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