Eastern
Automation Systems (Farmingdale, NJ) designs and builds custom assembly and
inspection machines. The machines range from simple manual tools to fully
automated assembly lines. Eastern places a large emphasis on quality by
integrating inspection tasks directly into the assembly process. All machine
motion is controlled with endpoint sensors, and all components assembled are
100% inspected in-process for presence, color, orientation and function.
A
large part of the machine control software is dedicated to validating all
inputs of each cycle of the assembly process, which ensures robust machine
operation for many cycles within an industrial environment. A recent automated
assembly line for a plastic over-molded, spring steel, automotive glove box
latch was designed with a latch effort inspection station that required
measuring small forces in-process. However, the machine and operators can cause
high overloads as part of normal machine use. Thus the customer required a
force sensor that was durable enough for the environment, yet sensitive enough
for the required measurement.
Inspection Station
The inspection station is one of 12 placed around a carousel-style, asynchronous
assembly machine. The assembly must pass several inspection criteria including
an automatic pull test to measure the human effort required to pull up on the
latch, just as one would encounter when they open up the glove box in an
automobile. In order to perform this inspection, a strain gage load cell with
an analog output is typically used.
Pneumatic actuators present the load cell tooling fingers to the latch. The
latch is then cycled a few times to “break-in” the components. On the last pull
cycle, the analog reading of the load cell input is sampled, compared to
controls limits and stored in the system’s controller.
The customer stated the following inspection criteria: an inspection rate of
one inspection every 3.5 seconds; inspection load range of 8 to 25 Newtons,
with ±1 resolution (latch handle style dependent); control parameters of lower
limit, upper limit, scale (calibration) factor, quantity bad in a row to stop
machine.
How to Measure?
An ideal sensor choice was a 45 Newton, 2 millivolts per volt (mV/V) strain
gage load cell. However, in normal machine use, an intermittent condition
occurred that generated high-impact loads perpendicular to the measuring axis
of load cell. This impact force exceeded the 150% safe overload range of the
load cell and resulted in failure.
The most common failure mode of a strain gage load cell is the application of
force beyond the yield point of the strain gage flexure (safe overload range).
A typical 2.225 kilonewtons strain gage load cell has an overload limit of 3.3
kilonewtons, equivalent to 150%. Overloading the load cell usually causes
permanent damage to the flexure, which results in a zero shift, nonlinearity or
complete failure.
Quartz piezoelectric force sensors are typically an order of magnitude stiffer
than strain gage load cells of an equivalent full-scale capacity. A quartz
piezoelectric force sensor reacts to stress, resulting in a miniscule strain,
to produce its charge output. They have stiffness on the order of 1.05 to 23 kilonewtons
per micron (kN/µm), which means there is virtually no deflection during
measurement. Most have a compressive strength of 4.351 x 104 psi, which allows
massive overloading without the risk of crushing the sensor. Even when the
sensor is overloaded beyond its stated capacity, they suffer no ill effects,
zero-shift, fatigue or linearity change. For example, PCB Piezotronics Inc.’s
(PCB, Depew, NY) model 208C03, with a capacity of 2.2 kilonewtons and a
diameter of 16 millimeters, the maximum compression of 22 kilonewtons is
equivalent to 1,000% over-range protection. Additionally, the sensors are
designed for harsh industrial environments with hermetically sealed, stainless
steel housings.
“The main reason that we use PCB is for their excellent technical support,”
says Scott Bellows, president of Eastern Automation Systems. “The sensors were
easy to use and interface with, robust and industrialized, accurate and
repeatable.”
For the customer’s latch effort application, a 2.2 kilonewton quartz piezoelectric
force sensor was therefore used in order not to damage the force sensor. Why
not simply select a 2.2 kilonewton strain gage load cell? It is not the best
solution for the application because of the low output sensitivity of a strain
gage load cell. A strain gage load cell typically has a 2 mV/V sensitivity, and
with a 10 Volt DC power supply, the full scale strain gage output would only be
20 millivolts. While the quartz piezoelectric technology allowed use of a
sensor with a much higher capacity, hence a much higher tolerance for breakage,
it also featured ICP sensor output. This type of output is a 5-volt signal
directly from the sensor. The high voltage output of the ICP force sensor
provides a significant benefit in terms of signal to noise ratio, especially
since the application required a low force capacity—25 Newton—compared to the
rated capacity, 2.2 kilonewtons.
Using the ICP sensor circuit, which is built inside the sensor, there is
excellent measurement resolution. The 2.2 kilonewton force sensor that was
required to survive the impacts had a broadband resolution of 0.022 Newton.
Calibration on the Machine
Another issue common to in-process monitoring applications is the complexity of
a calibration routine. A calibration on the machine was simply not possible
because of mechanical constraints of the tooling.
The quartz ICP quartz force sensor does not require the use of a dead-weight
style calibration and a lengthy setup routine. Instead a master latch was used.
This master latch was calibrated with a separate tension-measuring device. The
master latch was then used to check the machine measurement and scale the ICP
force sensor input in the machine controller.
A common sensor characteristic that makes most machine builders shy away from
piezoelectric sensors is zero drift. Drift is a long-term zero-shift phenomenon
encountered with traditional charge output piezoelectric force sensors. These
older style piezoelectric force sensors require remote charge amplifiers, which
are the source of the drift.
The ICP voltage output force sensor actually eliminates issues associated with
this drift through AC coupling. Low frequency response for ICP force sensors
acts like a highpass filter and may be tailored to a specific value to
accommodate most high-speed in-process inspection machines. This AC coupled
signal not only eliminates the drift issue, but it also allows for control
simplification. Charge amplifiers require reset and control signals from the
machine controller. Thus, since extra relays, wiring and programming are not
required, the machine control becomes less confusing.
During machine runoff at the customer’s site, several sets of data were taken
with the master latch and then both good and bad parts to verify the measuring
system performance. Additionally, a comparison between the 45 Newtons and 2.2
kilonewtons ICP sensors was performed to verify that the higher capacity sensor
would prove useful.
Using the master latch in the inspection station, the standard deviation for
the 2.2 kilonewtons sensor was ±0.03 Newton, which was within the machine’s
required resolution of ± 1.
The customer was able to use a sensor that survived the harsh industrial
environment and provided the required resolution to ensure 100% in-process
latch inspection.
PCB Piezotronics Inc., Force/Torque Division
(888) 684-0004
www.pcb.com
Reply 13
Benefits- Eastern places a large
emphasis on quality by integrating inspection tasks directly into the assembly
process.
- All machine motion is
controlled with endpoint sensors, and all components assembled are 100%
inspected in process for presence, color, orientation and function.
- The sensor survived the harsh
industrial environment and provided the required resolution to ensure 100%
in-process latch inspection.
