ROTATION SPEED ASSESSMENT DEVICE

Abstract

A rotation speed assessment device (1) assesses suitability of a rotation speed of a motor (11) in a rotation device (10) having the motor (11) connected to an object to be rotated (20) to rotate the object to be rotated (20) and having a rotation control unit (13) controlling operation of the motor (11) and carrying out emergency stopping of the motor (11) when the motor (11) becomes uncontrollable. The rotation speed assessment device (1) has an inertia estimation unit (2) estimating an inertia of the motor (11) and object to be rotated (20), an assessment unit (3) assessing suitability of a planned rotation speed based on the inertia estimated by the inertia estimation unit (2), and a notification unit (4) providing a notification when the planned rotation speed is assessed as unsuitable by the assessment unit (3).

Claims

1. A rotation speed assessment device assessing suitability of a rotation speed of a motor of a rotation device having the motor and a rotation control unit, the motor being connected to an object to be rotated to rotate the object to be rotated, the rotation control unit controlling operation of the motor and carrying out emergency stopping of the motor when the motor becomes uncontrollable due to interruption of electricity supply to the motor while the motor is in operation, the rotation speed assessment device comprising: an inertia estimation unit configured to estimate an inertia of the motor and object to be rotated; an assessment unit configured to assess, based on the inertia estimated by the inertia estimation unit, whether a planned rotation speed of the motor planned as a control target in the rotation control unit is suitable or unsuitable in terms of whether the planned rotation speed allows the emergency stopping of the motor to be carried out within an allowable time; and a notification unit configured to, when the planned rotation speed is assessed as unsuitable by the assessment unit, provide a notification that the planned rotation speed is unsuitable.

2. The rotation speed assessment device according to claim 1, wherein the assessment unit is configured to assess the planned rotation speed as unsuitable when the inertia estimated by the inertia estimation unit exceeds an allowable inertia allowed with respect to the planned rotation speed.

3. The rotation speed assessment device according to claim 1, wherein the assessment unit is configured to assess the planned rotation speed as unsuitable when the planned rotation speed exceeds an allowable rotation speed allowed with respect to the inertia estimated by the inertia estimation unit.

4. The rotation speed assessment device according to claim 1, wherein the inertia estimation unit is configured to obtain from outside three-dimensional model data set for the object to be rotated and estimate the inertia of the object to be rotated based on the obtained three-dimensional model data.

5. The rotation speed assessment device according to claim 1, wherein the inertia estimation unit is configured to obtain from outside an image of the object to be rotated captured by a camera and estimate the inertia of the object to be rotated based on the obtained image.

6. The rotation speed assessment device according to claim 1, wherein the inertia estimation unit is configured to obtain an acceleration time taken for the motor to reach the planned rotation speed as well as the planned rotation speed from the rotation device when the motor is rotated at the planned rotation speed and estimate the inertia of the motor and object to be rotated based on the obtained acceleration time and planned rotation speed.

7. The rotation speed assessment device according to claim 1, wherein the assessment unit is configured to, when the planned rotation speed is assessed as unsuitable, estimate a rotation speed allowed with respect to the estimated inertia and transmit data on the estimated rotation speed to the rotation control unit of the rotation device.

8. The rotation speed assessment device according to claim 2, wherein the inertia estimation unit is configured to obtain from outside three-dimensional model data set for the object to be rotated and estimate the inertia of the object to be rotated based on the obtained three-dimensional model data.

9. The rotation speed assessment device according to claim 3, wherein the inertia estimation unit is configured to obtain from outside three-dimensional model data set for the object to be rotated and estimate the inertia of the object to be rotated based on the obtained three-dimensional model data.

10. The rotation speed assessment device according to claim 2, wherein the inertia estimation unit is configured to obtain from outside an image of the object to be rotated captured by a camera and estimate the inertia of the object to be rotated based on the obtained image.

11. The rotation speed assessment device according to claim 3, wherein the inertia estimation unit is configured to obtain from outside an image of the object to be rotated captured by a camera and estimate the inertia of the object to be rotated based on the obtained image.

12. The rotation speed assessment device according to claim 2, wherein the inertia estimation unit is configured to obtain an acceleration time taken for the motor to reach the planned rotation speed as well as the planned rotation speed from the rotation device when the motor is rotated at the planned rotation speed and estimate the inertia of the motor and object to be rotated based on the obtained acceleration time and planned rotation speed.

13. The rotation speed assessment device according to claim 3, wherein the inertia estimation unit is configured to obtain an acceleration time taken for the motor to reach the planned rotation speed as well as the planned rotation speed from the rotation device when the motor is rotated at the planned rotation speed and estimate the inertia of the motor and object to be rotated based on the obtained acceleration time and planned rotation speed.

14. A rotation speed assessment method assessing suitability of a rotation speed of a motor of a rotation device having the motor and a rotation control unit, the motor being connected to an object to be rotated to rotate the object to be rotated, the rotation control unit controlling operation of the motor and carrying out emergency stopping of the motor when the motor becomes uncontrollable due to interruption of electricity supply to the motor while the motor is in operation, the rotation speed assessment method comprising: estimating an inertia of the motor and object to be rotated; assessing, based on the inertia estimated, whether a planned rotation speed of the motor planned as a control target in the rotation control unit is suitable or unsuitable in terms of whether the planned rotation speed allows the emergency stopping of the motor to be carried out within an allowable time; and providing a notification that the planned rotation speed is unsuitable when the planned rotation speed is assessed as unsuitable.

15. A program recording storage that stores a rotation speed assessment program used for a rotation device having a motor and a rotation control unit, the motor being connected to an object to be rotated to rotate the object to be rotated, the rotation control unit controlling operation of the motor and carrying out emergency stopping of the motor when the motor becomes uncontrollable due to interruption of electricity supply to the motor while the motor is in operation, the program comprising: an inertia estimation unit configured to estimate an inertia of the motor and object to be rotated; an assessment unit configured to assess, based on the inertia estimated by the inertia estimation unit, whether a planned rotation speed of the motor planned as a control target in the rotation control unit is suitable or unsuitable in terms of whether the planned rotation speed allows the emergency stopping of the motor to be carried out within an allowable time; and a notification unit configured to, when the planned rotation speed is assessed as unsuitable by the assessment unit, provide a notification that the planned rotation speed is unsuitable.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0031] FIG. 1 is a block diagram illustrating a rotation speed assessment device according to a first embodiment of the present invention together with a rotation device;

[0032] FIG. 2 is a circuit diagram illustrating a spindle motor circuit including an emergency stop circuit provided in a rotation control unit in the first embodiment;

[0033] FIG. 3 is a diagram showing a relationship between an allowable inertia and an allowable rotation speed allowing emergency stopping of a motor and an object to be rotated to be carried out within a predetermined time;

[0034] FIG. 4 is a block diagram illustrating a rotation speed assessment device according to a second embodiment of the present invention together with the rotation device; and

[0035] FIG. 5 is a block diagram illustrating a rotation speed assessment device according to a third embodiment of the present invention together with the rotation device.

DESCRIPTION OF EMBODIMENTS

[0036] Hereinafter, specific embodiments of the present invention will be described with reference to the drawings.

First Embodiment

[0037] First, a rotation speed assessment device according to a first embodiment of the present invention is described with reference to FIGS. 1 to 3. As illustrated in FIG. 1, the rotation speed assessment device 1 according to this embodiment assesses whether a rotation speed in a rotation device 10 is suitable or unsuitable.

[0038] The rotation device 10 in this embodiment is a component of an NC lathe and drives and rotates an object to be rotated 20 that is also a component of the NC lathe. The object to be rotated 20 is constituted by a spindle 21 rotatably supported by a headstock (not illustrated), a chuck 22 mounted on the distal end of the spindle 21, and a workpiece 23 clamped by the chuck 22. Although the rotation device 10 and the object to be rotated 20 to be driven by the rotation device 10 in this embodiment are components of an NC lathe, the present invention is not limited thereto. The rotation device 10 and the object to be rotated 20 may be components of a machining center or any other machine tool or may be components of any other type of machine.

[0039] The rotation device 10 has a spindle motor (synchronous motor) 11, a rotary encoder 12, a rotation control unit 13, and an NC program storage 14. The spindle motor 11 is connected to the spindle 21 to rotate the spindle 21. The rotary encoder 12 detects the rotation speed and the like of the spindle motor 11. The rotation control unit 13 controls operation of the spindle motor 11.

[0040] The NC program storage 14 is a functional unit that stores an NC program to be used in machining in the NC lathe. The NC program storage 14 is composed of an appropriate storage medium such as a RAM. Note that the NC program contains positional commands for feed axes, rotational commands (rotation direction and rotation speed) for the object to be rotated 20, and other commands.

[0041] The rotation control unit 13 is a functional unit that controls rotational operation of the spindle motor 11. When the NC program stored in the NC program storage 14 is executed, the rotation control unit 13 rotates the spindle motor 11 with a commanded rotation direction and a commanded rotation speed in accordance with the rotational commands contained in the NC program. In this process, the rotation control unit 13 feedback controls the spindle motor 11 based on the rotation speed of the spindle motor 11 detected by the rotary encoder 12. The commands other than the rotational commands contained in the NC program are executed for control by control units other than the rotation control unit 13. For instance, the positional commands for the feed axes are executed for control by a feed-axis control unit. The same applies to the other commands.

[0042] The rotation control unit 13 includes a circuit (emergency stop circuit, conventionally called “dynamic braking circuit”) for carrying out emergency stopping of the spindle motor 11 when electricity supply to the spindle motor 11 is interrupted by power outage or any other cause. This emergency stop circuit is to be connected to the spindle motor 11 in the event of interruption of electricity supply to form a short circuit as illustrated in FIG. 2 with the spindle motor 11 in the event of interruption of electricity supply. When electricity supply is interrupted, an induced current occurs in the spindle motor 11 due to idling of the spindle motor 11 and the object to be rotated 20. The short circuit causes this induced current to flow into a coil, so that the rotational energy of the spindle motor 11 and object to be rotated 20 is converted into heat and thereby consumed (i.e., copper loss occurs). Thereby, the spindle motor 11 is stopped. Note that the emergency stop circuit is not limited to such a dynamic braking circuit and may be, for example, a short circuit which is configured to, in the event of interruption of electricity supply, short-circuit a power line of the spindle motor 11 and a power line of an amplifier driving the spindle motor 11 so as to cause electricity to be consumed using only an internal resistance of the spindle motor 11.

[0043] The rotation control unit 13 is composed of a computer including a CPU, a RAM, and a ROM and an electric or electronic circuit or any other circuit which has an appropriate configuration.

[0044] Note that the copper loss P.sub.loss [J/s] in the emergency stop circuit illustrated in FIG. 2 can be calculated in accordance with Equation 1 below:

[00001]$\begin{array}{cc}{\text{P}}_{\text{loss}}=3{\text{RI}}^2=\text{R}\cdot {\left({\text{K}}_{\text{m}}\cdot {\omega }_{\text{m}}\right)}^2/\left({\text{R}}^2+{\left({\omega }_{\text{m}}\cdot \text{p}\cdot \text{L}\right)}^2\right),& \text{­­­(Equation 1)}\end{array}$

where R is the resistance per phase [Ω], L is the inductance per phase [H], K.sub.m is the line-to-line induced voltage constant [Vrms/(rad/s)], ω.sub.m is the rotational angular speed [rad/s] (the rotational angular speed is equivalent to the rotation seed [m/s]; therefore, the rotational angular speed is referred to as “rotation speed” hereinafter), and p is the number of pole pairs (½ of the number of poles).

[0045] The rotation speed assessment device 1 consists of an inertia estimation unit 2, an assessment unit 3, and a notification unit 4. Note that the rotation speed assessment device 1 is composed of a computer including a CPU, a RAM, and a ROM. The inertia estimation unit 2, the assessment unit 3, and the notification unit 4 are functionally implemented by a computer program to execute the processes described later.

[0046] The inertia estimation unit 2 obtains data on the time taken for the spindle motor 11 to reach a target rotation speed after starting rotation (acceleration time) and on the rotation speed reached from the rotation control unit 13 and estimates an inertia of the spindle motor 11 and object to be rotated 20 based on the obtained data.

[0047] Between the acceleration time of the spindle motor 11, the inertia of the spindle motor 11 and object to be rotated 20, the output of the spindle motor 11 during acceleration, and the rotation speed, the following relational equation (Equation 2) holds:

[00002]$\begin{array}{cc}\text{I}=\left(2{\text{P}}_{\text{motor}}\cdot {\text{t}}_{\text{ac}}\right)/{\omega }^2,& \text{­­­(Equation 2)}\end{array}$

where t.sub.ac [s] is the acceleration time of the spindle motor 11, l [kgm.sup.2] is the inertia of the spindle motor 11 and object to be rotated 20, P.sub.motor [w] is the output of the spindle motor 11 during acceleration, and ω [rad/s] is the rotation speed.

[0048] The inertia estimation unit 2 obtains data on the acceleration time t.sub.ac of the spindle motor 11 and on the rotation speed ω of the spindle motor 11 from the rotation control unit 13 and estimates the inertia l of the spindle motor 11 and object to be rotated 20 in accordance with Equation 2 above.

[0049] Based on the inertia l of the spindle motor 11 and object to be rotated 20 estimated by the inertia estimation unit 2, the assessment unit 3 assesses whether the rotation speed ω (a rotation speed planned as a target rotation speed) of the spindle motor 11 is suitable or unsuitable. The assessment whether the rotation speed ω of the spindle motor 11 is suitable or unsuitable is made in terms of whether the rotation speed ω allows the spindle motor 11 and the object to be rotated 20 to be stopped within a predetermined time (allowable time) by the emergency stop circuit in the rotation control unit 13 in the event of interruption of electricity supply to the spindle motor 11. Note that the allowable time is a time during which a load being applied to the emergency stop circuit does not bring about seizing of the emergency stop circuit. The allowable time may use a time experimentally determined for individual circuit.

[0050] The time (emergency stop) t.sub.s needed to stop the spindle motor 11 and the object to be rotated 20 with the emergency stop circuit can be calculated in accordance with Equation 3 below using the above-described copper loss P.sub.loss in the emergent stop circuit:

[00003]$\begin{array}{cc}{\text{t}}_{\text{s}}={\text{r}}_{\text{e}}/{\text{P}}_{\text{loss}},& \text{­­­(Equation 3)}\end{array}$

[0051] where r.sub.e is the rotational energy of the spindle motor 11 and object to be rotated 20 and satisfies the following equation:

[00004]${\text{r}}_{\text{e}}=\left(\text{I}\cdot {\omega }^2\right)/2.$

[0052] Therefore, the following equation holds:

[00005]${\text{t}}_{\text{s}}=\left(\text{I}\cdot {\omega }^2\right)/\left(2\cdot {\text{P}}_{\text{loss}}\right).$

[0053] In order to allow the spindle motor 11 and the object to be rotated 20 to be stopped within the allowable time t.sub.aw, the inertia l of the spindle motor 11 and object to be rotated 20 needs to satisfy Equation 4 blow:

[00006]$\begin{array}{cc}{\text{t}}_{\text{aw}}{\text{> t}}_{\text{s}}=\left(\text{I}\cdot {\omega }^2\right)/\left(2\cdot {\text{P}}_{\text{loss}}\right),& \text{­­­(Equation 4)}\end{array}$

[0054] Therefore, the following equation holds:

[00007]$\text{I}<2\cdot {\text{P}}_{\text{loss}}\cdot {\text{t}}_{\text{aw}}/{\omega }^2.$

[0055] In other words, an allowable inertia l.sub.aw for the spindle motor 11 and object to be rotated 20 for allowing the spindle motor 11 and object to be rotated 20 rotating at the rotation speed ω to be stopped within the allowable time t.sub.aw is represented by Equation 5 below:

[00008]$\begin{array}{cc}{\text{I}}_{\text{law}}=2\cdot {\text{P}}_{\text{loss}}\cdot {\text{t}}_{\text{aw}}/{\omega }^2.& \text{­­­(Equation 5)}\end{array}$

[0056] Accordingly, the assessment unit 3 compares the estimated inertia l of the spindle motor 11 and object to be rotated 20 estimated by the inertia estimation unit 2 with the allowable inertia (threshold) l.sub.aw. When the estimated inertia l does not exceed the allowable inertia l.sub.aw, the assessment unit 3 assesses the rotation speed ω of the spindle motor 11 as suitable. When the estimated inertia l exceeds the allowable inertia l.sub.aw, the assessment unit 3 assesses the rotation speed ω of the spindle motor 11 as unsuitable.

[0057] In an alternative configuration, the assessment unit 3 judges whether the estimated inertia l satisfies Equation 4 above. When the estimated inertia l satisfies Equation 4, the assessment unit 3 assesses the rotation speed ω of the spindle motor 11 as suitable. When the estimated inertia l does not satisfy Equation 4, the assessment unit 3 assesses the rotation speed ω of the spindle motor 11 as unsuitable.

[0058] In another alternative configuration, the focus is placed on the rotation speed ω of the spindle motor 11. On the basis of Equation 4 above, where the allowable time t.sub.aw for the emergency stopping of the spindle motor 11 is determined and the inertia l of the spindle motor 11 and object to be rotated 20 is estimated, the rotation speed ω of the spindle motor 11 needs to satisfy Equation 6 below so as to allow the spindle motor 11 and the object to be rotated 20 to be stopped within the allowable time t.sub.aw:

[00009]$\begin{array}{cc}\omega <{\left(2\cdot {\text{P}}_{\text{loss}}\cdot {\text{t}}_{\text{aw}}/\text{I}\right)}^{1/2}.& \text{­­­(Equation 6)}\end{array}$

[0059] In other words, an allowable rotation speed ω.sub.aw for the spindle motor 11 and object to be rotated 20 for allowing the spindle motor 11 and the object to be rotated 20 with the inertia l to be stopped within the allowable time t.sub.aw is represented by Equation 7 below:

[00010]$\begin{array}{cc}{\omega }_{\text{aw}}={\left(2\cdot {\text{P}}_{\text{loss}}\cdot {\text{t}}_{\text{aw}}/\text{I}\right)}^{1/2}& \text{­­­(Equation 7)}\end{array}$

[0060] Accordingly, the assessment unit 3 compares the rotation speed ω of the spindle motor 11 with the allowable rotation speed ω.sub.aw. When the rotation speed ω does not exceed the allowable rotation speed ω.sub.aw, the assessment unit 3 assesses the rotation speed ω as suitable. When the rotation speed ω exceeds the allowable rotation speed ω.sub.aw, the assessment unit 3 assesses the rotation speed ω as unsuitable.

[0061] In an alternative configuration, the assessment unit 3 judges whether the rotation speed ω of the spindle motor 11 satisfies Equation 6 above established based on the estimated inertia l. When the rotation speed ω satisfies Equation 6, the assessment unit 3 assesses the rotation speed ω of the spindle motor 11 as suitable. When the rotation speed ω does not satisfy Equation 6, the assessment unit 3 assesses the rotation speed ω of the spindle motor 11 as unsuitable.

[0062] The relationship between the inertia l of the combination of the spindle motor 11 and the object to be rotated 20 and the allowable rotation speed ω.sub.aw for the spindle motor 11 and object to be rotated 20 with respect to the inertia l, in other words, the relationship between the rotation speed ω of the spindle motor 11 and object to be rotated 20 and the allowable inertia l.sub.aw for the combination of the spindle motor 11 and the object to be rotated 20 with respect to the rotation speed ω, is represented by a limit curve shown in FIG. 3. With respect to the inertia l of the spindle motor 11 and object to be rotated 20, the rotation speed ω of the spindle motor 11 is suitable as situated below the limit curve and is unsuitable as situated above the limit curve.

[0063] Upon assessing the rotation speed ω as unsuitable, the assessment unit 3 transmits to the rotation control unit 13 a command for deceleration to the allowable rotation speed ω.sub.aw that is to be or has been calculated in accordance with Equation 7 above, so that the rotation speed of the spindle motor 11 is reduced to the allowable rotation speed ω.sub.aw.

[0064] The notification unit 5 is connected to a display 5. The notification unit 5 displays on the display 5 whether the rotation speed ω of the spindle motor 11 assessed by the assessment unit 3 is suitable or unsuitable. In particular, it is preferred that the displaying of the rotation speed ω being assessed as unsuitable is performed in a highlighted manner or together with an alarm so that it is distinctly recognized from the outside.

[0065] With the rotation speed assessment device 1 according to this embodiment having the above-described configuration, once the spindle motor 11 is driven by the rotation control unit 13 and starts rotation, the inertia estimation unit 2 obtains data on the acceleration time t.sub.ac taken for the spindle motor 11 to reach a target rotation speed after the start of the rotation and on the rotation speed ω reached from the rotation control unit 13, and estimates the inertia l of the spindle motor 11 and object to be rotated 20 based on the obtained acceleration time t.sub.ac and rotation speed ω.

[0066] Based on the inertia l of the spindle motor 11 and object to be rotated 20 estimated by the inertia estimation unit 2, the assessment unit 3 assesses whether the rotation speed ω of the spindle motor 11 is suitable or unsuitable, i.e., whether the rotation speed ω allows the spindle motor 11 and the object to be rotated 20 to be stopped within a predetermined time (the allowable time t.sub.aw) by the emergency stop circuit in the rotation control unit 13 in the event of interruption of electricity supply to the spindle motor 11. The result of the assessment is displayed on the display 5 through the notification unit 5.

[0067] Further, upon assessing the rotation speed ω as unsuitable, the assessment unit 3 transmits to the rotation control unit 13 a command for deceleration to the allowable rotation speed ω.sub.aw that is to be or has been calculated in accordance with Equation 7 above. Consequently, the rotation speed ω of the spindle motor 11 is reduced to the allowable rotation speed ω.sub.aw under control by the rotation control unit 13.

[0068] As described above, with the rotation speed assessment device 1 according to this embodiment, it is possible to assess whether a rotation speed ω planned for the spindle motor 11 allows the emergency stopping of the spindle motor 11 to be carried out within the allowable time t.sub.aw. Further, the rotation speed assessment device 1 is configured such that the result of the assessment is displayed on the display 5 through the notification unit 4, which enables a person involved such as an operator to notice if the rotation speed ω is unsuitable.

[0069] Further, the rotation speed assessment device 1 according to this embodiment is configured such that, when the rotation speed ω is assessed as unsuitable, a command for reducing the rotation speed ω to the allowable rotation speed ω.sub.aw is transmitted from the assessment unit 3 to the rotation control unit 13 and the rotation speed ω of the spindle motor 11 is reduced to the allowable rotation speed ω.sub.aw by the rotation control unit 13, which enables the spindle motor 11 to be always rotated at a suitable speed.

[0070] Further, the rotation speed assessment device 1 according to this embodiment is configured such that the inertia l of the spindle motor 11 and object to be rotated 20 to be driven by the spindle motor 11, instead of the inertia of only the spindle motor 11, is estimated by the inertia estimation unit 2 and the assessment whether the rotation speed ω of the spindle motor 11 is suitable or unsuitable is made based on the Inertia l of the spindle motor 11 and object to be rotated 20, which enables the assessment to be made accurately.

Second Embodiment

[0071] Next, a rotation speed assessment device according to a second embodiment of the present invention is described with reference to FIG. 4. This rotation speed assessment device 101 is different from the rotation speed assessment device 1 according to the first embodiment in that the inertia l of the spindle motor 11 and object to be rotated 20 is estimated differently and the assessment whether the rotation speed ω is suitable or unsuitable is made differently. In terms of the differences in configuration, the rotation speed assessment device 101 further has a model data storage 104 and has an inertia estimation unit 102 and an assessment unit 103 that carry out processes different from those carried out by the inertia estimation unit 2 and the assessment unit 3. Except for these differences, the rotation speed assessment device 101 is identical to the rotation speed assessment device 1 according to the first embodiment. Therefore, the components of the rotation speed assessment device 101 identical to those of the rotation speed assessment device 1 are denoted by the same reference numerals as used for the rotation speed assessment device 1. The rotation device 10 and the object to be rotated 20 are also identical to those in the first embodiment.

[0072] The model data storage 104 is a functional unit that stores model data (shape and material) on a rotor unit of the spindle motor 11, the spindle 21, and the chuck 22 and modal data (shape and material) on each workpiece to be machined in the NC lathe. These model data are input and stored in advance into the model data storage 104 from the outside.

[0073] When the NC program stored in the NC program storage 14 is to be executed, the inertia estimation unit 102 obtains from the NC program storage 14 information on the type of the workpiece contained in the NC program. The inertia estimation unit 102 recognizes the shape and material of the workpiece by referring to the model data storage 104 on the basis of the obtained workpiece information. The inertia estimation unit 102 also recognizes the shapes and materials of the rotor unit of the spindle motor 11, spindle 21, and chuck 22 by referring to the model data storage 104.

[0074] The inertia estimation unit 102 subsequently estimates the masses of the spindle motor 11, spindle 21, chuck 22, and workpiece based on the obtained shapes and materials of them and theoretically estimates the total inertia l of them. Thereafter, the inertia estimation unit 2 transmits information on the inertia l to the assessment unit 103.

[0075] The assessment unit 103 obtains from the NC program storage 14 information on the rotation speed ω of the spindle motor 11 (rotational commands) contained in the NC program to be executed. Based on the inertia l of the spindle motor 11 and object to be rotated 20 estimated by the inertia estimation unit 102 and the obtained rotation speed ω of the spindle motor 11, the assessment unit 103 assesses in the same manner as that in the first embodiment whether the rotation speed ω of the spindle motor 11 is suitable or unsuitable. This assessment unit 103 is different from the assessment unit 3 in the first embodiment in that the rotation speed ω of the spindle motor 11 used in the assessment process is the commanded speed contained in NC program.

Third Embodiment

[0076] Next, a rotation speed assessment device according to a third embodiment of the present invention is described with reference to FIG. 5. This rotation speed assessment device 111 is different from the rotation speed assessment device 101 according to the second embodiment in that the inertia of the spindle motor 11 and object to be rotated 20 is estimated differently. In terms of the differences in configuration, a camera 113 is provided in the machine tool and the rotation speed assessment device 111 has an inertia estimation unit 112 that carries out a process different from that carried out by the inertia estimation unit 102. Except for these differences, the rotation speed assessment device 111 is identical to the rotation speed assessment device 101 according to the second embodiment. Therefore, the components of the rotation speed assessment device 111 identical to those of the rotation speed assessment device 1 and rotation speed assessment device 101 are denoted by the same reference numerals as used for the rotation speed assessment devices 1 and 101. The rotation device 10 and the object to be rotated 20 are also identical to those in the first and second embodiments.

[0077] The camera 113 images the interior of a machining area of the NC lathe. In particular, the camera 113 is configured to be able to capture an image of the appearances of the chuck 22 and workpiece 23.

[0078] When the NC program stored in the NC program storage 14 is to be executed, the inertia estimation unit 112 obtains from the camera 113 an image captured by the camera 113. The inertia estimation unit 112 recognizes the workpiece shape by processing the obtained image and recognizes the material corresponding to the recognized workpiece shape by referring to the model data storage 104. Further, the inertia estimation unit 112 also recognizes the shapes and materials of the rotor unit of the spindle motor 11, spindle 21, and chuck 22 by referring to the model data storage 104.

[0079] The inertia estimation unit 112 subsequently estimates the masses of the spindle motor 11, spindle 21, chuck 22, and workpiece based on the obtained shapes and materials of them and theoretically estimates the total inertia l of them. Thereafter, the inertia estimation unit 112 transmits information on the inertia l to the assessment unit 103.

[0080] This third embodiment is different from the second embodiment in that the workpiece shape is recognized based on an image of the workpiece captured by the camera 113.

[0081] Above have been described specific embodiments of the present invention. However, it should be noted that the present invention is not limited to the above-described embodiments and can be implemented in other manners. For example, the rotation speed assessment devices according to the above-described first, second, and third embodiments are configured such that the assessment unit 3 or the assessment unit 103 calculates the allowable rotation speed ω.sub.aw with respect to the inertia l or the allowable inertia l.sub.aw with respect to the rotation speed in accordance with an equation. However, the present invention is not limited to such a configuration. As illustrated in FIGS. 1, 4, and 5, the rotation speed assessment devices may have a limit information storage 6 into which the allowable rotation speed ω.sub.aw with respect to the inertia l or the allowable inertia l.sub.aw with respect to the rotation speed is stored in advance. In this case, the assessment unit 3 and the assessment unit 103 use the allowable rotation speed ω.sub.aw or allowable inertia l.sub.aw stored in the limit information storage 6 to assess whether the rotation speed ω of the spindle motor 11 is suitable or unsuitable.

[0082] Further, the above-described rotation speed assessment devices are configured such that, when the rotation speed ω is assessed as unsuitable by the assessment unit 3 or the assessment unit 103, a deceleration command for reducing the rotation speed ω to the allowable rotation speed ω.sub.aw is transmitted from the assessment unit 3 or the assessment unit 103 to the rotation control unit 13 so that the rotation speed ω of the spindle motor 11 is reduced to the allowable rotation speed ω.sub.aw. However, the present invention is not limited to this configuration, and the rotation speed assessment devices may be configured such that such a deceleration command is not transmitted from the assessment unit 3 or the assessment unit 103 to the rotation control unit 13. In this case, a person involved such as an operator can take an action, such as stopping the spindle motor, upon recognizing the assessment result displayed on the display 5.

[0083] Further, the notification by the notification unit 4 is not limited to the above-described manners and may be performed in any manner as long as the assessment result is notified to the outside.

[0084] Further, the object to be rotated 20 in the above-described embodiments is constituted by the spindle 21, the chuck 22, and the workpiece 23. It is to be understood that the present invention is not limited to such an object to be rotated. By way of example, in the field of machine tools, the object to be rotated may be constituted by a spindle and a tool attached to the spindle.

[0085] As already mentioned above, the foregoing description of the embodiments is not limitative but illustrative in all aspects. One skilled in the art would be able to make variations and modifications as appropriate. The scope of the invention is not defined by the above-described embodiments, but is defined by the appended claims. Further, the scope of the invention encompasses all modifications made from the embodiments within a scope equivalent to the scope of the claims.

TABLE-US-00001 Reference Signs List 1 Rotation speed assessment device 2 Inertia estimation unit 3 Assessment unit 4 Notification unit 5 Display 10 Rotation device 11 Spindle motor 12 Rotary encoder 13 Rotation control unit 20 Object to be rotated 21 Spindle 22 Chuck 23 Workpiece