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Force Measurement 101

Mon, 10/24/2011 - 2:10pm
Nick Tolley, FUTEK Advanced Sensor Technology, www.futek.com
Although force measurement has become commonplace in most manufacturing environments, many of us may still appreciate being refreshed on its rudiments from time to time. Many who have attempted to educate themselves on this topic will tell you that the reading material can be a bit unpalatable. Because of this, we are pleased to offer an overview to force measurement specifically covering: an introduction to load sensors and load sensor calibration; the basics of force interactions and measurement applications; the impact of alignment and loading mode; and why particular sensor designs are better for some applications than others.

Introduction to the Load Sensor and Its Calibration

Although it may sound unglamorous, it is fair to compare a load sensor to a common spring – well, a very stiff and very repeatable spring. As is the case with a spring, a load sensor will deflect (either compress or elongate) when a force is applied to it. If it is well-engineered, a load sensor will deflect the same amount every time a given force is applied, which is what is meant by repeatability. An obvious functional difference between a common spring and a load sensor is that the deflection of a load sensor is converted into an electrical output that can be measured.

The process of calibration involves subjecting a load sensor to a series of known forces and recording its electrical output at each force. Based on this data, an equation is solved to reliably predict the electrical response of the sensor to an applied force. Using this equation, a digital display can be configured so that the output of the sensor is interpreted to quantify what force is being applied.

Force Interactions & Measurement Applications

Newton’s third law of motion famously explains that the force interaction between two objects is always equal and opposite. In accordance with this law, even as you stand there fantasizing about load sensors, the floor is dutifully supporting your feet with a force that is exactly equal to your body weight. If one wanted to measure this force interaction, a load sensor could be placed between you and the floor. (The most courageous among us actually do this most mornings using a bathroom scale.)

An analog scenario that is common in many manufacturing environments is when an arbor press is used to apply a compressive force onto a specimen, like when a widget is crushed to test its strength. If one wanted to accurately measure the force being applied onto that specimen, a load sensor could be situated in line with both the specimen and the force being applied. Since the force generated by the press would act equally on both the specimen and the load sensor, whatever forces the specimen was subjected to would be simultaneously measured by the sensor.

Alignment and Loading Mode

Often overlooked, misalignment of the load sensor with respect to the applied force is one of the most common sources of measurement error. The load sensor, the specimen and the applied force must all be collinear (along the same axis) in order to consistently capture accurate force measurements. If a load sensor was properly calibrated, the forces applied during the calibration process were always centered about its designated loading axis. Because of this, forces measured by that sensor should also always be centered about its designated loading axis to maximize measurement precision.

force measurementAlthough it may seem counterintuitive, force measurement applications often have different alignment challenges based on their loading mode (tension or compression). Typically, alignment issues are reduced when loading in tension and exaggerated when loading in compression. This occurrence mainly results from the tendency of most objects (specimens and load sensors included) to straighten when pulled by a tensile load and to buckle when squeezed by a compressive load.

Which Sensor When?

When it comes to force measurement applications, there is no one-sensor-fits-all. Fortunately, there is an amazing number of load sensor designs and each one has its own particular strengths; FUTEK, for instance, has over 400 sensor designs in our product portfolio. Let us consider two very popular but very different load sensor designs: the FUTEK LSB350 S-beam sensor and the FUTEK LCF455 pancake sensor.

During their respective calibrations, an S-beam sensor will often outperform a pancake sensor because the simplistic geometry of the S-beam excels in situations of ideal alignment. For this reason, the LSB series has been extremely popular for measuring tensile loads (as tensile loads tend to reduce alignment issues). In the meantime, a pancake sensor will demonstrate superior repeatability when measuring compressive loads. This is largely because the cylindrical design of the pancake sensor is radially symmetric about its designated loading axis, which mitigates the effects of twisting and off-axis loading. Because of this, the LCF series has performed exceptionally well when mounted within the loading space of a press or when supporting a large tank or hopper.

Because there are so many sensor designs each with its own application strengths, it would be impossible to effectively address them all in any one article. But, if you do have a specific application of interest, we highly recommend that you consult the sensor manufacturer of your choice to discuss your product application in detail. We strongly believe that each application is unique and it’s the role of the sensor manufacturer to guide you to the best product solution. 
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