arm 5 houses a drive mechanism for driving the wrist 51. Although not shown in Fig. 1, the wrist 51 usually has fingers for gripping tools or objects to be moved. Designated by B in Fig. 1 is a robot control system comprising a numerical control device in the form of a computer. The robot control system B has a programmable sequence controller B1 for controlling power electric units such as various servomotors in the robot mechanism A, and a teaching control unit B2. As shown in Fig. 2, the first and second arms 3, 5 are angularly movable in prescribed horizontal planes around the center O of rotation of the post 1. Since an object M gripped by the wrist 51 can be moved along a horizontal path without being affected by gravity, when the object M is turned about the center O of rotation, a torque T applied to a servomotor of the turning mechanism 21 is expressed by: T = I x (αω) (1) where I is the moment of inertia of the object M and (αω) is an angular acceleration. Therefore, as shown in Fig. 2, when the first and second arms 3, 5 move the object M while they are extending in line, the torque T applied to the post 1 as expressed by the equation (1) is a maximum, and when the first and second arms 3, 5 are collapsed to minimize the distance from the center O of rotation to the object M, the torque T is a minimum. In the robot of this type, the output of the servomotor is determined to meet the maximum torque to ensure operational stability. Fig. 3 shows how the rotational angular velocity varies from a movement starting point when the robot is given a command speed F. When the object M is at a remote point, the maximum angular acceleration (αω1) applied to the object M is set as follows: (αω1) = F/τ₁ (2) and when the object M is at a closest point, the maximum angular acceleration (αω2) applied to it is: (αω2) = F/τ₂ (3) These angular accelerations (αω2), (αω1) are thus dependent on time constants τ₁,τ₂ (τ₁ > τ₂). The angular acceleration αω2 at the closest point can be increased to a maximum value within an allowable range by making it equal to a value obtained by the same servomotor when working at full output. Fig. 4 shows an acceleration table with the horizontal axis indicating the distance from the center O of rotation to the object M and the vertical axis indicating the permissible acceleration. This acceleration table can be prepared by using accelerations calculated from command speeds F found from the equations (2), (3). Therefore, if the distance from the center O of rotation to the object M is given, an acceleration that can be imposed on the object M can be determined according to the distance. Acceleration/deceleration operations for moving the object M from a point A to a point B by turning the arms as shown in Fig. 5 will be described with reference to the flowchart of Fig. 7. Fig. 6 shows acceleration/deceleration characteristics with respect to the command speed F. The numerical control device ascertains in a step a whether a movement command has been issued for the object M. If it has, a distance D1 between the center O of rotation and a present position A is measured in a step b . The distance D1 can quickly be measured by referring to a present position counter which indicates the present position of the reference point at the wrist. The measured distance D1 is stored in a temporary memory in the numerical control device in a step c . Then, a distance D2 between the center O of rotation and a next (target) position of the object M is measured in a step d . The distance D2 can be measured by referring to a movement command position of commanded next block data. The measured value D2 is stored in the temporary memory in the numerical control device in a step e . Thereafter, an acceleration α1 is found from the acceleration table based on the measured value D1 stored in the memory, in a step f , and a deceleration -α2 is found from the acceleration table based on the measured value D2 stored in the memory in a step g . In a next step h , the acceleration α1 and the deceleration -α2 are input from the numerical control device to the programmable controller B1 or an acceleration/deceleration circuit in the robot mechanism A. The acceleration/deceleration circuit applies a pulse having acceleration characteristics as shown in Fig. 6 based on the input acceleration α1 to the servomotor in the turning mechanism 21 for acceleration control in a step i . Once the acceleration range (phase) has passed, the acceleration control is finished in a step j , and the servomotor is operated at a constant speed in a step k . If the object M approached the target point B, entering a deceleration range in a step l , then the acceleration/deceleration circuit applies a pulse having deceleration characteristics as shown in Fig. 6 based on the input deceleration -α2 to the servomotor in the turning mechanism 21 for deceleration control in a step m . Upon completion of the deceleration control or arrival at a stop position in a step n , the servomotor is de-energised in a step o . While the acceleration/deceleration circuit has been described as being a known linear acceleration/deceleration circuit of the variable time-constant type, an acceleration/deceleration circuit of the exponential function type may be used, and acceleration and deceleration may be determined as described above using such a circuit. Moreover, while acceleration and deceleration have been found using an acceleration table in the above embodiment, they may be computed from the distances from the starting and ending points of the object M to the center of rotation and acceleration and deceleration control may be carried out using such computed distances. With a system for controlling acceleration and deceleration of a horizontally articulated robot according to the present invention, acceleration and deceleration of a reference point at the distal end of arms of the robot can be varied dependent on the distance from the reference point to the center of rotation of the arms in horizontal planes. A servomotor which rotates the arms can therefore be utilized effectively, and the arms can be moved quickly and smoothly.

Claims: A system for controlling acceleration and deceleration of a horizontally articulated robot having a plurality of interconnected swing arms (3, 5) that are angularly movable in horizontal planes, said system comprising:- processing means for computing the distance from the centre of rotation of the arms (3, 5) to a reference point at the distal end of the arms (3, 5); and accelerating/decelerating means responsive to the computed distance for setting an acceleration for movement of the distal end of the arms (3, 5); characterised in that:- said processing means is operable to compute a first distance (D1) between the centre of rotation (O) and a present position (A) of the reference point, and to compute a second distance (D2) between the centre of rotation (O) and a target position (B) of the reference point; and wherein said accelerating/decelerating means is operable to determine a first acceleration (α₁) corresponding to the first distance (D1), and a second acceleration (∝₂) corresponding to the second distance (D2); whereby the movement of the distal end of the arms (3, 5) from the present position (A) to the target position (B) includes an acceleration phase at the rate of said first acceleration (α₁), a constant speed phase, and a deceleration phase at the rate of said second acceleration (∝₂). A system according to claim 1, wherein said reference point is positioned at a wrist (51) attachment portion located at the distal end of the interconnected swing arms (3, 5). A system according to claim 1 or 2, wherein said accelerating/decelerating means includes an acceleration table storing absolute values of accelerations which are inversely proportional to the computed distance. A system according to claim 1, 2 or 3, wherein said accelerating/decelerating means comprises a linear acceleration/deceleration circuit of the variable time-constant type for controlling said acceleration phase and said deceleration phase. A system according to claim 1, 2, 3 or 4, further comprising a temporary memory for storing said first distance (D1) and said second distance (D2) for use by said accelerating/decelerating means. A system according to any preceding claim, wherein a turning mechanism of an innermost arm (3) of the interconnected swing arms (3, 5) is controlled in accordance with the first acceleration (α₁) and second acceleration (∝₂) determined by said accelerating/decelerating means.

Inventors: ITO, SURUMU, TERADA, AKIHIRO, TORII, NOBUTOSHI
Applicants: FANUC LTD.
Application year: 1986
Main classification: G05B19
IPC codes: B25J13, G05B19, B25J9