Figure 1-20 Operation of a Three-roll Wrist.

the three wrist orientation motions. The tool plate (left side of the ball in Figure 1-20) rotates to provide a roll motion. The line or separation through the sphere shaped object indicates that it is assembled from two hemispheres that can rota te with respect to one another. The entire sphere can rotate at the end of the arm where the top hemisphere (labeled A) connects to the arm shaft. The lower half of the sphere (labeled B) can rotate with respect to the upper half of the sphere to change the orientation of the tool plate. Note in Figure 1-20 that a 180 rotation of the lower hemisphere (B) would move the tool plate from position 1 to position 2. As a result, pitch and yaw motions can be achieved by a combination of rotations by the upper hemisphere (A) at the arm and the motion of the lower hemisphere (B) with respect to the upper hemisphere. All three orientation motions are achieved with a small wrist geometry.

Degree of Freedom. Every joint or movable axis on the arm is a degree of freedom. Amachine with six movable joints, such as the one in Figure 1-10, is a robot with 6 degrees of freedom or axes. Orientation of the tool by the wrist involves a maximum of 3 degrees of freedom, and up to 4 degrees of freedom are used far positioning within the work envelope. A range of 4 to 7 degrees of freedom is typical far industrial robots.

Coordinate Systems. Ali points programmed in the work cell are identified by a base coordinate system that consists of three translation coordinates-X, Y, and Zand three rotational coordinates-A, B, and C. The number of work-cell coordinates required to define a programmed point is determined by the number of degrees of freedom present on the robot. Figure 1-21 shows the coordinate systemfrequently used by robots with 6 degrees of freedom. Accuracy. Accuracy is best explained by an example from target shooting with a rifle. The targets in Figure 1-22 indicate the results when two different rifles are fired at the center of the targets. The rifle producing the results in Figure 1-22a is

Figure 1-21 Robot CoordinateSystem.

Position in work envelope is described by three coordinatevalues (X, Y, Z) and three wrist angles (A, B, C)z (X, Y, Z, A, B, C)------ - YXWork area coordinate system

not accurate because ali shots missed the center; however, the rifle is repeatable because all shots hit the target in the same area. The second rifle was both accurate and repeatable, because it placed all shots (Figure 1-22b) in a close group in the center of the target.In robotics, accuracy is the degree to which a robot arm can move to a specific translation or position point in the work cell when the point coordinates are: (1) entered from an off-line programming station, (2) calculated inside the program, (3) received from a vision system, or (4) generated in a work-cell simulator. For example, a vision system could specify that the robot tooling should move to a point described as X = 50.00 inches, Y = 45.30 inches, Z = 10.01 inches (tooling position), A = 0.00 degrees, B = 90.00 degrees, and C = 100.00 degrees (tooling orientation).

This move implies that the tool center point moves from the robot's O, Q, O point to a point 50.00 inches along the X axis, 45.30 inches along the Y axis, and 10.01 inches along the Z axis. The wrist then changes by the degrees specified. The accuracy specification describes how close the tool center point will be to the point described by the vision system. In general, robot accuracy is the difference w ~

Figure 1-22 Rifle Targets: (a) Repeatable and (b) Accurate and Repeatable.lntroduction to Industrial Robots 37/

\

between actual location of the robot tool and the location specified by the translation point entered by one of the four methods specified earlier. Robot accuracy is usually an order of ten worse than the arm' s repeatability.Industry standards are in place to assist robot manufacturers in determining the accuracy of robot systems. The ISO standard uses a test cube with an inclined test plane containing five programmed points (Figure 1-23). The test results for three ABB robots are shown in Figure 1-24.

Observing the robots in the chapter figures indica tes that robots have a base on which the arm is built. The geometry requires that each axis or degree of freedom be built on the previous axis, with axis 1 attached to the base, axis 2 attached to axis 1, and so on, until the tool plate is attached to the end of the last axis. The degree of inaccuracy is a result of the buildup of the mechanical tolerances on the arm elements as they are assembled. For example, the Adeptne robot in Figure 1-16 has an accuracy of 0.003 in., which implies that the arm elements are manufactured and assembled with less than 0.003 in. variation from the base to the toolplate on every robot produced.

Repeatability. Repeatability is the degree to which a robot system can return to a specific programmed position point in the work cell. Frequently, a robot is taught the required gripper location by moving the arm to the location with the teach pendant and then pushing the program point button. The repeatability specification indicates how well the robot arm can return to the taught point on each subsequent cycle of the program execution. The best repeatability on assembly robots is 0.0005 inches. A robot's repeatability is not affected by the tolerance buildup that affects accuracy because the tool-plate reference point is aligned with a desired work envelope point either visually during programming or with mechanical devices. Any manufacturing or assembly variation in the arm linkages is eliminated by the visual aligriment of the tooling with the desired work envelope location. The errors associated with repeatability are produced by robot axes that do not have tight joint movements or when looseness or sloppiness is introduced in the mechanical linkages dueto wear.

Tool Center Point. The point aj action for the tool mounted to the robot tool plate is called the offset or too/ center point (TCP), see Figure 1-25. With an offset of O, O, O for L, A, and B, respectively, the TCP is located at point TCPl in Figure 1-25. With L = 10.00 inches, A = 5.00 inches, and B = 4.00 inches, the TCP is located at TCP2. When a tool is mounted to the tool plate, the distance from TCPl to theFigure 1-25 Tool Center Point.(Courtesy of Cincinnati Milacron Corp)Tool plate

Figure 1-26 Robot with Welding

action point on the tool is included in the robot program. Far example, the welding torch in Figure 1-26 has a TCP of L = 14.00 inches, A = -4.00 inches, and B = 0.00 inches. In the welding example, the robot will control the motion at the welding electrode as the arm moves through the programmed points. In sorne robot systems the offset is part of the programming language. In the Seiko DARL language, far example, the define offset commandis DEF TL < 1 - 9 > Xlt Y1, X2, Y2, Z1. Up to nine define offset tool statements can be used to specify the X, Y, and Z coordinates far the TCP.

Precise identification of the tool center point (TCP) or offset is critical to systemaccuracy and repeatability. If the robot tooling collides with part of the work cell during production, the offset of the TCP can be changed. Calibration of the new offset causes lost production time. Res robot systems overcome this problem with a special function called TCP measurement, which automatically determines the hand length, tool length, and tool angle of unknown tools. In the case of tool collision, a simple move to a reference point from faur different dire~tions establishes the new TCP (Figure 1-27).

Velocity. The rate at which the robot can move each axis and the TCP under program control is a measure of the machine' s velocity. Velocity is expressed in linear or angular English and metric units. Arm velocity between programmed points is much higher than the average velocity over a number of programmed points because of the deceleration and acceleration of the TCP as it passes through a programmed point. Direct-drive robots like the Adeptne have excellent too! velocity; note the velocity specification in Figure 1-16.

Example 1-5The velocity (VNL) far a robot is rated at 20 feet per second no load with a 25 percent reduction at maximum payload. The robot moves through 200 inches with close to maximum payload and 125 inches with no part inthe gripper. There are 23 programmed points and 6 seconds of programmed delays. Test data indicates that each programmed point takes an average of 0.1 seconds and the velocity (VcL) with the gripper empty is 18 feet per second. Find the fastest possible time for one cycle.

Payload. The rated payload is the weight that the robot is designed to manipula te under the manufacturer's specified performance conditions of speed and acceleration-deceleration over the entire work envelope. The center of gravity of the payload must be within offsets or locations specified by the manufacturer. The maximum payload is the maximum weight that the robot can manipula te ata specified speed, acceleration-deceleration, center of gravity location (offset or location), and repeatability under continuous operation overa specified work envelope. The payload or load capacity of a robot (Figure 1-16) often determines if the machine is suitable for a specific task. The payload is the total combined weight of the gripper or end-of-arm tooling and the part to be moved (payload = tooling weight + part weight).

Example 1-6A robot must move a 14-pound load in a material handling application. If the maximum payload (PM) for the robot is 25 pounds, what is maximum allowed weight for the gripper (Wc) if a 25 percent safety factor is required?

Additional terms associated with robot hardware and software are introduced as needed throughout the text.

1-1 O ROBOT SAFETY GUIDELINESThe safety of personnel and equipment is an important consideration in any manufacturing area and is especially critical when machines like robots are present. Safety results from a detailed plan to opera te in a safe manner; as a result, safety considerations must be part of every machine and work-cell design. Safety design considerations are covered in great detail in Chapter 10; however, safety guidelines are included here because it is important to introduce safe robot operating procedures at the start of any robot study.

Work Cell Safety Design RequirementsMuch can be said about the design requirements for a safe work cell; most of the issues are addressed in Chapter 10. Sorne fundamental issues, however, should be known from the start. The maximum reach or work envelope of the robot should be clearly marked on the floor with high contrast safety tape or paint. A drive-active warning device should be clearly visible from any point around the total perimeter of the robot work cell. The warning device should be a flashing light or strobe that is automatically triggered anytime the drive motors, valves, or actuators that produce robot motion are powered.Most robot controllers provide a set of contacts that can be used to control a drive-active warning device. No hardware should be in the robot work envelope except that portion of the process machines that must be accessed by the robot tooling. Personnel should be protected from inadvertent exposure to the robot arm by barriers placed around the work cell. The barriers can be work-cell equipment or standard safety fence. In situations where production requirements prevent hard barriers from being erected, safety curtains using sensors should be used to warn of an intrusion and power down the robot to a safe condition. Emergency stop switches should be located at strategic points around the work cell to power down the robot to a safe condition when a problem situation is detected. Signs should be used to clearly indicate that a programmable device is present and movement of the device could occur at anytime. Provision should be made to allow maintenance workers to lock off (sometimes using lock-out procedures) master control power panels and device controllers when they are making changes to the system.

Guidelines far Safe Robot UseThere are a number of ways that a robot can cause injury to personnel, but the two most common are injury from a blow to sorne part of the body by the robot arm or tooling, and having sorne part of the body trapped between the robot arm and a fixed object. While both types of accidents are serious, the second usually causes the most serious injury and can be avoided. The general rule to avoid serious injury is to avoid pinch points. This means that you must be constantly alert to avoid any position in the work cell that puts any part of your body between the robot arm and sorne fixed or unmovable object. Most impact accidents from a robot happen because personnel in the work envelope make the following incorrect assumptions:

If the arm is not moving it is assumed that the program has been halted. In many Robot applications the robot program halts robot motion until a signal from sorne external machine indicates that the robot should proceed with the next motion. Far example, a robot program to move parts to a conveyor could have a pause programmed in the routine if the previous part was still in the drop position. If the previous part gets jammed on the conveyor, the robot would halt ata position sorne distance from the conveyor. The situation may not occur far months, then one day the cell seems to be shut clown due to a jam. An operator who moves between the robot and conveyor to remove the jam could be injured when the robot proceeded after the jammed part was removed. The operator made two mistakes: first, assuming that the robot was off beca use it was not moving, and, second, walking into a pinch point.

If the arm is repeating one pattern of motion, then it is assumed that the same pattern will continue. Robot programs often have subroutines that allow the robot motion to be changed by an external signal. Far example, a robot may have a program that moves castings from a die cast machine to a trim press. What may not be apparent is that a sensor determines if the casting is good befare the move to the trim press is executed.

If the casting is bad, a subroutine is used to move the part to a scrap bin. Most parts are good, so the repeated motion to the trim press could be incorrectly assumed until that one bad part out of a hundred is generated.

If the arm is moving slowly, it is assumed that slow movement will continue. Programmers have a wide range of speeds that they can use in a robot program. The slow speed far one move is no guarantee that the same speed will be used far the next move.

If you program the arm to move, you assume it will move the way you wanted. Robot programs are developed using two steps: (1) the points or the desired locations in the work cell are taught and given names to identify the locations, (2) the program commands are organized into the program structure to move the robot to the programmed points at the correct speed. It is not uncommon to get taught points interchanged or incorrectly identified. When you think the robot is going to one location, it actually moves toan entirely different one.

Robot accidents can occur during three different activities: regular operation, programming, and maintenance. A study, Figure 1-28, conducted by the Japanese indicates the frequency of accidents in eight different situations that cover these three areas. Sorne of the causes involve failure or incorrect action of the robot system or sorne peripheral machine or device. System failure cannot be completely eliminated; however, a good preventive maintenance program can eliminate sorne accidents attributed to machine or component failure. A better-educated work force would reduce the injury rate in many of the groups where incorrect

action or operation was due to the operator or maintenance personnel not following correct procedures. There are few reasons for an operator to be in the work envelope of a robot.Production systems and procedures can be developed that keep the operator out of harm's way. Most maintenance procedures do not require the maintenance personnel to be in the work envelope during testing, so with good maintenance procedures and education the maintenance-related accidents can be greatly reduced.The only activity that requires a person to be clase to the robot arm and tooling is programming taught points.During the programming process it is often n ecessary for the programmer to be visually clase to the tooling to assure that aligrtment with the part or fixture is present befare the point is programmed. During this time the programmer is moving the tooling with motion buttons on the teach pendan t. When manually taught points is the programming technique required for the robot cell, the programmer must be clase to the robot with the drive system active. To minimize the risk of injury the following guidelines should be followed. Always have an operator or other programmer present with a hand on the emergency stop when working clase to the robot tooling. Never stand in a pinch point when performing any programming activity. Always u se a low-arm-move speed when programming requires clase proximity to the tooling. Never change electrical cables or input/output signa! wires with the controller power on. Never stand in the work envelope when you send the tooling to a taught point to verify location accuracy. Signa! the move to the programmed point from outside the work envelope and check the point accuracy afterall motion has stopped. Never stand in the work envelope when testing a new or edited robot program. If possible, keep one hand on the robot arm as you move the robot with the teach pendant. If a failure in the system causes a sudden motion of the robot in your direction, your hand and arm will cause you to be pushedaway minimizing direct contact with the moving robot arm.While accidents can never be completely eliminated, the following four guidelines will reduce them significantly. Use effective perimeter warning devices, barriers, and interlocks around work cells and robot-type devices Provide general training for ali shop floor employees concerning the dangers inherent in robot systems. Provide machine-specific safety training for operators who work in robot work cells. Establish a comprehensive preventive maintenance program for the robot and work cell implemented by maintenance personnel trained in the hardware and software used in the cell. In addition to safety guidelines for the robot, a good industrial safety program must also consider the hazards associated with injury from electrical shock and the dangers from broken or leaky high pressure air and hydraulic lines. lnjury from a robot can often be avoided by remembering these guidelines and the three Rs of robotic safety. Robots Require Respect.

1-11 ROBOT STANDARDSIn 1984 the Robotic Industries Association (RIA) established the R15 Executive Committee on Robotic Standards and the Automated Imaging Association (AIA). One year later the A15 Standards Committee in the AIA was formed to