THE ORIENTATION OF PARTS FOR ROBOT ASSEMBLY
An assembly robot will never have the dexterity or intelligence of its
human counterpart. A manual worker is able to pick a part, in
random orientation, from a storage bin and orientate it - ready for
insertion. The human senses of sight and touch are used for this
purpose. Whilst a manual worker can perform these tasks with
comparative ease, a robot requires a large amount of computer
processing power and many feedback devices to achieve any form of
intelligence and, even then, the cycle time of the operation is so long
as to make it uneconomic to use robots for bin picking. Automatic
feeders for robot assembly must, therefore, present parts to the
workhead in a known orientation, or in a limited number of known
orientations. The attitude of the part on the feed track or
pallet influences the number of robot degrees of freedom
required. More degrees of freedom are required for those parts
which are inserted in a different attitude to which they are
presented. In the case of parts which cannot be presented in one
known orientation, the final orientation must be carried out by a robot
with extended capabilities. This involves sensing and part
manipulation, to achieve the required insertion orientation.
The robot work envelope poses limitations on the automatic feeder types
that can be used in robot assembly. Each part is presented to the
workhhead at the end of a track or on a pallet. The space
occupied by the material of these devices must also be considered when
determining the maximum number of parts that can be fed to any one
robot. Robots that can only access parts in the vertical axis
must have parts arranged so that they can all be seen in plan view at
the part presentation points. Another problem arises when turret
mounted grippers are used. The grippers can occupy a large volume
in space and this makes the avoidance of collision very
important. This situation can be investigated before the design
of the robot assembly system is finalised. The complete assembly
process can be studied using computer simulation and there are many
three-dimensional graphic simulation packages available that can
identify if a collision is likely to occur.
THE INSERTION OF PARTS FOR ROBOT ASSEMBLY
An insertion operation is defined as being the action whereby a part is
added to a work fixture, another part, or part-built assembly.
This may involve a simple vertical downwards motion where the part is
added to the part-built assembly, without being immediately
secured. Alternatively, it may be a complex motion, such as that
required for the application of an adhesive to a part. Each
insertion process may require a different type of end effector and each
process takes a certain amount of time to be executed. It’s
possible to categorise each type of insertion process to define the
type of end effector required and to estimate the time it would take to
carry out the operation.
The end effectors may be accessed by the robot arm in many ways.
The design of end effector, and the method of mounting it onto the arm,
influences both the cycle time of the process and the cost associated
with the insertion of a particular part. The simplest, yet most
expensive and time consuming, method of accessing an end effector, is
to use an
individual gripper, or tool, for each part or insertion process.
The grippers and screwdrivers are stored in a rack within the work
envelope of the robot arm. The relevant tool is picked from the
rack, used for the insertion process, and then returned. The
action of picking up the tool, and returning it, can often take longer
than the insertion process itself.
Another method of inserting many different designs of parts is to use a
multi-functional gripper. Only one gripper with a multitude of
faces is used, for the internal or external gripping of parts.
The time involved with gripper changing is eliminated, but the design
of the gripper is complex and other tools cannot be mounted onto the
same unit. Problems may also occur because only one set of jaws
is being used for the insertion of many parts. The gripper
designer has to ensure that the gripping force is sufficient to hold
the part and yet not too excessive as to cause damage to the
part. The varying force requirements can be met by additional
gripper sensing. This, of course, increases the cost of this
design of end effector.
The most efficient method of accessing a multitude of end effectors is
to mount them onto an indexing turret. Between eight and twelve
tools can be housed on one unit, depending on their size.
Grippers, screwdrivers and othertools are mounted in a circle.
This may be about a vertical, horizontal or inclined axis. The
use of universal mounting plates, between the turret and the end
effectors, allows interchange-ability of grippers and tools for product
changeover. The time lost, due to gripper changing, is minimised
because indexing of the turret occurs between movements to, and from,
the parts feeders.
Most products, or sub-assemblies, have many possible sequences of
assembly and it is important to recognise the most appropriate
sequence, particularly in robot assembly. In all forms of line
assembly, where moving work carriers are employed, it is good practice
to secure parts as soon as possible because subsequent work carrier
movements may cause a part to be displaced. This suggests certain
precedences. If no movement of the part-built product occurs
during assembly then the securing of parts is not important and a
sequence of assembly can be chosen which involves a minimum number of
Consideration also has to be given to the appropriate action needed
when a malfunction occurs. The decision to scrap, rectify or
dismantle depends on the; value of the part-built assembly, frequency
of the malfunction, labour cost and sequence of assembly. In
single station robot assembly, an overriding consideration is the cost
of gripper changing. The
optimal sequencing, linked with appropriate product design, can
significantly reduce this cost. Computer software applications
are available which, given the precedence constraints, identify the
optimal sequence to minimise gripper changes. The cost of error
recovery is important. The alternative actions need to be
examined at each stage in the assembly build and the cost of these
actions should be determined for all possible sequences. This
activity is influenced by the chosen criteria of; minimum cost, maximum
production or maximum profit.