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  For a review of the basic principles and application design considerations of tension measurement click on the link below.

Application design information   (How Sensors Work) 

Frequently asked questions:

- What is a Tension Sensor?

- How does the Sensor Measure the Tension?

- How much deflection is required in the
  material to measure the tension?

- Are there restrictions in the mounting
  position of the tension sensor?

- Why do I need a signal conditioner?

- Is calibration of the sensor required?

- How do I calibrate the tension sensor?

What is a Tension Sensor?
TMI's sensors are electro-mechanical assemblies designed to measure the tension forces applied to a wide variety of materials. These sensors are most commonly used to measure and control running-line tension in materials as they are moved dynamically through a manufacturing or test process. The active tension measuring element is a strain gaged transducer, or loadcell, in a housing designed to install on a machine frame. Attached to the transducer is a roller, wheel, sheave, pulley, or similar, that typically rotates on precision bearings.

How does the Sensor Measure the Tension?

Tension is the pulling force applied to a material when moving it from one location to another. This could be winding electrical coils, processing optical fibers, electrical wire extrusion, twisting processes, medical tubing manufacturing, web converting, etc. The longitudinal pulling force is deflected by a series of at least 3 rollers to create a fixed force vector, typically perpendicular to the tension force vector. The tension sensor is attached to the middle of these 3 rollers. As the tension force increases, the sensor element is deflected causing a change in the voltage output signal. This deflection is small, typically from .000" to .004" .


How much deflection is required in the material to measure the tension?

The deflection angles (a1 and a2 shown above) can be typically from 15° to 90° per side. The sum of angles a1 and a2 is called the WRAP ANGLE. The tension sensor capacity is designed for a specified wrap angle. As the wrap angle increases the deflection force on the sensor also increases. TMI's sensors
will be rated for the tension capacity not the deflected force value. When ordering a sensor you will need to provide the maximum tension in your material and the approximate total material wrap angle.  When adding a sensor to your process, the sensor can be a single roller sensor or a 3-roller faceplate sensor. The 3-roller option provides a complete sensor system. The single roller option lets you use your existing rollers by placing the sensor in the material path between two of the rollers to create the necessary fixed deflection points.

Are there restrictions in the mounting position of the tension sensor?

The tension sensor may be mounted in any position or orientation as long as the indicated direction of force is maintained. With a three roller faceplate sensor the force direction is fixed at the factory and can simply be placed anywhere in your materials path. With a single roller sensor the force vector direction is marked on the sensor and the sensor must be placed between two (2) fixed rollers to create a 3-roller system. Angles a1 and a2 should be approximately equal. The sensor is designed to deflect in one direction only. The sensor should be mounted to a rigid surface that will not deflect or bend.
                                                   Click here for mounting option illustrations.

Why do I need a signal conditioner?
The tension sensor is not an active electronic device by itself. A regulated precision power source must be applied to one side of the Wheatstone bridge. This power is typically 5 Vdc to 10 Vdc and is referred to as the Excitation Voltage. The signal side of the bridge will output between +/- 20 mV dc. This signal changes as the force/tension to the sensor changes. This signal must be measured by the signal conditioner. The signal is then calibrated to corresponding real world values such as grams, pounds, kg, Newton, etc. The calibrated signal can then be used to provide a digital display of the tension and/or to output a higher level voltage to send to a PLC for tension control. The strain gage signal conditioning unit provides the excitation voltage for the sensor and calibration of the sensors signal.

Is calibration of the sensor required?

For many applications sensor calibration after installation is suggested. Calibration with the actual material used in your process will provide the most accurate tension data.  When a 3-roller faceplate sensor is purchased with a signal conditioner, the sensor has been calibrated at the factory as a system. This calibration is adequate for most users, which means you can simply install and use without recalibrating the system. With a single roller sensor calibration will be required. After installing the sensor on your machine frame the calibration will correct for the actual wrap angle created. Single roller sensors are tested and calibrated at the factory based on the anticipated wrap angle.

How do I calibrate the tension sensor?

The tension sensor is calibrated by applying known tension loads to the sensor. The tensioned material is threaded through the actual material path and a known weight is suspended from one end of the material. This will create a known tension. The signal conditioner will measure the output signal and use this value to create a calibration point. Two calibration points are required to create a signal span that equates to the sensor's range. The 1st point is the no tension point, which creates the -0- calibration point. The 2nd point is with a known load applied to the material that is at least 50% to 100% of the sensors full scale range. A digital indicator/signal conditioner will use these 2 points to create a calibration slope. After calibration the sensor's output signal is continuously measured and the corresponding tension value is displayed. If you have purchased a sensor with a signal conditioner from TMI, a complete manual will be provided showing how to pair, calibrate and use the tension measurement system.
For more information see the following 2 documents.
Tension Sensor Calibration with Know Loads                 Tension Sensor Calibration Definitions

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