Pull wire displacement sensor, also known as pull rope Displacement sensor Is to use the circumference of the winding wheel to measure the length distance of the object. To put it bluntly, the guyed displacement sensor is like a tape measure. By changing the length of the pulling rope, it can measure the corresponding displacement and convert it into the corresponding precision pulse output, that is, the conversion from displacement to digital quantity. Guyed displacement sensor is used in a wide range of engineering fields. There are cable products in all kinds of occasions that need to detect linear displacement, speed and direction, such as linear guide system, hydraulic cylinder system, etc.
Working principle of rope displacement sensor
Through a series of mechanical structures, the angle change of the inner single or multi turn potentiometer is converted into the linear displacement change of the external connecting end, and the change is output in the form of various signals as analog or digital quantity. Because its output end is soft steel wire, so it can be transited through pulley, turntable and other mechanisms, and can be installed in the place not along the movement direction of the measured object. Therefore, in the installation and use, the pull rope sensor has the flexibility and adaptability that cannot be compared with the electronic ruler.
The function and application range of pulling rope displacement sensor
Converting mechanical motion into electrical signals that can be measured, recorded, or transmitted. The sensor can be an encoder, a rotary potentiometer, a synchronizer or a resolver. The common parameters include measuring stroke, output signal form, resolution, working voltage, etc. Signal output mode usually matches the output mode of encoder, such as common digital pulse output.
Application scope: universal testing machine position control, guide rail position control, printing machine sensor, storage Device sensor Industrial automation position and range detection system, such as gate opening control, cylinder position control, cylinder position control, etc.
Signal output mode of pulling rope displacement sensor
For digital signal output and analog signal output, incremental rotary encoder and absolute value encoder can be selected for digital output type. The output signals are square wave ABZ signal, sine cosine signal, CANopen signal, free RS485 signal, MODBUS signal, PROFIBUS signal or gray code / binary system signal. The measurement stroke is long (100-15000mm), high precision (～% FS), and protection grade is IP65. The rope is made of 316 stainless steel, which can be used in harsh environment. The optional output modes are: resistance type, voltage type, current type, incremental pulse type and absolute pulse type.
The analog output type can choose precision potentiometer, hall encoder, absolute value encoder, etc. the output signal can be 4-20 mA, 0-5 V, 0-10 V and resistance signal, etc., the maximum stroke can reach 12500 mm, the maximum use environment can reach the protection grade of IP65, and the wide temperature environment from - 45 ℃ to + 105 ℃.
An example of design scheme of Guyed displacement sensor
The traditional pull wire displacement sensor adopts potentiometer displacement sensor, which converts mechanical displacement into resistance or voltage output with linear or arbitrary function relationship through potentiometer element. Ordinary linear potentiometer and circular potentiometer can be used as linear displacement sensor and angular displacement sensor respectively. However, the potentiometer designed for the purpose of measuring displacement requires a definite relationship between the change of displacement and the change of resistance.
The movable brush of the potentiometer displacement sensor is connected with the measured object, and the displacement of the object causes the resistance change of the moving end of the potentiometer. The change of resistance value reflects the value of displacement, and the increase or decrease of resistance value indicates the direction of displacement. Usually, the potentiometer is connected with a power supply voltage to convert the resistance change into voltage output.
Because the resistance of the traditional pull wire displacement sensor changes step by step when the brush moves, its output characteristic is also ladder shaped. If this kind of displacement sensor is used as displacement feedback element in servo system, the excessive step voltage will cause system oscillation. The resistance in each turn should be reduced as much as possible. At the same time, another main disadvantage of potentiometer sensor is easy to wear, poor resolution, low resistance, poor high frequency characteristics, which leads to the decline of measurement accuracy. It has the advantages of simple structure, large output signal, convenient use and low price.
The pull wire displacement sensor based on the magnetic angle technology takes the magnetic field as the transmission carrier, converts the displacement into the magnetic field angle displacement, and returns the displacement signal to the application system through the communication interface.
1. Overall design scheme
The function of the pull wire displacement sensor based on the magnetic sensitive angle technology is to change the mechanical displacement of the stay wire into the electrical signal that can be measured, recorded or transmitted. It is mainly composed of automatic recovery spring, wheel hub, magnet and data processing unit. The structure is shown in Fig. 1.
It can be seen from Figure 1 that the pull wire displacement sensor based on the magnetic sensitive angle technology is mainly composed of six parts, which changes the shortcomings of the traditional pull wire displacement sensor, such as contact type, easy wear and poor high frequency characteristics. The pull wire displacement sensor based on the magnetic angle technology takes the magnetic field as the medium to convert the mechanical displacement change into the magnetic field angle change, so as to solve the traditional pull wire displacement on the one hand The contact mode of the sensor, on the other hand, reduces the wear and improves the high frequency characteristics of the system, so as to ensure the accuracy of displacement detection. The data processing arithmetic unit is used to calculate the received magnetic sensitive angle signal into the displacement signal of stay wire through mathematical model. The communication interface communicates with the equipment of the application system through the communication interface, receives the command from the application system equipment, and feeds back the collected displacement signal to the application system. The accuracy, stability and reliability of data acquisition are improved, and the application threshold of displacement sensor is reduced.
The function of each component is described as follows:
(1) The wire rope of the stay wire is wound on the wheel hub, and the hub is connected with a magnet. When the cable is displaced, it drives the wheel hub to rotate. The rotation of the hub causes the magnet connected with the hub shaft to rotate, so that the magnetic field of the magnet produces a change angle. When the cable movement occurs, the automatic return spring ensures that the cable has a certain tension, and ensures the proportional relationship between the displacement of the stay wire and the magnetic sensitive angle.
(2) The magnetic angle sensor and the magnet are installed on the same central axis to sense the change of the magnet angle. A microprocessor is selected. The processor reads the magnetic angle information and calculates the magnetic angle as the displacement of the pull wire by establishing a mathematical model.
(3) Communication interface: the microprocessor receives the command from the application system through the communication interface and returns the displacement information to the application system through the communication interface.
2. Hardware interface circuit design
The data processing unit consists of magnetic angle sensor, microprocessor unit, communication interface and output module. The specific function frame is shown in Fig. 2.
Through the analysis of Figure 2, MLX90316 is selected as the magnetic angle sensor, which converts the magnetic field rotation angle caused by the displacement of stay wire into the magnetic sensitive angle. The microprocessor unit selects 32-bit embedded arm to process the received magnetic angle data and complete the receiving of the magnetic angle data. Since the received angle is the angle of magnetic field conversion, the magnetic angle is converted into the displacement of the stay wire by establishing a mathematical model and combining with the diameter of the wheel hub and other factors. Therefore, lpc2136 is selected as the data processing unit in order to quickly realize the data receiving and model building. The input and output control module is responsible for the processing of various external interfaces, such as receiving commands from the application system through the communication interface, and returning the collected displacement results to the application system, so that the microprocessor unit can execute the commands of the application system and send the acquisition results to the application equipment safely and reliably through the interface, mainly including 1-way RS 485 and 4-20 Ma current output.
2.1 magnetic angle receiving interface
MLX90316 is a kind of linear Hall chip, which adopts planar Hall sensing technology. The chip can be used to measure the magnetic flux density coplanar with the chip surface, and can obtain the rotation position value from 0 to 360 ° and output the linear absolute position signal with high accuracy through various modes. It has the advantages of low cost and easy installation.
The front end of MLX90316 chip adopts triaxis hall technology sensor 。 The two orthogonal analog signals obtained by Hall sensor are amplified and then sent to the chip microprocessor (DSP) through 14 bit differential A / D converter, and then processed by 16 bit DSP. The digital signals are output in three ways. The output of MLX90316 has 12 bit angle resolution and 10 bit angle accuracy, and can avoid the influence of peripheral temperature change on the output accuracy to a certain extent. MLX90316 has three kinds of output: 12 bit D / a conversion to analog output; 100-1000 Hz frequency PWM output; and serial communication protocol output (SPI) in digital mode.
Since the output signal of serial communication comes directly from the internal DSP output of MLX90316, SPI output mode is more stable, error is smaller, and has higher anti-interference ability. In this design, SPI interface is selected. The specific hardware interface circuit is shown in Figure 3. The three lines of xc2136 are connected with SPL 903 of arm. SPI (serial protocol interface) is a synchronous serial peripheral interface. This is a 4-wire serial interface protocol, including master and slave modes. The four signal lines are clock line (SCK), data input line (MISO), data output line (MoSi) and slave device enable line (SS).
Stay wire Displacement sensor The wiring diagram is as follows:
The above is about the wire displacement sensor principle, design scheme, wiring knowledge introduction, Xianji network small editor friendly reminder: displacement sensor installation boss positioning, must use screws, nuts or pressure plate fixed on the metal plate. When installing the sensor, it is strictly forbidden to turn or drill the shaft and shell to avoid external impact force and pressure. The axial and radial direction of the shaft are not allowed to be impacted and pressed (the static pressure should be less than 300N). Do not loosen the screw on the sensor and turn the position of the fastening ring. The external wiring of displacement sensor should be welded at the waist groove of the leading end, and should not be welded on the top of the leading end as far as possible. The electric ferrochrome of no more than 45W should be used in welding, and the welding time should be less than 5 seconds. The wire shall not be pulled during welding and cooling.