1. Mechanism of commutation impact

Because in its hydraulic system, when the hydraulic transmission surface grinder reverses. The valve port of the reversing valve is closed instantaneously, and the oil circuit is suddenly disconnected, so that the oil in the return oil chamber cannot be discharged.

The bigger m and v are, you can see that. The greater the kinetic energy, the greater the reversing impact. For the surface grinding machine with large inertia and high speed operation, the reversing impact is huge, which not only affects the machining accuracy of the machine tool, but also hampers the normal operation and service life.

People all want the machine tool to achieve the ideal commutation. The so-called ideal commutation means that under any working condition, the machine tool speed can be smoothly reduced according to an ideal curve without mutation, and the valve is closed at the moment, the speed is just reduced to zero, that is, all the kinetic energy is converted into heat energy is lost. The ideal commutation process is shockless

2, commonly used hydraulic transmission reversing method analysis

The following is a simple analysis and comparison. At present, there are many hydraulic transmission reversing methods applied to surface grinding machines.

2.1 The reversing method of stroke reversing valve is adopted

The reversing valve core is connected with a pull rod to use the stroke reversing valve to reverse. Use the travel stop on the work table to push the pull rod to realize automatic reversing. When the table is moving slowly, when the reversing valve reaches the middle position, whether the left and right chambers of the hydraulic cylinder are either through the pressure oil, or through the oil, or are closed, at this time, the two chambers of the hydraulic cylinder are not driven by liquid pressure, which will stop the movement of the table, so the reversing valve can not reach the other end, and there is a so-called "dead point"; In addition, when the workbench moves at a high speed, the stop block pushes the pull rod to make the direction of the reversing valve change very fast, and the pressure of one chamber of the hydraulic cylinder suddenly decreases from the working pressure p to 0, and the other chamber suddenly rises from 0 to p, which results in a great reversing impact. At present, this kind of system is used more in small grinding machines.

2.2 The reversing method of electromagnetic reversing valve is adopted

The reversing signal is sent by the stroke stop to push the stroke switch, and the stroke reversing valve is changed to the reversing method of the electromagnetic reversing valve. Make the electromagnet action to push the slide valve to reverse, can prevent the "dead point", but it is a switch type hydraulic valve, according to the instruction to open or close instantaneously, that is, the instantaneous switch or cut off the oil channel, such a hydraulic reversing system will have a great impact in the reversal.

2.3 The reversing method of electro-hydraulic reversing valve is adopted

Then the control oil drives the main valve to change direction. The pilot valve is not changed forward, and the electro-hydraulic reversing valve is replaced by the electromagnetic reversing valve, which constitutes a new reversing method. The electro-hydraulic directional valve is composed of the pilot valve electromagnetic slide valve and the main valve hydraulic slide valve. The system controls the oil circuit through the pilot valve reversing. The direction of the oil flow of the control oil circuit does not change, the reversing valve always remains at the original end, the direction of the main oil circuit does not change, and the workbench can always continue to move forward. Once the direction of the control oil circuit is switched, the main valve spool moves to another working position according to the preset speed, the direction of the main oil circuit is changed, and the workbench is reversed to prevent the "dead point" of the reversal.

In this way, the kinetic energy of the large inertia table can be converted into heat energy and consumed through the throttling effect, and the control oil port size of the electro-hydraulic reversing valve main valve is adjustable, that is, the reversing time △t can be extended. It can effectively reduce the reversing impact, so this reversing method has been dominant for a long time. However, its reversing parameters can only be adjusted in advance, and can not be changed according to the change of working conditions, which is impossible to achieve ideal reversing for the system that the working conditions change at any time.

3. Electro-hydraulic proportional commutation system

When the working condition changes, the above commutation methods are active control commutation. It can not adapt to the requirements of commutation in new working conditions, and certain commutation impact will occur. Therefore, the hydraulic pressure reversing system with electro-hydraulic servo valve and closed-loop control came into being. This system is a kind of intelligent control reversing system, which realizes the automatic control of its reversing mechanism by a microcomputer. The scheme not only makes full use of the advantages of electro-hydraulic servo valve, but also applies a variety of mature microcomputer control technology to the system. The commutation precision of this hydraulic commutation system is very high, and the impact is small, which makes the commutation smoothness of large high-speed surface grinding machine greatly improved.

Therefore, this commutation system is generally only suitable for systems with higher precision requirements. However, the electro-hydraulic servo valve has complex structure, high cost and poor anti-pollution ability.

The electro-hydraulic proportional valve is introduced into the hydraulic reversing system of a large high speed surface grinder. Electro-hydraulic proportional valve is a hydraulic component between a switch type hydraulic valve and a servo valve. In addition to the control accuracy and response speed is not as good as the servo valve, so. Other aspects of the performance and control level is comparable to the servo valve, its dynamic and static performance is enough to meet the requirements of most industrial applications, and its pollution resistance is strong and low cost.

Similar to the working principle of the hydraulic reversing system using electro-hydraulic servo valve, the hydraulic reversing system using electro-hydraulic proportional valve. Through the microcomputer input control of the opening size of the proportional valve spool, to achieve the movement of the surface grinding machine table or speed control, and the valve displacement of this system is adjusted by the control curve of the microcomputer input, so it can achieve timely adjustment of the proportional valve spool opening size, can adapt to a variety of different working conditions. When the surface grinder uses the hydraulic reversing system for reversing, the ideal smooth and impact-free reversing process can be realized with the ideal control curve. In this process, the kinetic energy of the table is also converted into heat energy through the throttling effect and consumed. For large high-speed surface grinding machines, the reversing system has high use value.

4. Research on control strategy

The movement law of the main spool of the reversing valve has a decisive role in the reversing impact and the reversing performance. This spool movement rule is called the control curve. For the design of electro-hydraulic proportional commutation system, it is important to select the control curve. The study of control strategy is to find an ideal control curve to make the electro-hydraulic proportional commutation system achieve smooth and impact-free commutation. Under normal conditions, the ideal control curve is a smooth high-order curve, and the acceleration at the initial moment of commutation gradually increases from 0, which eliminates the impact caused by sudden acceleration at the beginning of the radical deceleration curve. When the speed is fast approaching 0, the acceleration decreases, and the smooth transition to 0 is a curve that well overcomes the limitations of the radical deceleration curve, and also solves the contradiction between improving the working speed and improving the commutation accuracy. No matter how big the muzzle velocity is, as long as the hydraulic cylinder is controlled according to the ideal curve, zero impact and zero error can be theoretically achieved at the end of the commutation.