Load Cell Minimum Weight

What is the minimum weight a load cell can sense?

The minimum weight that a load cell can sense is determined by the sensitivity and tolerance ratings of the load cell [7]. Specifications for load cells can be found in data sheets which are provided by the manufacturer. Load cells are given a division range which can be used to determine the minimum weight that a load cell can measure. Division ranges vary from load cell to load cell, usually between 500-10,000 divisions. However, the sensitivity of a system used to measure a load cells output is also an important factor in displaying an accurate minimum weight. Systems that have low sensitivity ranges or are of lower quality will not be able to detect small changes that occur when a load cell experiences a force from a minimal weight. A block diagram of a typical load cell system can be seen in Figure 1.

Figure 1: Load Cell System Block Diagram. 

The analog portion of load cell system comprises of a several strain gauge sensors and a Wheatstone bridge (both built into the load cell), an amplifier and offset stage, an ADC converter, and a power supply. Each of these stages can affect the reliability and accuracy of a load cell system.

How a load cell system works

The sensing portion of a load cell is a Strain Gauge. A strain gauge is a passive transducer which changes its resistance based upon the stress which is placed on it. These type of sensors allow us to create load cells, which respond to different weights by a change in resistance. To measure this resistance change, which is typically very small, a Wheatstone Bridge is used. A Wheatstone bridge is a circuit with two voltage dividers. example of  a Wheatstone bridge can be seen in Figure 2.

Figure 2: Unbalanced Wheatstone Bridge 

Figure 2 shows an unbalanced Wheatstone bridge with a measurable difference between the two terminals. To power a load cell an excitation voltage is placed across the cell power rails, typically in the range of 5-15V. When a force is applied on the load cell, a voltage difference can be detected between the output terminals from the Wheatstone bridge. This difference is typically small, usually in the µV range. There is an upper limit to this measurable change, which is referred to as the Full Scale Output. The maximum voltage difference or full scale output of a load cell is provided by the manufacturerIt should be noted however that the maximum voltage difference between the terminals is relative to the excitation voltage being placed across the Wheatstone bridge. The maximum output will be the full scale output multiplied by the excitation voltage. A full scale output of 3 mV/V and an excitation voltage of 10 V will result in a maximum measurable difference of 30 mV. The smallest measurable difference between the terminals will be the maximum output divided by the number of Divisions from the load cell. If the load cell in the above example has a division range of 10,000, the smallest detectable voltage change would be 3 µV. Since this is such a small voltage difference, sensitive systems are required to measure this change. To boost this output signal, Signal Amplification is performed. A typical method for signal amplification is an instrumentation amplifier. This amplifier makes use of two input buffers for high input impedance, and a difference amplifier. is placed in between the Wheatstone bridge output and the ADC converter. A standard instrumentation amplifier circuit can be seen in Figure 3.

Figure 3: Instrumentation Amplifier 

The instrumentation amplifier stage needs to have high differential gain, high power supply rejection ratio, low drift, low offset and low input bias current [2]. If the instrumentation amplifier has all of these qualifications, the signal to noise ratio will be large enough for accurate conversion by the ADC. ADC (Analog to Digital Conversion) takes an analog signal and converts it into a discrete signal. This signal can then be displayed on a digital display. A direct correlation exists between the number of divisions of an ADC from an indicator and the number of divisions of a load cell. An ADC must have enough high enough resolution, i.e. enough bits, to detect small changes from a high division load cell. Manufacturers of load cell indicators will provide the number of bits of the ADC, and sometimes even provide details on which load cells should be used with the system.

Load Cell Specifications and Definitions

Load cells have many important specifications that determine reliability and application. Load cells are typically given error ranges, which is a sum total of the errors a load cell can experience. This term is referred to as the Maximum Possible error. The maximum possible error is important when considering accurate division measurements [3]. If a load cell experiences its maximum possible error, its minimum weight divisions be less reliable. For this reason, load cells are given accuracy classes. If a load cell is given a 0.1 % accuracy class, its minimum divisions will be accurate with a  maximum error of ±0.1%. The total error is determined by a number of varying factors, from temperature deviations to magnetic error. Figure 4 displays typical specifications found on a load cell data sheet.

Figure 4: Specifications from a Load Cell Data Sheet

The following definitions are important determinants of the accuracy class of a load cell. Understanding the source of error can help increase accuracy when testing a load cell system. These definitions are provided from the Handbook of Electronic Weighting.

  1. Zero Balance: This is the electrical output signal of the load cell when no weight or load is placed on it.
  2. Non-Linearity: This expresses the maximum deviation of the calibration curve that is obtained by gradually increasing the applied weight from the zero balance level to the rated output of the load cell. The smaller the non-linearity, the more accurate measurement we obtain.
  3. Hysteresis: This is the maximum deviation of the output signal for the same applied load. The first value is obtained by increasing the applied weight from zero balance to the rated output, while the second reading is obtained by decreasing the rated output to the zero balance level. The smaller this numerical difference is, the more accurate measurement we obtain.
  4. Non-Repeatability: This is the maximum difference between the electrical output signal of the load cell for repeated loads under identical environmental and loading conditions. A small value depicts a high system accuracy and reliability.
  5. Creep: This specification becomes very important when the weight is a constant load to be placed on the load cell for a long time, maybe for monitoring purposes. Creep is the change in the load cell output signal level with respect to time under a constant load, with all environmental conditions being constant.
  6. Temperature Effect on Output: This is the effect of temperature shifts on the output of the load cell as it tends to introduce errors that affect system accuracy.
  7. Temperature Effects on Zero Balance: Temperature shifts also affect the output signal of the load cell under no-load. To cater for both types of temperature shifts, ensure the load cell design you are using incorporates a temperature compensation technique.

Regulations on Load Cell Specifications

Multiple organizations exist which maintain legal standards for measurement purposes. These include organizations such as the International Organization of Legal Metrology (OIML) and the National Type Evaluation Committee (NTEP). These organizations have their own standards and tolerances for different load cell classes. An example chart of some tolerances given by OIML can be seen in Figure 5.

Figure 5: Tolerances from R60 OIML-CS Rev. 2

Highlighted in Figure 5 are the minimum and maximum load cell divisions for class IIIL3 load cells. The IIIL3 class includes weights over 5 lb. and over 2 kg.  As can be seen, the minimum and maximum number of divisions for class IIIL3 load cells are 2000 and 10000 respectively. To find the minimum weight that a load cell can detect, simply divide the maximum weight by the number of divisions. For example, if a load cell is said to have 10,000 divisions with a capacity of 50,000 lb., the minimum weight that is measurable by the cell will be 5 lb. Load cells are typically given a classification which indicates the number of divisions the load cell has. An example of this can be seen in Figure 6.

Figure 6: OIML Certificate of Conformity for Anyload 102BH Load Cells

Figure 6 shows the characteristics of the 102BH class of load cells, which are produced by Anyload. As can be seen from Figure 2, the maximum number of divisions for this load cell class is 3000. The maximum capacity range for this load cell class is between 11 t and 55t. Similar data sheets for different load cell classes can be found on manufacturers websites.

Summary

The minimum weight that a load cell can sense is determined its number of divisions. This range is important when determining a minimum measurable weight of a load cell system. However, the sensitivity ratings and the quality of the system being used to detect the changes from a load cell are also a critical factor in displaying accurate weight measurements.

Sources

  1.  Brusamarello, V., Machado de Brito, R., Muller, I., Pereira, C. E., “Load Cells in Force Sensing Analysis – Theory and a Novel Application”, ResearchGate, Jan. 2010.
  2.  Franco, S. “Design with operational amplifiers and analog integrated circuits”. New York: McGraw-Hill. pp. 87, 2015
  3. K. Elis Nordon, “Handbook of Electronic Weighting”, Wiley-VCH, pp. 24-29, Jul. 1998.
  4. “Load Cell Accuracy in Relation to the  Conditions of Use”, Technical Note VPGT-02, Jan. 8 2015. Retrieved from http://www.vishaypg.com/docs/11864/11864.pdf
  5. “Load Cell and Weight Module Handbook”,  Rice Lake Weighing Systems, pp. 9-10, 2010
  6. “OIML Certificate of Conformity”, Number R60/2000-NL1-10.27 , Dec. 2010
  7. “R 60 OIML-CS rev.2”, NIST Handbook 44, pp. 1-4, Jan 5 2018.