What is the Extended Range option and when would we need it?
Extended Range output, or XR as we abbreviate it, is the name
of an option that DFE offers with its tension transducer (load cell) and corresponding electronics
products. By doubling the excitation voltage to a given transducer from
5V dc to 10V dc, the transducer produces twice the signal output. This
improves the signal output quality at the low end of the transducer's
operating load range. Transducers with the XR option must be used in
conjunction with electronics with the XRE option.
To better understand the concept of Extended Range let's discuss the
function of the basic tension transducer. The transducer functions by
the deflection or distortion of an internal load beam; as an applied
increased, the deflection increases. The load beam is designed so
that as it
deflects from ‘no load' to ‘full load' it provides a linear
from the transducer.
The illustration above shows a vertical cut-away cross section of one of DFE's model C tension transducers.
The maximum load allowed by the transducer is set by the
load stop, which limits the deflection of the beam to protect it and the strain
gages from damage due to excessive deflection. The maximum load of the
transducer is a function of this load stop and increasing the range is not
The sensitivity of a given transducer is also a
function of the load beam. DFE offers a broad range of transducer beam 'load ratings' to cover all levels of tension that we encounter in industrial applications. When comparing transducers with higher load ratings to those with lighter load ratings, beam load sensitivity increases as the
load rating decreases. In other words, the lighter load transducers are more responsive and provide greater signal
output at lighter tensions than the higher load-rated transducers do. The "sensitivity"
of a given transducer is determined by the
minimum amount of applied force needed to deflect the load beam. The greater
the beam deflection, the higher the sensor output.
Now let's address the electrical characteristics of the
transducer. The output of the transducer at zero load will be approximately
zero volts. While the output at full load will be approximately 250 mVolts,
when configured as a half-bridge device. (A transducer pair wired as a full
bridge will provide 500 mVolts at full load.)
As tension is applied, causing the deflection of the load
beam, the output voltage will change by an amount corresponding to the amount
of deflection. Both the deflection and the voltage change are linear throughout
the mechanical range of the transducer.
Another component of the
signal output of the transducer is electrical noise. If the signal
output from a transducer is fairly high and the calibration of our electronics (amplifiers, indicators, or controllers)
is set to something in the low- to mid-range of the gain, then we don't need to worry
about electrical noise interfering significantly with our tension signal.
But what happens when we're forced to work at the top end of our amplifier gain (a situation we find ourselves in when working with lower applied tensions on higher load-rated transducers)?
When working at the lower end of a transducer's operating range, we can realize the benefit of a system with the Extended Range option. Here's an
Let's say we are using a transducer with a 400 lb load rating and applying 65.26 lbs
of tension and a wrap angle of 100 degrees. The change in transducer signal
output, what we call ‘delta', ends up as 62.49 mV, or 124.98 mV (2 x 62.49 mV)
for the pair of transducers mounted on the idler roll. (This can be calculated using DFE's tension transducer sizing formulas.)
Let's assume that we've decided that our current tension should be about 25% of the maximum
tension readout of the system. This means that the output of the amplifier at this tension
needs to be about 2.5 volts DC (equaling 25% of 0-to-10V). So, in order to provide 2.5
volts at this tension the gain of the amplifier must be set during calibration to about 40X. (40 x
62.49mV = 2,500 mV = 2.5V)
Unfortunately, along with the 62.49 mV of transducer output generated by the applied
load there will be a noise component in our transducer signal that is also amplified by 40X. While the
electrical noise from this relatively lower level of electronic gain won't be significant enough to degrade
our tension signal output in this instance, let's consider another example when our application demands that we operate at a significantly lower tension than our 400-lb load-rated transducer was designed for.
Let's adjust the running tension
from 65.26 lbs in our example above to something much lighter, like 6.26 lbs of tension. While using DFE
electronics allows multiple ranges of tension measurement from a transducer pair, we must still provide
amplification of the signal to a usable level. Normally, in this case, we would calibrate the amplifier for a maximum tension of at least 16 lbs so as to provide at least a 30 mV
output (the minimum signal required to calibrate for full scale in DFE
amplifiers) at full load. In this instance the 6.26 lbs will provide a delta of
just 5.99 mV. Since the 6.26 lbs is 39% of the full range calibration (if 16
lbs = 100% full scale) we must amplify this voltage to 3.9 VDC at the output of
the amplifier. This requires us to apply an amplifier gain of greater than 600X. The noise component
of the tension signal also becomes amplified by the same factor. The problem? A GAIN this
substantial increases the possibility of unstable tension and ZERO readings.
So, what do we do to reduce the need for a huge amount of
amplifier GAIN when our applied tension is at the lower end of our transducer's operating range? By adding the Extended Range option to the transducer pair we
increase the transducer output at 6.26 lbs from 5.99 mV to 11.98 mV thus reducing the
required gain by 50%. In fact, in our example, the required amplifier gain is reduced from 651X
to 325X with a corresponding reduction in the amplification of the noise
portion of the signal.
With the increasing demand on converters to be able to
operate over a wider range of tensions to better serve their customers, Extended
Range transducers are experiencing increasing popularity. For the informed converter
Extended Range is a simple and dependable way to provide extended capability to
the tension system without the need for changing web paths or adding more
The Product Section of
our web site highlights major features and benefits of each of our standard
products. Please inquire if there does not appear to be a product to fit
your application. DFE delivers tension solutions for almost any continuous