Automatic assembly; how to make a robot as good as a housewife working to pay off a mortgage and three sets of braces Essay

Automatic assembly



When you’re a world leader in robotics, the sight of a group
of women putting together relay contactors on an assembly line at your
home plant is a great temptation. You soon find yourself asking,
“Why can’t this operation be automated? Gee, if I only had a
robot with 12 hands?’

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The classic problems with using robots to replace people on an
assembly line are speed and dexterity–the time it takes the robot arm
to move into position is never as fast as you would like, and the task
of replicating that evolutionary masterpiece, the remarkably engineered
human hand, seems nearly impossible.



But ASEA’s Industrial Robot Div, Vasteras, Sweden, knows no
fear. Their Control Group seemed like the ideal testing ground. The
manufacture of control relays and contactors was an ideal product. The
volumes were moderate, the number of different models in the product
family was low, the number of components to assemble was reasonable, and
they seemed grippable and feedable.



The incentive was obvious–to eliminate human error. The most
common mistake was assembling a contactor with a coil of the wrong
voltage. There was also the potential here to incorporate a completely
computerized checkout of the relay as a final assembly operation.



Orange clockwork



The answer ASEA came up with was the robotic equivalent of 12 human
hands. A floor-monted IRb-6 robot has a mirror-image robot suspended
overhead so that each has equal access to a radial array of pickup and
assembly stations. Each robot arm has a special turret with six finger
pairs so that it can pick up six different components at one time and
dispense them at an assembly station with minimum arm movement.


There are 12 relay components. In the work-cell plan view you will
note that the lower half of the assembly circle is devoted to incoming
major elements. The molded-plastic body enters left via belt conveyor.
The relay core, coil, armature and leaf spring are offered in palletized
feeder trays. A preassembled contact package enters, right.



The assembly cycle begins when one of the robots makes the rounds
of these six stations, picking up one each of these components with its
six gripping-finger pairs.



Meanwhile, in the upper half of the assembly circle, the second
robot is moving through five assembly stations, essentially setting down
the relay body, positioning coil, core, armature, etc and inserting
locking pins and clamps to complete the relay. One station is an array
of proximity sensors to mechanically donfirm that everything is in
place, and they hope to incorporate a complete electrical checkout
station later. When assembly is completed, the robot deposits the relay
on a conveyor, left, parallel to the incoming body conveyor. Then the
robot arms change sides.



Results look good



Cycle time at the pickup and assembly stations varies between 2.3
sec and 3 sec, and with the dueling robots, the cell yields 2.5
contactors/min. This matches the production rate achieved previously
with three manual assemblers. ASEA feels that with a little refinement,
production rates for the automated unit could be boosted to 3 relays/min
if that volume becomes necessary.



The gripper turret is a new design being marketed in this country
by ASEA Robotics Div, New Berlin, WI. It’s pneumatically driven
with mounting plates for each set of gripping fingers. The fingers are
humanlike in their flexibility; they adapt to different part sizes and
shapes; there is no need to match finger characteristics to specific
parts.



At the moment, the cell is designed to handle two basic sizes of
relay. By varying the coil voltages and contact preassemblies, this
yields five different products and this could be extended to as many
more without refixturing. Changeover time between the smaller and
larger body sizes is only five minutes, consisting of exchanging part
pallets and key fixtures and changing programs. The dual vibratory pin
and clamp feeders are positioned so that they can dispense the large and
small size fasteners as required, so they do not need to be changed over
between model runs.



One feeder problem ASEA encountered was with separating coil
springs. After no success at devising a system to untangle a box full
of coil springs and feed them accurately into the assembly stations,
they developed a machine to wind them on demand.



Payback period for the assembly cell was originally estimated at 2
1/2 years, based on achieving 65-percent uptime. So far the system has
worked even better than they expected, achieving an 80-percent figure,
so they are now hoping to boost that to 90 percent.



For more information from ASEA Robotics on assembly robots, circle
E3.



Photo: Control panel for the assembly cell is relatively simple,
consisting primarily of the two portable panels for the two IRb-6 robots
that feature simplified (multilanguage) programming with a two-line
alphanumeric display and positioning joystick. One operator runs the
cell and handles all the changeover between model sizes.



Photo: Six-position turret on robot arm holds six components to
reduce pickup time.



Photo: Vibratory feeders supply two sizes of locking pins.

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