Several methods for identifying sensor faults in electronic crane scales: Resistance method: Use a multimeter to measure the resistance between the two output terminals and the input terminal of the sensor, as well as the insulation resistance between each core wire of the signal cable and the shielding layer. If the measured resistance exceeds the value given in the sensor's certificate of conformity or the insulation performance deteriorates, the sensor is considered faulty. If normal, this method can be used for further inspection. First, power on the instrument, then disconnect the output signal lines of each sensor one by one. Under no-load conditions, use a multimeter to measure its mV output value. Assuming the rated excitation voltage is U (V), the sensor sensitivity is M (mV/V), the sensor's rated capacity is F (kg), and the load weight the sensor can bear is K (kg), then the rated output voltage of each sensor should be: U × M × K / F (mV).
A relatively simple yet effective method for troubleshooting is to disconnect each sensor from the terminal block one by one until the meter displays a stable reading when a particular sensor is disconnected. This indicates that the sensor is faulty. Alternatively, disconnect all sensor wires, connect the simulator to the terminal block to supply power to the meter, and if the display is stable, connect each sensor in parallel with the simulator, observing the meter's display for stability after each connection. The sensor that causes the display to become unstable after connection is the faulty sensor.
1. Under normal circumstances, we can determine the specific location of the fault by observing the instrument display.
1.1. Large zero-point drift
1. Insufficient foundation causes unstable zero position of the scale (large temperature difference between winter and summer, low winter temperature places high requirements on the foundation).
2. Improper cleaning and maintenance can cause the scale to be unstable at zero position (due to friction between the limit switch and the scale base).
3. Poor grounding causes unstable zero position of the scale (electronic instruments require a grounding resistance of less than 4Ω and an independent ground electrode).
If the zero point of the electronic crane scale drifts, when weighing a truck weighing more than ten tons, it will weigh more than one ton less than another scale. By pressing the corners with weights, it was found that the two middle sensors had the largest errors, one weighing less than the standard weight and the other even less, while the four sensors at both ends were basically normal. It is estimated that the sensor at the corner with the largest error is damaged.
1.2. The displayed weight has a large error.
(1) There is a foreign object stuck in the scale body or the limit bolt is in contact with the scale body.
(2) The connector is broken and the sensor is displaced.
(3) The core wire of the signal cable is broken.
(4) The instrument amplifier circuit is damaged.
(5) The levelness of the scale body is compromised.
(6) The sensor changes linearly.
Solution:
(1) After ensuring the weighing platform is in good condition and the instrument is confirmed, first perform an off-center load test to preliminarily determine which sensor is damaged. If the off-center load test is qualified, recalibrate the weighing; if the off-center load test is unqualified, it indicates that a certain sensor is damaged and must be replaced.
(2) Check whether the signal cable is broken and whether the wiring of the terminal block is disconnected.
(3) Check the sensor using the method described above.
1.3. The instrument display is unstable.
(1) Instrument malfunction.
(2) Loose signal cable wiring or poor soldering of solder joints, or cable damage causing poor contact.
(3) The junction box is damp and the terminal block is wet, causing the output signal to be unstable, sometimes large and sometimes small.
(4) There is a foreign object stuck in the scale body or the limit bolt is in contact with the scale body.
(5) Short circuit between sensor signal line and shielding layer or decreased insulation performance.
(6) The sensor is damaged.
(7) Check if the sensitivity setting is too low, if the AD conversion rate is set too high, if there is any interference such as vibration or wind, and if the weighing platform is touched.
Solution:
(1) First, use the simulator to send a signal to the instrument. If the display is normal, the instrument malfunction can be ruled out.
(2) Check if there are any foreign objects stuck in the scale body and whether the limit gap is normal. If there are no problems, check if there is moisture, water droplets, dust, etc. in the junction box, and whether the wiring is loose or disconnected. If so, re-solder and rewire to make the connection reliable.
(3) Check the signal cable for damage. If it is damaged, cut it off and reconnect it, and wrap it with waterproof tape.
