How to use a multimeter to test a load sensor
Weighing sensors are widely used in industrial weighing (such as belt scales, floor scales, electronic scales, human body scales, etc.), force measurement, and tension and pressure measurement. The faults of load cells during on-site use are generally as follows.
1. The sensor is overloaded. There is no clear communication between the user and the manufacturer. The sensor range does not match the actual force value and weight, causing the sensor to be overloaded, causing the sensor bridge arm resistance to deform and causing circuit imbalance. The sensor does not work properly, the output signal fluctuates, the resistance is infinite, etc.
2. The sensor lead is broken. The user does not take protective measures during use. The sensor lead is usually broken at the sensor lead interface, causing the sensor to become unresponsive or the measurement value to change suddenly.
3. Improper use of the sensor. Impact, shearing force, torsion, etc. during use of the static sensor can seriously damage the sensor and render it impossible to repair.
So how can we effectively use a multimeter to detect common faults in load cells in the field?
1. The sensor manufacturer provides sensor output sensitivity and power supply voltage before delivery. We detect the sensor output signal based on these two parameters. The strain gauge load cell outputs an analog signal in millivolts. For example, the sensor output sensitivity is 2.0mV/V, and the power supply voltage is DC10V. The two parameters can provide us with the linear relationship that the sensor excitation working voltage requires DC10V, and the sensor output signal corresponds to the output of 2.0mV for every 1V of excitation voltage. For example, if the sensor's full-scale range is 50KG, then the sensor's DC10V voltage full-scale output is 20mV. Based on this relationship, we use the mV range of a multimeter to measure the sensor output signal. The no-load output of the sensor is 0mV, which is normal and is greater than this value. However, if it is close to this value, the change in value means that the sensor has zero drift. If the value is very large, it means that the sensor is damaged or the internal bridge is a circuit and the bridge arm resistance is asymmetrical.
2. According to the sensor parameters, input resistance and output resistance provided by the sensor at the factory, determine whether the sensor strain gauge is damaged. Sensor input and output resistance values vary from manufacturer to manufacturer. Therefore, this should be tested according to the manufacturer's labeling. Use the ohm range of a multimeter to check the resistance of the power supply and power ground, as well as the resistance of the signal line and signal ground. If the resistance value is larger than the factory value, it means that the sensor has been overloaded and the strain gauge has deformed. If the resistance value is infinite, the sensor strain gauge is seriously damaged and cannot be repaired.
3. Because the lead wire is often pulled off during use of the sensor, and the outer layer of the sheathed wire is intact, it is visually inspected that the sensor wire is intact. We use the ohm range of the multimeter to detect the continuity of the sensor wire. If the resistance is infinite, it is determined to be broken. If the resistance changes, the contact is poor.
