Continuous weighing — differential loss-in-weight balance

Release Date:

2022-05-26

Author:

Zero-gravity scales are gradually replacing traditional equipment such as belt scales and screw scales, and their use is becoming increasingly widespread. They calculate material flow by continuously monitoring changes in the weight of the weighing unit in real time, and utilise PID algorithms to control material flow with precision; as they are unaffected by mechanical fluctuations, they offer higher measurement accuracy. Combining the advantages of both static and dynamic scales, the system design must take into account conveying rate, sensor range, mechanical structure and transmission stability to ensure accurate and stable measurement.

Loss-in-weight scales are gradually replacing belt scales, screw scales and even totalising scales; as a completely new method of measurement, they are increasingly being applied to a wider range of material handling applications.

1. Basic Principle:

The weighing hopper and feeding mechanism constitute the entire weighing unit. A display or host computer continuously samples the weight signals from the unit, calculating the rate of change in weight per unit time as the instantaneous flow rate. This is then processed using various software and hardware filtering techniques to derive the ‘actual flow rate’, which serves as the control variable. The accurate determination of this flow rate is crucial, forming the basis for the loss-in-weight scale’s ability to measure precisely. The FC then employs a PID feedback algorithm to perform control calculations aimed at approximating the target flow rate, outputting a control signal to regulate the feeder controller, such as a variable-frequency drive.

2. Practical Applications of Differential Weight Loss Scales (Loss-in-Weight Scales):

From a theoretical perspective, it is evident that this method is unaffected by mechanical variations in the weighing structure or feeding mechanism; it merely calculates the difference in weight (weight difference). Compared with traditional dynamic measurement methods, its advantages are self-evident. Where the control objective is flow rate (t/h, kg/min) and the material has good conveyability whilst high measurement accuracy is required, the loss-in-weight method can be adopted as a viable solution.

3. Design considerations for loss-in-weight scales and factors affecting accuracy: As loss-in-weight scales combine the characteristics of both static and dynamic scales, the following requirements must be met when designing the system:

3.1. Correct conveying rate range: Generally, the actual operating range is 60–70 per cent of the rated conveying capacity. If AC speed control is used, the corresponding frequency should be 35–40 Hz. This ensures a wide adjustment range. Furthermore, system stability is poor when the conveying rate is too low.

3.2. Appropriate selection of sensor range: According to the formula

Dead weight + maximum weighing range ÷ (0.6–0.7) ≈ sensor range

The number of sensors should be determined such that they operate within 60–70 per cent of their full scale range, as a wide signal variation range is highly beneficial for improving accuracy.

3.3 The mechanical design must ensure good material flow whilst minimising replenishment time; replenishment should not be too frequent, with a general requirement of once every 5–10 minutes.

3.4 The associated drive system must ensure smooth operation and good linearity.

Keywords:

Mixing of lithium-ion battery slurry

The working principle and application of continuous weightless scale

Mixing of lithium-ion battery slurry

The working principle and application of continuous weightless scale

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