The Review of Reliability Factors Related to Industrial Robo- Juniper Publishers
Juniper Publishers- Journal of Robotics
Abstract
Although, the problem of industrial robot reliability
is closely related to machine reliability and is well known and
described in the literature, it is also more complex and connected with
safety requirements and specific robot related problems (near failure
situations, human errors, software failures, calibration, singularity,
etc.).Compared to the first robot generation, the modern robots are more
advanced, functional and reliable. Some robot’s producers declare very
high robot working time without failures, but there are fewer
publications about the real robot reliability and about occurring
failures. Some surveys show that not every robot user has monitoring and
collects data about robot failures. The practice show, that the most
unreliable components are in the robot’s equipment, including grippers,
tools, sensors, wiring, which are often custom made for different
purposes. The lifecycle of a typical industrial robot is about 10-15
years, because the key mechanical components (e.g. drives, gears,
bearings) are wearing out. The key factor is the periodical maintenance
following the manufacturer’s recommendations. After that time, a
refurbishment of the robot is possible, and it can work further, but
there are also new and better robots from modern generation.
Keywords: Industrial robot;Reliability; Failures; Availability; Maintenance; Safety; MTTF; MTBF; MTTR; DTDTRF
Introduction
Nowadays, one can observe the increasing use of
automation and robotization, which replaces human labor. New
applications of industrial robots are widely used especially for
repetitive and high precision tasks or monotonous activities demanding
physical exertion (e.g. welding, handling). Industrial robots have
mobility similar to human arms and can perform various complex actions
like a human, but they do not get tired and bored. In addition, they
have much greater reliability then human operators. The problem of
industrial robot reliability is like machine reliability and is well
known and described in the literature, but because of the complexity of
robotic systems is also much more complex and is connected with safety
requirements and specific robot related problems (near failure
situations, hardware failures, software failures, singularity, human
errors etc.). Safety is very important, becausethere were many accidents
at work with robots involved, and some of them were deadly. Accidents
were caused rather more often by human errors than by failures of the
robots.
The research about robot reliability was started in
1974 by Engleberger, with publication, which is a summary of three
million hours of work of the first industrial robots–Unimate[1]. A very
comprehensive discussion over the topic is presented by Dhillon in the
book, which covers the problems of robot reliability and safety,
including mathematical modelling of robot reliability and
some examples[2]. An analysis of publications on robot reliability up to
2002 is available in Ref. Dhillon et al.[3], and some of the important
newer publications on robot reliability and associated areas are listed
in the book [4].The modern approach to reliability and safety of the
robotic system is presented in the book, which includes Robot
Reliability Analysis Methods and Models for Performing Robot Reliability
Studies and Robot Maintenance[5]. The reliability is strongly connected
with safety and productivity, therefore other researches include the
design methods of a safe cyber physical industrial robotic manipulator
and safety-function design for the control system or simulation method
for human and robot related performance and reliability[6-7]. There are
fewer publications about the real robot reliability and about occurring
failures [8]. The surveyshows that only about 50 percent of robot users
have monitoring and collect data about robot failures.
Failure analysis of approximately 200 mature robots
in automated production lines, collected from automotive applications in
the UK from 1999, is presented in the article, considering Pareto
analysis of major failure modes. However, presented data did not reveal
sufficiently fine detail of failure history to extract good estimates of
the robot failure rate[9-10].
In the article11. Sakai et al.[11], the results of
research about robot reliability at Toyota factory are presented. The
defects of 300 units of industrial robots in a car assembly line were
analyzed, and
a great improvement in reliability has been achieved. The authors
consider as significant activities that have been driven by robot
users who are involved in the management of the production line.
Nowadays, robot manufacturers declare very high reliability of
their robots [12]. The best reliability can be achieved by the robots
with DELTA and SCARA configuration. This is connected with lower
number of links and joints, compared to other articulated robots.
Because each additional link with serial connection causes an
increase of the unreliability factors, therefore, some components
are connected parallel, especially in the Safety Related Part of the
Control System (SRP/CS), which have doubled number of some
elements, for example emergency stops. Robots are designed in
such way that any single, reasonably foreseeable failure will not
lead to the robot’s hazardous motion [13].Modern industrial
robots are designed to be universal manipulating machines, which
can have different sort of tools and equipment for specific types of
work. However, the robot’s equipment is often custom made and
may turn out to be unreliable as presented in, therefore, the whole
robotic system requires periodic maintenance, following to the
manufacturer’s recommendations [14-15].
operators and robots in cooperative tasks, therefore, the safety
plays a key role. Safety can be transposed in terms of functional
safety addressing the functional reliability in the design and
implementation of devices and components that build the robotic
system [16].
Robot Reliability
The reliability of objects such as machines or robots is defined
as the probability that they will work correctly for a given time
under defined working conditions. The general formula for
obtaining robot reliability is [2]:
Where:
Rr(t) is the robot reliability at time t,
λr(t) is the robot failure rate.
In practice, for description of reliability, in most cases the
MTTF (Mean Time to Failure) parameter is used, which is the
expected value of exponentially distributed random variable with
the failure rate λr [2].
In real industrial environments, the following formula can be
used to estimate the average amount of productive robot time,
before robot failure [2]:
Where:
PHR – is the production hours of a robot,
NRF – is the number of robot failures,
DTDTRF – is the downtime due to robot failure in hours,
MTTF – is the robot mean time to failure.
In the case of repairable objects, the MTBF (Mean Time
Between Failures), and the MTTR (Mean Time to Repair)
parameters, can be used.
The reliability of the robotic system depends on the reliability
of its components. The complete robotic workstation includes:
A. Manipulation unit (robot arm),
B. controller (computer with software),
C. equipment (gripper, tools),
D. workstation with workpieces and some obstacles in the
robot working area,
E. safety system (barriers, curtains, sensors),
F. human operator (supervising, set up, teaching,
maintenance).
The robot system consists of some subsystems that are
serially connected (as in the Figure 1) and have interface for
communication with the environment or teaching by the human
operator.The robot arm can have different number of links and
joints N. Typical articulated robots have N=5-6joints as in the
Figure 2, but more auxiliary axes are possible.
For serially connected subsystems, each failure of one
component brings the whole system to fail. Considering complex
systems, consisting of n serially linked objects, each of which has
exponential failure times with rates λi, i= 1, 2, …, n, the resultant
overall failure rate λSof the system is the sum of the failure rates of
each element λi[2]:
Moreover, the system MTBFS is the sum of inverse MTBFi, of
linked objects:
There are different types of failures possible:
A. Internal hardware failures (mechanical unit, drive, gear),
B. Internal software failures (control system),
C. External component failures (equipment, sensors,
wiring),
D. Human related errors and failures that can be:
a. Dangerous for humans (e.g. unexpected robot
movement),
b. Non-dangerous, fail-safe (robot unable to
move).
Also possible are near failure situations and robot related
problems, which require the robot to be
stopped and human intervention is needed (e.g. recalibration,
reprograming).Because machinery failures may cause severe
disturbances in production processes, the availability of means
of production plays an important role for insuring the flow of
production. Inherent availability can be calculated with the
formula 7 [2].
For example, the availability of Unimate robots was about 98
% over the 10-years period with MTBF=500h and MTTR=8 hours
[2].
The reliability of the first robot generation represents the
typical bathtub curve (as in Figure3), with high rate of early
“infant mortality” failures, the second part with a constant failure
rate, known as random failures and the third part is an increasing
failure rate, known as wear-out failures (it can be described with
the Weibull distribution).
Therefore, the standard [17] was provided, in order
to minimize testing requirements that will qualify a newly
manufactured (or a newly rebuilt industrial robot) to be placed
into use without additional testing. The purpose of this standard
is to provide assurance, through testing, that infant mortality
failures in industrial robots have been detected and corrected
by the manufacturer at their facility prior to shipment to a user.
Because of this standard, the next robot generation has achieved
better reliability, without early failures, with MTBF about 8000
hours [16].In the articleSakai&Amasaka[11], the results of
research about robot reliability at Toyota are presented. Great
improvement was achieved with an increase of the MTBF to about
30000 hours.
Nowadays, robot manufacturers declare an average of MTBF =
50,000 - 60,000 hours or 20 - 100 million cycles of work [12]. The
best reliability is achieved by the robots with SCARA and DELTA
configuration. This is connected with lower number of links and
joints, compared to other articulated robots.Some interesting
conclusions from the survey about industrial robots conducted in
Canada in year 2000 are as follows [9]:
A. Over 50 percent of the companies keep records of the
robot reliability and safety data,
B. In robotic systems, major sources of failure were
software failure, human error and circuit board troubles from
the users’ point of view,
C. Average production hours for the robots in the Canadian
industries were less than 5,000 hours per year,
D. The most common range of the experienced MTBF was
500–1000h (from the range 500-3000h)
E. Most of the companies need about 1–4h for the MTTR of
their robots (but also in many cases the time was greater than
10h or undefined).
The current industrial practice show that the most unreliable
components are in the robot’s equipment, including grippers,
tools, sensors, wiring, which are often custom made for different
purposes. This equipment can be easily repaired by the robot
user’s repair department. But the failure of critical robot
component requires intervention of the manufacturer service and
can take much more time to repair (and can be counted in days).
Therefore, for better performance and reliability of the robotic
system, periodic maintenance is recommended.
Robot Maintenance
Three basic types of maintenance for robots used in industry
are as follows [4]:
Preventive maintenance
This is basically concerned with servicing robot system
components periodically (e.g. daily, yearly. …)
Corrective maintenance
This is concerned with repairing the robot system whenever
it breaks down.
Predictive maintenance
Nowadays, many robot systems are equipped with
sophisticated electronic components and sensors; some of them
are capable of being programmed to predict when a failure
might happen and to alert the concerned maintenance personnel
(e.g. self-diagnostic, singularity detection).Robot maintenance
should be performed, following to the robot manufacturer’s
recommendations, which are summarized in the Table 1[15].
Preventive maintenance should be provided before each automatic
run, including self-diagnostic of the robot control system, visual
inspection of cables and connectors, checking for oil leakage or
abnormal signals like noise or vibrations. The replacement of the
battery, which powers the robot’s positional memory, is needed
yearly. If the memory is lost, then remastering (recalibration,
synchronization) is needed.Replenishing the robot with grease
every recommended period is needed to prevent the mechanical
components (like gears) from wearing out. Special greases are
used for robots (e.g. Moly White RE No.00) or grease dedicated for
specific application like for the food-industry. Every 3-5 years a
fully technical review (overhaul) with replacement of filters, fans,
connectors, seals, etc. is recommended.
Performing daily inspection, periodic inspection, and
maintenance can keep the performance of robots in a stable state
for a long period. The lifecycle of typical robot is about 10-15
years, because the wear of key mechanical components (drives,
gears, bearings, brakes) causes backlash and positional inaccuracy.
After that time a refurbishment of the robot is possible, and it can
work further for long time. Refurbished Robots are also called remanufactured,
reconditioned, or rebuilt robots.
Conclusion
Nowadays modern industrial robots have achieved high
reliability and functionality;therefore, they are widely used. This is
confirmed by more than one and half million of robots working
worldwide. According to the probability theory, in such large
robot population the failures of some robots are almost inevitable.
The failures are random, and we cannot predict exactly where
and when, they will take place. Therefore, the robot users should
be prepared and should undertake appropriate maintenance
procedures. This is important, because industrial robots can highly
increase the productivity of manufacturing systems, compared to
human labor, but every robot failure can cause severe disturbances
in the production flow,therefore periodic maintenance is required,
in order to prevent robot failures. High reliability is also important
for the next generation of collaborative robots, which should work
close to human workers, and safety must be guaranteed without
barriers. Also, some sorts of service robots, which should help
nonprofessional people (e.g. health care of disabled people)
must have high reliability and safety. There have already been
some accidents at work, with robots involved, therefore, the
next generation of intelligent robots should be reliable enough
to respect the Asimov’s laws and do not hurt people, even if they
make errors and give wrong orders.
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