Problems
237
[11] W
, P., “Robotics Evolution,” Manufacturing Engineering, February 1999, pp. 40–50.
[12] W
, P., “Masters of Manufacturing: Joseph F. Engelberger,” Manufacturing
Engineering, July 2006, pp. 65–75.
[13] W
, P., “Flexible Automation for Automotive,” Manufacturing Engineering,
September 2012, pp. 103–112.
[14] www.abb.com/robotics
[15] www.fanucrobotics.com
[16] www.ifr.org/industrial-robots
[17] www.kuka-robotics.com
[18] www.wikipedia.org/wiki/Delta_robot
[19] www.wikipedia.org/wiki/Industrial_robot
[20] Z
, R. G., Jr., “Guided by Vision,” Assembly, September 2005, pp. 52–58.
REVIEW QUESTIONS
8.1
What is an industrial robot?
8.2
What was the first application of an industrial robot?
8.3
What are the five joint types used in robotic arms and wrists?
8.4
Name the common body-and-arm configurations identified in the text.
8.5
What is the work volume of a robot manipulator?
8.6
Robotic sensors are classified as internal and external. What is the distinction?
8.7
What is a playback robot with point-to-point control?
8.8
What is an end effector?
8.9
In a machine loading and unloading application, what is the advantage of a dual gripper
over a single gripper?
8.10
What are the general characteristics of industrial work situations that tend to promote the
substitution of robots for human workers?
8.11
What are the three categories of robot industrial applications, as identified in the text?
8.12
What is a palletizing operation?
8.13
What is a robot program?
8.14
What is the difference between powered leadthrough and manual leadthrough in robot
programming?
8.15
What is control resolution in a robot positioning system?
8.16
What is the difference between repeatability and accuracy in a robotic manipulator?
PROBLEMS
Answers to problems labeled (A) are listed in the appendix.
Robot Anatomy
8.1
Using the joint notation system for defining manipulator configurations (Section 8.1.2),
sketch diagrams (similar to Figure 8.1) of the following robots: (a) TRT, (b) VVR, (c) VROT.
8.2
Using the joint notation system for defining manipulator configurations (Section 8.1.2),
sketch diagrams (similar to Figure 8.1) of the following robots: (a) TRL, (b) OLO,
(c) LVL.
Chap. 8 / Industrial Robotics
238
8.3
Using the joint notation system for defining manipulator configurations (Section 8.1.2),
sketch diagrams (similar to Figure 8.1) of the following robots: (a) TRT:R, (b) TVR:TR,
(c) RR:T.
8.4
Using the robot configuration notation scheme discussed in Section 8.1, write the configuration notations for some of the robots in your laboratory or shop.
8.5
Describe the differences in orientation capabilities and work volumes for a TR and a RT
wrist assembly. Use sketches as needed.
Cycle Time and Cost Analysis
8.6
(A) An articulated robot loads and unloads a CNC machine tool. The cell is scheduled to produce a batch of 300 parts. Setting up the cell for this part style takes 30 min,
programming the robot takes 75 min, and programming the CNC machine tool takes
55 min. For safety reasons, these three setup activities must be done sequentially. The
programmed machining cycle takes 3.75 min. Cutting tools wear out and must be periodically changed, which takes 6.0 min every 20 cycles and is performed by a human worker.
At the end of each machining cycle, the robot reaches into the machine and removes the
just-completed part, places it in a parts storage carousel, then reaches for a starting work
part from the same carousel and places it in the machine tool fixture. This sequence of
handling activities takes 45 sec. The storage carousel holds 25 parts. At the beginning of
the production run, it is full of raw work parts. As each part is retrieved by the robot, the
carousel indexes one position to present a new raw part. The robot is programmed to
place the completed part in the empty position in the carousel and take the next raw part.
Periodically, workers visit the carousel to collect finished parts and replace them with
starting work parts. This is accomplished without loss of production time. Determine (a)
the average production time, (b) production rate of the cell, and (c) how many hours are
required to complete the production run. (d) What is the proportion of the total time that
the robot is working?
8.7
Solve the previous problem, except that the following changes apply: (1) Setting up the
cell, programming the robot, and programming the CNC machine can be performed at the
same time rather than sequentially; (2) an automatic tool changer and tool storage unit are
used so worn tools can be exchanged from the tool storage unit with no lost production
time; and (3) a dual gripper is used rather than a single gripper, which reduces the part
handling time from 45 sec to 15 sec.
8.8
A large overhead gantry robot loads and unloads three CNC lathes in an automated work
cell. The three lathes are dedicated to the mass production of the same part style, so their
semiautomatic turning cycles are the same: 3.30 min. Assume setup time can be ignored.
The cell includes an automated parts storage unit from which the robot retrieves starting
work units and deposits finished parts using a dual gripper. The storage unit has an indexing system that presents starting work units at one position and accepts finished parts at
another position. To perform the loading and unloading sequence for each lathe requires
48 sec of the robot’s time; however, only 18 sec of lost production time are experienced
by each lathe. The loading and unloading sequence for the three lathes is synchronized
so there is no machine interference. The tooling of each lathe must be changed once each
hour, and this takes 6.0 min, which is lost production time. Determine (a) the average production time for each lathe, (b) production rate of the cell, and (c) the proportion of time
that the robot is working.
Problems
239
8.9
A robot performs a loading and unloading operation for a machine tool. The work cycle
consists of the following sequence of activities:
Seq.
Activity
1
Robot reaches and picks part from incoming conveyor and loads
into fixture on machine tool.
Machining cycle (automatic).
Robot reaches in, retrieves part from machine tool, and deposits
it onto outgoing conveyor.
Move back to pickup position.
2
3
4
Time
5.5 sec
33.0 sec
4.8 sec
1.7 sec
The activities are performed sequentially as listed. Every 30 work parts, the cutting tools
in the machine must be changed. This irregular cycle takes 3.0 min to accomplish. The
uptime efficiency of the robot is 97%; and the uptime efficiency of the machine tool is
98%, not including interruptions for tool changes. These two efficiencies are assumed not
to overlap (i.e., if the robot breaks down, the cell will cease to operate, so the machine
tool will not have the opportunity to break down, and vice versa). Downtime results from
electrical and mechanical malfunctions of the robot, machine tool, and fixture. Determine
the hourly production rate, taking into account the lost time due to tool changes and the
uptime efficiency.
8.10
Seq.
1
2
3
4
5
In the previous problem, suppose that a double gripper is used instead of a single gripper
as indicated in that problem. The activities in the cycle would be changed as follows:
Activity
Robot reaches and picks raw part from incoming conveyor in one
gripper and awaits completion of machining cycle. This activity
is performed simultaneously with machining cycle.
At completion of previous machining cycle, robot reaches in,
retrieves finished part from machine, loads raw part into fixture,
and moves a safe distance from machine.
Machining cycle (automatic).
Robot moves to outgoing conveyor and deposits part. This activity
is performed simultaneously with machining cycle.
Robot moves back to pickup position. This activity is performed
simultaneously with machining cycle.
Time
3.3 sec
5.0 sec
33.0 sec
3.0 sec
1.7 sec
Steps 1, 4, and 5 are performed simultaneously with the automatic machining cycle. Steps 2
and 3 must be performed sequentially. The same tool change statistics and uptime efficiencies are applicable. Determine the hourly production rate when the double gripper is used,
taking into account the lost time due to tool changes and the uptime efficiency.
8.11
Because the robot’s portion of the work cycle requires much less time than the machine
tool in Problem 8.9, the possibility of installing a cell with two machines is being considered. The robot would load and unload both machines from the same incoming and outgoing conveyors. The machines would be arranged so that distances between the fixture and
the conveyors are the same for both machines. Thus, the activity times given in Problem 8.9
Chap. 8 / Industrial Robotics
240
are valid for the two-machine cell. The machining cycles would be coordinated so that the
robot would be servicing only one machine at a time. The tool change statistics and uptime
efficiencies in Problem 8.9 are applicable. Determine the hourly production rate for the
two-machine cell. Assume that if one of the two machine tools is down, the other machine
can continue to operate, but if the robot is down, cell operation is stopped.
8.12
Determine the hourly production rate for the two-machine cell in Problem 8.11, only the
robot is equipped with a double gripper as in Problem 8.10. Assume the activity times from
Problem 8.10.
8.13
Arc-on time is a measure of efficiency in an arc welding operation. Typical arc-on times
in manual welding range between 20% and 30%. Suppose that a certain welding operation
is currently performed using a welder and a fitter. Production requirements are steady
at 500 units per week. The fitter’s job is to load the component parts into the fixture and
clamp them in position for the welder. The welder then welds the components in two
passes, stopping to reload the welding rod between the two passes. Some time is also lost
each cycle for repositioning the welding rod on the work. The fitter’s and welder’s activities are done sequentially, with times for the various elements as follows:
Seq.
Worker and activity
Time
1
2
3
4
5
6
Fitter: load and clamp parts
Welder: weld first pass
Welder: reload weld rod
Welder: weld second pass
Welder: repositioning time
Delay time between work cycles
4.2 min
2.5 min
1.8 min
2.4 min
2.0 min
1.1 min
Because of fatigue, the welder must take a 20 min rest at mid-morning and mid-afternoon,
and a 40 min lunch break around noon. The fitter joins the welder in these rest breaks.
The nominal time of the work shift is 8 hr, but the last 20 min of the shift is nonproductive
time for cleanup at each workstation. A proposal has been made to install a robot welding
cell to perform the operation. The cell would be set up with two fixtures, so that the robot
could be welding one job while the fitter is unloading the previous job and loading the next
job. In this way, the welding robot and the human fitter could be working simultaneously
rather than sequentially. Also, a continuous wire feed would be used rather than individual
welding rods. It has been estimated that the continuous wire feed must be changed only
once every 40 weldments and the lost time will be 20 min to make the wire change. The
times for the various activities in the regular work cycle are as follows:
Seq.
Fitter and robot activities
Times
1
2
3
4
Fitter: load and clamp parts
Robot: weld complete
Repositioning time
Delay time between work cycles
4.2 min
4.0 min
1.0 min
0.3 min
A 10 min break would be taken by the fitter in the morning and another in the afternoon,
and 40 min would be taken for lunch. Clean-up time at the end of the shift is 20 min. In
your calculations, assume that the proportion uptime of the robot will be 98%. Determine
the following: (a) arc-on times (expressed as a percent, using the 8-hr shift as the base) for
the manual welding operation and the robot welding station, and (b) hourly production
rate on average throughout the 8-hr shift for the manual welding operation and the robot
welding station.
Problems
241
8.14
(A) A work cell is currently operated 2,000 hr/yr by a human worker who is paid an hourly
rate of $23.00, which includes applicable overhead costs. One work unit is produced in a
cycle time of 4.8 min. Management would like to increase output to meet increasing demand and a robot cell is being considered as a replacement for the present manual cell. The
cycle time of the proposed cell would be reduced to 4.0 min. The installed cost of the robot
plus supporting equipment is $120,000. Power and other utilities to operate the robot will
be $0.30/hr, and annual maintenance costs are $2,500. Determine (a) the number of parts
produced annually by the manually operated cell and (b) cost per part produced. (c) How
does the cost per part of the robot cell compare with your answer in part (b), given a 4-year
service life, 10% rate of return, and no salvage value.
8.15
A manual arc welding cell uses a welder and a fitter. The cell operates 2,000 hr/yr. The
welder is paid $30/hr and the fitter is paid $25/hr. Both rates include applicable overheads.
The cycle time to complete one welded assembly is 15.4 min. Of this time, the arc-on time
is 25%, and the fitter’s participation in the cycle is 30% of the cycle time. A robotic arc
welding cell is being considered to replace this manual cell. The new cell would have one
robot, one fitter, and two workstations, so that while the robot is working at the first station, the fitter is unloading the other station and loading it with new components. The
fitter’s rate would remain at $25/hr. For the new cell, the production rate would be eight
welded assemblies per hour. The arc-on time would increase to almost 52%, and the fitter’s
participation in the cycle would be about 62%. The installed cost of the robot and workstations is $158,000. Power and other utilities to operate the robot and arc welding equipment
will be $3.80/hr, and annual maintenance costs are $3,500. Given a 3-year service life, 15%
rate of return, and no salvage value, (a) determine the annual quantity of welded assemblies that would have to be produced to reach the break-even point for the two methods.
(b) What is the annual quantity of welded assemblies produced by the two methods working 2,000 hr/yr?
Robot Programming Exercises
Note: The problems in the following group are all programming exercises to be performed
on robots available to students. The solutions depend on the particular programming methods or languages used. They represent suggestions for laboratory exercises to instructors
using the book.
8.16
The setup for this exercise requires a felt-tipped pen mounted to the robot’s end-of-arm
(or held securely in the robot’s gripper). Also required is a thick cardboard, mounted on
the surface of the worktable. Pieces of plain white paper will be pinned or taped to the
cardboard surface. The exercise is the following: Program the robot to write your initials
on the paper with the felt-tipped pen.
8.17
As an enhancement of the previous programming exercise, consider the problem of programming the robot to write any letter that is entered at the alphanumeric keyboard.
A textual programming language is required for this exercise.
8.18
Apparatus for this exercise consists of two wood or plastic blocks of two different colors
that can be grasped by the robot gripper. The blocks should be placed in specific positions
(call the positions A and B on either side of a center location (call it position C). The robot
should be programmed to do the following: (1) pick up the block at position A and place it
at position C; (2) pick up the block at position B and place it at position A; (3) pick up the
block at position C and place it at position B. (4) Repeat steps (1), (2), and (3) continually.
8.19
Apparatus for this exercise consists of a cardboard box and a dowel about 4 inches long
(any straight thin cylinder will suffice, pen, pencil, etc.). The dowel is attached to the robot’s end-of-arm or held in its gripper. The dowel is intended to simulate an arc welding
Chap. 8 / Industrial Robotics
242
torch, and the edges of the cardboard box are intended to represent the seams that are to
be welded. The programming exercise is the following: With the box oriented with one of
its corners pointing toward the robot, program the robot to weld the three edges that lead
into the corner. The dowel (welding torch) must be continuously oriented at a 45° angle
with respect to the edge being welded. See Figure P8.19.
Robot
end-of-arm
Dowel
45
Cardboard
box
Figure P8.19
8.20
Orientation of arc welding torch for Problem 8.19.
This exercise is intended to simulate a palletizing operation. The apparatus includes six
wooden (or plastic or metal) cylinders approximately 20 mm in diameter and 75 mm in
length, and a 20 mm thick wooden block approximately 100 mm by 133 mm. The block is
to have six holes of diameter 25 mm drilled in it as illustrated in Figure P8.20. The wooden
cylinders represent work parts and the wooden block represents a pallet. (As an alternative to the wooden block, the layout of the pallet can be sketched on a plain piece of paper
attached to the worktable.) The programming exercise is the following: Using the powered
leadthrough programming method, program the robot to pick up the parts from a fixed
position on the worktable and place them into the six positions in the pallet. The fixed position on the table might be a stop point on a conveyor. (The student may have to manually
place the parts at the position if a real conveyor is not available.)
6 Holes (25mm dia.)
Figure P8.20 Approximate pallet dimensions for Problem 8.20.
Accuracy and Repeatability
8.21
(A) The linear joint (type L) of an industrial robot is actuated by a piston. The length of
the joint when fully retracted is 800 mm and when fully extended is 1,400 mm. If the robot’s
controller has a 10-bit storage capacity, determine the control resolution for this joint.
Problems
243
8.22
(A) In the previous problem, the mechanical errors associated with the linear joint
form a normal distribution in the direction of the joint actuation with standard
deviation = 0.11 mm. Determine the accuracy and repeatability for the robot.
8.23
The rotational joint (type R) of an industrial robot has a range of 170° rotation. The mechanical errors in the joint and the input/output links can be described by a normal distribution with its mean at any given addressable point and a standard deviation of 0.15°.
Determine the number of storage bits required in the controller memory so that the accuracy of the joint is as close as possible to, but less than, its repeatability. Use six standard
deviations as the measure of repeatability.
8.24
An articulated robot has a T-type wrist axis that can be rotated a total of 2 rotations (each
rotation is a full 360°). It is desired to be able to position the wrist with a control resolution
of 0.25° between adjacent addressable points. (a) Determine the number of bits required
in the binary register for that axis in the robot’s control memory. (b) Using this number of
bits, what is the actual control resolution of the joint?
8.25
One axis of an RRL robot is a linear slide with a total range of 750 mm. The robot’s control
memory has a 10-bit capacity. It is assumed that the mechanical errors associated with the
arm are normally distributed with a mean at the given taught point and a standard deviation of 0.10 mm. Determine (a) the control resolution for the axis under consideration,
(b) accuracy, and (c) repeatability.
8.26
(A) Link 3 of a TRR robot has a rotational joint and its outer end is connected to a wrist
assembly. Considering the design of this joint only, the link 3 is 600 mm long, and the
total range of rotation of the joint is 60°. The control resolution of this joint is expressed
as a linear arc measure at the wrist, and is specified to be 1.0 mm or less. It is known that
the mechanical inaccuracies in the joint result in an error of {0.018° rotation, and it is
assumed that the output link is perfectly rigid so as to cause no additional errors due to
deflection. (a) Determine the minimum number of bits required in the robot’s control
memory to achieve the control resolution specified. (b) Using this number of bits, what is
the actual control resolution of the joint?