[Introduction] A displacement sensor, also known as a linear sensor, is a linear device based on metallic induction. The function of the sensor is to convert various measured physical quantities into electrical signals. This article explains in detail the selection of displacement sensors and common troubleshooting methods.

Selection of Displacement Sensors

The selection of displacement sensors must meet the requirements of the following indicators:

1, technical indicators regarding sensitivity

For an instrument, generally, the higher the sensitivity, the better. This is because higher sensitivity makes it easier to detect changes in acceleration in the surrounding environment. With large acceleration changes, the corresponding change in output voltage naturally becomes larger, making measurement easier and more convenient, and the measured data more precise.

2, zero-point temperature

Zero-point balance change caused by changes in ambient temperature. It is generally expressed as the percentage of the zero-point balance change relative to the rated output for every 10 °C change in temperature, i.e., the drift of the sensor's input (when not under pressure) caused by temperature changes.

Do You Really Understand Displacement Sensors? Introduction to Selection and Common Troubleshooting Methods

3, technical indicators regarding bandwidth

Bandwidth refers to the effective frequency band that the sensor can measure. For example, a sensor with a bandwidth of hundreds of HZ can measure vibration, while a sensor with a bandwidth of fifty HZ can effectively measure tilt angles.

4, technical indicators regarding output mode

There are two modes: digital output and analog output. Digital sensors input digital signals to the instrument, such as count and weight; analog sensors input analog signals to the instrument, such as voltage and current.

5, technical indicators regarding measuring range

The measuring range required for measuring the motion of different objects varies and must be evaluated based on actual conditions.

6, ultimate overload

The maximum load the sensor can withstand without losing its operational capability. This means that if operation exceeds this value, the sensor will suffer permanent damage.

7, sensor gain

This is the amplification factor of the sensor's original signal output.

Common Faults and Troubleshooting Methods

The working principle of a linear displacement sensor is similar to that of a sliding rheostat. It is used as a voltage divider, presenting the actual measured position through a relative output voltage.

1, If the electronic ruler has been used for a long time, and the seal has aged, mixing in many impurities, water mixtures and oil will seriously affect the contact resistance of the brush, causing the displayed numbers to jump continuously. In this case, it can be said that the electronic ruler of the linear displacement sensor is damaged and needs to be replaced.

2, If the power supply capacity is very small, many situations may occur: the movement of the plasticizing unit will cause fluctuations or changes in the display of the mold-clamping electronic ruler, or the mold-clamping movement will cause fluctuations in the display of the injection-molding electronic ruler, resulting in large measurement errors. If the drive power of the solenoid valve and the power supply of the electronic ruler are shared, the above situations are more likely to occur. In severe cases, voltage fluctuations can even be measured using the voltage setting of a multimeter. If the situation is not caused by high-frequency interference, electrostatic interference, or poor neutral grounding, it may be due to insufficient power supply capacity.

3, Frequency modulation interference and electrostatic interference can both cause the displayed numbers on the electronic ruler of a linear displacement sensor to jump. The signal cables of the electronic ruler must be routed in separate cable trays from the high-power lines of the equipment. The electronic ruler must mandatorily use a grounding bracket, ensuring good contact between the housing of the electronic ruler and the ground. Shielded cables must be used for signal lines, and one end of the shielded cable should be grounded at the electrical cabinet. When there is high-frequency interference, voltage measurements with a multimeter usually appear normal, but the displayed numbers will keep jumping; the situation with electrostatic interference is similar to that of high-frequency interference.

To determine whether the interference is electrostatic, you can temporarily short the housing screw of the electronic scale to a metal part of the machine using a power cable. Once shorted, the electrostatic interference will be eliminated immediately. However, it is difficult to eliminate high-frequency interference using the above method. Variable frequency drives and robotic arms often generate high-frequency interference. Therefore, you can try stopping the high-frequency drive or the robotic arm to verify whether the issue is caused by high-frequency interference.

4. If the display data of the linear displacement sensor's electronic scale jumps regularly at a certain point during operation, or if there is no display data, check whether the insulation of the connection cable is damaged and causing a regular contact with the machine housing, resulting in a ground short circuit.

5. The supply voltage must be stable. Industrial voltage stability should comply with ±0.1 [%]. For example, if the reference voltage is 10V, a fluctuation of ±0.01V is allowed. Otherwise, it may cause significant display fluctuations. However, if the amplitude of the display fluctuation does not exceed the amplitude of the voltage fluctuation, the electronic scale is functioning normally.

6. The alignment of the linear displacement sensor must be precise. However, a parallelism error of ±0.5mm is permissible, and an angular error of ±12° is allowed. If both the parallelism and angular errors are excessive, the display numbers may jump. In such cases, the parallelism and angle must be adjusted.

7. Pay close attention during the connection process. The three wires of the electronic scale must not be connected incorrectly, and the power line and output line must not be swapped. Incorrect wiring will result in significant linearity errors, making control difficult and reducing control precision. Additionally, the display may easily exhibit jumping phenomena.

There are many types of displacement sensors, and their application fields are continuously expanding. Meanwhile, more innovative technologies are being applied to sensors, such as OEM-based LVDT technology, ultrasonic technology, magnetostrictive technology, fiber optic technology, and time-grating technology. Displacement sensor technology has achieved breakthrough progress. Due to technological advancements, the performance of various sensors has significantly improved, and costs have been substantially reduced, greatly expanding the range of applications and forming a rapidly growing industry.