Weighing scales must remain fine-tuned to deliver the most precise and consistent reading measurements. Some important considerations for attaining accuracy in weighing are given below.
A digital weighing scale is the most accurate and precise analog front-end (AFE) instrument that uses force sensors to measure the load of an object. These scales find application in myriad areas, including wide-ranging use in industry.
Weight measurement challenges
Generally, a resistive load cell is used in weigh scale designs. The need for precision makes sensor interface complex. Load cells are sensitive to the effects of noise and low levels of signals. Pertinent issues include:
- Attaining accurate measurement of the signal to ensure that precise weighing requirements are met
- Taking into consideration the load cells’ numerous parameters responsible for the inaccuracies in digital weighing scales
- Establishing a well-defined user interface and boost circuitry to handle a low battery situation
- Setting up a communications protocol in digital weighing sets to interface with the host controller
- Integrating features that contribute to greater accuracy.
Accurate weighing
The need for high accuracy and user-friendly features in digital weighing scales has led to an increase in demand by industrial and nonindustrial users. Accurate weight measurement is vital in the food industry, pharmacology, chemistry and many other fields. In addition, there has been increased emphasis on implementing uniform regional weighing standards.
Precise and accurate weight control measures can translate into higher product quality and packaging and greater manufacturing efficiency.
Analog-to-digital converters
Some high-resolution, advanced digital weighing scales are based on 24-bit sigma-delta ADC with embedded systems. High-resolution measurement ADC has revolutionized the entire area of precision sensor signal conditioning and data acquisition. Modern ADCs offer high-grade code resolutions to 24 bits and greater than 19 bits of noise-free code resolution. The inclusion of on-chip PGAs in ADC coupled with the high resolution virtually eliminates the need for signal conditioning circuitry. The precision sensor can interface directly with the ADC in many cases and can be accessed by a microcontroller to enhance measuring capabilities with embedded platforms.
Higher accuracy: the stumbling blocks
Most weigh scales output the final weight value at a resolution of 1:3000 or 1:10000. This can be easily met by 12-bit to 14-bit to ADC. It is essential but not easy to attain a resolution of 1:200000 for high-precision measurement. Experts recommend an ADC accuracy closer to 24 bits to meet the requirement for highly precise weight measurement.
Extraneous noise on useful signal impedes higher accuracy and increases the throughput rate of passing products. Common sources of obstacles are 1) constant values; 2) noises, i.e., electromagnetic pickup, power harmonics, thermally unstable circuits and gain programmable by software (via the PWM’s output of the microcontroller) and 3) strain-gage based sensor (load cells), which are particularly sensitive to vibration.
These obstacles are amplified before being transferred to the ADC, and the amplified noise introduces an error to the system.
The solution is a signal processing module (SPM). The SPM gets electrical signals from the weighing machine and calculates the product’s weight as its output. The improvement in SPM increases the speed of weighing and enables high measurement accuracy. This brings significant benefit to the static weighing system.
Digital filters
Digital filters are an important consideration for the following reasons:
- Improved filtering increases the speed of weighing and enhances measurement accuracy
- Digital filtering is used to remove measurement noises from the extremely low-frequency noise of the static weighing system
- Digital filters are suitable for the treatment of low-frequency noise. They can be installed in real time or as postprocessed applications. The amplitude spectra reveal major components of noise emanating from the weighing system.
Load cells
A load cell is an uncontrollable weighing device. A major concern in a load cell interface is that it is prone to gain error because the output signal range is dependent on the excitation voltage.
Any variation in the excitation voltage can cause a similar percentage of gain error in the measurements. However, this can be avoided if the signal measurements are made as a ratio against the excitation voltage. This can be achieved by two means:
- Measuring the signal and excitation voltage separately and then calculating the ratio, thus removing the gain error
- Multiplexing the ADC between the two signals.
Load cell sensor: The weigh scale uses a resistive bridge-type load-cell pressure sensor. The voltage output is directly proportional to the pressure placed on the sensor.
A typical resistive load-cell sensor contains a resistor bridge circuit of four resistors with two variable arms, where the resistance varies with weight applied and generates a differential voltage at a reference level of 2.5 V. Each resistance is 350 ohm.
Load cell electrical sensitivity: The load cell’s electrical sensitivity is defined as the ratio of the full load output to the given input voltage. It is in units of mV/V.
For example, if the input voltage given to bridge is +5 V, then at full load the output voltage is 10 mV (if the electrical sensitivity of the load cell is 2 mV/V). Most of the linear portion of the load cell is two-thirds of the full rated load.
Only 6.66 mV as the most linear output of load cell’s span can be used. The challenge is to measure small signal changes within this 6.66-mV full-scale range to get the highest achievable performance—not an easy task in industrial environments where weigh scales are typically used.
Load cell total error: The total error is the ratio of the output error voltage to the rated output voltage. A typical load cell has a total error specification of about 0.02%. It is a very important parameter because it affects the accuracy, and an improved signal conditioning circuit is required.
Controlling inaccuracies
If the current ADC value differs from its previous value by a predefined threshold, then there is a substantial change in weight and 10 new samples are collected. In this way, sudden weight changes are taken into account.
All weight, count values and operating modes of the weigh scale are shown on the LCD. Weights of less than 1 kg are displayed in grams, while those greater than 999 g are displayed in kg with three decimal places.
As the ADC count value read from the controller corresponds to the load cell output, the load cell characteristic is derived by plotting the ADC count values against the standard weights. An appropriate equation is used to calculate the weight at run time.
Embedded platform-based weighing system
Embedded platform-based digital weighing systems enhance measuring capabilities. Calibrated weights are used to confirm accuracy and reliability.
Static weighing system
The conventional filtering method used in static weighing systems is lacking in its ability to improve accuracy and throughput rate. For this reason, an alternative technique is being explored.
Conclusion
The following are important design considerations for weigh scales:
- Ratiometric measurement: For best performance, ratiometric measurement techniques are used. The output accuracy of the load cell is determined by the excitation voltage of the bridge. The ratiometric connection removes the effect of drifts and very low-frequency noise in the excitation source. In order to filter out noise from the load cell at the inputs to the ADC, a simple first-order RC filter can be used.
- Layout: For best noise performance it is critical that a high-precision sigma-delta ADC is used. The most important aspects are grounding and power-supply decoupling.
- Hardware and software: The weigh-scale design can be interfaced to any PC using an interface. This allows the user to save and process data when evaluating the system.
Kevin Hill heads up the marketing efforts and provides technical expertise to the sales and service teams at Quality Scales Unlimited in Byron, Calif.; e-mail: [email protected]; www.scalesu.com