Controller of machine tool

Abstract

In a controller of a machine tool having a converter that converts an AC power supply to DC and a plurality of inverters that drive a feed axis and a spindle, a power outage preparation mode is judged when an AC power supply voltage becomes lower than a predetermined value, and power outage is judged when the power outage preparation mode continues for a time period exceeding a predetermined time period or when a DC voltage which is output from the converter becomes lower than V.sub.AL1. During the power outage preparation mode, a threshold V.sub.AL2 of a low voltage alarm provided at each inverter is changed to a value lower than V.sub.AL1. In addition, when the power outage is judged, a stopping operation or a retracting operation of the feed axis is executed.

Claims

1. A controller of a machine tool, comprising: a converter that converts an alternating current power supply voltage to a direct current voltage and that outputs the converted voltage to a direct current bus; a plurality of inverters that convert the direct current voltage which is supplied from the converter into an alternating current and that drive a plurality of feed axis motors and a spindle motor; an alternating current voltage detection circuit that detects a voltage value of an alternating current power supply which is input to the converter; a first direct current voltage detection circuit that detects a direct current voltage which is output from the converter; a second direct current voltage detection circuit that is provided at each of the plurality of inverters and that detects a direct current voltage which is input; a low voltage alarm judgment unit that is provided at each of the plurality of inverters and that detects an alarm when a detection value of the second direct current voltage detection circuit becomes lower than a first threshold; a power outage preparation judgment unit that judges a power outage preparation mode when a power supply voltage value detected by the alternating current voltage detection circuit becomes lower than a predetermined value; and a power outage judgment unit that judges power outage when a detection value of the first direct current voltage detection circuit becomes lower than a predetermined second threshold while the power outage preparation judgment unit judges the power outage preparation mode, or when a state of judgment of the power outage preparation mode is continued for a time period exceeding a predetermined time period, and that commands a stopping operation or a retracting operation of a feed axis to the plurality of inverters, wherein the first threshold at the low voltage alarm judgment unit is switched from a first value during a normal time to a second value lower than the predetermined second threshold at the first direct current voltage detection circuit during a time period in which the power outage preparation judgment unit judges the power outage preparation mode.

2. The controller of machine tool according to claim 1, wherein during a period in which the power outage preparation judgment unit judges the power outage preparation mode, in the plurality of inverters, a motor output is limited to an output reduction rate P.sub.Lim given by following Equation (1) according to a difference between a detection value V.sub.DC of the second direct current voltage detection circuit and an output limit voltage V.sub.PL which satisfies a relationship of V.sub.PL>V.sub.AL1:
P.sub.Lim=K.sub.L(V.sub.DC−V.sub.PL)  (1) wherein 0≤P.sub.Lim≤1 and K.sub.L is an arbitrary constant.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) Embodiment(s) of the present disclosure will be described based on the following figures, wherein:

(2) FIG. 1 is a block diagram showing an embodiment of the present disclosure;

(3) FIG. 2 is a first time chart showing an operation of an embodiment of the present disclosure;

(4) FIG. 3 is a second time chart showing an operation of an embodiment of the present disclosure;

(5) FIG. 4 is a third time chart showing an operation of an embodiment of the present disclosure;

(6) FIG. 5 is a block diagram showing a controller of a machine tool according to related art; and

(7) FIG. 6 is a block diagram showing a controller of another machine tool according to related art.

DESCRIPTION OF EMBODIMENTS

(8) FIG. 1 is a block diagram showing an apparatus according to an embodiment of the present disclosure. Elements identical to those of the related art shown in FIG. 5 and FIG. 6 are assigned the same reference numerals, and will not be described again. In addition, for the purpose of simplification, there will be described a structure having two feed axis motors and one spindle motor, but the present disclosure is not limited to such a configuration.

(9) An AC voltage detection circuit 13 detects an AC power supply voltage which is input to a converter, and outputs the detected voltage to a power outage preparation judgment unit 18. The power outage preparation judgment unit 18 judges a current operation mode of the apparatus to be a power outage preparation mode when the power supply voltage is reduced to a predetermine value or lower, and outputs a power outage preparation mode signal to a power outage judgment unit 19 and commands a change of an alarm threshold to low voltage alarm judgment units 20a, 20b, and 20c provided on the feed axis inverters 3 and 4 and the spindle inverter 5.

(10) The low voltage alarm judgment units 20a, 20b, and 20c and the alarm threshold V.sub.AL2 will now be described. When the AC power supply is converted to DC by a full-wave rectifier circuit, a DC voltage V.sub.DC is:
V.sub.DC=√2×V.sub.rms
with respect to an effective value V.sub.rms of the AC power supply voltage. For example, for a power supply based on 200V, the DC voltage is approximately 283V. Typical inverters are designed presuming the DC voltage of 283V, and the motor cannot be appropriately controlled when the DC voltage is reduced. Thus, normally, for example, the threshold V.sub.AL2 is set to about −20˜−30%.

(11) In addition, in usages which use a plurality of motors such as in machine tools, generally, a plurality of inverters are connected by a DC bus. When connection deficiency occurs in the DC bus wiring, heat may be generated in the deficiency portion, and burnout accidents may occur. Therefore, the low voltage alarm judgment unit is provided on each of the inverters.

(12) During a period when the power outage preparation judgment unit 18 judges the power outage preparation mode, because the power supply voltage is reduced, there is a possibility that the DC bus voltage may be reduced to a voltage lower than a normal low voltage alarm level. However, statistically, it is known that most of the instantaneous voltage reduction takes place for a few ms to 100 ms, and, with such a short time, position control of the feed axis and velocity control of the spindle motor are almost unaffected. Thus, in the power outage preparation mode, the low voltage alarm threshold V.sub.AL2 is changed to a low value satisfying V.sub.AL1>V.sub.AL2, to permit reduction of the DC bus voltage. V.sub.AL1 will be described later in detail.

(13) The power outage judgment unit 19 judges power outage when the power outage preparation mode continues for a predetermined period of time (for example, 100 ms), or when the detection value of a DC voltage detection circuit 16 becomes a value below the predetermined value V.sub.AL1, and commands a retracting operation or stoppage to the feed axis inverters 3 and 4.

(14) When the low voltage alarm judgment unit 20a or 20b individually detects a low voltage alarm for the feed axis inverter 3 or 4 in the power outage preparation mode, there is a possibility that, due to a difference in timings of detection, an operation trajectory of the tool deviates from a commanded trajectory, resulting in possible damages to the workpiece and the tool. Specifically, when the low voltage alarm is detected for a voltage which is input to a certain feed axis inverter, the numerical controller 6 stops the feed axis inverter, and the feed axis motor driven by the inverter is stopped. If there is a time difference in detection of the low voltage alarms for two feed axis inverters 3 and 4 in the exemplary configuration, one of the feed axis motors 7 and 8 first stops, and then, with elapse of a certain time, the other of the feed axis motors 7 and 8 stops. When there is such a time difference in stopping of the feed axis motors 7 and 8, there is a possibility of a situation where the operation trajectory of the tool is deviated from the commanded trajectory.

(15) Thus, during the power outage preparation mode, the low voltage alarm threshold V.sub.AL2 is set to a value lower than the threshold V.sub.AL1 with which the power outage judgment unit 19 judges the power outage. With this configuration, the timing for the retracting operation or the stop operation can be collectively controlled by the power outage output of the power outage judgment unit 19, the plurality of feed axes can operate in synchronization with each other, and the tool trajectory does not deviate from the commanded trajectory.

(16) FIG. 2 is a time chart showing an operation of the embodiment of FIG. 1, and shows an operation when the instantaneous voltage reduction is continued for a time period exceeding a predetermined time (for example, 100 ms).

(17) At a time t1, it is detected that the power supply voltage became lower than the power outage preparation mode threshold due to the instantaneous voltage reduction, the power outage preparation mode signal is switched ON, and the threshold V.sub.AL2 of the low voltage alarm of the DC bus voltage is changed from the value at the normal time to a low value.

(18) In the period in which the instantaneous voltage reduction occurs, the DC bus voltage is reduced by loads of the spindle motor and the feed axis motor. FIG. 2 shows a case where these loads are small, and the DC bus voltage is gradually reduced from the time t1. However, because the threshold V.sub.AL2 of the low voltage alarm is changed to the low value as described above, the low voltage alarm is not generated.

(19) When a time of 100 ms elapses from the time t1, the power outage judgment unit judges the power outage, and outputs the power outage signal. A factor which determines the time period of 100 ms is retention times of a control circuit power supply and a hydraulic pressure system. When the time exceeds 100 ms, even if the spindle and the feed axis continue the operations, other constituting elements of the machine tool may cause an inappropriate behavior, and thus, the machining is stopped for the purpose of safety. Therefore, this time is not limited to 100 ms, and is suitably set according to the retention times of the control circuit power supply and the hydraulic pressure system.

(20) When the power outage signal is switched ON, the retracting operation of the feed axis is executed. The axis to be retracted is determined according to a positional relationship between the workpiece and the tool, and in general, an axis which operates in an up-and-down direction is retracted upward.

(21) FIG. 3 is a time chart showing an operation in a case where loads of the spindle and the feed axis are large in comparison to the case of FIG. 2, and in which an output limit process of the motor to be described later is not executed. In this example configuration, after the instantaneous voltage reduction occurs at the time t1, the DC bus voltage is reduced by the loads of the spindle and the feed axis, and becomes lower than the threshold V.sub.AL1 at the power outage judgment unit at a time t3. At this point, the power outage judgment unit judges the power outage, and switches the power outage signal ON. Then, the retracting operation of the feed axis is executed similar to the case of FIG. 2.

(22) FIG. 4 is an operation time chart for a case in which output limitation of the spindle and feed axis motors is executed according to the DC bus voltage. After the instantaneous voltage reduction occurs at the time t1, the DC bus voltage is reduced by the loads of the spindle and the feed axis. In this example configuration, a limit process of the motor output as will be described below is executed at the inverters 3, 4, and 5 according to the DC bus voltage. Specifically, when a regular motor output is 1.0, an output reduction rate P.sub.Lim is calculated, for example, as:
P.sub.Lim=K.sub.L(V.sub.DC−V.sub.PL)
wherein V.sub.PL is an output limit voltage, which is set to a value larger than the threshold V.sub.AL1 of the power outage judgment unit.

(23) With such an output limitation, as the DC bus voltage is reduced in FIG. 4, the motor output is limited, and, when the DC bus voltage reaches V.sub.PL at a time t2, the motor output is set to 0. As a result, a power consumption from the DC bus becomes minimal, including the heat generation of the winding of the motor and loss of the inverter, and almost no voltage reduction occurs. Because V.sub.PL is set higher than the threshold V.sub.AL1 of the power outage judgment unit, the operation is continued without judging the power outage. Then, at a time t4, the instantaneous voltage reduction is resolved, and the operation can be returned to the normal operation.

(24) The above-described limitation applies a limit on the motor output, and thus, does not apply a limit on a torque command when the velocity is stopped. Therefore, for example, at an up-and-down axis to which a gravitational load is applied, a retention torque can be maintained even during the output limitation, and dropping due to the force of gravity does not occur.

(25) In the embodiment described above, in one example configuration, the power outage preparation judgment unit 18, the power outage judgment unit 19, and the low voltage alarm judgment units 20a, 20b, and 20c are realized as hardware circuits such as a logical circuit. In another example configuration, the power outage preparation judgment unit 18, the power outage judgment unit 19, and the low voltage alarm judgment units 20a, 20b, and 20c are realized by a computer executing programs describing functions of these parts. The computer used in this configuration has, as hardware, a circuit structure, for example, in which a processor such as a CPU, a memory (primary storage device) such as a random access memory (RAM), a controller which controls an auxiliary storage device such as a flash memory, an SSD (solid state drive), and an HDD (hard disk drive), an interface with various input/output devices, a network interface for controlling connection with a network such as a local area network, or the like are connected via a data transmission path such as, for example, a bus. Programs describing contents of the processes of the functions of these parts are installed on the computer via the network or the like, and are stored in the auxiliary storage device. The program stored in the auxiliary storage device is executed by the processor using the memory, to realize the functions of these parts.

(26) A controller of a machine tool according to the present disclosure can execute a cooperative stopping process by judgment of the power outage by the power outage judgment unit without each of the inverters driving the feed axis and the spindle being stopped by the reduction of the DC bus voltage during a period in which the instantaneous voltage reduction occurs. In addition, the AC voltage detection circuit monitors the voltage of the AC power supply which is input to the converter, and the operation returns to the normal operation without judging the power outage when the instantaneous voltage reduction is resolved. Because of this, the machining can be continued without interruption.

(27) Further, because the DC bus voltage which is input to each inverter is monitored by the second DC voltage detection circuit and the low voltage alarm judgment unit during a period in which the power supply voltage is normal, excessive heating or the like due to a voltage reduction caused by deficiency of the DC bus wiring or the like can be prevented in advance.

(28) Moreover, because the motor output is limited when the DC bus voltage is reduced to a value close to the threshold .sub.AL1 during the period in which the instantaneous voltage reduction occurs, the reduction of the DC bus voltage can be suppressed, and the operation can be returned to the normal operation after the AC power supply voltage is recovered.