Torque detecting apparatus for reducing torque ripple in an AC motor

A torque detecting apparatus for an AC motor is provided which includes a current detecting circuit, an angular position detecting circuit, a torque detecting circuit, and a ripple reducing circuit. The current detecting circuit detects a phase current flowing through each winding of the motor. The torque detecting means determines a torque value indicative of the amount of torque of the AC motor based on the phase currents detected by the current detecting circuit according to an angular position detected by the angular position detecting circuit. The ripple reducing circuit reduces ripple contained in the torque value according to the angular position of the motor for determining an accurate amount of torque of the motor.

BACKGROUND OF THE INVENTION 
1 Technical Field of the Invention 
The present invention relates generally to a torque detecting apparatus for 
an AC motor, and more particularly to an improved circuit structure of a 
torque detecting apparatus for measuring torque of an AC motor based on 
phase current flowing through each winding of the motor. 
2 Background Art 
Such a type of torque detecting apparatus is conventionally designed to 
detect a current flowing through each winding of an AC motor and output 
the greatest one of the currents detected as indicating a torque value of 
the AC motor. 
In general, the current flowing through each winding of an AC motor varies 
in sine wave form according to motor rotation. For example, a three-phase 
motor has a given phase angle of 120 deg. in electrical angle, while a 
two-phase motor has a given phase angle of 180 deg. in electrical angle. 
Thus, a conventional torque detecting apparatus measures torque of the AC 
motor by selecting based on a motor angular position a-winding through 
which a maximum current flows, to detect the current of the selected 
winding as indicating a torque value. 
Such a prior art torque detecting apparatus, however, encounters the 
drawback in that a variation component of current flowing through each 
winding will be contained as ripple in a torque value measured since the 
current of each winding varies in sine wave form. 
Accordingly, when a deviation between measured motor torque and target 
torque is determined to correct energization current flowing through each 
winding so as to maintain a constant value of zero (0) under feedback 
control, a control signal provided based on the deviation will undesirably 
include ripple components, thus leading to unstable feedback control. 
SUMMARY OF THE INVENTION 
It is therefore a principal object of the present invention to avoid the 
disadvantages of the prior art. 
It is another object of the present invention to provide a torque detecting 
apparatus for an AC motor which is designed to reduce ripple contained in 
a phase current flowing through each winding of the motor so as to derive 
an accurate torque value based on the ripple-reduced phase current. 
According to one aspect of the present invention, there is provided a 
torque detecting apparatus for an AC motor which comprises a current 
detecting means for detecting phase currents flowing through windings of 
the AC motor, an angular position detecting means for detecting an angular 
position of the AC motor, a torque determining means for determining a 
torque value indicative of the amount of torque of the AC motor based on 
the phase currents detected by the current detecting means according to 
the angular position detected by the angular position detecting means, and 
a ripple reducing means for reducing ripple contained in the torque value 
determined by the torque determining means according to the angular 
position detected by the angular position detecting means. 
In the preferred mode of the invention, the torque determining means 
selects the greatest phase current from the phase currents flowing through 
the windings of the motor to provide the greatest phase current as 
indicating the torque value. 
The ripple reducing means includes a memory which stores therein ripple 
correction values determined for every given angular position of the motor 
according to variations in the phase currents. The ripple reducing means 
reads out of the memory one of the ripple correction values corresponding 
to the angular position of the motor to correct the torque value 
determined by the torque determining means based on the ripple correction 
value read. 
The ripple correction values may be determined based on a value of current 
flowing through each winding of the AC motor at each of the given angular 
positions of the motor.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, wherein like numbers refer to like parts in 
several views, particularly to FIG. 1, there is shown a motor control 
system 100 designed to control an angular position and torque of a 
three-phase motor 10 which is, in this embodiment, used for actuating a 
screw-tightening device mounted on the end of an arm of a robot. 
The motor control system 100 generally includes a rotary encoder 12, a 
signal processing circuit 14, a current detecting circuit 16, and a torque 
determining circuit 18. 
The rotary encoder 12 provides an angular position signal to the signal 
processing circuit 14 every rotational displacement of a given angle of 
the three-phase motor 10. The signal processing circuit 14 receives the 
angular position signal from the rotary encoder 12 to derive a rotational 
speed VM, an angular position (actual position), and an electrical angle 
.theta. indicative of a rotor angular position relative to each stator 
position of the three-phase motor. The current detecting circuit 16 
measures currents iMU, iMV, and iMW flowing through windings of phases U, 
V, and W of the three-phase motor 10, respectively. The torque determining 
circuit 18 derives torque TM of the three-phase motor 10 based on the 
currents flowing through the phases U, V, and W detected by the current 
detecting circuit 16. 
The motor control system 100 further includes a position control circuit 
20, a speed control circuit 22, a torque control circuit 24, a current 
control circuit 26, and a driving circuit 28. 
The position control circuit 20 is responsive to a position control signal 
issued from an external controller (not shown) and an angular position 
signal outputted from the signal processing circuit 14 to determine a 
positional deviation ep between a target angular position of a controlled 
object specified by the angular position signal and an actual angular 
position of the three-phase motor 10. The speed control circuit 22 
determines, based on the positional deviation ep, a target rotational 
speed of the three-phase motor 10, and calculates a speed deviation ev 
between the target rotational speed and the actual rotational speed VH 
derived by the signal processing circuit 14. 
The torque control circuit 24 determines a target torque T of the 
three-phase motor 10 based on the speed deviation ev, and a torque 
deviation eT between the target torque T and the torque TM of the 
three-phase motor 10 determined by the torque determining circuit 18. The 
current control circuit 26 determines, based on the torque deviation eT, a 
target current to be supplied to the windings of the phases U, V, and W 
(practically, the phases U and W) to derive a current deviation el between 
the target current and current flowing through the windings of the phases 
U, V, and W. 
The driving circuit 28 controls a current supply to the winding of each 
phase U, V, and W based on the current deviation ei determined by the 
current control circuit 26. 
The torque control circuit 24, as shown in FIG. 2, is composed of a target 
torque determining circuit 24a and a torque deviation determining circuit 
24b. The target torque determining circuit 24a determines, based on the 
speed deviation ev provided by the speed control circuit 22 and a 
predetermined control parameter Kv, the target torque T of the three-phase 
motor 10 which is required to compensate for the speed deviation ev, or to 
decrease it to zero (0). The torque deviation determining circuit 24b 
derives the torque deviation eT (=T-TM) between the target torque T and 
the actual torque TM of the three-phase motor 10 determined by the torque 
determining circuit 18, and outputs it to the current control circuit 26. 
The target torque determining circuit 24a, as apparent from the drawing, 
receives a torque control signal To which is provided by the external 
controller to determine the target torque T of the three-phase motor 10 so 
as to set screw-tightening torque produced by the screw-tightening device 
to a preselected torque To. 
The current control circuit 26 is formed with a target current determining 
circuit 30 and current deviation determining circuits 32U, 32V, and 32W. 
The target current determining circuit 30 is composed of phase current 
determining circuits 30U and 30V which respectively determine target 
currents iU and iV to be supplied to the windings of the phases U and V of 
the three-phase motor 10 based on the torque deviation eT provided by the 
torque control circuit 24, the predetermined control parameter Kt, and the 
electrical angle 0 according to the following equations. 
EQU iU=Kt.multidot.sin .theta. (1) 
EQU iV=Kt.multidot.sin (.theta.+120 deg.) (2) 
The current deviation determining circuits 32U and 32V determine current 
deviations eiU (=iU-iMU) and eiV (=iV-iMV) between the target currents iU 
and iV calculated by the target current determining circuits 30U and 30V 
and actual currents iMU and iMV, measured by the current detecting circuit 
16, which have flowed through the windings of the phases U and V. The 
current deviation determining circuit 32W then determines a current 
deviation eiW for the phase W using the equation below. 
EQU eiW=-eiU-eiV (3) 
The current deviations eiU, eiV, and eiW are all sent to the driving 
circuit 28. 
The three-phase motor 10 is so designed as to change the electrical angle 
.theta. twice every revolution. The currents iMU, iMV, and iMW flowing 
through the windings of the phases U, V, and W must be timed to vary, as 
shown in FIG. 3, according to the electrical angle .theta.. The current 
control circuit 26, thus, determines the target currents iU and iV to be 
supplied to the windings of the phases U and V, and outputs the current 
deviations eiU, eiV, and eiW determined in the above manner to the driving 
circuit 28 so that the currents iMU, iMV, and iMW flowing through the 
windings of the phases U, V, and W are controlled to the target currents 
iU, iV, and iW. 
The current deviation eiW, as discussed above, may be determined based on 
the current deviations eiU and eiV of the phases U and V without deriving 
a target current iW to be supplied to the winding of the phase W. This is 
based on the fact that the sum of current following through all the 
windings the three-phase motor 10 is zero (0). Accordingly, the target 
current iW may be expressed according to the relation below. 
EQU iW=Kt.multidot.sin (.theta.-120 deg.)=-iU-iV (4) 
Additionally, the current iMW actually flowing through the winding of the 
phase W may be determined based on the currents iMU and iMV inputted to 
the current deviation determining circuits 32U and 32V according to the 
following relation. 
EQU iMW=-iMU-iMV (5) 
Thus, the current deviation eiW is 
EQU eiW=iW-iMW=-(iU-iMU)-(iV-iMV)=-eiU-eiV 
This embodiment, as apparent from the above discussion, uses the equation 
(3) to determine the current deviation eiW based on the current deviations 
eiU and eiV without deriving the target current iW itself. 
The current detecting circuit 16 is mounted on circuit lines extending from 
the driving circuit 28 to the windings of the phases U and V of the 
three-phase motor 10, and includes current detectors 16U and 16V and a 
W-phase current determining circuit 15 16W. The current detectors 16U and 
16V detect the current iMU and iMV flowing through the windings of the 
phases U and V. The W-phase current determining circuit 16W determines the 
current iMW flowing through the winding of the phase W based on the 
currents iMU and iMV measured by the current detectors 16U and 16V. 
The current detecting circuit 16, as mentioned above, directly measures the 
currents iMU and iMV flowing through the windings of the phases V and W 
through the current detectors 16U and 16V and mathematically projects the 
current iMW flowing through the winding of the phase W using the equation 
(5) based on the fact that the sum of the currents iMU, iMV, and iMW is 
zero (0). 
The torque determining circuit 18 includes a ripple reducer 36 and a 
selection circuit 38. The selection circuit 38 is designed to select the 
greatest one of the currents iMU, iMV, and iMW based on the electrical 
angle .theta. of the three-phase motor 10 found in the signal processing 
circuit 14. The ripple reducer 36 reduces a ripple component contained in 
the current selected by the selection circuit 38. 
The currents iMU, iMV, and iMW flowing through the windings of the phases 
U, V, and W of the three-phase motor 10, as already shown in FIG. 3, are 
varied according to an angular position, or the electrical angle .theta. 
of the three-phase motor 10. However, the torque of the three-phase motor 
10 may be determined easily based on a maximum value of current flowing 
therethrough. The torque determining circuit 18 is based on this fact to 
first determine in which winding a maximum value of current is flowing 
based on the electrical angle .theta. and to sample a current flowing 
through the winding so determined. 
While the currents iMU, iMV, and iMW flowing through the windings of the 
phases U, V, and W are, as shown in FIG. 2, reversed alternately in 
positive and negative directions, the selection circuit 38 finds a phase 
having a maximum absolute current value based on the electrical angle 
.theta. to provide an absolute current value of that phase as indicating 
torque of the three-phase motor 10. Specifically, as clearly shown in FIG. 
3, within a range from 0 to 60 deg. in the electrical angle .theta., an 
absolute value of the current iMV of the phase W is the greatest one, 
while an absolute value of the current iMU of the phase U becomes greatest 
within a range from 60 to 120 deg. In this way, the phase having a maximum 
absolute current value is changed every rotation of 60 deg. in the 
electrical angle .theta., and thus the current selected by the selection 
circuit 38 is changed at each 60 deg. of the electrical angle .theta.. 
A value of the current thus selected by the selection circuit 38 
corresponds to torque of the three-phase motor 10, and is, as discussed 
above, changed at each 60 deg. in the electrical angle .theta., that is, 
every rotation of 30 deg. of the three-phase motor 10. Therefore, if a 
current value is inputted directly to the torque control circuit 24 as 
indicating torque of the three-phase motor 10, it will cause the torque 
deviation eT determined in the torque control circuit 24 to be varied 
periodically in synchronization with rotation of the three-phase motor 10, 
leading to unstable motor control. For overcoming this problem, the ripple 
reducer 36 is provided for reducing ripple contained in the current 
selected by the selection circuit 38 to produce a ripple-reduced current 
to the torque control circuit 24 as representing torque of the three-phase 
motor 10. 
The determination of which of the currents flowing through the windings of 
the phases U, V, and W has been selected by the selection circuit 38 may 
be easily accomplished based on the electrical angle .theta., and its 
current value is varied according to the equation employed in determining 
a target current in the current control circuit 26. Accordingly, the 
ripple reducer 36 is designed to determine, based on the electrical angle 
.theta., which phase the current value selected by the selection circuit 
38 corresponds to and calculate ripple-reduced torque TM using one of the 
following equations corresponding to the phase so determined. 
EQU U-phase TM=iMU/(Ki.multidot.sin .theta.) (6) 
EQU V-phase TM=iMV/(Ki.multidot.sin .theta.)+120 deg.) (7) 
EQU W-phase TM=iMV/(Ki.multidot.sin .theta.)-120 deg.) (8) 
where Ki is a correction value. 
As explained above, the determination of the torque TM of the three-phase 
motor 10 through the torque determining circuit 18 using the currents iMU, 
iMV, and iMW flowing through the windings of the phases U, V, and W 
determined by the current detecting circuit 16 is achieved by not only 
having the selection circuit 38 select a maximum current from among the 
currents iMU, iMV, and iMW, but also using the ripple reducer 36 to 
minimize the ripple contained in the maximum current selected so that the 
stable torque TM having no ripple is derived. This allows the three-phase 
motor 10 to be controlled in torque and angular position with high 
accuracy. 
FIG. 4(a) illustrates a variation in screw-tightening torque, when the 
three-phase motor 10 is employed to tighten 100 screws through a 
screw-tightening device, measured by a conventional motor control system 
which does not have the ripple reducer 36. Similarly, FIG. 4(b) 
illustrates a variation in screw-tightening torque, when the three-phase 
motor 10 is employed to tighten 100 screws through a screw-tightening 
device, measured by the motor control system 100 of the present invention. 
The graphs show that the use of the ripple reducer 36 in the torque 
determining circuit 18 eliminates the variation in screw-tightening torque 
effectively. 
FIG. 5 shows a modification of the ripple reducer 36 which is designed to 
reduce ripple contained in the current prodded by the selection circuit 38 
without use of the above equations (6), (7), and (8). 
The ripple reducer 36 as shown in the drawing includes a memory 36a and a 
correction circuit 36b. The memory stores therein correction values 
corresponding to the electrical angle .theta. for compensating for ripple 
components. The correction circuit 36b retrieves one of the correction 
values out of the memory 36a according to the electrical angle .theta., 
and corrects a value of the current selected by the selecting circuit 38 
using the correction value retrieved. These arrangements achieve a simple 
circuit structure of the ripple reducer 36. 
Additionally, ripple components contained in the current selected by the 
selection circuit 38 may be measured experimentally by activating the 
three-phase motor 10 and storing them in the memory 36a for each given 
value of the electrical angle .theta.. This permits the ripple components 
to be removed from the current provided by the selection circuit 38 
without the influence of structural variation of the three-phase motor 10 
for improvement of motor control accuracy. 
In the above embodiment, the signal processing circuit 14 determines the 
electrical angle .theta. of the three-phase motor 10 based on a detection 
signal outputted from the rotary encoder 12 , and the target current 
determining circuit 30, the selection circuit 38, and the ripple reducer 
36 operate according to the electrical angle .theta.. However, since the 
electrical angle .theta. corresponds to an angular position of the 
three-phase motor 10, the need of determining the electrical angle .theta. 
may be eliminated by operating the target current determining circuit 30, 
the selection circuit 38, and the ripple reducer 36 according to an 
angular position of the three-phase motor 10. 
Further, the motor control system 100 of the invention may be used with a 
two-phase motor. 
While the present invention has been disclosed in terms of the preferred 
embodiment in order to facilitate better understanding thereof, it should 
be appreciated that the invention can be embodied in various ways without 
departing from the principle of the invention. Therefore, the invention 
should be understood to include all possible embodiments and modifications 
to the shown embodiments which can be embodied without departing from the 
principle of the invention as set forth in the appended claims.