Method of automatically adjusting thread tension in a sewing machine

A method of automatically adjusting thread tension in a sewing machine including detecting changes of load subjected on the needle bar caused when the needle penetrates the fabric to be sewn and adjusting the electric driving part of the thread tension device in accordance with the values of the changing components of the load.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of automatically adjusting thread 
tension in a sewing machine. 
The prior art belonging to this field will be explained in reference to the 
attached drawings. With respect to the thread tension, taking the tension 
of the upper thread, for example, it is preferable to make adjustments for 
thin fabric materials, middle materials and thick materials in proper 
ranges of the thread tension as shown with arrows in FIG. 1. It is in 
general required to increase the tension as thickness of the fabric 
becomes large and as hardness becomes high. 
For automatically responding to these characteristics of the fabric, there 
has been proposed an adjustment of an electric driving part of the thread 
tension device in accordance with the data of the fabric thickness. 
However, for the fabric hardness, there have not been any measures. 
SUMMARY OF THE INVENTION 
This invention is to adjust the thread tension in response to load 
subjected on a needle bar when a needle penetrates the fabric material, 
thereby to automatically provide exact thread tension. 
The load is detected by the premise that the load is relative to the 
characteristics of the fabric thickness and hardness. The detection is 
made in that electric current of the load on a motor driving a main shaft 
of the sewing machine is momentarily increased at the penetration, or in 
that an electric driving part of the thread tension device is controlled 
by deviation signals expressed with deviation between ordinary speed 
designating signals (corresponding to objective values of the speed) and 
speed feedback signals (corresponding to surveyed values of the speed), or 
by signals from a stress detector provided on the needle bar or others.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 2, a microcomputer is composed of a central processing unit (CPU), 
a read-only-memory (ROM), a read-access-memory (RAM) and an input-output 
port (I/O). 
The sewing machine is, though not shown, provided with a driving part for 
controlling stitchings, which is controlled by the above mentioned 
microcomputer for producing desired stitching patterns. A motor (SM) for 
driving a main shaft of the sewing machine is provided with an armature 
(A) and a series field (FC), and is connected to a commercial power source 
(AC). The load current is effected with a full wave phase control by mixed 
bridge comprising diodes (D1)(D2) and thyristors (SCR1) (SCR2). 
A gate controller (CC) makes an ignition phase control on the thyristors 
(SCR1) (SCR2) in accordance with signals processed as later mentioned from 
the central processing unit (CPU). A machine controller (CONT) designates 
the speed of the main shaft driving motor (SM), and gives digitalized 
designation signals to the central processing unit (CPU). A speed detector 
(SD) is provided on the main shaft (not shown) of the sewing machine for 
issuing and giving pulse signals of numbers in proportion to the rotation 
speed of the main shaft to the central processing unit (CPU), and feeding 
back the speed signal. 
A transformer (TF) for detecting the load current supplies the current wave 
of the main shaft driving motor (SM) to a pick-up circuit (PU) and 
transmits the waves of the full wave controlled currents as shown in FIG. 
3 for driving the sewing machine. The pick-up circuit (PU) rectifies the 
input signal wave in accordance with an order from the central processing 
unit (CPU) and holds peak values (P1) (P2) (P3) (P4) shown in FIG. 3 which 
are produced when the needle passes the fabric, and issues a read order of 
peak hold in synchronism with the phase penetrating the needle through the 
fabric, in response to the signal of a detector (PDp) of needle moving 
phase provided on the main shaft of the sewing machine. 
Analog-digital converter (A/D) converts the peak hold value into a digital 
signal, and supplies it to the central processing unit (CPU). DC motor 
(DM) is controlled by a driver (DV) to adjust the upper thread tension 
effected by a thread tension device 1, and rotates a gear 3 secured on a 
thread shaft 2 to axially move an actuator 4 screwed on the shaft 2 for 
controlling the pressure exerted by thread tension discs 5 holding a 
thread. The driver (DV) rotates DC motor (DM) forward and backward and 
stops it in response to signals (+)(-) and (STOP) issued from a comparison 
circuit (COMP) for the period of issuing the signals. 
A pulse generator (PG) issues pulse signals in proportion to the rotation 
or the rotation phase angle of the shaft 2, and and supplies these signals 
to a polarity discriminating circuit (Z). In combination of the signals 
(+)(-) of the comparison circuit (COMP) and the signals of the pulse 
generator (PG), and when the combination is the signal (+), the circuit 
(Z) counts up (UP) counting of a counter (COUNT) per each of the signals 
from the pulse generator (PG), and when it is the signal (-), the circuit 
(Z) counts down (DOWN). 
The comparison circuit (COMP) is supplied with digital data for setting the 
thread tension at the starting of the sewing machine when the control 
power source is supplied, or digital data for setting the thread tension 
during driving of the sewing machine (called "thread tension designation 
data X" hereinafter) from the central processing unit (CPU), and the 
comparison circuit is supplied with the counting data for setting the 
thread tension at the starting of the driving of the sewing machine when 
the control power source is supplied, or the counter data counted by this 
data (called "counting data Y" hereinafter), from the counter (COUNT). 
These data X and Y are compared, and the case of Y&lt;X, the signal (+) is 
the output, and in the case of Y&gt;X, the signal (-) is output, and in the 
case of Y=X, the signal (STOP) is the output. 
The next explanation will be concerned with actuation of the above 
mentioned structure. When the control electric source is supplied, the 
control circuit shown in FIG. 2 starts to work. The comparison circuit 
(COMP) is supplied with binary data 0 0 0 0 as an initial value of the 
data X from the central processing unit (CPU), and supplied with binary 
data 1 1 1 1 as an initial value from the counter (COUNT). Since Y&gt;X is 
obtained, the comparison circuit (COMP) issues an output of the signal (-) 
to the driver (DV), so that the DC motor (DM) is reversely rotated and the 
actuator 4 of the thread tension device 1 is moved to the right side in 
FIG. 2 to loosen the thread tension. 
The counter (COUNT) is successively counted down, and when Y=X is obtained 
the comparison circuit (COMP) outputs the signal (STOP) and the DC motor 
(DM) is stopped. During this period the actuator 4 engages a stopper (not 
shown) and stops at a scale 0 of the thread tension. The DC motor (DM) is 
idle in rotation after engagement, and the initial setting is finished by 
this stopping. On the standard of the initial setting position where the 
thread tension device 1 of X at the finishing being 0 0 0 0 and the DC 
motor (DM) are combined, the following thread tension is controlled. 
Subsequently, the central processing unit (CPU) issues a determined value, 
e.g., X=0 0 1 0, as a standard setting value of the thread tension. When 
Y&lt;X is obtained, the comparison circuit (COMP) outputs the signal (+), so 
that the DC motor (DM) is normally rotated and the actuator 4 moves to the 
left in FIG. 2 to increase the thread tension. When the counter (COUNT) is 
counted up by 2, Y=X is obtained and the DC motor (DM) is stopped. The 
thread tension at this time of the thread tension device 1 is set at the 
standard. 
The fabric is set on the sewing machine and the machine controller (CONT) 
is operated to drive the sewing machine by designating, e.g., the low 
speed. The current waves of the load effected with the full wave control 
of the motor (SM), which is given to the pick-up circuit (PU) via the 
transformer (TF), are as shown in FIG. 3. Since friction resistance is 
large when the sewing machine starts to rotate, the load of the needle bar 
is increased and current value (I) becomes remarkably large to generate a 
peak (Ps) at the beginning of rotation. Since the friction resistance is 
decreased as time (t) passes, an evnelope (E) of the waves effected with 
the full wave control rapidly falls. After passing a transient period (A), 
the rotation speed of the sewing machine goes up as the time (t) passes by 
successively moving from the low speed designation to the high speed 
designation by means of the controller (CONT) and the envelope (E) also 
goes up. When the needle passes through the fabric per each of the 
rotation period (T) in an interval (B) of relatively low speed, the load 
of the needle momentarily increases due to the friction resistance, so 
that the current of the load increases accordingly and each of the peak 
values (P1) (P2) (P3) (P4) appears. The larger are these peak values, the 
thicker and the harder is the fabric. When the rotation speed of the 
sewing machine becomes higher, and enters an interval (C) of relatively 
higher rotation speed, these peak values do not appear, even if the load 
of the needle bar increases momentarily by inertia of the rotation, 
because the rotation speed of the sewing machine is equalized so that the 
load of the motor driving the main shaft is hardly changed. 
As is seen from the above, in this invention, a calculation is made on each 
of the peak values (P1) to (P4) in the interval (B) having interrelation 
with the fabric characteristics, and the thread tension is controlled with 
the results of this calculation. 
The pick-up circuit (PU) outputs the peak values (P1) to (P4) whose peaks 
are held, and these values are converted into the digital values by means 
of the analog-digital converter (A/D) and given to the central processing 
unit (CPU). The central processing unit (CPU) makes a calculation in the 
interval (A) for starting rotation of one or two stitchings by means of 
the signal from a speed detector (SD), and does not adopt the peak (Ps). 
Thus, the stitching is begun at the thread tension which has been in 
advance determined at the standard. The thread tension is not required to 
be of a high precision at the beginning of rotation. 
The peak values (P1) to (P4) in the interval (B) of the normal low rotation 
are amended by the central processing unit (CPU), or judged successively 
as to whether the normal values or noises, or calculated to obtain mutual 
average values, so that these results are stored in the 
random-access-memory (RAM) as data ranking the fabric characteristics and 
are re-written appropriately. 
The read-only-memory (ROM) stores the data X of designating the thread 
tension in response to the data of the fabric characteristics, and the 
data X are newly output instead of X=0 0 1 0. If the new data X is 
different from the previous condition, and since it is different from the 
counting data Y from the counter (COUNT), the comparison circuit (COMP) 
actuates and the thread tension device 1 is controlled to the thread 
tension based on a new data X. In the interval (C) of the comparatively 
high speed, the central processing unit (CPU) makes the calculation from 
the speed signal and does not adopt each of the peak values, but adopts 
the last data X in the interval (B), so that the thread tension based on 
this data is maintained, while the controlling electric source is 
continuously supplied the last data X is adopted also in the interval (A), 
so that this thread tension is maintained. 
A further reference will be made to another embodiment where the thread 
tension is controlled by detecting the load of the needle bar not 
depending on the load electric current but depending on the speed 
controlling signal of the motor driving the main shaft of the sewing 
machine. 
In the control circuit in FIG. 2, a speed control system of the motor (SM) 
driving the main shaft of the sewing machine may be simplified as shown in 
FIG. 4. A digital signal Cs(t) (t is for time) corresponding to the speed 
objective value is supplied to one of the terminals of the comparator 
(CP). The rotation period calculator (SR) counts the signal of the speed 
detector (SD), and supplies to the other terminal of the comparator (CP) 
the digital signal Cn(t) based on the counting value per unit time of the 
counted signal. The comparator (CP) compares these digital signals, and 
deviation signal .DELTA.Cn(t)=Cs(t)-Cn(t) is calculated per each small 
period of time t and gives it to an ignition time calculator (AT). 
The ignition time calculator (AT) amends a new ignition phase in accordance 
with the deviation signal .DELTA.Cn(t) with respect to the present 
ignition phase for the gate controller (CC), and produces a negative 
feedback in order to lower the next deviation signal .DELTA.Cn(t). In the 
present invention, this deviation signal .DELTA.Cn(t) is amended in the 
speed similarly as in the previous embodiment and is stored as the data 
ranking fabric characteristics in the random-access-memory (RAM). If the 
rotation of the sewing machine rapidly decreases concerning the speed 
designation due to the rapid increase of the load of the sewing machine or 
for other reasons, the deviation signal .DELTA.Cn(t) increases to amend 
this rapid decrease as the peak values (P1) to (p4) in FIG. 3. In such a 
manner, the data X for designating the thread tension in response to the 
data of the fabric characteristics is stored in this embodiment, so that 
the thread tension is controlled as mentioned above. The present case does 
not need the transformer (TF) and the pick-up circuit (PU). 
A further explanation will be made to a method of adjusting the thread 
tension by detecting the load on the needle bar by means of a stress 
detector. In FIG. 5, the needle bar 6 is secured with an elastic member 8 
of U shape serving to hold the needle 7 and is provided with a stress 
gauge 9 in a bottom of an inner side. With respect to the stress gauge 9, 
leads 10 pass through the hollow needle bar 6 and reach sliding plates 12 
which are insulated from each other. Brushes 11 contacting the sliding 
plate 12 issue detected values, and these values are amplified or 
digital-converted and given to the central processing unit (CPU). The 
stress gauge 9 detects the stress of the elastic member 8 when the needle 
7 penetrates the fabric, and the detected data are stored in the 
random-access-memory (RAM) as data ranking the fabric characteristics. The 
data X for designating the thread tension in response to the data of the 
fabric characteristics are read out to adjust the thread tension. This 
case adopts all of the detected data all over the intervals (A), (B), (C). 
As having mentioned above, according to the invention, the fabric 
characteristics concerning the thread tension are detected with respect to 
the thickness and hardness of the fabric to automatically adjust the 
thread tension without being troublesome, so that the thread tension 
suitable to each of the fabric characteristics may be obtained.