I. Electrical Connection Precautions for Weight Sensors
Sensor signal cables should not be run parallel to high-voltage power lines or control lines (e.g., do not place sensor signal lines, high-voltage power lines, or control lines in the same conduit). If they must be run parallel, the distance between them should be at least 50 cm, and the signal lines should be encased in metal conduits.
Under all circumstances, power and control cables should be twisted together at a twist rate of 50 revolutions per meter. If the sensor signal cable needs to be extended, a specially designed sealed cable junction box should be used. If this junction box is not used, and cables are directly connected (soldered ends), special attention should be paid to sealing and moisture protection. After connection, the insulation resistance should be tested and must meet the standard (2000-5000 MΩ). If necessary, the sensor should be recalibrated.
If the signal cable is very long and high measurement accuracy is required, a cable compensation circuit with a repeater amplifier should be considered.
All wires leading to or from the display circuit should be shielded cables. The connections and grounding points of the shielding cables should be appropriate. If grounding is not achieved through the mechanical frame, then external grounding is necessary, but the interconnected shielding cables should not be grounded and should remain suspended.
Note: Three sensors are connected in parallel. The sensors themselves are 4-wire, but they are replaced with 6-wire connections in the junction box.
The sensor output signal readout circuit should not be placed in the same enclosure as equipment that can generate strong interference (such as SCRs, contactors, etc.) or equipment that generates considerable heat. If this cannot be guaranteed, a baffle should be installed between them for isolation, and a fan should be installed inside the enclosure.
Electronic circuits used to measure the output signal of weight sensors should be equipped with independent power supply transformers whenever possible, and should not share the same main power supply with devices such as contactors.
II. How to determine if a weight sensor is good or bad
(I) How to determine the quality of a weight sensor
Check the sensor for deformation, cracks, or other defects. If any of these are found, contact the manufacturer to replace the sensor.
Locate the sensor connection terminal in the weighing controller (weighing instrument) and determine the sensor connection circuit.
Under normal circumstances, the excitation voltage (between EXC+ and EXC-) is 5-10V, and the output voltage (between SIG+ and SIG-) is close to 0 when the equipment is unloaded, which is less than the sensor's maximum output. (Maximum sensor output = excitation voltage * sensor sensitivity; the sensitivity of checkweigher sensors is mostly 2mV/V). If it exceeds this range, contact the service provider to apply for sensor replacement.
(II) How to use a multimeter to measure the quality of a weight sensor
1. Sensor manufacturers provide the sensor's output sensitivity and power supply voltage. We use these two parameters to test the sensor's output signal. Strain gauge load cells output analog millivolt signals. For example, if the sensor's output sensitivity is 2.0mV/V and the power supply voltage is DC10V, these two parameters provide a linear relationship: the sensor requires a DC10V excitation voltage, and the sensor output signal corresponds to a 2.0mV output for every 1V of excitation voltage. For example, if the sensor's full-scale range is 50KG, then a DC10V voltage will result in a 20mV full-scale output. Based on this relationship, we use a multimeter in the mV range to measure the sensor's output signal. A zero-load output of 0mV is normal. A value greater than this but close to it indicates zero-point drift. A very large value suggests sensor damage or an imbalance in the internal bridge circuit's resistance.
2. Determine if the strain gauge is damaged based on the sensor parameters provided by the manufacturer, specifically the input and output resistances. The input and output resistance values vary between manufacturers, so this must be checked according to the manufacturer's specifications. Use a multimeter in ohm mode to check the resistance between the power supply and power ground, as well as the resistance between the signal lines and signal ground. If the resistance is higher than the manufacturer's specified value, the sensor has been overloaded and the strain gauge is deformed. If the resistance is infinite, the strain gauge is severely damaged and cannot be repaired.
3. Because sensor leads often break during use, while the outer sheath remains intact, the sensor wires appear healthy to the naked eye. We use the ohmmeter setting on a multimeter to check the continuity of the sensor wires. If the resistance is infinite, the wire is definitely broken; if the resistance changes, there is poor contact. The above are simple methods for testing common faults in weight sensors using a multimeter.
