International Journal of Informatics Society, VOL.12, NO.1 (2020) 69-78

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Industrial Paper

Optimum Method of Spindle control for Multi-tasking Machine
Kazuhiko Tsutsui*✝, Jun Sawaki*, Naoya Chujo✝✝, and Tadanori Mizuno✝✝
*✝

Mitsubishi Electric Corporation
Graduate School, Aichi Institute of technology, Japan✝✝
Aichi Institute of technology, Japan
Tsutsui.Kazuhiko@ay.MitsubishiElectric.co.jp

Abstract - Recent machine tools control quite an increased
number of axes for multiple cutting processes. Furthermore,
more advanced functions have become necessary for machine
tools: sharing data with the host system or using AI to improve machining accuracy, supporting collaboration with robots, and so forth. Therefore, one of the most critical issues
for CNC controllers of machine tools is to enhance the CPU
performance.
To address this issue, we have established distributed control of servo/spindle position/speed feedback loops in the
power amplifier independently from the CNC controller.
In the past, only speed control was necessary for spindles.
However, switching between position control and speed control has become necessary to allow for various machining
modes for the increasing number of systems with advanced
spindle functions. The time and complexity required forswitching control loops are bottlenecks for improvement inproductivity of machine tools.
In this research, we propose an approach to perform consecutive position control for spindles.

Keywords: Spindle motor control, Multi-tasking machine,
Consecutive position control, Tracking delay compensation

1

INTRODUCTION

CNC equipment consist of the CNC controller, actuators,
amplifiers, and detectors. To run the user's machining program, the CNC controller generates position/speed commands sent to the feed axes and spindles (tools) that constitutethe machine coordinate system. Actuators such as servo motors and spindle motors are used to drive the feed axes andspindles. Servo amplifiers and spindle amplifiers are used tosupply variable power to the actuators. Detectors such as encoders and linear scales are used to feed back the position/speed of operating parts of machines or motors.
The CNC controller, i.e. computerized numerical control
controller, is the brain that controls the entire operation ofmachine tools. The CNC controller sequentially analyzes a
G-code program that include description of the machining
path, feed speed, and number of rotations of the tool, and generates commands of the travel distance per unit time for eachfeed axis. The CNC controller also generates speed commands for spindle motors according to the cutting conditions.
The CNC controller is the open-loop control in which the con-trol action is not dependent on feedback from the path or position of the actual machine, and the control to ensure correct
positions is not performed.
Meanwhile, machine tools are required to have performanceto minimize the reaction forces (disturbances) during heavy
cutting and the effects of friction and stiffness in machines toachieve micrometer or nanometer level cutting accuracy in as
short time as possible. High-speed, high-accuracy tracking of
position/speed commands generated by the controller is critical for the actual machines and motors. Hence, high-response
feedback loop control is one of the most essential factors indeveloping CNC equipment.
There are two ways to establish the feedback loop: one is toestablish the loop of the system as a whole using the CNC
controller, servo/spindle amplifiers, motors, and detectors,
and the other is to establish the loop without the CNC controller. The design philosophy differs between the two models
because the performance required for each component andthe interface between components are different for eachmodel.
While servo/spindle amplifiers are connected one-on-one to
motors with power lines, serial communication through one
network interface is used between the CNC controller andmultiple amplifiers. When the feedback loop control is processed by the CNC controller, the communication cycle time
becomes longer in the control loop with increasing difficulty
in seeking high-level response performance due to the massive amount of exchanged data. For that reason, we developed
CNC equipment in which servo amplifiers perform position/speed feedback loop control for servo motors that movethe feed axes.
On the other hand, the spindle motor control has been more
complex to achieve high productivity. The servo axes don’t
have to change the control mode. It means that the servo axes
are controlled by position loop consecutively. On the other
hand, spindle motor control is required to use properly the
speed loop control and position loop control. Although spindle amplifier is only needed to control the spindle motor
speed and output power on the process of turning and milling,
it is needed to control the angle position of cutting tool and tosynchronize with other servo axes on the process of tapping
C-axis mode for multi-tasking machine.
In the former systems, the mechanical angle of spindles had
to be adjusted every time when the machining mode waschanged. As it was necessary to recalculate a designated position, it was not possible to reduce the machining time.

ISSN1883-4566 © 2020 - Informatics Society and the authors. All rights reserved.

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K. Tsutsui et al. / Optimum Method of Spindle control for Multi-tasking Machine

Figure 1: System configuration example ofmulti-tasking machine
Besides, speed control operations were more susceptible todisturbances during cutting than position control operations.

Figure 2: Centralized control of Servo/Spindle.

Hence, we propose an approach to perform consecutive position control in spindle amplifiers for spindle motors regardless of the machining mode.
This approach reduces extra sequential control to change
the machining mode, ensures robustness against disturbances,and enables establishment of the spindle control system forhigh-accuracy machining.

2
2.1

RELATED TECHNOLOGIESBasic Configuration of CNC Equipment

Multi-tasking machines such as the one shown in Fig. 1 areincreasing in recent years.
Each axes are driven by the servo motors connected to theball screw in the machine. The tool used for cutting is attached to the spindle head and driven by the spindle motor.
The CNC controller and the amplifiers which is used to control the speed and position in addition to power supply for the
servo and the spindle motors are all installed in an electricalenclosure.
The path of the cutting tool affects accuracy of the workpiece directly. Therefore, it is significant for servo motors toensure robust command tracking for position and feed speed
control commands with minimal errors by suppressing load
disturbances such as cutting reaction force or friction in machines. It is also important to keep the synchronous accuracy
and the same response level for X, Y, and Z axes. Otherwise,
the tool may follow the wrong path not intended by the CNC
controller, resulting in unsatisfactory machining accuracy.

2.2 Basic Servo/Spindle Control Architecture(Distributed Control)
In some cases, positions and speeds of servo/spindle motors
are controlled by the CNC controller showed by Fig. 2. Byusing the CNC controller, it becomes easier to develop equipment with synchronous control of multiple axes or to perform
compensation for multiple axes. However, this architecture
has a drawback: the controller is connected to the amplifiers

Figure 3: Response level of the
speed control loop.

Figure 4: Distributed control of Servo/Spindle
over network, and it is not possible to allow high-speed control loops due to the dead time.
Figure 3 shows the frequency response characteristics that
affect disturbance suppression and command tracking forservo and spindle speed control. Generally speaking, it is necessary to increase the loop gain and broaden the response
range to improve disturbance suppression and commandtracking. However, the phase margin is reduced as the deadtime increases, and the feedback loop becomes unstable andInternational Journal of Informatics Society, VOL.12, NO.1 (2020) 69-78

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In order to address the above-mentioned issues, we adopt
the architecture shown in Fig. 4 for servo and spindle control
loops. The position feedback and speed loops are implemented inside the amplifiers (distributed control) and the network communication that involves a substantial amount ofdead time is established outside of the control loops.

2.3

Figure 5: Switching fromturning to milling (C-axis).

Figure 6: Switching fromspindle-linked turning toC-axis-linked milling.

Issues Identified with Spindle Control

Multi-tasking machines that perform multiple operations
prevail in recent years. Position control and speed control forthe spindle motors are switched in multi-tasking machines according to the machining program (refer to the spindle amplifier section in Fig. 4). In addition to operations such as turning,
milling, or drilling by spindle rotation control, it is necessary
for multi-tasking machines to perform operations such as Caxis control or synchronous tapping (C-axis control: synchronous position control with the servo axes).

2.3.1 Time required for switching the control
methods

Figure 7: Normal acceleration/deceleration waveform of position loop control.

When the spindle requires only speed control in the machining mode such as turning, neither the CNC controller nor thespindle amplifier handles position data such as tool tip angles.
In this case, to change the control mode from turning tomilling (C-axis control), it is necessary to stop the spindle
once, perform home position return, and detect the tool tip
angle as shown in Fig. 5.
Another example is shown in Fig. 6. To change the machining mode from turning one workpiece chucked by two spindles to milling (C-axis control), home position return is required for both spindles. The chuck of each spindle is opened
to release the workpiece and home position return is performed one after another for two spindles. Then, the chuck isclosed and the workpiece is held by the spindles again, the Caxis angle is calculated, and milling is started. It takes several
to tens of seconds each time to switch between two modes.
Therefore, repeating switching has significant impact on
productivity.
Moreover, the switching process sequence control involves
complex programming.

Figure 8: Abnormal acceleration/deceleration waveform of position loop control.

2.3.2 Consecutive position control in spindle
amplifier
The following section describes the issues involved withconsecutive position control in spindle amplifiers in the same
way as in servo amplifiers.

(1) Acceleration and deceleration time

Figure 9: Acceleration/deceleration waveform of
speed loop control.
causes oscillation. Furthermore, the load applied to the controller increases along with increase in the total number ofcontrol axes in recent machine tools.

The tool attached to the spindle is frequently changed according to the machining conditions. The spindle motor must
be decelerated and stopped once every time the tool ischanged, and accelerated back to the maximum speed after
the tool change. The number of spindle rotations per minute
ranges from several thousands to tens of thousands and it
takes several to tens of seconds each time to accelerate or decelerate the spindle. One of the requirements demanded for72

spindle motor control is to reduce the acceleration and deceleration time as much as possible.
Figure 7 is a diagram to show the maximum motor torque.
The horizontal axis of the graph represents the motor speed.
In general, the maximum torque generated by motors is reduced due to saturation of the induced voltage in the highspeed region even though the maximum current value of theamplifier is not changed.
Under these circumstances, when acceleration/decelerationis performed with the unchanged position loop, it is necessary
to build a program with a time constant that would not allow
the required torque to exceed the maximum motor torque asshown by the dashed line in Fig. 7.
If a program is built with a time constant that would require
the torque exceeding the maximum motor torque as shown in
Fig. 8, position control to follow the command is not achieved.
Then, position errors are accumulated to cause overshooting,resulting in vibration or mechanical impact.
On the other hand, acceleration to the maximum speed or
deceleration to a stop in a shortest possible time is desirable
in the machining mode without position control as mentioned
above. Therefore, the utmost motor torque must be applied
for acceleration/deceleration.
As shown by the solid line in Fig. 9, in the case of speed
loop control, the utmost motor torque can be applied for acceleration/deceleration without overshooting since position
deviations are not accumulated even though a program is builtwith a time constant that would allow the torque require foracceleration/deceleration to exceed the maximum motor
torque. However, it is not possible to control the cutting tool’s
angle position on the spindle since position control is not performed in this case. For that reason, position control andspeed control are switched according to the machining modefor spindle control in preceding systems.

(2) Spindle encoder
Spindle motors require speeds exceeding 30,000 r/min (revolutions per minute). Besides, cutting fluid or other liquids
may enter from the spindle end. For the reason of durability
and speed, optical encoders for servo motors cannot be used
for spindle motors. Therefore, magnetic detection type encoders are used in general.
In the preceding systems, it is difficult for magnetic encoders to keep tracking the spindle end angle accurately without
deviation in the range over several tens of thousands revolutions per minutes. To switch to position control for C-axis
control or the like, installation of an encoder for position control is required in addition to the encoder for speed detection.
However, we modified the spindle encoder which is able touse for both speed control and C-axis control without additional encoder.
We could achieve this new encoder by adopting mainly two
attempt.
One is detection method. We adopted high accuracy cutting
method of gear and also high resolution Magneto Resistive
Sensor inside the encoder.
Another one is data transmission method. We had made theposition data in amplifier by interpolating AD convertor data
which was converted from analog output signal through thecable from encoder before. But it was limited by the analog

K. Tsutsui et al. / Optimum Method of Spindle control for Multi-tasking Machine

Figure 10: Tracking delay compensation control for
consecutive position control.
output frequency and easily effected by noise. If we wanted
to obtain high resolution data in the condition of high speed,
analog frequency was far beyond the frequency of AD convertor performance. So we developed the serial communication interface which is able to send the high resolution data
without any effect of noise.
By using these technologies, we could create the spindle
end position data accurately without deviation even if thenumber of revolutions exceeds several tens of thousands per
minute. The next section describes the control method toachieve consecutive position control with this spindle encoder.

3

PROPOSED METHOD

3.1 Outline of Tracking Delay Compensation
Control
As mentioned above, a major issue for consecutive position
control is overshooting. Overshooting occurs when position
control to follow the command is not achieved, resulting in
position deviation, during acceleration/deceleration with theutmost motor torque. Thus, we propose the use of spindle amplifier with the tracking delay compensation control as shownin Fig. 10.
Thus, consecutive position control can be performed in thespindle amplifier regardless of the machining mode. Consequently, the machining mode switching time can be reduced
and disturbances during cutting can be suppressed further
more.

3.2

Tracking Delay Compensation Control

Figure 11 is the block diagram to show the details of thecontrol in the spindle amplifier which performs the tracking
delay compensation control we propose.

(1) CNC controller
A CNC controller 1 gives speed command signals as output
to the spindle during ordinary turning, machining, or milling
operations. For tracking delay compensation control, speedcommand signals are integrated to generate position commands. On the other hand, position command signals are out-International Journal of Informatics Society, VOL.12, NO.1 (2020) 69-78

73

put according to the spindle-end rotation angle during synchronous tapping or C-axis control. In the CNC controller 1,
the position of a switch 4d is changed according to the change
in the machining mode, and a position command signal θr forthe spindle amplifier is generated. Meanwhile, a command
switching control part 4e outputs a position/speed operation
switching command MOD, which contains data to showwhether position control operation or speed control operationis performed at the time of switching.

(2) Position/Speed/Current control part
Position, speed and current control parts are used typicalmethod. However, current limiter rolls as important functions.
The current control part 12 controls a current of a motor 13
based on a current limit value output by the current limiter 11.
While a current is limited by the current limiter 11, the current limiter 11 outputs a current limit command Il to the integral speed controller 10 to stop the integral action. The integral speed controller 10 stops the integral action to avoid unnecessary integral action of the speed deviation signal Ve
while the current is limited, and to prevent overshooting fromoccurring for the speed command value after the current limit
is released. The current limiter 11 outputs the current limit
command Il also to the position deviation control part 21.

(3) Position deviation control part
Figure 11: Tracking delay compensation.
Position deviation control processing part
Current limited?

S101

Yes
S102
Speed control operation mode?
MOD = 1
Yes
S103
Positive acceleration value (Acc ≥ 0)
Yes

S104
Last value of (θf - θg)
≥0
Yes
S105
θg
= Last value of θg

No

S107
θg
= θf

S106

S108
Vh
=0

Vh
= θf - θg

S109
Negative acceleration value (Acc < 0)Yes
Last value of (θf - θg)<0
Yes S111
θg= Last value of θg
S112
Vh= θf - θg

S110

No

No

S113
θg
= θf
S114
Vh
=0

End

Figure 12: Operation flowchart in the position.

A position deviation signal θe represents the difference between the position command signal θr output by the CNC
controller and a position signal θs generated by a motor-end
or spindle-end encoder. In typical position loop control, θe is
input to position control part directly. However, this proposed
method uses position deviation control part 21.
This position deviation control part 21 uses below input signals.
・θf: Deviation input signal created by θe and θcd which is
generated by command compensation part 19.
・θr: Which is converted to a command speed signal Fdt by
a differentiator 22.
・Acc: Command acceleration signal.
・MOD: Position/speed operation switching command.
・Fdt: Command speed signal.
After the predetermined operation is completed, the position deviation control part 21 outputs a deviation limiting output value θg to a position control part 5.
Figure 12 shows the flowchart of the operation carried out
by the position deviation control part 21. In the position deviation control part 21, while the current limit command Il isgiven (S101) and the operation mode is speed control during
which the position/speed operation switching command(MOD) does not require absolute position tracking (S102),
when the command acceleration signal Acc has a positive
value (Acc ≥ 0) (S103) and the deviation input signal θf increases in the positive direction (S104), it is assumed that theposition deviation control section output value θg equals tothe last value of θg (S105), and that the input/output deviation
signal Vh has a (θf - θg) value (S106). Meanwhile, in order tosend a command to the position compensation amount control
part 19a in the position compensation part 19 to turn on the74

switch 19b (Fig. 11), the input/output deviation signal Vh output from the position deviation control part 21 is accumulatedby the compensation position deviation amount θcd. The
compensation position deviation amount θcd is subtracted
from the position deviation signal θe to generate the deviation
input signal θf.
When the command acceleration signal Acc has a negativevalue (Acc < 0) (S109) and the deviation input signal θf increases in the negative direction (S110), it is assumed that theoutput value θg equals to the last value of θg (S111), and thatthe input/output deviation signal Vh has a (θf - θg) value(S112).
When the input/output deviation signal Vh output from theposition deviation control part 21 has a (θf - θg) value, theposition deviation amount control part 19a of the positioncompensation part 19 shown in Fig. 11 turns on the switch
19b. Then, the input/output deviation signal Vh of the position deviation control part 21 is accumulated by the integrator
20 to output the compensation position deviation amount θcd.
The compensation position deviation amount θcd is subtracted from the position deviation signal θe. The input/output
deviation signal Vh (base of θcd) represents the difference
between θcd and the signal θe that represents the difference
between the position command signal θr and the motor 13 position signal θs. Therefore, when the current command value
reaches the limit in the motor controller due to saturation of
the motor output voltage during acceleration/deceleration of
the motor, insufficient torque for the commanded acceleration, or other reason, it is possible to prevent the gap between
the speed command calculation signal Vr (output from theposition control part 5) and the actual motor speed Vs fromincreasing even if the position command signal θr value (converted from the speed command signal) is too large while thespeed command 2 is selected with the switch 4d. Consequently, it is possible to reduce the delay in returning to position control when the current limit is removed.

(4) Position compensation partA position-within-one-revolution compensation control
part 16 is present on the output side of the position compensation part 19. The position-within-one-revolution compensation control part 16 normalizes the compensation position
deviation amount θcd output by the position compensation
part 19, and calculates a position-within-one-motor-revolution deviation signal Vrh (the data for two or more revolutions
are discarded and the motor deviation amount within one revolution (difference between the position command and the actual motor position) is calculated). When it is confirmed thatthe current command is within the current limit after the current limit is removed, a position-within-one-revolution compensation amount Vrh is calculated so that the position within
one motor revolution deviation becomes zero. Vrh is added
to the integrator 20 in the position compensation part 19.
Figure 13 (b) shows the acceleration/deceleration slope profiles of the position-within-one-revolution deviation signal
Vrh. The horizontal axis represents the speed, the vertical axis
the slope (acceleration), the solid line the case 1 profile, thechain line the case 2 profile, and the dash-dotted line the motor torque characteristics.

K. Tsutsui et al. / Optimum Method of Spindle control for Multi-tasking Machine

As shown in Fig. 13 (a), the maximum speed in the Vrh
compensation pattern is determined at a certain ratio γ (e.g.
10%) of the speed feedback value at the start point of actual
compensation. The slope profile (acceleration) to the maximum speed in the Vrh compensation pattern is determinedaccording to the motor output torque characteristics as shownby the dash-dotted line in Fig. 13 (b). The slope profile line
may be straight as shown by the solid line (case 1) in Fig. 13(b) by leaving a margin for the motor output torque characteristics if the controller's processing time and the memory
capacity allow. Otherwise, the profile may be set in stages asshown by the chain line (case 2). Thus, the compensation is
performed in a stable and fast way for the position within one
revolution after the current limit is removed.

(5) In case of speed control mode
In the CNC controller 1 in Fig. 11, a command PHS, whichshows that compensation is not required for the position
within one motor revolution, is input to the motor controller
from a position-within-one-revolution alignment control part
26 in speed operation mode that does not require absolute position tracking. The command PHS is transmitted to a switch
27 in the position compensation part 19 and a switch 28 between the position deviation control part 21 and the position
control part 5. The switch 27 usually connects to the input/output deviation signal Vh input from the position deviation control part 21. However, when the command PHS is input, the switch 27 is switched to connect to the opposite sideof the Vh signal. On the opposite side, the switch connects tothe differential value input of the deviation of the actual motor 13 position measured by the encoder 6 from the ideal motor 13 position calculated from an equivalent control systemmodel that has the characteristics of the controlled object bya position loop model 15 based on the position command signal θr. Ideally, motor 13 position is controlled to follow thecharacteristic of position control part 5. So position loop
model 15 is designed based on parameters of position control
part 5.
When the position/speed operation switching command
MOD instructs speed control operation mode, the ideal motor
13 position is calculated and the deviation of the actual motor
13 position measured by the encoder 6 from the ideal position
is used for generation. When the command PHS is input, theswitch 28 is also switched and the deviation input signal θf isdirectly input to the position control part 5 by a route passing
through the position deviation control part 21.
Thus, when compensation is not required for the position
within one motor revolution, acceleration/deceleration time
can be minimized according to the motor output torque.

(6) The signal waveform in the proposed control
Figure 13 also shows waveforms of the operation signals
when the proposed approach is taken. In the topmost graph,
the horizontal axis represents the time, the vertical axis the
speed, the chain line the speed command signal Fdt, the dashdotted line the speed command calculation signal Vr, and thesolid line the motor speed Vs. In the second graph from thetop, the horizontal axis represents the time, the vertical axis
the current, and the solid line the current command value. The

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represents the gap between the speed command calculation
signal Vr output by the position control part 5 and the actual
motor speed Vs, from increasing beyond the predetermined
level. Thus, transition to position compensation is performed
faster when the motor output torque characteristics are restored and the current limit is removed, preventing overshooting from occurring for speed or position control.
Furthermore, after the current limit is removed, the position-within-one-revolution compensation amount Vrh isadded to the integrator 20 in the position compensation part
19 to enable consecutive control of the spindle position within
one revolution to follow the command. The compensation position deviation amount θcd is reduced during accelerationand increased during deceleration to make the position within
one revolution zero. Therefore, it takes less time to determine
the compensation amount and perform the compensation forthe position within one revolution.

4 VERIFICATION OF THE PROPOSEDAPPROACH BY SIMULATION
The following shows the result of verification by simulationof the effect of the proposed tracking delay compensation
control.
Figure 14 shows the acceleration/deceleration waveform
for an ordinary position loop without the tracking delay compensation control we propose. When a program is built witha time constant that would allow the torque to exceed the
maximum motor torque, the intended acceleration/ deceleration torque becomes unachievable. Consequently, deviations
from position commands are accumulated, and control loop
works for recovery, resulting in overshooting.

Figure 13: Tracking delay compensation control signal waveform.
third graph shows the position-within-one-revolution compensation signal Vrh. The horizontal axis represents the time,
and the vertical axis the compensation amount. In the fourthgraph, the horizontal axis represents the time, the vertical axis
the position deviation, and the solid lines represent the compensation position deviation amount θcd and the deviation of
the position within one motor revolution. In our approach,even if the motor cannot be fully accelerated due to the current limit and the motor speed Vs deviates significantly froma command speed signal Vrv output by the command generation part 1, the position deviation control part 21 limits the
position deviation control part output value θg under certain
conditions to prevent the speed deviation signal Ve, which

Figure 14: Acceleration/deceleration waveform
without tracking delay compensation control.

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K. Tsutsui et al. / Optimum Method of Spindle control for Multi-tasking Machine

Figure 15: Acceleration/deceleration waveformwith tracking delay compensation control enabled.
Moreover, when the torque reaches the maximum motor
torque region, the current is limited in the amplifier and thecontrol system enters nonlinear regions. Consequently, thecurrent command behavior becomes unstable and causes reciprocation between upper and lower current limits. This phenomenon causes increase in mechanical impact and significant damage on the machine.
Figure 15 shows the acceleration/deceleration waveformwith the tracking delay compensation control we propose. In
order to apply the utmost motor torque, the program is builtwith an acceleration/deceleration time constant to exceed it,
enabling acceleration/deceleration along the diagram of the
maximum motor torque. Since the motor torque is restrictedagainst the command, the current is limited at the maximum
current in the amplifier. Overshooting does not occur and thecontrol loop remains stable.

5

EVALUATION

5.1 Reduction in the Time Required for Control Mode Switching by Performing Consecutive Position Control
The following shows the result of verification of the effectof implementing the proposed approach with an actual
spindle amplifier.
The system for verification is a multi-tasking machine withtwo main spindles for turning (spindle 1 and spindle 2) and atool spindle for milling. Each main spindle has feed axes X1and Z1, and X2 and Z2 respectively. Each main spindle canbe used for machining of different workpieces, or a workpiececan be pre-cut by spindle 1, re-chucked by spindle 2 formachining without set-up change operation by an operator.
Spindle 1 and spindle 2 can be also used to chuck one
workpiece for heavy cutting.
[Reduction in the C-axis switching time]

Figure 16: Time reduction for switching from turning
to milling (C-axis control).

Figure 17: Time reduction for switching from spindle-linked turning to C-axis-linked milling (spindle
synchronous control)
Since consecutive position control in the system we propose
enables position control with main spindles, it is possible toeliminate processes such as stopping the operation once,
home position return of the C-axis, and C-axis angle calculation required for transition from speed control forturning to position control for milling with the tool spindle asshown in Fig. 16.
[Reduction in the spindle synchronization time]
As shown in Fig. 17, basically the same processes as shownin Fig. 16 are followed when the main spindle 1 and main
spindle 2 chuck a workpiece for heavy cutting (turning) andC-axis control is required in the next process in former systems. In this case, position control is not performed for thespindle 1 and spindle 2 during turning. To perform home position return, each spindle must release the workpiece. Therefore, the chuck of each spindle is opened and the workpieceis held by the other spindle during home position return. The

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Machining conditions
Milling spindle: spindle rotation speed： 9000 r/min,
Main spindle: machining surface speed： 900 mm/min
 Tool: C2MAD160 (Mitsubishi Materials), material: A7075 ALQUEEN- Cutting fluid: Finecut 1000 (NEOS)

Figure 18: Result of spindle orientation time reduction.
Table 1: Effect of time reduction for operation of the multi-task machining program

Figure 19: Improvement of cutting performance byintegrating servo control and spindle control [reference].
Along with adoption of consecutive position control, we
standardized the processor and algorithm between spindle
control and servo control, and improved the accuracy of thespindle encoder.
As a result, the feedback control performance level is raised
to the same level required for servo axis control that demands
high-precision path control.
Figure 19 shows comparison of machining results for Caxis control between the spindle amplifier mainly used forspeed control operation and the spindle amplifier with thecontrol performance equivalent to servo control, which isadopted along with adoption of consecutive position control.

proposed approach reduces the no-cutting time required inexisting systems.

6[Stopping time reduction for spindle orientation]
The tool to be used must be changed according to the machining process. To change the tool automatically by a programusing an Auto Tool Changer (ATC) without tool change operation by an operator, the spindle must be stopped at a predetermined angle.
In existing systems, the speed of revolutions is changed
from a high speed to a certain low speed once for a while forswitching to position control, the spindle (tool) angle is detected, a distance command is generated in the amplifier tostop the spindle at a predetermined angle, and then the spindle
is stopped.
These processes can be eliminated when the proposed approach is used since consecutive position control is established to keep tracking the spindle (tool) angle in the amplifier, resulting in reduction in the spindle orientation time. Figure 18 shows the test result of the proposed approach.

In the past, a focus is put on high-speed revolutions of thetool for spindle motors and spindle amplifiers in machine
tools. Therefore, the critical issues are to generate the maximum torque to reduce the acceleration time to reach highspeed revolutions, and to generate high power for heavy cutting.
However, in the recent progress in pursuing high productivity for cutting machining, multi-tasking machines for both
turning and milling have become more and more dominant.
As a consequence, position control such as C-axis control has
become essential for spindles (motors and amplifiers) in increasing number of cases and how to reduce the switchingtime has become a controversial issue.
However, the other ways except for switching had not existed before our proposal. Because no other way but our proposal couldn’t solve the overshooting issue of spindle’s position loop during acceleration/deceleration with the utmost
motor torque.
In this research, we proposed consecutive position control
in spindle amplifiers in the same way as in servo amplifiers
to address the issue, and verified its validity. This control
method has been adopted in our products, which differentiates our products from previous spindle amplifiers and controllers. Furthermore, we are in the process of promoting
standardization of amplifiers for motors driving machine

Table 1 also shows reduction in time required for abovementioned switching of the machining mode.

5.2 Improvement of Cutting Accuracy by Integrating Servo Control and Spindle Control

CONCLUSION

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K. Tsutsui et al. / Optimum Method of Spindle control for Multi-tasking Machine

tools to remove the barriers between different types of amplifiers.

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(Received Octorber 29, 2019)
Kazuhiko Tsutsui received the B.E.
and M.E. degrees in Mechanical Engineering from Kumamoto University, Japan in 1990 and 1992 respectively. In
1992, he joined Mitsubishi Electric
Corp. Since when he joined Mitsubishi
Electric Corp, he has been engaged indevelopment of Servo and Spindle drive
system.
Jun Sawaki received the B.E. and M.E.
degrees in Electrical Engineering from
Meiji University, Japan in 1995 and1997 respectively. In 1997, he joined
Mitsubishi Electric Corp. Since when
he joined Mitsubishi Electric Corp, hehas been engaged in development ofServo and Spindle drive system.
Naoya Chujo received his B.E. degree
in applied physics and his M.S. degree
in information science and his Ph.D.
degree in electrical engineering from
Nagoya University in 1980, 1982 and2004. He joined Toyota Central R&D
Labs. in 1982. He has been a professor
at Aichi Institute of Technology since2010. His research interests are in thearea of embedded system and automotive electronics. He isa member of IEEE, IPSJ, IEICE, IEEJ, and Informatics Society.
Tadanori Mizuno received the B.E.
degree in Industrial Engineering fromthe Nagoya Institute of Technology in1968 and received the Ph.D. degree inEngineering from Kyushu University,
Japan, in 1987. In 1968, he joined
Mitsubishi Electric Corp. From 1993 to2011, he had been a Professor at
Shizuoka University, Japan. From 2011
to 2016, he had been a Professor at the Aichi Institute ofTechnology, Japan. Since 2016, he is an Affiliate Professor
at the Aichi Institute of Technology, Japan. His researchinterests include mobile computing, distributed computing,computer networks, broadcast communication andcomputing, and protocol engineering. He is a member ofInformation Processing Society of Japan, the Institute ofElectronics, Information and Communication Engineers, theIEEE Computer Society and Consumer Electronics, andInformatics Society.