December 24, 2021

Troubleshooting Scheme for Load Cell

Once the signal provided by the load cell is interrupted, the zero point is abnormal, or the indicated value jumps and drifts are found on the digital display, we can use an ordinary digital multimeter to find the fault link of the symmetrical load cell.


Measure the load cell with a multimeter


  1. Fault judgment of cable end


(1) If the four wires of the cable have lost any marks, the following methods can be used to roughly judge the bridge structure and fault links, clamp the wire head of any line with the alligator pliers at one end of the multimeter, sequentially detect the resistance between it and the other three wires, record the value, analyze the value, and make a series of conclusions.


(a) If all three groups have exact resistance values, one group is larger, such as 350 Ω, and the other two groups are smaller and equal to each other, such as 270 Ω, it can be determined that the bridge network is basically normal, and 350 Ω group is the input or output end of the bridge.


(b) Copy the impedance between the above two pairs of outgoing lines (input and output). If both are equal to 350 Ω, it indicates that the input and output impedance of the sensor have been adjusted (or the elastic modulus compensation coefficient and linear compensation may not be adjusted).


(c) If one group of open circuit is found in the above measurement, and one group is 350 Ω and the other group is 278 Ω, it indicates that the strain bridge is not damaged, and the problem is that one outgoing line or the device on the outgoing line is desoldered. At this time, it is necessary to follow the open wire head and search from the end of the cable from the outside to the inside to find the fault link.


If it is found that all three groups of measurement results have resistance values, but they are approximately 350 ~ 700 ~ 1050 Ω respectively, it is proved that the bridge has been broken.


(e) Screw the four leads at the end of the cable into one strand, that is, the insulation resistance between the bridge network and the shell can be measured with a megger or 100V megger.


(f) For the ordered sensors, the same method can be used to check the input and output impedance from the four outgoing lines at the end of the cable, judge whether the input impedance has been adjusted, whether the coefficient temperature compensation and linear compensation have been carried out, and measure the most insulation resistance. If there is a weight, the psychic coefficient can be roughly measured. If there is a reliable secondary instrument, the time drift of the sensor can be observed within a few hours. If there is a power source, even if the detection accuracy cannot be detected, the key technical index of the sensor – zero reset characteristic can be learned through loading and unloading.


  1. Fault search inside the load cell


Generally speaking, the load cell is a disposable instrument. In case of internal failure, it shall be handed over to the manufacturer for treatment or scrapping. However, based on the principle of economy, we can continue to find the fault link for reuse, especially for imported sensors.


The repair work can be carried out according to the following steps:


Melt off the solder of the sealed terminal, pull out the lead and check whether it is desoldered. If the fault is not on the back of the terminal, melt off the lead, and then process and remove the lower half jacket to find the internal fault.


Most of the faults are due to wire head off welding or zero temperature compensation wire (or sheet) breaking or off welding. After welding firmly, the zero indication can be re evaluated and the temperature compensation can be redone. If the compensation condition is normal, the newly processed section of outer cover can be sleeved, vacuum and nitrogen filling can be conducted again after welding, the zero point can be adjusted in the junction box again, and the sealing filler can be melted and poured again before it is restored to use.


If it is found that the compensation plate, linear correction plate or strain gauge have been damaged, it is necessary to re patch and re evaluate each process sequence.


According to the above knowledge, a digital multimeter can be used to find and judge a variety of fault items.


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