Application of proximity switch in motor speed measurement based on single chip microcomputer control

1 Introduction

In the real-time control of industrial processes, the slip monitoring of couplings and the detection of motor speed are very important, which have a vital impact on the safety protection of the motor and even the smooth operation of the entire device. Therefore, a high-resolution, fast and accurate monitoring system is essential. With the continuous innovation of single-chip microcomputer and sensor technology, especially the emergence of cost-effective single-chip microcomputer chips, speed measurement and control generally adopt digital and intelligent systems with single-chip microcomputers and high-performance sensors as the core. This article introduces a high-precision intelligent instrument and proximity switch sensor to measure the motor speed and monitor the same speed of the friction clutch.

2 Process Introduction

The friction clutch is a clutch that uses friction to transmit torque. When engaged, the master and slave friction parts are pressed together under a certain pressure. When the driving shaft rotates, a sufficiently large friction force is generated between the joint surfaces to drive the driven shaft to rotate. When separated, the pressing force disappears, the joint surfaces separate, the friction force disappears, and the driven shaft does not move. The operation of the friction clutch can be divided into three stages: engagement, normal operation, and separation. In the engagement and separation stages, the speed of the driven friction part is lower than that of the active friction part, which will cause slippage, resulting in heating and wear of the working surface. In the normal rotation process, the friction surfaces are required to be in close contact, otherwise the friction surfaces will be damaged and even cause production accidents.

Now, we will take the polypropylene granulator in the refinery as an example to introduce the application of proximity switches in sliding monitoring. A friction clutch is used between the main motor of polypropylene extrusion granulation and the screw compressor coupling. During the start-up process, the instrument controls the solenoid valve to open, the power wind drives the friction clutch to engage, and the main motor (master) drives the screw coupling part (slave) to rotate. The screw compressor coupling directly controls the screw speed after speed change. Under normal circumstances, the motor speed is 1000prm/m and the screw speed is 200prm/m. In the normal production process, it is very important that the main motor and the screw compressor coupling rotate at the same speed. Therefore, high-precision instruments are installed on the main motor of the friction clutch and the screw compressor coupling part to monitor the same speed state and the motor speed online. The instrument displays the speed of the main motor and the screw coupling part, and introduces the detection control signal into the PLC. Once sliding occurs, the granulator will stop immediately to protect the equipment safety. The instrument installation is shown in Figure 1.

3. Introduction to measurement principle

3.1 Introduction to the measurement principle of proximity switches

The proximity switch uses an inductive proximity switch. The inductive proximity switch is a position sensor with a switch output. It consists of an LC high-frequency oscillator and an amplification processing circuit. When a metal object approaches the oscillating induction head that can generate an electromagnetic field, an eddy current is generated inside the object. This eddy current reacts to the proximity switch, causing the proximity switch's oscillation ability to decay and the parameters of the internal circuit to change. This is used to identify whether a metal object is approaching, and then control the switch on or off. The inductive proximity switch consists of three parts: an oscillator, a switching circuit, and an amplification output circuit. The oscillator generates an alternating magnetic field. When a metal target approaches this magnetic field and reaches the sensing distance, an eddy current is generated inside the metal target, causing the oscillation to decay and even stop. The changes in the oscillation and stop of the oscillator are processed by the post-amplification circuit and converted into a switch signal, triggering the drive control device, thereby achieving the purpose of non-contact detection. The detection principle is shown in Figure 2. [page]

3.2 Speed ​​measurement

There are two detection metal blocks on the front and rear shafts of the friction clutch. The detection metal blocks also rotate synchronously with the shaft. A proximity switch is installed on each side of the shaft. When the proximity switch is facing the detection metal block, the output side of the proximity switch sends a pulse flip signal. The motor speed is inversely proportional to the pulse period. The longer the pulse square wave signal period, the slower the speed. The period and the motor speed have the following relationship:

Where: n is the motor speed; p is the number of pulses per motor revolution; t is the output square wave signal period. The speed of the DC motor can be calculated according to the formula. For a speed detection system, the key is to make the accuracy and resolution of the speed measurement results in the entire speed range the best and meet the requirements of fast dynamic response. The pulse signals on both sides of the main motor and the screw compressor shaft are input into the instrument equipment at the same time. While displaying the respective speeds, the instrument compares the pulse difference, and according to the needs of safe production and protection of equipment, the equipment interlock protection signal is input into the plc. If the difference in the number of pulses on the two channels is relatively large (the granulator unit is set to have a pulse difference of more than 7 per second), the control instrument will be considered as slipping, and the instrument will output a control signal to the plc to control the granulator to stop, and send an audible and visual alarm on the control panel.

3.3 Instrument selection

The granulation sliding monitoring instrument uses the German ifm monitor fs-2 control instrument, with a counting frequency of up to 400hz (minimum pulse length> 1.2ms), and a very fast scanning speed, which has great advantages compared with the mainstream large-scale controllers of industrial control. For example, the scanning speed of plc is generally 650ms/time, and the fastest is 50ms/time, and the scanning speed of dcs is generally 1 second/time, and the fastest is 200ms/time. Therefore, plc and dcs cannot be used to measure the speed of high-speed rotating motors. The control instrument with a single-chip microcomputer as the core is very suitable for measuring the speed of high-speed running motors. ifm inductive proximity switches have high reliability, strong anti-interference ability and fast measurement speed. Compared with mechanical switches, inductive sensors have perfect prerequisites: contactless and wear-free working mode, high switching frequency and switching accuracy. In addition, they are resistant to shock, dust and moisture. Inductive sensors can detect all metals without contact.

4 Slide detection process

After the unit is turned on, the instrument solenoid valve is energized, and the instrument power wind pushes the friction clutch to engage. The motor starts running and drives the screw compressor to run. The sliding detection instrument timely monitors the synchronous rotation of the main motor and the screw compressor shaft, and inputs the control node signal into the plc as a shutdown condition. The number of revolutions of the granulation main motor is generally 1000prm/m, that is, 33 pulses/second (two detection blocks on one axis). The instrument always detects the number of revolutions on both sides of the clutch. When the difference in the pulse signals on both sides is greater than 7 per second, the control instrument node is disconnected, the signal input to the plc disappears, and the granulation unit is chained and stopped. During the detection process, the instrument can simultaneously display the number of revolutions on the motor side and the screw compressor shaft side. And it can be refreshed in time according to the scanning speed of the instrument.

5 Conclusion

The online detection system composed of proximity switches and high-precision detection instruments has fast frequency response and strong anti-interference ability. It is widely used in motor speed measurement and sliding monitoring. It can accurately monitor the running status of the motor and play a role in protecting the equipment.