Robotic Accuracy Improves Aerospace Manufacturing
Where accuracy is concerned, robots have traditionally relied on repeatability.
In the past, robotic accuracy has not been published or developed to a level
of maturity acceptable to standard production processes. Critical aerospace
manufacturing techniques such as fastening, and drilling, were historically
not held to tight tolerances. Typical tolerances for airframe assembly fastening
were in the + 0.030” range. The standard is set by the positional requirement
for drilling of fastener holes which has been taken as a key target application
for robotics in aerospace manufacturing. Because there are many factors that
influence robot accuracy, it is important to define the accuracy requirements
for the system. Different levels of accuracy require different solutions;
the higher the accuracy required the more factors that must be considered,
adding to the cost and complexity of the solution. The level of accuracy
should be defined according to the process requirements. Some processes will
only require positional accuracy while others require path accuracy, and some
applications will require both.
Recently, for example, manufacturers are demanding high wear parts that
require frequent maintenance and replacement, be replaced seamlessly with
identically manufactured parts. Inconsistent and inaccurately machined or
assembled replacement parts might traditionally have meant time lost due to
trimming, deburring or other adjustments. Reducing fastener tolerances not
only improves the reproducibility of an assembled component, it also allows
for a reduction in overall structure weight due to reduced fastener size and
weight. Eliminating these adjustments by machining or assembling precisely
formed parts allows for predictable and timely part replacement, reducing
costs and downtime, and allowing for parts to be interchanged repeatedly without
any interruption in production. The introduction of robotic accuracy into
the manufacturing process guarantees that this replacement is smooth, does
not interrupt the manufacturing process, is cost-effective, and highly accurate.
High accuracy is also critical in data driven applications — those
developed using off-line programming methods. For example, an advanced deburring
process can begin off-line with products such as FANUC America' ROBOGUIDE
PC-based simulation software. ROBOGUIDE allows you to import 3D representations
of robots, parts, and system peripherals, to create realistic “virtual”
workcells. Using ROBOGUIDE’s built-in utilities, part features can be
selected from 3D CAD data of the part. Robot programs can then be generated
automatically from these selected features. Even vision programming can be
accomplished off-line. Until recently, it was not easy to off-line program
a robot without having to perform extensive manual touch-up of the programs
on the production floor. Because of the large number of points required for
airframe drilling, it is mandatory to off-line program without extensive program
Robot Accuracy and Repeatability
Robot accuracy is a measure of how
close a robot can attain a known position. It is required for systems where
the paths are taught off-line or if the process requires changing the robot
position dynamically using vision or another means. Robot
repeatability is a measure of the robot’s ability to return
to a known position. High robot accuracy during manufacturing ensures that
parts are precisely manufactured with predictable results even after changes
are made to the process. High accuracy robots are becoming valuable tools
for many processes in aerospace manufacturing such as drilling and fastening,
deburring and trimming, and a variety of others such as non-destructive inspection,
coatings, and composite layup. Manufacturers can enjoy significant cost savings
as a result.
Systems that combine processes like drilling, routering, and material
removal require both positional accuracy and path accuracy. Positional accuracy is a measure of
how accurately the robot can achieve a commanded position. Positional accuracy
is required for processes like drilling, where the robot moves to a position,
stops, and holds that position while the process is completed. Path accuracy
is a measure of how accurately the robot follows a line between two points.
Path accuracy is required for processes like laser cutting where the process
is taking place while the robot is moving between points.
Robot accuracy is improved when the work zone is defined as localized
as possible. It is important to define where in the robot’s work envelope
the process will takes place. This is called the process
work zone. A higher level of accuracy is achievable if the process
work zone is defined and the calibration is restricted to this zone. When
defining a process work zone, there are three considerations to follow. First
is that the process work zone needs to include all processes that require
accuracy. Second, make the zone only as large as the process requires. Third,
limit robot configuration (orientation) changes in the process work zone as
much as possible.
Different levels of accuracy require different solutions. The required
level of robot accuracy determines the number of options and calibration tools
required to achieve that accuracy. The more calibration tools required, the
more complex and expensive the solution will be. FANUC America uses several
software applications and robot model combinations to maintain accuracy in
aerospace manufacturing solutions. Some of those calibration tools include
robot joint axis mastering, tool center point calibration, part frame calibrations,
robot to robot calibrations for coordinated motion, and multi-robot applications.
Many of these calibrations use FANUC intelligent based tools like touch sense
or iRVision. Some more advanced calibration
options available include iRCalibration
Signature (re-modeling of the true parameters of the robot based on metrology
feedback), secondary encoder feedback, and deflection compensation tools.
For even tighter tolerances you can use a metrology tool such as a laser tracker
to guide the robot to precision better than .1mm.
Repeatable robot paths and tool execution means critical material cost
savings in removal applications. An added benefit of using accurate robots
for aerospace manufacturing is the inherent repeatability of robotic processes
allowing for better predictability and control of process parameters. This
makes it easier to identify and refine process parameters that affect component
quality. In addition, robots can execute complex or repetitive processes at
very high speeds.
The most significant game-changing process in aerospace manufacturing
is carbon fiber layup. In this process, carbon fibers are combined with a
resin or epoxy material to create a lightweight but strong composite. This
material is highly suited for the aerospace industry because it can reduce
the weight of the airplane in order to achieve better fuel economy without
sacrificing strength or durability. Robotic accuracy is important in this
process because the placement of the carbon fiber strands relative to each
other is critical to the structural integrity of the component.
One important feature used to achieve high accuracy in FANUC robots
is the use of secondary encoders. “Secondary encoders reduce omni-directional
repeatability to nearly zero and has been validated via laser tracker while
exploiting the combined effects from moving all axes.” (1) Secondary
encoders connected directly to the controller are installed on the output
side of each axis drive train to measure and control the true position of
each axes. This allows the robot to control position eliminating errors due
to backlash and essentially improving the ability of the robot to achieve
a commanded position. This is ideal for applications that require high precision
or need to compensate for external forces.
Deflection compensation and advanced motion planning tools
are also critical in the manufacture of large aerospace parts where large
tools are mounted to the robot, the robot tooling is required to contact the
part, and where off-line programming is critical. Some additional applications
that have benefited from high accuracy robots are aerospace engine components
manufacturing and airframe painting/depainting.
Contributors to Achieving High Robotic Accuracy
Several factors contribute to robot accuracy and must be considered.
Foundation, Mounting, and Environmental Considerations
One of the first considerations when designing a highly accurate system
is how the robot is mounted or anchored to the floor; this is a critical consideration
and cannot be underestimated. If the robot is not securely anchored to the
floor, the robot can pitch or yaw in different directions or shift from the
inertia of the robot’s movement and payload. The amount of effort spent
on making the robot accurate will not matter if the very foundation to which
it is mounted is not secure. This includes the floor thickness, how it is
anchored, if it is isolated, and the number of anchors, floor flatness, and
the thickness of the base plate. The riser construction needs to be as ridged
as possible to ensure only minor deflections. Some options to consider are
to create the riser as a hollow tube and fill the tube with concrete. Be sure
to use ample gussets and the proper thickness for the base plates and robot
The effects of thermal expansion should be considered for the applications
that require the highest level of accuracy. There are two factors that directly
influence thermal expansion. The first is thermal expansion due to ambient
temperature which is the temperature in the atmosphere surrounding the robot.
Controlling the ambient temperatures in which the robot works can help to
control thermal expansion. The second is thermal expansion due to self-heating;
reducers and motors will heat up during robot operation causing the robot
castings to expand which could alter robot accuracy. The ambient temperature
cannot control thermal expansion due to self-heating. There is one factor
that will directly affect the magnitude of the thermal expansion in both cases.
The coefficient of thermal expansion of the material used to construct the
robot is a major factor when allowing for thermal expansion. The coefficient
of linear thermal expansion of aluminum is 23 where the coefficient of linear
thermal expansion of steel is 11 to 13 depending on composition. This means
that the effects of thermal expansion for a robot constructed of aluminum
will be approximately double that of a robot constructed out of steel.
End of Arm Tooling (EOAT)
The next consideration is the design of the End of Arm Tooling (EOAT),
payload, and dress out. The mass properties of the payload must be accurately
defined. This is also true for any valve packages that are mounted on the
robot’s arm. The better the payload is defined, the better the robot
software will be able to correct the robot's position due to gravitational
effects. For calibration proposes, it is best to do the calibration with
the actual EOAT and dress out.
When designing the EOAT, considerable attention needs to be taken to
minimize EOAT deflection. The design needs to eliminate the possibility of
the EOAT shifting or moving either from the robot’s own inertia, or
from any process forces that will transfer back to the robot. This is done
by designing an EOAT that uses dowel pins to eliminate the possibility of
the EOAT moving or shifting. Also, the effects of the dress out cannot
be ignored. Large and improperly designed dress outs affect robot accuracy
by pulling on the EOAT.
Software defines the relationship between the part or fixture and the
robot base frame. It is important that it is established accurately to account
for any variance in the location of either the part or the fixture. This variance
will be important when trying to maintain an accurate system. Depending on
the application, several software packages can be used to set up this relationship
Maintenance and Repair
A maintenance and recovery plan is an essential part of an accurate
robotic system. Just like there are three components that contribute to the
overall accuracy of a robotic system (the process work zone needs to include
all processes that require accuracy, make the zone only as large as the process
requires, and limit robot configuration (orientation) changes in the process
work zone as much as possible), the maintenance and recovery plan must address
all three of these components. FANUC has calibration tools that can validate
and even adjust some of these parameters automatically as needed.
Not all options and not all applications will require accuracy validation,
but accuracy validation is achieved using metrology such as a laser tracker
to measure the robot's actual position, and compare it to the commanded position.
A laser tracker, tooling ball reflectors, and a knowledgeable user will be
required for most validations.
High Accuracy Positioning Using LVC and an R-2000iB/165F
FANUC America' Learning Vibration Control (LVC) allows the robot to
learn its vibration characteristics for higher accelerations and speeds After
a path is taught, an accelerometer is added to the tool, and the path is run
several times in order for LVC to collect learning data. Using the retrieved
data, the robot optimizes the motion to have shorter accelerations while keeping
vibration to a minimum The accelerometer is only used during the learning
LVC, along with integrated iRVision
and secondary encoders, work together to provide a highly accurate solution
suitable for the aerospace industry. LVC, when used, can decrease cycle times
while maintaining accuracy within or beating industry-standard acceptable
FANUC America uses iRVision® for part location and evaluation. iRVision® is Integrated Robot Vision which is
the integration of a camera interface built into the robot controller. One
or more cameras can be attached to the robot, or they can be in a remote location.
In traditional processes, if you want the robot to manipulate every workpiece
in the same way, you need to place every workpiece at exactly the same position. iRVision® is a visual sensor system designed
to eliminate such restrictions. iRVision®
measures the position of each workpiece by using cameras, and it adjusts the
robot motion so that the robot can manipulate the workpiece in the same way
as programmed even if the position of the workpiece is different from the
workpiece position set when the robot program was taught. All of the application-specific
tools developed to simplify the use of the camera as a guidance, identification,
or inspection tool are integrated with the robot.
Can handle multiple parts at one time.
Reduces floor space.
Makes part changeovers a breeze.
Can identify parts in multiple orientations.
Can identify parts in 2D or 3D.
Can error proof the parts as they enter assembly, as well
as the assembly itself.
Can increase throughput.
Reduces or eliminates fixturing costs.
Robotic Aerospace Applications
LVC is a breakthrough product for operations that require increased
accuracy at high speeds allowing the robot to operate with less vibration,
even at maximum speed. In this example, a FANUC R-2000iB
robot simulates drilling an aerospace panel. FANUC America' high accuracy
solution provides absolute accuracy in a given area allowing the robot to
be accurately positioned. Secondary encoders allow the robot to further enhance
the accuracy by being able to control the robot position within the backlash
band, iRVision snaps an image to acquire
a local frame for further increased accuracy and to locate the part. The edge
positions create a path to guide the robot around the object. Two paths used
in this example simulate cutting with a waterjet or plasma cutter, and applying
a tool offset to the path for material removal.
For more information about working with FANUC America in the aerospace
industry, go to Aerospace
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(1) http://www.electroimpact.com/research/2010-01-1846.pdf; accessed 3/11/11.