Robot Programming

       ROBOT PROGRAMMING

To do useful work, a robot must be programmed to perform its motion cycle. A robot program can be defined as a path in space 10 be followed by the manipulator, combined with peripheral actions that support the work cycle, Examples of the peripheral actions include opening and closing the gripper, performing logical decision making, and communicating with other pieces of equipment in the robot cell. A robot is programmed by entering the

programming commands into its controller memory. Different robots use different methods of entering the commands

. In the case of limited sequence robots, programming is accomplished by setting limit switches and mechanical stops to control the endpoints of its motions. The sequence in

Lead through Programming

Powered Lead through Versus Manual Leadthrough. There are two methods of performing the leadthrough teach procedure: (1) powered leadthrough and (2) manual leadrhrough. The difference between the two is in the manner in which the manipulator is moved through the motion cycle during programming. Powered leadthrough is commonly used a~ thc programming method for playback robots with point-to-point control. It in· valves the use of J teach pendant (handheld control box) that has toggle switches and/or contact buttons for controlling the movement of the manipulator joints. Figure 7.13 illustrates the important components of a teach pendant. Using the toggle switches or buttons, the programmer power drives the robot arm to the desired positions, in sequence, and records the positions into memory. During subsequent playback, the robot moves through the sequence 01 positions under its own power.

Manual leadthrough is convenient for programming playback robots with continuous path control where the continuous path is an irregular motion pattern such as in spray painting. This programming method requires the operator to physically grasp the end-of-arm or tool attached to the arm and manually move it through the motion sequence, recording the path into memory. Because the robot arm itself may have significant mass and would therefore he difficult to move, a special programming device often replaces the actual robot for the teach procedure. The programming device has the same joint configuration as the robot. and it is equipped with a trigger handle (or other control switch), which is activated when the operator wishes to record motions into memory. The motions arc recorded a~ a series of closely spaced points' During playback, the path is recreated by controlling the actual robot arm through the same sequence of points.

Motion Programming. The lendthrough methods provide a very natural way of programming motion commands into the robot controller. In manual leadthrough, the operator simply moves the arm through the required path to create the program. In powered leadthrough the operator uses a teach pendant to drive the manipulator. The teach pen b equipped with switch or a pair of contact buttons for each joint By activating these switches or in a coordinated fashion for the various joints. the programmer moves the manipulator to The required positions in the work space.

Coordinating the individual joints with the teach pendant is sometimes an awkward way to enter motion commands to the robot, For example, it is difficult to coordinate the individual joints of a jointed-arm robot (TRR configuration) to drive the end-of-arm in a straight line motion. Therefore, many of the robots using powered leadthrough provide two alternative methods for controlling movement of the manipulator during programming, in addition to individual joint controls. With these methods, the programmer can control the robot's wrist end to move in straight line paths. The names given to these alternatives are (1) world coordinate system and (2) tool coordinate system. Both systems mak., use of a Cartesian coordinate system. In/he world coordinate system. the origin and frame of reference are defined with respect to some fixed position and alignment relative to the robot base. This arrangement is illustrated in Figure 7,14(a). In the tool coordinate system, shown in Figure 7 .14(b), the alignment of the axis system is defined relative to the orientation of the wrist faceplate (to which the end effector is attached). In this way, the programmer call orient the tool in a desired way and then control the robot to make linear moves in directions parallel OT perpendicular to the tool.

The world coordinate system and the tool coordinate system are useful only if the robot has the capacity to move its wrist end in a straight line motion, parallel to one of the axes of the coordinate system. Straight line motion is quite natural for a Cartesian coordinate robot (LOO configuration) but unnatural for robots with any combination of rotational joints (types R, T, and V). To accomplish straight line motion for manipulators with these types of joints requires a linear interpolation process to be carried out by the robot's controller. In straight line interpolation. the control computer calculates the sequence of addressable points in space that the wrist end must move through to achieve a straight line path between two points.

There are other types of interpolation that the robot can use. More common than straight line interpolation is joint interpolation. When a robot is commanded to move its wrist end between two points using joint interpolation, it actuates each of the joints simultaneously at its own constant speed such that all of the joints start and stop at the same time The advantage of joint interpolation over straight line interpolation is that there is usually less total motion energy required to make the move. This may mean that the move could be made in slightly less time. It should be noted thai in the case of a Cartesian coordinate robot, joint interpolation and straight line interpolation result in the same motion path.

Still another form of interpolation is that used in manual leadthrough programming In this case, the robot must follow the sequence of closely space points that are defined during the programming procedure. In effect, This is an interpolation process for a path that usually consists of irregular smooth motions.

The speed of the robot is controlled by means of a dial or other input device, located on the teach pendant and/or tile main control panel. Certain motions in the work cycle should be performed at high speeds (e.g., moving parts over substantial distances in the work (ell), while other motions require low speed operation (e.g. motions that require high precision in placing the workpart). Speed control also permits a given program to be tried out at a safe slow speed and then at a higher speed to be used during production.

There are several inherent disadvantages of the lcadthrough programming methods. First, regular production must he interrupted during the leadthrough programming procedures. In other words, leadthrough programming results in downtime of the robot cell or production line. The economic consequence of this is that the lead through methods must be used for relatively long production runs and are inappropriate for small batch sizes

Second. the teach pendant used with powered leadthrough and the programming devices used with manual leadthrough arc limited in terms of the decision-making logic that can he incorporated into the program. It is much easier to write logical instructions using the computer like robot languages than the lead through methods

Third, since the leadthrough methods were developed before computer control became common for robots, these methods are not readily compatible with modem computer-based technologies such as CAD/CAM, manufacturing data bases, and local communications networks. The capability to readily interface the various computer-automated subsystems in the factory for transfer of data is considered a requirement for achieving computer integrated manufacturing.

          Robot Programming  languages

The use of textual programming languages became an appropriate programming method as digital computers took over the control function in robotics. Their use has been stimulated by the increasing complexity of the tasks that robots are called on to perform, with the concomitant need to imbed logical decisions into the robot work cycle. These computer-like programming languages are really-online/off-fine methods of programming, because the robot must still be taught its locations using the leadthrough method. Textual programming languages for robots provide the opportunity to perform the following functions that Ieadthrough programming cannot readily accomplish:

   enhanced sensor capabilities. including the use of analog as well as digital inputs and outputs

   improved  output  capabilities  for controlling  external  equipment

   program  logic that is beyond  the capabilities  of leadthrough  methods

   computations   and data processing  similar  to computer  programming   languages

   communications   with other  computer  systems

This section reviews some of the capabilities of the current generation robot program ming languages. Many of the language statements are taken from actual robot programming languages.

Motion Programming. Motion programming with robot languages usually requires a combination of textual statements and leadthrough techniques. Accordingly, this method of programming is sometimes referred to as online/offline programming. The

MOVE                      PI

which commands the robot to move from its current position to a position and orientation defined by the variable name Pl. The point P1 must be defined, and the most convenient way to define P1 is to use either powered leadthrough or manual leadthrough to place the robot at the desired point and record that point into memory. Statements such as

HERE                       PI

or

LEARN                     PI

are used in the lcadthrough procedure to indicate the variable name for the point. What is recorded into the robot's control memory is the set of joint positions or coordinates used by the controller to define the point. For example, the aggregate

(236.158,65.0.0,O)

could be utilized to represent the joint positions for a six-jointed manipulator. The first three values (236.158.65) give the joint positions of the body-and-arm, and the last three values (0,0.0) define the wrist joint positions. The values are specified in millimeters or degrees. Depending on the joint types.

There are variants of the MOVE statement. These include the definition of straight line interpolation motions, incremental moves, approach and depart moves, and paths. For example, the statement

MOVES                     PI

denotes a move that is to be made using straight line interpolation, The suffix S on MOVE designates straight line motion.

An incremental move is one whose endpoint is defined relative to the current position of the manipulator rather than to the absolute coordinate system of the robot. For example, suppose the robot is presently at a point defined by the joint coordinates (236, 158, 65,0,O,0),and it is desired to move joint 4 (corresponding to a twisting motion of the wrist) from 0 to 125,The following form of statement might be used to accomplish this move'

DMOVE                   (4, 125)

The new joint coordinates of the robot would therefore be given by (236, 158, 65, 125, 0, 0). The prefix D is interpreted as delta, so DMOVE represents a delta move, or incremental move.

Approach and depart statements are useful in material handling operations. The APPROACH statement moves the gripper from its current position 10 within a certain distance

of the pickup (or drop-off) point, and then a MOVE statement is used to position the end effector at the pickup point. After the pickup is made. a DEPART statement IS used to move the gripper away from the point. The following statements illustrate the sequence:

APPROACH  P1,40 MM

MOVE                      PI

(actuate gripper)

DEPART 40 MM

The final destination is point Pl. but the APPROACH command moves the gripper to a safe distance (40 mm) above the point. This might be useful to avoid obstacles such as other parts in a tote pan. The orientation of the gripper at the end of the APPROACH move is the same as that defined for the point PI, so that the final MOVE Pi is really a spatial translation of the gripper. This permits the gripper to be moved directly to the part

for grasping.

A path in a robot program is a series of points connected together in a single move. The path is given a variable name, as illustrated in the following statement:

DEFINE             PATHl23      =  PATH(Pl.P2,P3)

This is a path that consists of points PI. P2, and P3. The points are defined in the manner described above. A MOVE statement is used to drive the robot through the path.

MOVE                     PA.TH123

The speed of the robot is controlled by defining either a relative velocity or an absolute velocity. The following statement represents the case of relative velocity definition:

SPEED                      75

when this statement appears within the pr ograrn, it is typically interpreted to mean that the manipulator should operate at 75% of the initially commanded velocity in the statements that follow in the program. The initial speed is given in a command that precedes the execution of the robot program, For example,

SPEEDO.5MPS

EXECUTE              PROGRAMI

indicates that the program named PROGRAM} is to be executed by the robot, and rhar the commanded speed during execution should he 0.5 m/sec.

Interlock and Sensor Commands. The two basic interlock commands (Section 4.3.2) used for industrial robots are WAIT and SIGNAL. The WAIT command is used to implement an input interlock. For example,

WAIT20,ON

would cause program execution to stop at this statement until the input signal coming into the robot controller at port 20 was in an "on" condition. This might be used to cause the robot to want for the completion of an automatic machine cycle in a loading and unloading application.

The SIGNAL statement is used to implement an output interlock. This is used to communicate to some external  piece of equipment. For example,

SIGNAL                   10. ON

would switch on the signal at output port 10, perhaps to actuate the start of an automatic machine cycle.

Both of the above examples indicate on/off signals. Some robot controllers possess the capacity to control analog device, that operate at various levels. Suppose it were desired to turn on an external device that operates on variable voltages in the range 0 to 10 V. The command

SIGNAL                   10,6.0

is typical of a control statement that might be used to output a voltage level of 6.0 V to the device from controller output port 10.

All of the above interlock commands represent situations where the execution of the statement occurs at the point in the program where the statement appears. There are other situations in which it is desirable for an external device to be continuously monitored for any change that might occur in the device, This might be useful, for example, in safety monitoring where a sensor is set up to detect the presence of humans who might wander into the robot's work volume. The sensor reacts to the presence of the humans by signaling the robot controller. The following type of statement might he used for this case:

REACT                  25. SAFESTOP

This command would be written to continuously monitor input port 25 for any changes in the incoming signal. If aud when a change in the signal occurs, regular program execution is interrupted, and control is transferred to a subroutine called SAFESTOP. This subroutine would stop the robot from further motion and/or cause some other safety action to be taken.

End effectors arc devices that, although they are attached to the wrist of the manipulator. are actuated very much like external devices. Special commands are usually written for controlling the end effector. In the case of grippers, the basic commands are

OPEN

and

CLOSE

which came the gripper to actuate to fully open and fully closed positions, respectively. Greater control over the gripper is available in some sensored and servo-controlled hands.

For grippers that have force sensors that can be regulated through the robot controller, a command such as

CLOSE2.0N

controls the dosing of the gripper until a 2.0N force is encountered by the gripper fingers. A similar command used \0 close the gripper to a given opening width is:

CLOSE25MM

A special set of statements is often required to control the operation of tool-type end effectors, such as spot welding guns, arc welding tools, spray painting guns, and powered spindles (for drilling, grinding, etc.]. Spot welding and spray painting controls are typically simple binary commands (e.g., open/close and on/off), and these commands would be similar to those used for gripper control. In the case of arc welding and powered spindles, a greater variety of control statements is needed to control feed rates and other parameters of the operation

Computations and Program Logic. Many of the current generation robot languages possess capabilities for performing computations and data processing operations that are similar to computer programming languages. Most present-day robot applications do not require a high level of computational power. As the complexity of robot applications grows in the future, it is expected that these capabilities will be better utilized than at present,

Many of today's applications of robots require the use of branches and subroutines in the program. Statements such as

GOTO                       150

and

IF (logical  expression)  GO TO 150

cause tile program TO branch to some other statement in the program [e.g., to statement number 150 in the above illustrations).

A subroutine in a robot program is a group of statements that are to be executed separately when called from the main program. In a preceding example, the subroutine SAFESTOP was named in the REACT statement for use in safety monitoring. Other uses of subroutines include mak.ing calculations or performing repetitive motion sequences at a number of different places in the program. Rather than write the same steps several times in the program, The use of a subroutine is more efficient.

          Simulation   and Offline  Programming

The trouble with leadthrough methods and textual programming techniques is that the robot must be ta.ken out of production for a certain length of time to accomplish the programming. Off-line programming permits the robot program to be prepared at a remote computer terminal and downloaded to the robot controller for execution. In true offline programming. there is no need to physically locate the positions in the workspace for the robot as required with present textual programming languages. Some form of graphical computer simulation is required to validate the programs developed offline, similar to offline procedures used in NC part programming. The advantage of true offline programming is that new programs can be prepared and downloaded to the robot without interrupting production

The off-line programming procedures being developed and commercially offered use graphical simulation to construct a three-dimensional model of a robot cell for evaluation and offline programming. The cell might consist of the robot, machine tools, conveyors, and other hardware. The simulator permits these cell components to be displayed on the graphic, monitor and for the robot to perform its work cycle in animated computer graphics. After the program has been developed using the simulation procedure, it is then converted into the textual Language corresponding to the particular robot employed in the cell. This is a step in the offline programming procedure that is equivalent to post-processing in NC part programming.

In the current commercial offline programming packages, some adjustment must be performed to account for geometric differences between the three-dimensional model in the computer system and the actual physical cell. For example, the position of a machine tool in the physical layout might be slightly different than in the model used to do the off-line programming. For the robot to reliably load and unload the machine, it must have an accurate location of the load/unload point recorded in its control memory. This module is used to calibrate the 3D computer model by substituting location data from the actual cell for the approximate values developed in the original model. The disadvantage with calibrating the cell is that time is lost in performing this procedure.

In future programming systems, the offline procedure described above will probably be augmented hy means of machine vision and other sensors located in the cell. The vision and sensor systems would be used to update the three-dimensional model of the workplace and thus avoid the necessity for the calibration step in current offline programming methods. The term sometimes used 10 describe these future programming systems in which the robot possesses accurate knowledge of its three-dimensional workplace is world modeling. Associated with. the concept of world modeling is the use of very high-level language statements, in which the programmer specifies a task to be done without giving details of the procedure used to perform the tusk. Examples of this type of statement might be

ASSEMBLE    PRINTING      MECHANISM     TO BRACKET

or

WELD             UPPER   PLATE      TO LOWER         PLATE

The statements are void of any reference to points in space or motion paths to be followed by the robot. Instead, the three-dimensional model residing in the robot's control memory would identify the locations of the various items to be assembled or welded. The future robot would possess sufficient intelligence to figure out its own sequence of motions and actions for performing the task indicated.