How to adjust when the weighing error of the ton-bag packaging machine is too large?

First, clarify the standard: What constitutes a "large deviation"?
The weighing accuracy of the ton-bag packaging machine is usually measured as a percentage of the full scale. High-precision equipment typically has an error range of plus or minus 0.1% to 0.2% of the full scale, suitable for chemical and food-grade fine powders. The industrial-grade accuracy is plus or minus 0.3% to 0.5% of the full scale, suitable for most mineral powders, building materials, and plastic particles. If the error exceeds plus or minus 0.5% of the full scale, it exceeds the allowable range and requires immediate intervention.
For example: The target weight is 1,000 kilograms per bag. According to the conventional standard of plus or minus 0.5 percent, the error for a single bag should be controlled within plus or minus 5 kilograms. If it often exceeds plus or minus 8 to 10 kilograms, it indicates a larger error and requires a systematic investigation.
II. The Five Major Causes of Large Errors (Ranked by Priority of Investigation)
Field experience indicates that weighing errors are often not just a problem with the scale, but rather a systemic issue caused by the materials, equipment and environment. It is recommended to troubleshoot in the following order:
Mechanical installation and interference issues (common and often overlooked)
Contact or jamming between the scale body and the surrounding structure is a common cause. If the weighing hopper or hanging bag frame touches fixed components such as chutes, guardrails, or dust collection pipes, the force on the sensor will be stolen or overloaded, resulting in reading drift. Inaccurate equipment installation or loose anchor bolts will generate resonance and additional forces during operation, causing the weighing data to shake. When shock absorption is insufficient, vibrations from the motor, screw conveyor, and adjacent equipment will also be transmitted to the sensor through the base.
Quick judgment method: Observe the instrument reading in the no-load state. If the number keeps fluctuating slightly, such as by plus or minus three to five digits or more, there is likely an interference or vibration problem.
② Weighing sensors and their associated systems
The sensor is the brain that collects data, and its condition directly determines the correctness of all subsequent control operations. Zero drift may be caused by long-term load, overload shock, moisture, or dust accumulation, resulting in a deviation in the output baseline. Loose wiring or oxidation can cause intermittent signal transmission, causing the display value to fluctuate or even appear with false numbers. Uneven installation force, such as uneven loading, can lead to a skewed load on a weighing hopper supported by multiple sensors. If one corner of the hopper is higher or lower, the local force becomes concentrated, resulting in systematic deviations in the overall weighing.
③ The feeding or discharging system is unstable (one of the true culprits behind the error)
Many sites simply rely on the scale to handle the feeding, but the actual problem is that the ingredients are not placed firmly. When the blades of the screw feeder wear out or the gap of the bushing becomes larger, the actual flow rate fluctuates greatly at the same rotational speed, causing the material to be unable to be collected properly at the end stage. The delayed action or unstable air pressure of the pneumatic gate or butterfly valve will cause the shutdown timing to drift, resulting in an additional few kilograms per bag. Accumulated material, scaling, or walling at the feeding port can cause intermittent collapse or ejection of the material, resulting in an instant overloading. Leakage from the filling nozzle becomes an invisible increment for the last bag.
④ Fluctuations in material properties
This is a factor that is not easily detectable but has a profound impact. Changes in moisture content cause hygroscopic powders to form clumps after becoming damp, resulting in reduced fluidity and causing bridging and pulsating fluctuations in discharging, along with significant fluctuations in metering. When the particle size or density is uneven, the initial part of the batch is dense while the latter part is loose, resulting in a significant difference in weight for the same volume. During the high-speed discharging of ultrafine powders, air flow entrainment is prone to occur, leading to uncontrollable self-flowing discharge phenomena.
⑤ Inappropriate or drifting settings of control parameters
The incorrect setting of the advance quantity for fast addition and slow addition is a common problem. If the advance quantity is too small, it will cause the coarse material to stop too late, resulting in excessive inertial discharge weight; if the advance quantity is too large, too much material will be replenished by slow addition, leading to low efficiency and the possibility of over-discharge during the slow addition stage. An inappropriate filter coefficient will cause the filtered data to be too light, resulting in data jitter; or the system response will be too slow, missing the critical shutdown points. Additionally, some people accidentally modify the parameters without re-calibrating and verifying, which can also cause errors.
III. Systematic Adjustment Steps (Follow these steps carefully and complete them all at once)
First: Safety preparations and visual inspection (five to ten minutes)
First, stop the machine, mark it and lock it to ensure safety. Then, visually inspect: the weighing hopper has no contact with the surrounding area. Push the hopper body with your hand and it should be able to swing freely to a slight extent. The sensor cable is undamaged, and the connectors are free from oxidation and loosening. The bag-hanging arm or the hanger is not deformed, and the fasteners are not loose. The discharge port and the transition section have no large-area accumulation of materials or scaling. Then, use a level gauge to recheck the level of the machine base. If necessary, add or remove shock-absorbing rubber pads on the base, with a thickness of no less than twenty millimeters, and tighten the anchor bolts.
Step 2: Zero-point calibration (This must be done and should be conducted in a clean and unloaded state)
The prerequisite for zero-point calibration is that there are no ton bags hanging on the scale bin, no residual materials, no personnel leaning against it, and no strong vibration sources nearby. The operation steps are as follows: Power on and connect the power supply, preheat for fifteen minutes to stabilize the temperature of the instrument and the sensor. Clear all residual materials in the scale bin. Perform zero-point calibration on the control panel and confirm that the displayed value is stable within the allowable range of zero plus or minus. If it cannot be zeroed no matter how you adjust, then check the sensor wiring, whether there is a continuous lateral force or jamming, and whether the sensor itself is damp or aged in sequence.
Step 3: Sensor status verification and load imbalance calibration (the core of the core)
Perform three-point calibration using standard weights. Prepare a set of known standard weights, covering three points near the empty scale, half-load, and full-load areas, loading them in ascending and descending order respectively. The empty scale is used to confirm the stability of the zero point, approximately 40% of the full-scale range is used to test the mid-range linearity, and approximately 70% to 100% of the full-scale range is used to check whether the full-load section is bowed down or tails off. If the reading at each point deviates from the standard value in the same direction, for example, all are 8 kilograms heavier, then it is an overall calibration coefficient offset, and a new range calibration needs to be performed in the instrument. If the deviations at different angles are not the same, then it is a load imbalance problem, and the output balance of each sensor needs to be adjusted. This can be achieved by adjusting the bias micro-adjustment resistor or the bias coefficient in the instrument.
For multi-sensor scales, a load imbalance check must be conducted. Equal weights should be placed near each supporting point of the scale bin. The display values at each position should be recorded. The difference between each point should be less than or equal to the specified tolerance, for example, not exceeding 0.05% to 0.1% of the full scale. Please refer to the instrument manual for specific details. If the tolerance is exceeded, the installation height or signal gain of the corresponding sensor needs to be adjusted.
Sensor quality determination: Use a multimeter to measure the bridge circuit impedance. Usually, the input is approximately 400 ohms and the output is approximately 350 ohms. Refer to the nameplate for reference. If there is an open circuit or short circuit, replace it directly. After loading a known weight, if the output signal is non-linear and has large hysteresis, it indicates that the sensor is damaged internally or the strain gauge is damp, and it also needs to be replaced.
Step 4: Rectification of the consistency of the feeding system
This step is crucial for transforming the fluctuating errors into stable and controllable ones. Firstly, clean the screw or vibrating feeder, disassemble it for inspection to check if there is any scaling or bridging residue on the inner wall. Replace if the spiral blades are severely worn or the bushing clearance is uneven. Secondly, check the air pressure system to confirm if the air source pressure is stable. It is recommended to install a pressure stabilizing tank. Check if the electromagnetic valve response is quick and decisive, and if the cylinder rod is stuck. Then, ensure that the discharge port is centered. The discharge center should be as close as possible to the geometric center of the hopper to reduce asymmetric impact forces.
Step 5: Parameter Optimization (The advance amounts for fast or slow addition are the crucial knobs)
Take the common dual-speed feeding logic as an example, namely coarse addition and fine addition. When the current weight is greater than or equal to the target weight minus the fine addition lead time, the system switches from coarse addition to fine addition. This lead time needs to be determined through repeated experiments based on the material characteristics and the equipment response time. The general approach is to first set a conservative lead time, such as 5% of the target weight, and then weigh ten bags continuously to calculate the average overshoot. If the average overshoot is positive and too large, then appropriately increase the lead time; if it is negative and too small, then reduce the lead time. Each adjustment should not be too large. It is recommended to adjust by 0.5 kilograms or 1% increments until the error stabilizes within the allowable range.
The adjustment of filtering parameters is also very important. If the instrument reading fluctuates severely during the coarse addition stage, the filtering depth can be appropriately increased, but do not go too far to avoid affecting the response speed of the fine addition stage. Usually, a medium-level filtering setting is sufficient.
After completing all the above adjustments, it is essential to conduct a complete calibration verification again, including zero point, range and overload. Also, test package at least ten bags with actual materials and record the net weight of each bag. Calculate the average value and standard deviation, and confirm that it is qualified before resuming production.
IV. Daily Maintenance and Preventive Measures
Regularly clean the dust on the hopper, discharge port and sensor surface to prevent accumulated dust from affecting the payload. Conduct zero-point and overload checks once a month. Perform a full-scale calibration verification using standard weights every quarter. Pay attention to changes in environmental temperature and humidity. Sensors and instruments have working temperature ranges; in extreme environments, measures such as insulation or air conditioning should be taken. Establish a parameter modification ledger. After each parameter modification, sign the record and make verification notes to prevent accidental operations.
As long as the above steps are followed to systematically investigate and adjust, the weighing errors of the majority of ton bag packaging machines can be restored to the ideal level. If the problem still cannot be solved after a comprehensive investigation, it is recommended to contact the equipment manufacturer or a professional metrology engineer for on-site diagnosis.