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 load is increased, the deflection increases. The load beam is designed so that as it deflects from ‘no load' to ‘full load' it provides a linear electrical signal from the transducer.

Cross Section of Model C Tension 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 possible electrically.

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.)

Principles of Operation

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 example application:

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.)

[Download a spreadsheet for calculating correct Model C transducer sizing. Right click and 'save as' on the link.]

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 equipment.

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 process machine.

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