1. Principle and application of rope displacement sensor The rope displacement sensor uses a highly flexible composite steel wire to measure displacement. The steel wire is wound on the drum by a spring, and Yuci transforms the displacement into a rotary motion. The drum is directly connected with a precision rotary encoder, and the displacement is converted into current, voltage or digital pulse output.
When in use, the displacement sensor is installed in a fixed position, the draw rope is connected to the moving object, and the linear movement of the draw rope is aligned with the movement axis of the moving object. When the object moves, the displacement sensor outputs a digital signal. The rope displacement sensor can be used to measure the movement interval, movement speed, sensing position or limit sensor, and a calculator that records the number of mechanical movements. Application areas include testing, instrument calculation, transportation, robotics, hydraulic devices, pneumatic devices, industrial automation, motion control, etc. The measurement interval ranges from tens of millimeters to tens of meters, and it is widely used.
2. Sensor factors
According to JJF 1305-2011 "Line Displacement Sensor Calibration Specification", the basic error of a typical rope displacement sensor is generally between ±0.05% and ±0.5%. Pay attention to parameters such as linearity, repeatability, and return error when choosing. And pay attention to the difference between the percentage of the full scale and the percentage of the measured value in terms of linearity. If the error cannot be confirmed, it should be calibrated with an instrument such as a length measuring machine.
In the choice of resolution, it is recommended to select the resolution according to the needs, and it is enough. The theory of analog signal sensor is unlimited, but in actual use, it is affected by the accuracy, robustness and repeatability of the instrument. Some digital signal sensors claim to have a resolution of 0.001mm, but a sensor with a high resolution may not match the display instrument with a low reading speed, causing data loss.
The rope displacement sensor is divided into analog signal and digital signal according to the output signal. The analog signal is divided into voltage output and current output. The analog signal should pay attention to the issue of transmission interval. If the transmission interval is too large, the accuracy will be severely affected. The analog signal will have problems such as signal interference, stability, drift and load size.
The digital output is divided into square wave pulse output and Gray code signal. Digital signals generally do not have problems such as signal interference, drift, etc., and digital output sensors should generally choose incremental types, otherwise the direction cannot be distinguished.
On the internal structure of the sensor, pay attention to whether the cable is laminated. The non-laminated structure can ensure the same stroke and high repeatability in each circle.
3. Factors of instrument
When choosing the secondary instrument, pay attention to the problem of matching the sensor. First of all, the meter should meet the requirements of overall accuracy in the display digits and the digits of the ratio setting. Some display instruments or the use of single-chip microcomputers have fewer ratio digits, which affects the accuracy of adjustment. For example, the maximum allowable error of the sensor is ±0.5%, and the ratio digits can only be adjusted by 1%, so that the accuracy of the impact can only reach ±0.1%.
Secondly, pay attention to the problem of the reading speed of the counting device. Digital output type sensors may lose data severely when the movement speed is too fast. The reason for this phenomenon is the slow reading of data from the display instrument or counting device. Some instruments are designed for counters. This kind of instrument cannot be used on pull-wire displacement sensors. In addition, the factor of resolution is also very important. If it is required to be used in faster applications such as movement speed> 0.5m/s, it is recommended to choose a sensor with a low resolution or a display instrument with a high reading speed. Generally, for the sensor with 0.01mm/pulse, the reading speed of the meter is recommended to be ≥30Hz.
4. The influence of installation location and environmental factors
The installation should conform to the Abbe principle as far as possible to reduce system errors. Abbe's principle means that in length measurement, the scale length (scale line) should be placed on the extension line of the measured length (measured line). In addition, try to make the sensor's measuring direction and movement direction level. If the measurement interval is longer and the installation is horizontal, you should pay attention to the influence of the weight of the cable.
On environmental issues, attention should be paid because the material expansion coefficient is different from the impact on accuracy. If it is applied outdoors and the measurement interval is long, the influence of crosswind should also be considered.
However, the influence of the above factors on the displacement sensor is relatively low. If the measuring direction of the sensor is not parallel to the direction of movement during installation, the direction perpendicular to the measuring length of 1m is offset by 10mm (observable by the naked eye). But the error has only increased by 0.05mm, which is 0.005%, which is very small compared to the 4-0.05%～4-0.5% error of the ordinary rope displacement sensor.
In addition, the corresponding protection level should be selected according to the use environment, including sensors, meters, converters and connecting wires. If there is no corresponding protection level in harsh environments, the use cannot be guaranteed, let alone accuracy. Choosing stainless steel, corrosion-resistant cable is guaranteed for long-term use.
Conclusion
Generally speaking, sensor factors and instrument factors have a greater impact on the overall accuracy and should be considered. Readers can refer to the above points according to their own situation, choose sensors and display instruments scientifically and fairly, try to eliminate all kinds of factors that affect errors, and improve the overall accuracy.