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Automatic checkweighers are widely used in pharmaceutical laboratories and production environments to verify the weight of products, packages, bottles, blister packs, and other pharmaceutical materials. By automatically detecting products that are underweight or overweight, a checkweigher helps maintain product consistency, reduce material waste, and support quality-control requirements.
However, inaccurate weighing, unstable readings, product jams, conveyor problems, and rejection failures can affect the reliability of the entire inspection process. Correct fault diagnosis is therefore important for maintaining stable operation. This article introduces common automatic checkweigher faults, their possible causes, and practical troubleshooting methods.
One of the most common problems is unstable or fluctuating weight readings. The displayed value may continuously move even when the product weight is relatively consistent.
Possible causes include environmental vibration, air movement, unstable installation, contamination around the weighing platform, or problems with the load cell.
First, check whether the checkweigher is installed on a stable and level surface. External vibration from nearby machines, conveyors, pumps, or motors can affect measurement accuracy. If necessary, isolate the checkweigher from vibration sources.
Airflow can also influence lightweight products. Doors, ventilation systems, fans, and air-conditioning outlets should not directly blow across the weighing area.
The weighing conveyor and platform should then be inspected for foreign material. Dust, product residues, packaging fragments, and spilled materials may interfere with the weighing mechanism. Clean the weighing section according to the manufacturer's maintenance procedure.
If the problem remains, inspect the load cell, cable connections, and electrical connectors. A damaged or poorly connected load cell can produce unstable signals and should be tested or replaced by qualified personnel.
If the displayed weight is consistently higher or lower than the actual reference weight, calibration should be checked.
Begin by verifying that the reference test weight is accurate and suitable for the checkweigher's measurement range. Place the test weight on the weighing area according to the manufacturer's calibration procedure.
If the displayed value differs significantly from the reference value, perform zero adjustment and calibration where permitted by the equipment manufacturer.
Before calibration, make sure the weighing conveyor is clean and free from mechanical interference. Calibration performed while the weighing system is contaminated or mechanically restricted may not solve the underlying problem.
If calibration cannot be completed successfully, inspect the load cell, signal cable, analog-to-digital conversion system, and weighing controller.
Zero-point drift occurs when the checkweigher does not return to zero after a product has passed through the weighing section.
Several factors can cause this problem. Product residue may remain on the conveyor, the conveyor may be touching another component, or the load cell may be affected by mechanical stress.
Check whether the conveyor belt is correctly installed and properly tensioned. The belt should move freely without rubbing against the frame or other components.
Temperature changes may also cause gradual measurement drift. Allow the equipment to reach its normal operating temperature before performing calibration.
If the zero value continues to drift after cleaning and mechanical inspection, the load cell or associated electronic components may require professional testing.
A checkweigher normally uses photoelectric sensors or other detection devices to identify when a product enters and leaves the weighing area.
If products pass through the machine without being detected, inspect the sensor first. Dust or product residue on the sensor surface can reduce detection reliability.
Check the sensor alignment and make sure the product passes through the intended detection area. Loose cables, damaged connectors, incorrect sensor settings, or communication faults can also cause detection failures.
For transparent or reflective packaging, sensor sensitivity may need to be adjusted according to the equipment manufacturer's specifications.
If the conveyor does not move, first check the emergency-stop status and system alarms. An active safety interlock may prevent the conveyor from starting.
Next, inspect the motor, drive system, belt tension, and mechanical components. A belt that is too tight or mechanically blocked may overload the motor.
Check whether the conveyor is obstructed by packaging material, product residue, or foreign objects. If the motor produces abnormal noise but the belt does not move, the drive mechanism may require inspection.
Electrical troubleshooting should only be performed by appropriately trained personnel, particularly when checking power supplies, motor drives, and control cabinets.
A false rejection occurs when a product with an acceptable weight is incorrectly rejected.
Possible causes include incorrect target-weight settings, inappropriate tolerance limits, unstable weighing signals, incorrect product spacing, or improper rejection timing.
Verify the product recipe and weight parameters stored in the control system. Confirm that the nominal weight, upper limit, lower limit, and other relevant parameters correspond to the current product.
Product spacing is also important. If two products are too close together, the weighing system may interpret them incorrectly as one item or may not have sufficient time to complete the measurement.
Check the synchronization between the weighing conveyor and reject mechanism. Incorrect timing can cause a correctly measured product to be rejected because the rejection device activates at the wrong position.
If an overweight or underweight product is correctly identified but is not removed, inspect the reject mechanism.
Depending on the equipment design, the rejection system may use an air jet, pneumatic cylinder, mechanical arm, flap, or other actuator.
Check the air pressure and pneumatic connections for systems using compressed air. Low pressure, leaking tubing, blocked valves, or damaged cylinders can prevent reliable rejection.
For electrical actuators, inspect the actuator connection, control signal, and mechanical movement.
The rejection area should also be kept clean. Product accumulation can restrict actuator movement and create repeated rejection failures.
Modern pharmaceutical checkweighers often communicate with PLCs, production management systems, printers, or laboratory information systems.
Communication errors may result from loose network cables, incorrect communication settings, software configuration problems, or controller faults.
Check cable connections first and confirm that the communication parameters match the connected system. If the equipment displays a specific alarm code, record the code before restarting the system.
Avoid repeatedly resetting the machine without identifying the cause, because doing so may make intermittent faults more difficult to diagnose.
Preventive maintenance is one of the most effective ways to reduce checkweigher failures.
Daily maintenance should include cleaning the weighing conveyor, checking the sensor area, inspecting the conveyor belt, and confirming that the equipment returns correctly to zero.
Periodic maintenance should include inspection of load-cell connections, conveyor tension, motor condition, pneumatic components, electrical connections, and calibration performance.
For pharmaceutical applications, cleaning and maintenance procedures should follow the site's validated procedures and the equipment manufacturer's instructions. Maintenance records should include the date, equipment identification, fault description, corrective action, calibration information, and technician.
When a fault occurs, technicians should avoid immediately replacing components. A systematic troubleshooting process is more efficient.
A practical sequence is:
Step 1: Record the alarm or abnormal phenomenon.
Step 2: Check emergency stops, safety interlocks, and power status.
Step 3: Inspect the weighing and conveyor areas for contamination or mechanical obstruction.
Step 4: Check product detection and sensor alignment.
Step 5: Verify zero stability and reference-weight accuracy.
Step 6: Check product parameters, tolerance limits, and rejection timing.
Step 7: Inspect electrical, pneumatic, and mechanical components.
Step 8: Perform calibration or component replacement only when the underlying cause has been identified.
After maintenance, run several test products and reference weights to confirm that weighing accuracy, product detection, conveyor movement, and rejection performance have returned to normal.
Automatic checkweighers play an important role in pharmaceutical quality control and automated inspection. Common problems include unstable readings, zero drift, inaccurate measurements, product detection failures, conveyor faults, false rejection, and rejection-system failures.
Effective troubleshooting should begin with simple external factors such as cleanliness, vibration, product spacing, sensor alignment, and mechanical interference before moving to load cells, electrical systems, controllers, and other internal components.
Regular cleaning, calibration, preventive maintenance, and accurate maintenance records can significantly improve equipment reliability. When a fault involves electrical safety, load-cell replacement, controller hardware, or other critical components, qualified maintenance personnel should follow the manufacturer's service procedures and the pharmaceutical site's applicable quality and validation requirements.