METHOD OF CONTROLLING TEMPERATURE OF MACHINE TOOL AND APPARATUS FOR PRACTICING SAME

Abstract: This invention relates to an apparatus for controlling the cooling of a heated unit in a machine tool, and a method of controlling the temperature of a machine tool by bringing a heating medium liquid into contact with a constituent element (41) constituting the machine tool and generating heat, so as to indirectly control the temperature of the constituent element (41) to a predetermined level. A heating medium liquid is brought into contact with the constituent element (41) to carry out heat exchanging, and the heating medium liquid is then cooled with a cooling medium in a heat exchanger (10), and the level of an increase of the sensible heat temperature of the cooled heating medium liquid is measured with a sensor (S2). The cooling temperature for the heating medium liquid is controlled by a PID temperature controller (3) in accordance with the level of an increase of this sensible heat.

Description: The present invention relates to a temperature control apparatus for a ma-chine tool or an industrial machine according to the preamble of claim 1. Background Art The body of a machine tool is deformed by the environmental temperature, heat from a heat generating portion, etc. The deformation of the machine body affects the machining accuracy and therefore it has heretofore been general practice to control the temperature of each part of the machine body to a constant temperature. Although a variety of temperature control methods for this purpose have been proposed, an indirect control method wherein a temperature-controlled fluid is arranged to flow through a heat generating portion of a machine tool to thereby cool the heat genarating portion is generally carried out. The set temperature for the fluid is controlled so as to follow up the room temperature or the one of the constituent elements that has the largest time constant. This method minimizes the thermal deformation and reduces machining errors. This is a known art, and these methods are called the room temperature follow-up control or the machine body temperature follow-up control. Hitherto, two-position control has been employed as a control action used when the indirect control of a machine tool by the fluid temperature is effected by the room temperature follow-up control or the machine body temperature follow-up control. In the case of two-position control, the temperature fluctuation range σ(k) of the fluid temperature controlled may be expressed as follows: σ=(QR/Kr){1-exp(-L/T)} +Δexp(-L/T) (1) wherein QR: the cooling capacity (Kcal/h) of the temperature control apparatus; Kr: the coefficient of heat dissipation (Kcal/h· o C); Δ: the differential gap ( o C) of the two-position (ON-OFF action) temperature controller; L: the dead time (h) from the time when the control action is initiated to the time when a temperature change is detected with a temperature detector (sensor); T: the time constant (h) of an object of temperature control; γ: the load factor (the ratio of the quantity of heat generated to the cooling capacity); and exp: natural logarithm. In two-position control, the steady-state deviation Δϑ 0 of the control variable (controlled fluid temperature) that may be expressed by the following equation is produced: Δϑ 0 =(γ-0.5)(QR/Kr){1-exp(-L/T)} (2) Both the temperature fluctuation range σ and the steady-state deviation Δϑ 0 increase as QR increases. As the quantity of heat generated in the machine tool increases, QR is increased, as a matter of course. Therefore, in recent machine tools in which a large quantity of heat is generated, the temperature fluctuation range σ and the steady-state deviation Δϑ 0 may exceed the allowable values. If PID control which includes an integral control action is employed, the steady-state deviation Δϑ 0 becomes 0, but the PID control has the problem that, if detection is not carried out at a point where the dead time is minimized (i.e., at the outlet of the heat exchanger in the case of the temperature control of a machine tool), the transient deviation of the control variable increases and the settling time becomes longer. Indirect control in which the fluid temperature at the outlet of the heat exchanger is detected to effect control, however, leads to the steady-state deviation Δϑw₁( o C) of the machine body (i.e., the difference between the temperature of the machine wall temperature and the temperature of the heat transfer fluid) expressed by the following equation: Δϑw₁=Q/(αF)+Q/(2W) (3) wherein Q: the heat load (kcal/h) transferred from the machine wall to the heat transfer fluid; α: the means heat transfer coefficient (kcal/m²h o C) between the machine wall and the heat transfer fluid; F: the heat exchanging surface area (m²); and W: the water equivalent of the heat transfer fluid, i.e., the product (kcal/h o C) of the volumetric rate of flow, density and specific heat of the fluid. If the fluid temperature at the outlet of the machine tool is detected to effect control, as performed by a known temperature control apparatus of the type mentioned at the beginning, the steady-state deviation Δϑw₂( o C) of the machine body is as follows: Δϑw₂=Q/(αF)-Q/(2W) (3) In other words, the steady-state deviation of the machine wall is Q/W( o C) smaller than in the case where the fluid temperature at the outlet of the heat exchanger is detected to effect control. This may be said that the thermal deformation of the machine tool is minimized and machining errors are reduced. With all the above-described factors taken into consideration, it is found that PID control is superior to ON OFF control in that the steady-state deviation of the fluid temperature is 0, but the conventional PIP control has the contradiction that if the transient deviation of the controlled fluid temperature is reduced, then the steady-state deviation of the machine body increases, whereas, if the steady-state deviation of the machine body is reduced, then the transient deviation of the controlled fluid temperature increases. The present invention solves the problems of the conventional indirect control that employs a two-position action and those of the conventional indirect control by PID control. Disclosure of Invention It is therefore an object of the present invention to provide a temperature control apparatus of the above type which is such designed that the steady-state deviation of the fluid temperature is made 0 (zero), that the steady-state deviation of the machine body is minimized and that the transient deviation of the fluid temperature is minimized to shorten the settling time. This object is achieved by the characterizing features of claim 1. Advantageous developments are described in the subclaims. Brief Description of Drawings Fig. 1 is a system diagram showing one embodiment of the temperature control method and apparatus therefor according to the present invention. Fig. 2 is a system diagram showing in detail the functions of the temperature controller section. Fig. 3 is a system diagram showing the flow path of an experimental apparatus in an experimental example. Best Mode for Carrying Out the Invention A preferred embodiment of the present invention will be described below in detail with reference to the drawings. This embodiment controls a spindle head of a machine tool as a controlled system and employs oil as a heat transfer medium. Fig. 1 is a system diagram showing one example of a temperature control apparatus for a machine tool on which the temperature control method of the present invention is carried out. An outline of the arrangement of the temperature control apparatus will first be explained with reference to Fig. 1. The figure shows a temperature control apparatus for a spindle head 41 of a machining center 40. The temperature control apparatus generally comprises a cooling medium flow section 1, a heat transfer medium flow section 2 and a temperature controller section 3. As the heat transfer flow, oil (cooling oil) is employed, and as the cooling medium, a refrigerant (fluorocarbon) is employed. The temperature controller section 3 directly controls the temperature of the heat transfer fluid by controlling the degree of opening of a solenoid-operated expansion valve 15 provided in a refrigerant passage 13 by proportional plus integral plus derivative action, i.e., PID action, in accordance with the quantity of heat generated (heat load) at the spindle head 41, thereby indirectly controlling the temperature of the spindle head 41. Each individual constituent section will next be described in detail. The heat transfer medium flow section 2 is arranged to pass cooling oil as being a heat transfer fluid through a jacket 25 provided inside the spindle head 41 so as to take away the heat generated at the spindle head 41 through heat exchange, thereby cooling the spindle head 41. The heat transfer medium flow section 2 comprises the following devices. A heat exchanger (evaporator) 10 cools the cooling oil heated by taking away the heat generated at the spindle head 41. The cooling is effected by performing heat exchange between the heated cooling oil and the refrigerant flowing through the cooling medium passage 13, thereby cooling the oil to a set temperature. The heat transfer medium flow section 2 further comprises a cooling oil pump 20 for circulating the cooling oil, a damping tank 22 and cooling oil pipes 21, 23, 24, 26 and 27. The damping tank 22 is provided for the purpose of increasing the time constant of a sensor S₃ for detecting the oil temperature at the spindle head inlet for the reason described later. The time constant of the sensor S₃ may be made greater than that of a sensor S₂ by bringing the sensor S₃ into contact with the wall of the passage or placing in the passage the sensor S₃ which is put in a sheath having a proper time constant instead of providing the damping tank 22. Alternatively, a sensor which has a large time constant may also be employed. To effect heat exchange between a machine tool and a heat transfer fluid, it is possible in the case of the spindle head 41 to employ a method wherein the jacket 25, which is a space, is provided as in the case of this embodiment or a method wherein cooling oil is showered on the inner wall surface of the spindle head 41 and this cooling oil is collected and discharged out of the spindle head 41. The cooling medium passage 1 is the same as the ordinary refrigerating circuit. However, a solenoid-operated expansion valve 15 which is capable of automatically controlling the flow rate of the refrigerant by force is employed as a means that functions as an expansion valve or a capillary tube. The arrangement may also be such that a capillary tube is employed in place of the solenoid-operated expansion valve 15 and a solenoidoperated evaporation pressure control valve is provided at the outlet of the heat exchanger (evaporator) 10 to control the flow rate of the refrigerant. Next, the temperature controller section 3 has four sensors S₁ to S₄ which are disposed as temperature detectors for detecting a reference temperature ϑ₁ (room temperature or machine body temperature), the oil temperature ϑ₂ at the outlet of the heat exchanger 10, the oil temperature ϑ₃ at the inlet 24 of the spindle head 41 and the oil temperature ϑ₄ at the outlet 26 of the spindle head 41. The outputs from the four sensors S₁ to S₄ are input to a Wheatstone bridge circuit, as shown in Fig. 2. The output ΔE(V) obtained at the detecting terminals may be expressed from the equation (well-known) used in the calculation for the bridge circuit as follows: wherein R: the resistance (Ω) of the fixed resistance; R₁: the resistance (Ω) of a potentiometer VR1 connected to the sensor S₁ for detecting the reference temperature; R₄: the resistance (Ω) of a potentiometer VR2 connected to the sensor S₄ for detecting the fluid temperature at the outlet of the machine tool; ϑ₁: the temperature ( o C) detected by the sensor S₁; ϑ₂: the temperature ( o C) detected by the sensor S₂; ϑ₃: the temperature ( o C) detected by the sensor S₃; ϑ4: the temperature ( o C) detected by the sensor S4; and ε: the temperature coefficient (1/ o C) of the sensor resistance. The main purpose of this control is to control the machine tool outlet temperature ϑ₄ with a temperature difference between the reference temperature ϑ₁ and the machine tool outlet temperature ϑ₄ being taken as a set point; however, ϑ₂ is also an object of control until the condition of ϑ₂=ϑ₃ is reached. In this case, as will be understood from the equation (4), since both ϑ₂ and ϑ₄ are contained in the first term of the numerator in the equation (4), an increase in ϑ₂ results in an increase in ΔE and an increase in ϑ₄ also results in an increase in ΔE. Accordingly, one single operation enables two different kinds of temperature to be controlled without any contradiction. Since both the sensors S₂ and S₃ are disposed between the heat exchanger and the inlet of the machine tool and the time constant of the sensor S₂ is smaller than that of the sensor S₃, if the heat load or the reference temperature changes, the output ΔE is such as that expresed by the equation (4), but after a certain period of time has passed, the condition of ϑ₂=ϑ₃ is reached. If the numerator and denominator of the equation (4) are divided by [R+R 0 (1+εϑ₂)] , the terms in the numerator of the equation (4) are as follows: The numerator of the equation (4) = R₄-R₁+R 0 ε(ϑ₄-ϑ₁) (5) Since a PID temperature control unit 32 operates so that ΔE is 0, control is effected so that the following condition is reached: R₄-R₁+R 0 ε(ϑ₄₋ϑ₁)=0 i.e., ϑ₄=ϑ₁+(R₁-R₄)/(εR 0 ) Thus, ϑ₄ constantly follows up ϑ₁ with an equal temperature difference. An input processing unit 31 takes in as input signals th values that are respectively obtained in the sensors S₁ to S₄ by converting temperatures into electric signals as described above, and the input processing unit 31 electrically processes these four signals and outputs a difference between a set point and a controlled variable. The voltage output ΔE representing the deviation is taken out in the form of an analog voltage, and after the noise component is removed in a CR filter 52, it is amplified by a DC amplifier unit 53 to a input level for control in the PID temperature control unit 32. The PID temperature control unit 32 performs a proportional plus integral plus derivative (PID) action so that the amplified deviation input signal becomes 0 (zero), as described above. Since this PID action is the action of a well-known control circuit which performs a control action by which the system is stabilized most promptly when there is a disturbance or a change in the set point, the circuit and the action thereof are not disclosed herein. The output of the PID temperature control unit 32 is input to a control output control unit 33. The control output control unit 33 is an output amplifier circuit which amplifies the output of the PID temperature control unit 32 to a signal level with which the solenoid-operated expansion valve 15 can be actuated and then delivers an output in the form of an output voltage or an output current. Operation of the Temperature Control Apparatus The temperature control action according to the present invention will next be explained with reference to Figs. 1 and 2. It is assumed that, in Fig. 1, the quantity of heat generated at the spindle head 41 increases and consequently the heat load with respect to the heat transfer oil increases. The rise in the oil temperature as a result of an increase in the heat load is immediately detected by the sensor S₄ and a cooling operation is initiated. More specifically, the amount of fluorocarbon flowing through the heat exchanger is increased to lower the oil temperature. By virtue of the four-sensor system of the present invention, the lowering in the oil temperature is detected by the sensor S₂ at the output of the heat exchanger 10 to make a correction without delay; therefore, the transient deviation is minimized and hence the settling time is shortened. Since in the prior art system the lowering in the oil temperature caused by the cooling operation is detected by the sensor S₄ to make a correction, there is a time delay and therefore the overshoot of temperature, that is, the transient deviation, increases. If there is no difference in the time constant between the sensors S₂ and S₃, then ϑ₂=ϑ₃, and since neither ϑ₂ nor ϑ₃ is contained in the numerator, as will be understood from the above-described equation (5), the sensors S₂ and S₃ are merely two equal fixed resistances, and any temperature change in ϑ₂ will not participate in the control. In other words, it is impossible to make a correction without delay. The condition of ϑ₂=ϑ₃ is reached in a short period of time after the starting of the control action, and at this time, ϑ₄ becomes stabilized with a predetermined temperature difference kept between the same and ϑ₁, as shown by the equation (6). More specifically, since the spindle head outlet temperature ϑ₄ is controlled with a predetermined temperature difference being constantly kept between the same and ϑ₁, the steady-state deviation of the machine body is small. The same consideration as in the above case where the heat load increases also applies to the case where the heat load decreases. Specifically, a lowering in the oil temperature due to a decrease in the heat load is immediately detected by the sensor S₄ and consequently the cooling capacity of the heat exchanger 10 is reduced, and a rise in temperature as a result of this is immediately detected by the sensor S₂ to make a correction. If the reference temperature ϑ₁ rises with the heat load being constant, this means that the set temperature for the spindle head outlet is raised, and the output of the Wheatstone bridge 51 increases to the negative side. Thus, the control action works in the direction in which the cooling capacity is reduced. In this case also, a correction is made without delay by means of the sensor S₂. If the reference temperature lowers with the heat load being constant, this means that the set temperature for the spindle head outlet is lowered, and the output of the Wheatstone bridge 51 increases to the positive side. Thus, the control action works in the direction in which the cooling capacity is increased. In this case also, a correction is made without delay by means of the sensor S₂ Other Embodiments Although the foregoing embodiment employs four sensors, the output of the sensor S₃ among the sensors eventually coincides with the output value of the sensor S₂ after a certain period of time has passed. Accordingly, it is not always necessary to provide the sensor S₃, provided that a substitute means, for example, a compensating circuit, which performs the same function as that of the sensor S₃ is employed. More specifically, it is possible to employ a method wherein a value which changes with time in accordance with the output value of the sensor S₂ is artificially produced by a microcomputer according to software or by a compensating circuit as being a hardware means to thereby effect a control action in the same way as in the case of the above-described operating principle. This method also makes it possible to obtain advantageous effects similar to those in the foregoing embodiment. The method enables the time constant of the sensor to be readily varied in accordance with the characteristics and heating capacity of the machine tool. In addition, although in the foregoing embodiment oil is employed as a heat transfer fluid, other substitutes, for example, a cutting fluid, water, gas, etc., may also be employed according to the application. As to the temperature controller also, although the foregoing embodiment employs a PID controller, it is also possible to employ a multi-position ON-OFF action temperature controller which enables multi-stage setting, that is, which is capable of stepwise control of the flow rate of a cooling medium. Although in the foregoing embodiment a solenoid-operated expansion valve is employed as an actuator which varies the flow rate of the cooling medium, it is also possible to employ a solenoid-operated evaporation pressure control valve which controls the flow rate of the low-pressure refrigerant gas at the outlet of the evaporator on the basis of a differential pressure. It is also possible to employ a method wherein the number of revolutions of the refrigerating machine is controlled by use of an inverter. Although the above-described temperature controller section 3 is of analog type, it may be a control apparatus which processes signals in a digital manner. For example, it is possible to employ a method wherein duty control is effected using the period of time during which the solenoid-operated expansion valve 15 is open. Although the foregoing embodiment employs a Wheatstone bridge circuit, other types of known circuit may be employed, for example, a bridged-T circuit, which is a circuit performing the same function as that of the Wheatstone bridge circuit. Experimental Example In the following experimental example, a model spindle head which is closer to an actual one is employed as a load. Fig. 3 shows an experimental machine. The specifications of the experimental machine and the experimental conditions are as follows: (1) Controlled system: model spindle head weight: 65kgf spindle diameter: 50mm⊘ bearings: front 7010CDBB (JIS) rear NU209 (JIS) (2) Number of revolutions: 8000rpm (3) Cooling oil: Kantone 16, 10 ℓ/min (trade name, available from KANTO SEIKI Co., Ltd.) (4) Refrigeration compressor output: 500w cooling capacity 820kcal/h (5) Room temperature: 22 to 25 o C (6) Control method: (a) Prior art method (I): two-position control detecting oil temperature at outlet of spindle head (b) Prior art method (II): PID control detecting oil temperature at outlet of heat exchanger (c) Method of this invention: PID control employing four sensors The experimental results are shown in Table 1. Experimental Results Control method Transient deviation Steady-state deviation of oil temperature Steady-state deviation of machine body σ( o C) Δϑ 0 ( o C) Δϑw( o C) Prior art (I) 1.24 -0.05 0.15 Prior art (II) 0.3 0 2.3 This invention 0.3 0 0.1 In this control, 0.5( o C) or less is targeted for all the three deviations, that is, the transient deviation, the steady-state deviation of the oil temperature and the steady-state deviation of the machine body. As will be understood from Table 2, in the two-position control by the prior art method (I), although the steady-state deviation of the machine body is small, i.e., 0.15( o C), the steady-state deviation of the oil temperature is -0.05( o C) and the transient deviation σ is large, i.e., 1.24 ( o C). In the prior art method (II), the steady-state deviation of the oil temperature is 0 and the transient deviation is also small, i.e., 0.3( o C), but the steady-state deviation of the machine body is large, i.e., 2.3 ( o C). In contrast to them, in the method of the present invention, the steady-state deviation of the oil temperature is 0, the transient deviation is small, i.e., 0.3( o C), and the steady-state deviation of the machine body is the smallest in the three, i.e., 0.1( o C). In other words, although the prior art methods (I) and (II) cannot attain the targets, the present invention can satisfactorily attain the targets. Industrial Applicability The present invention is not limited to cooling of a spindle head of a machine tool such as the machining center in the embodiment. The present invention is applicable to various kinds of machine tool and industrial machine, for example, NC (numerically controlled) lathes, (NC) grinding machines, electric discharge machines, etc. The constituent element of a machine that is subjected to cooling control is not limited to a spindle head, either, and the present invention is applicable to cooling of various portions of machine tools, for example, a driving system using a ball screw, a driving motor section, a column, a bed, etc.

Claims: A temperature control apparatus for a machine tool or an industrial machine including a constituent element which generates heat, comprising: a heat transfer fluid brought into contact with the constituent element (41) to cool it; a heat exchanger (10) for cooling the heat transfer fluid with a cooling medium to control the temperature of said heat transfer fluid, said heat exchanger (10) being connected to an outlet of said constituent element (41), differential temperature detecting means (51) comprising a first sensor (S₁) for detecting a reference temperature (ϑ₁) and a second sensor (S₄) for detecting the temperature (ϑ₄) of the heat transfer fluid at said outlet of the constituent element (41), said differential temperature detecting means (51) being adapted to produce an output signal (ΔE) depending on a difference between the temperature (ϑ₄) detected by said second sensor (S₄) and said reference temperature (ϑ₁); and control means (32) activated in accordance with said output signal (ΔE) to control the temperature of said heat transfer fluid to maintain a set temperature difference between said temperature (ϑ₄) detected by the second sensor (S₄) and said reference temperature (ϑ₁), characterized in that said differential temperature detecting means (51) further comprises a third sensor (S₂) for detecting the temperature (ϑ₂) of the heat transfer fluid at an outlet of said heat exchanger (10) and a fourth sensor (S₃) for detecting the temperature (ϑ₃) of the heat transfer fluid at an inlet of said constituent element (41), said fourth sensor (S₃) having a time constant being greater than that of the third sensor (S₂); said differential temperature detecting means (51) is adapted to produce said output signal (ΔE) further depending on a difference between the temperature (ϑ₂) detected by said third sensor (S₂) and the temperature (ϑ₃) detected by said fourth sensor (S₃), and said control means (32) is adapted to maintain said temperature (ϑ₂) detected by the third sensor (S₂) equal to said temperature (ϑ₃) detected by the fourth sensor (S₃). A temperature control apparatus according to claim 1, characterized in that said differential temperature detecting means is a bridge circuit (51) having all of said sensors (S₁ -S₄) incorporated therein, and said control means is constituted by a PID circuit (32) which performs a proportional plus integral plus derivative action. A temperature control apparatus according to claim 1 or 2, characterized in that a damping tank (22) for increasing said time constant of the fourth sensor (S₃) is provided in a heat transfer fluid passage (23) between said outlet of the heat exchanger (10) and said inlet of the constituent element (41).

Inventors: CHIGIRA JUNJI, URANO KOICHI, CHIGIRA, JUNJI, URANO, KOICHI
Applicants: KANTO SEIKI CO, KANTO SEIKI CO., LTD.
Application year: 1989
Main classification: B23Q11
IPC codes: F25D17, B23Q11, G05D23