Method for supplying a machine tool with cooling lubricant

Abstract

Method for supplying a machine tool (TM) with cooling lubricant comprising the following steps: a. Selection of a specific flow value according to the drill used by a machine control unit (MC) b. Measurement of flow in a lubricant circuit using a fast flow sensor (FM2) with a response time of less than 10 msec c. Generating a control signal for an electromagnetic flow controller to adjust the flow in the lubricant circuit (LC) according to the selected flow value d. Regulating the flow of the cooling lubricant with a movable part of the flow actuator to the selected flow value during the drilling process e. Generating a dithering signal superimposed on the control signal for the flow actuator to generate a small vibration of the moving part to increase the linearity of the flow actuator and improve the response and hysteresis f. direct detection of the vibrations of the moving part by measuring the flow changes with the fast flow sensor (FM2) g. Adjustment of the dithering signal to improve the control quality of the flow actuator according to the measured flow value.

Description

[0001] The invention relates to the field of flow control, whereby protection is sought for a method for supplying a machine tool with cooling lubricant.

[0002] In the prior art, US 7284570 B1 describes an electrically operated valve assembly. It applies to valves on seagoing vessels where a time-consuming inspection is required to correct a valve malfunction.

[0003] In the document, the opening and closing of the valve are measured by a magnetic position sensor for positioning, opening and closing of the valve stem, which relies on the signal sent to the drive coil.

[0004] US 8165762 B2 describes a valve arrangement for operating a transmission system in a vehicle.

[0005] This document mentions that “the dynamics and precision of a machine operated by an electro-hydraulic concept are generally determined by the precision and flow control of such actuators, also by trying to optimize the dynamics and precision of the flow control”.

[0006] The friction that is usually unavoidable when using pilot valves is often the main reason for a limited service life of the system and counteracts the desired system optimization of the drives.

[0007] As an example, a diagram of a proportional pressure-reducing valve in a figure, when describing the relationship between the output pressure at a control output of a valve under the condition of constant pressure input at the input and return paths and the current in the coil of an electromagnetic actuator, shows that there is an offset between the rising, increasing current edge and the falling, falling current edge. This offset is caused by mechanical friction in the valve and is commonly known in the art as hysteresis. The width of the hysteresis of an actuator acting as a valve limits the ability to reproduce the actuator's position and also restricts the functionality of the system components connected to the actuator that follow.

[0008] Pressure-controlled coolant flow is standard in the machine tool industry. A pump delivers a fluid volume based on a pressure set at the pump. Some lubrication circuits, such as those in WO 2012 / 119723 A2, employ volume control via a speed-controlled, constant-volume pump. The pump delivers a tool-specific amount of fluid to at least one machining tool. It is claimed that setting a volume via the pump ensures a constant flow—regardless of the coolant back pressure at the tool.

[0009] To overcome the fact that dither signal modulation and its low-frequency oscillations cause increased wear in an electrohydraulic valve system and thus shorten the service life of a valve, it is proposed to generate an amplitude and / or a pulse duration of the pulse-width modulated signal by superimposing the pulse width modulation with the dither modulation. While maintaining the pressure level, the pulse duration changes periodically, and the longitudinally movable actuator armature and the physically connected valve stem are exposed to the modulation and accordingly adopt the oscillation. The dither modulation is switched on or off according to existing, predefined operating conditions. A pressure reducing valve is described, but the same principles also apply to flow control valves.When a dithering signal is used with a flow controller, the amplitude of the dithering signal correlates with the uncertainty of the flow value. A large amplitude indicates high uncertainty, while a small amplitude indicates low uncertainty. If the flow is low and the dithering signal is significant, thus resulting in high oscillations, then the uncertainty of the flow value is also high.

[0010] The invention: The purpose of this invention is to provide a method for supplying lubricant to a machine tool with a controlled flow rate, where the accuracy of the flow control is based on verifying the actual effect of dithering on the valve by measuring the flow rate. In flow controllers such as electromagnetic valves, a sticking effect occurs, which leads to a delayed valve response and can generate valve overshoot, affecting the flow rate. Superimposing a dithering signal helps the valve avoid this sticking effect. Since dithering is a continuous slight change in the valve opening, it affects the flow of lubricant.By measuring the precise flow variation with a high-precision mechatronic flow meter (ifm electronic gmbh), the actual effect of dithering on the valve can be confirmed and the valve piston can be ensured to prevent sticking. The dither value can then be adjusted according to the measures taken to ensure a consistent laminar flow and limit negative effects on flow caused by abrupt flow control.

[0011] Furthermore, its industrial application is simple and does not require complex modifications to the hydraulic system, as it is implemented directly on the valve. This system can be used with any type of mechatronic valve with linear and non-linear properties. Description:

[0012] The hydraulic system for supplying lubricant to machine tools consists of a pump to generate a lubricant flow, an electromagnetic control valve for precise flow regulation, a fast-response flow meter (response time in the millisecond range compared to flow meters with a response time in the seconds range), a PLC that receives the flow measurements and controls the signals superimposed on the valve, namely a PWM signal responsible for opening and closing the valve and a dithering signal responsible for the slight vibration of the valve piston.

[0013] The entire system is controlled by a machine control unit.

[0014] Typically, a valve receives a PWM signal from a PLC. Theoretically, this should control the displacement of the valve spool. In practice, the spool does not always respond as expected, and consequently, the flow is not as expected either. This justifies the superposition of a dithering signal and its advantages. Dithering is used to increase the linearity of the flow controller and improve the valve's response and hysteresis. In the present invention, dithering, in combination with the very precise flow meter, serves to indicate the valve's proper operation. The slight fluctuations in flow caused by the slight vibrations of the dithering are used to verify that the signals received by the valve are actually causing spool operation, preventing it from sticking.The accuracy of the flow measurements allows for precise and consistent flow control, as the absence of the sticking effect prevents delayed piston response and flow overshoot, which could lead to flow instability. Thus, the invention contributes to keeping the flow laminar and the control linear. The advantages and practicality of laminar flow lie, for example, in energy or economic savings, among other things.

[0015] Fig.Figure 1 shows an example of an assembly for supplying a machine tool TM with the cooling lubricant LC. In this example, a pump P is driven by a motor M, which generates a specific flow and pressure. The pump is protected by a safety valve. The flow in the lubricant circuit LC is regulated by a bypass valve V, which receives a PWM signal from the PLC FC, onto which a dithering signal is superimposed. These signals are generated based on the flow data from the bypass flowmeter FM2 and / or the flowmeter FM, which are mechatronic flowmeters. The flowmeter FM is actually a group of flowmeters with different measuring ranges. It enables precise flow measurement depending on on the flow range (one for the smallest flow, i.e., the range of milliliters per minute, and one for a larger flow, i.e., the range of liters per minute). The FM2 bypass flowmeter is a precise, fast flowmeter (response time in the millisecond range compared to flowmeters with a response time in the seconds range). The fast response allows for quick and smooth flow adjustment.

[0016] The applied dithering signal is a sine wave signal. This type of signal smooths the valve pin's response to the applied PWM signal, preventing abrupt changes in flow, further enhancing the beneficial effect of using dithering.

[0017] The process for supplying a machine tool TM with cooling lubricant comprises the following steps: In a first step, the MC machine control system selects a specific flow rate based on the drill being used. This ensures that the lubricant quantity is precisely adjusted to the machine tool's task. In a second step, the flow in the lubricant circuit is measured using one or more flow meters. The preferred sensor type is a mechatronic flow sensor. The flow sensor is a fast-response flow sensor (< 10 milliseconds), allowing the entire system to respond more quickly to optimize flow. In a further step, the flow meter generates a control signal for an electromagnetically controlled flow controller, which adjusts the flow in the lubricant circuit LC according to the selected flow value. The flow controller, which is preferably an electromagnetic proportional valve V, is located in a bypass. Control is achieved by a PID controller FC with a response time of less than 50 ms, in particular less than 10 ms. The valve is controlled by FC, which also adjusts the pump motor M and receives feedback from the flow meters FM and FM2 as well as from the machine control system MC.

[0018] The cooling lubricant flow is regulated by a movable part of the flow regulator V to the selected flow value during the drilling process. The flow is adjusted to be optimal for the selected tool and working parameters. Tools can vary depending on the job they are intended for and the tool size / length / width. The flow can range from hundreds of milliliters per minute to several tens of liters per minute, with a low pump pressure of, for example, 20 bar, since the pressures used in this type of system generally range from 40 to 80 bar. The parameters used in the described method are preferred for reasons of savings, e.g., less energy consumption, reduced tool wear, etc.

[0019] In a further step, a dithering signal is superimposed on the control signal for the flow controller to create a slight vibration of the moving part in order to increase the linearity of the flow controller and improve response and hysteresis. The dithering prevents the stick effect of the flow controller and allows smooth transitions when a different flow is set by the PLC. The use of this dithering is also beneficial for the flow, as it helps keep the valve reactive, which prevents the stick effect on the moving part, also called the piston. This consequently prevents a delayed response from the moving part and overshoot, which in a classical system would result from the delay, where the FC would receive feedback from the system that the moving part has not moved.Consequently, the BZ would force the reaction of the moving part, and this repeated process would cause flow overshoot, which can be fatal for the entire system, as it can trigger too high or too low flow and pressure for the tools and the lubricant circuit, which can also cause system instability and stress the pump, etc.

[0020] The good operation of the system and the dithering lead to vibration of the moving part. The vibration is directly measured by the

[0021] Flow changes are detected using a very precise and fast flow meter with a response time of milliseconds. Every change and every error is also detectable in the flow changes. For example, an unadjusted dithering amplitude can lead to significant flow fluctuations, which can be observed in the flow measurement.

[0022] This type of feedback can be interpreted by the flow controller as an external disturbance that it tries to overcome, resulting in unnecessary additional work for the controller.

[0023] In a further step, the dithering is adjusted according to the feedback signal of the measured flow. Observing the influence of the dithering signal on the flow is only possible with a very fast-responding flow meter, as described previously. The dithering is adjusted to improve the control work performed by the valve. This can ensure the valve's proper operation, eliminate any negative consequences of the sticking effect, ensure a stable system, and also maintain laminar flow. The preferred dithering in this method is a sinusoidal wave signal, with the dithering signal in the range of 1 Hz to 1 kHz. The dithering method is not limited to machine tools.

Claims

[1] Method for supplying a machine tool (TM) with cooling lubricant comprising the following steps: a. Selection of a specific flow value according to the drill used by a machine control unit (MC) b. Measurement of flow in a lubricant circuit using a fast flow sensor (FM2) with a response time of less than 10 msec c. Generating a control signal for an electromagnetic flow controller to adjust the flow in the lubricant circuit (LC) according to the selected flow value d. Regulating the flow of the cooling lubricant with a movable part of the flow actuator to the selected flow value during the drilling process e. Generating a dithering signal superimposed on the control signal for the flow actuator to generate a small vibration of the moving part to increase the linearity of the flow actuator and improve the response and hysteresis f. direct detection of the vibrations of the moving part by measuring the flow changes with the fast flow sensor (FM2) g. Adjustment of the dithering signal to improve the control quality of the flow actuator according to the measured flow value. [2] Method according to claim 1, wherein the flow sensor (FM2) measuring the flow in the lubricant circuit (LC) is a mechatronic flow meter. [3] Method according to claim 1 or 2, wherein the control is achieved by a PID control with a response time of less than 50 msec, in particular less than 10 msec. [4] Method according to one of claims 1 to 3, wherein the flow control element is an electromagnetic proportional valve (V). [5] Method according to one of claims 1 to 4, wherein the dithering prevents the sticking effect of the flow actuator. [6] Method according to one of claims 1 to 5, wherein the dithering is adapted to the measured flow value in order to obtain a laminar flow. [7] A method according to any one of claims 1 to 6, wherein the dithering is a sine wave type signal. [8] A method according to any one of claims 1 to 7, wherein the dithering signal is in the range of 1 Hz to 1 kHz.

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