22, 1998 Design News
DESIGN APPLICATIONS From the regional editors
Sensor ensures proper clamp operation
Attaching the sensor to
the clamping arm shaft, instead of the cylinder, improves
Charles J. Murray, Senior Regional
Plymouth, MN--Before an automated system
welds two parts together, it must "know" whether
the parts are properly clamped. If they're not--and
if the system cannot distinguish that fact--it can be
a nightmare for manufacturers. Costly tooling can be
damaged and assembly lines shut down, resulting in losses
of tens of thousands of dollars.
To combat that, engineers from Turck Inc., have developed
a patented sensor that attaches directly to a clamping
arm's shaft. The new system reportedly is more compact
and more accurate than previous clamp-sensing techniques.
It also uses less hardware.
The key to the new sensing system is that Turck engineers
placed the sensor on the clamp arm's shaft, instead
of in the hydraulic or pneumatic cylinders that move
the arm. "Ideally, you want the sensor as close
to the action as possible," notes William Eaton,
chief engineer for Turck and co-inventor of the system.
"But it's not practical to put a sensor on the
clamping arm. So the next best place is on the shaft
that connects to the arm."
When the clamp arm rotates, programming wheels
inside the sensor housing move with it. Proximity
sensors in the housing then detect angular displacement
of the programming wheels.
Up until now, sensors were typically placed at each
end of the cylinder. When the cylinder's piston was
at one end of its stroke, the sensor recognized it as
clamped. At the other end, it was unclamped.
But by attaching the sensor to the clamping arm shaft,
Turck engineers say they've realized several advantages.
First, the sensor isn't subjected to high pressures
within the cylinder. Nor is it exposed to hydraulic
or lubricating oils. Furthermore, it's more accurate,
To accomplish all that, the new system, known as the
Turck PCS sensor, "watches" the angular displacement
of the clamping arm, rather than the linear displacement
of the piston. It includes a stationary sensor housing
and a rotating element, which moves with the clamping
To measure angular displacement, the rotating portion
of the sensor employs a pair of mechanical programming
wheels, which contain small metal sensing targets. During
operation, stationary sensors in the housing detect
the presence of these targets. To determine when the
clamp arm is retracted, one inductive proximity sensor
detects the presence of a target on the so-called "lower
programming wheel." To determine when the clamping
arm is extended, another sensor detects the presence
of a target on the upper programming wheel. The sensors
attach to a single cable, which sends their signals
back to a programmable logic controller or a computer.
As a result, the system "knows" if the arm
is retracted or extended. Hence, it also "knows"
if the part to be welded is clamped or unclamped.
One of the beauties of the design is that the system's
programming wheels and sensors can be used for a wide
variety of clamp openings, ranging from 18 to 118 degrees.
In the past, clamping sensors typically required a variety
of probes and probe lengths. The new system also eliminates
the use of special ports in the hydraulic or pneumatic
cylinders for placement of the sensors. As a result,
those ports can now be used for ducting of air or oil.
The new design also reduces hardware: Because it needs
only a single cable, the PCS sensor eliminates the need
for an extra cable and extra connector.
During operation, the new sensing system also offers
another advantage: In the unlikely event that a clamping
arm breaks, the system will "know" it. In
contrast, sensors placed on or near the piston have
no way of sensing a broken clamp, because they monitor
movement of the piston, not the clamp. Because the new
technique monitors movement of the clamp itself, however,
it immediately senses if the clamp is incapacitated.
For many users, that may be the key advantage of the
technology, because it enables them to stop the transfer
line before parts are damaged.
The sensor design does require a specially designed
clamp, however. Turck has teamed with De-Sta-Co Industries
(Madison Heights, MI) to incorporate the sensing mechanism
on clamps made for the auto industry.
For the user, the system's main advantage is that it
improves the accuracy of detection. "This system
is virtually foolproof," Eaton says. "You
don't have to tighten it to a certain torque and you
don't have to worry about pressures. You install it
with three screws, connect it to your cable, and you're