What is Load Cell Sensitivity?
Load cell sensitivity refers to the smallest force a pair of load cells can reliably detect. The greater a load cell’s sensitivity, the better a load cell is at detecting the slightest changes in tension. Resolution refers to the load cell’s ability to produce an output relative to the detected change.
Load cell sensitivity is defined as the minimum amount of force needed to cause a change in a load cells output.
Calculating Load Cell Sensitivity
When shopping for load cells, you are typically provided with two pieces of information about the load cell, excitation voltage (V orVDC), and output (mV).
If a load cell has a 5 Volt DC (VDC) excitation voltage and a 0-250 mV output, the resultant ratio is 250mV/5VDC. This equates to 50 mV per Volt or has a 50:1 ratio. The greater this ratio means the greater a load cell’s signal resolution; resulting in a higher signal to noise ratio, and thus greater sensitivity.
Excitation Voltage is voltage required for the excitation of a circuit
Noise is random, non-grounded electrical signals produced by the components on the machine that interfere with the tension signal produced by the load cell. Often, noise includes mechanical movement that is translated into an electrical signal through sensors, amplifiers, and other machine sensory equipment.
Factoring In Noise – As the signal coming from a load cell is amplified for the controller, so is any noise. The LOWER a load cells signal to noise ratio the less accurate your tension measurement becomes (ex: 20:1 provides significantly less sensitivity than 50:1). When the signal-to-noise ratio is lower, more noise is being measured/amplified rather than the load cell’s signal. The HIGHER the signal-to-noise ratio the better. This is because the load cell’s electrical signals are being measured and the control system is receiving less interference from machine noise.
Why is Sensitivity Important?
When making a machine as productive and profitable as it can be, choosing load cells with higher sensitivity ratios becomes imperative. To ensure your controller is receiving the most accurate tension measurements, a load cell needs to be highly sensitive to any changes in the tension being placed on them. Load cells with greater sensitivity are able to sense immediate and minute changes in tension through any noise that can be interfering with it. This results in an application that responds faster and maintains proper tension throughout its process, maximizing productivity and profitability.
This performance difference is what makes your control system faster, more accurate and reliable.
Sensitivity ratings will vary among manufacturers, as well as the types of load cells themselves. After you’ve determined which brand and type of load cell is best for your application you will need to determine which size load cell is best for your application. This is determined by the load cell’s load rating. Load Rating refers to the force you will be placing on the load cells.
How Montalvo Calculates Load Ratings: Montalvo’s calculation method for determining which load cells are best for your application varies from other manufacturers methods. Others calculate load rating based on the load cells maximum operating capacity, whereas Montalvo focuses on calculating the load rating to optimize a load cells performance and longevity.
Montalvo calculates load rating via the following formula:
LR= (4Psin(B/2)-Wcos(A)) / 2
LR: load rating
P: Tension (PLI)
B: Wrap Angle
W: Weight of Roller
A: Direction Angle of Tension
In other manufacturer equations “4Psin” appears as “2Psin”. Montalvo doubles the resulting load rating. For example, if your application will be handling a 150lb load, Montalvo’s equation will give you a 300lb load rating, whereas others will give you the 150lb load rating.
Load Rating can also be referred to as Tension Force. Here is a link to the Montalvo Tension Force Calculator to calculate your tension force/load rating.
Why Double Load Rating?
Years of testing the performance of our load cells has shown that best long term performance is achieved at 50% of the load rating of a load cell. The load cell is doing a lower proportion of work over the same period of time. Just as in the operation of a car, if it is driven to its maximum performance all the time, the expected life is greatly diminished. In the case of a load cell, accelerated wear is due mostly to the metal beam’s fatigue. Operating at 100% capacity accelerates the fatigue process. Electronics also have a tendency to run better at 50% than they do at 100% and so we design around this principle.
This optimizes performance while giving the load cell a longer service life.
This methodology also provides numerous other benefits for users including safety, and flexibility.
Safety- When calculating a load cell to work at 50% of its capacity, the safety factor of the load cell increases as it will not be forced to work at its maximum levels and risk causing damage to the load cell, the machine, or those working with the load cell. Montalvo calculates in a x2 Safety Factor
Flexibility- We’ve experienced quite often the instance where users, after sizing their initial application, want to run a wider variety of webs with higher tensions, and/or thicker materials. If they’ve calculated and purchased load cells that operate at maximum capacity for their initial application they are forced to purchase another pair of load cells with higher load ratings. By offering load cells that initially operate at 50% of their capacity Montalvo load cell’s give users greater flexibility to run a variety of webs. Increase productivity and profitability by automatically being provided a more flexible product.
Semiconductor Strain Gauges
The design of our load cell includes a semiconductor strain gauge. Compared to a foil gauge, the signal stability and sensitivity is far greater. We have seen that a foil gauge load cell needs to operate at greater proportions of its capacity for a more reliable signal.
*This is a brief statement on semiconductor strain gauges vs foil gauges, as much more can be said on this topic