A load cell is a device that converts a mass signal into a measurable electrical signal output. All kinds of measuring equipment produced by the load cell. It has been widely used in all walks of life to realize the fast and accurate weighing of the required measurement materials. The supporting weighing instrument and electric control cabinet realize the measurement The process is fully automated, and it is used in raw materials, cement batching systems, and finished product factories. It has become a necessary equipment for process control in the cement industry.
1 The working principle of the load cell
The load cell is the core component of the metering equipment. Most of the load cells used in the cement industry are resistance strain type load cells. The resistance strain gauge (conversion element) pasted on its surface is also deformed. After the resistance strain gauge is deformed, its resistance value will change (increase or decrease), and then the resistance value will be measured by the corresponding measurement circuit.
The change is converted into an electrical signal (voltage or current), thus completing the process of transforming an external force into an electrical signal.
2 The difference and advantages and disadvantages of four-wire and six-wire load cells
The load cell can adopt two different input and output wiring methods, one is four-wire connection, and the other is six-wire connection. Four-wire system: excitation positive, excitation negative, signal positive, signal negative. Six-wire system: excitation positive, feedback positive, excitation negative, feedback negative, signal
Positive, negative signal. For the four-wire sensor, there are only four cables shown in the thin line in Figure 1, where EXC+ and EXC- are the excitation of the weighing module to the sensor, and SIG+ and SIG- are the output of the load cell; for the six-wire sensor, it will The lines SENS+ and SENS- with two more thick lines are positive feedback and negative feedback. Each load cell has a mv/v characteristic, and the mv signal it outputs is closely related to the received voltage. SENS+ and SENS- In fact, it is a high-impedance loop in the load cell, which can feed back the voltage actually received by the weighing module to the weighing module, and the weighing module can adjust the output power supply voltage through the internal comparator. The weighing sensor of the four-wire connection method has no special requirements for the secondary instrument, and it is more convenient to use, but when the cable is long, it is easily affected by interference caused by factors such as ambient temperature fluctuations, thereby affecting the measurement performance of the measurement equipment. ; The weighing sensor of the six-wire connection method requires the secondary instrument used with it to have a feedback input interface. The scope of use has certain limitations, but it is not easily affected by factors such as ambient temperature fluctuations. It has certain advantages when measuring long distances. Whether the weighing sensor should be connected to four wires or six wires depends on the hardware requirements of the equipment. The principle is: if the sensor can be connected to six wires, it should not be connected to four wires, and if it must be connected to four wires, it must be shorted. The general load cell is a six-wire system. When it is connected to a four-wire system, the power lines (EXC+, EXC-) and the feedback lines (SENS+, SENS-) are shorted respectively. SENS+ and SENS-Yes
Used to compensate line resistance. SENS+ and EXC+ are access, SENS- and EXC- are access.
The sensor is a four-wire sensor, then short-circuit SENS+ and EXC+ of the sensor on the junction box, and short-circuit SENS- and EXC-, so that the sensor becomes a six-wire system after coming out of the junction box, and is connected through a dedicated sensor cable, just connect to the weighing module. If the distance between the weighing module and the sensor is very close, such as a few meters, then you only need to short-circuit SENS+ and EXC+, and short-circuit SENS- and EXC- on the weighing module.
The belt scale uses a four-wire weighing sensor, and the equipment is about 200m away from the control cabinet in the power room. During the operation of the belt scale, stalls and fluctuations in measurement feedback values often occur, which affect the quality of cement products. After on-site inspection, it was found that during the measurement process In the case of weighing signal loss, it has been confirmed by repeated tests that the weighing signal is lost and the input mv signal fluctuates greatly, which leads to the maximum speed operation and feedback fluctuation of the belt scale; the current use of the weighing sensor terminal of the measuring instrument is a six-wire system Port, by short-circuiting the weighing modules SENS± and EXC±, the stall fluctuation of the belt scale is solved, and the effect is remarkable after short-circuiting.
3 Troubleshooting of the load cell
For load cells, troubleshooting should follow the steps of observation (visual inspection)—analysis (cause of failure)—detection (determination of failure through testing)—repair (replacement or repair)—verification (calibration). According to Tengzhou Zhonglian Cement Co., Ltd. metering equipment weighing sensor failure situation summary method is used for analysis and judgment.
(1) Appearance observation method. Observe whether the appearance of the sensor is deformed, cracked, etc., or whether it is stuck by materials, and if so, replace or clean the sensor immediately.
(2) Test impedance method. Remove the output and input lines of the sensor, and use a multimeter to measure the impedance of the input line and output line and the insulation resistance between the signal cable and the shielding layer. If the test results do not meet the sensor specifications, it can be judged that the sensor is damaged.
(3) Detection signal output method. The signal output by the load cell is mv signal. In the state of empty scale, keep the power line at the sensor end, remove the signal line, and use a multimeter to detect the mv value of the output signal. Normally, this value is smaller than the maximum output value of the sensor. Each sensor has sensitivity (mv/v) and excitation voltage. Its maximum output value is: excitation voltage × sensitivity, and damage can be judged if it exceeds the range. In addition to the above situations, there may also be situations such as external interference, weighing module failure, etc., which requires our electrical personnel to have a clear mind, be careful, and actively summarize and accumulate experience during the debugging process.