Stall protection circuit for brushless motor control

This relates to the provision of a stall shutdown circuit for a brushless direct current motor wherein energization of the windings of the stator of the motor is controlled by transistors. By providing a stall shutdown circuit for shutting down power to the transistors when a stall condition exists, the capacity of the power supply transistor may be reduced to one-third of that presently required, thereby greatly reducing the cost of the motor. The brushless direct current motor utilizing the stall shutdown circuit of the present invention is particularly useful in a textile machine such as a take-up device where it is necessary to stop a spindle to doff and don yarn packages.

FIELD OF INVENTION 
This invention relates to electronic control of a brushless direct current 
motor. More particularly, the invention relates to the use of a brushless 
motor on a textile machine, such as on a feeder or on a yarn take-up 
device. 
BACKGROUND OF INVENTION 
A conventional brushless direct current motor as used on a textile machine 
includes a three-phase wire wound stator, a permanent magnet armature, and 
a fixed encoder assembly coupled to the magnetic signal of the armature. 
The encoder assembly provides logic level signals to indicate motor 
armature position. 
The brushless direct current motor has a power supply in the form of a 
power transistor switching circuit which is designed to provide current 
flow through the three stator windings in sequence. The direction of 
current flow is controlled by a signal from the encoder assembly. 
The current flow through the stator windings causes an incremental rotation 
of the armature and the encoder, deriving a magnetic signal from the 
armature, which in turn causes the power circuit to switch the current 
flow at appropriate positions of the armature to continue the rotation of 
the armature. The continuous switching of current flow through the stator 
windings, commonly referred to as commutation, supports the rotation of 
the motor armature. This is similar to a conventional direct current motor 
with fixed carbon brushes and a rotating commutator in the form of copper 
segments attached to lead ends of the armature windings. 
The brushless motor power supply conventionally includes six power 
transistors. At any one instance during armature rotation, only two of the 
six transistors are conducting. The total current flow to the motor is 
supported by all six transistors, and each of the transistors which are 
connected in series need only be selected to support one-third of the 
total motor current. 
Brushless direct current motors are particularly useful in a textile 
machine, such as a take-up device as described in Benjamin, United States 
application Ser. No. 609,113 filed May 11, 1984; and in textile feeders 
because of their small size and the ability to operate such motors in 
hazardous environments. However, in a take-up device as described in the 
Benjamin application, it is necessary in operation of the device to lock 
the spindle driven by the motor to doff and don yarn packages. In textile 
feeders oftentimes a yarn strand becomes entangled which can accidentally 
lock-up the motor. In such machine applications of the motor which cause 
the motor armature to be clamped in a stall condition, the armature 
current flow will be concentrated through only the two transistors which 
are selected by the encoder for that particular armature position. The 
conventional control circuit will cause full current to flow through these 
two transistors. This will represent a demand upon these two transistors 
which will be three times normal requirement. 
It will be readily apparent that the selection of transistors capable of 
receiving three times normal current requirement represents a great 
addition to the power supply course. It is, therefore, the object of this 
invention to eliminate the requirement for such high capacity transistors 
by implementing an alternative stategy in the control circuit design. 
It is to be understood that the condition in which the motor is clamped or 
stalled is an undesirable or unusual machine condition. Detection of the 
stall condition can appropriately be utilized for purposely faulting or 
shutting down the operation of the power supply. 
PRIMARY OBJECTS AND GENERAL DESCRIPTION OF INVENTION 
In accordance with this invention there is provided a mechanism to sense a 
motor stall condition, to combine such signal with the information that 
there is a demand for motor rotation, and effect a fault shutdown to occur 
in the power supply.

DESCRIPTION OF PREFERRED EMBODIMENT 
FIG. 1 illustrates the circuitry for a conventional brushless motor drive 
of the type to be provided with a stall shutdown circuit in accordance 
with this invention. It is to be understood that the motor will include a 
stator generally identified by the numeral 10 having a three-phase 
winding, including windings 12, 14, and 16 which are equally 
circumferentially spaced. There is a DC power supply for the stator 10 
which includes a positive lead 18 and a negative lead 20. It will be seen 
that power is directed to the winding 14 from the positive lead 18 by 
means of a transistor 1Q and from the negative lead 20 through a 
transistor 4Q. In like manner, power is supplied to the winding 16 from 
the positive lead 18 through a transistor 2Q and from the negative lead 20 
through a transistor 5Q. Finally, power is supplied to the winding 12 from 
the positive lead 18 through a transistor 3Q and from the negative lead 20 
through the transistor 6Q. 
There is coupled to the armature (not shown) for incremental rotation 
therewith an encoder generally identified by the numeral 22. The encoder 
22 is provided with leads 24, 26, and 28 from each of the three elements 
thereof. There is also a common 40. 
The brushless direct current electric motor is also provided with a 
conventional drive control which is generally identified by the numeral 
32. The conventional drive controls are described in the literature, for 
example in DC Motors, Speed Controls, Servo Systems, An Engineering 
Handbook by Electro Craft Corp., Chapter 6, which is incorporated herein 
by reference. The drive control 32 is actuated in a conventional manner to 
sequentially turn on the sets of transistors to effect sequential pulsing 
of electrical energy through the coils 12, 14, and 16 so as to effect 
incremental armature rotation. 
The sequential turn on of the transistors is 1Q-5Q, 1Q-6Q, 2Q-6Q, 2Q-4Q, 
3Q-4Q, 3Q-5Q, back to 1Q-5Q, and so on. 
The drive control 32 will be provided with a conventional on-off switch 34 
and an adjustable speed control 36 so as to control both motor operation 
and motor speed. 
The drive control has leading therefrom six leads. These leads, as is shown 
in FIG. 1, include a positive lead 38, a common 40, and a negative lead 
42. The leads also include a speed signal lead 44 which is coupled through 
suitable resistors to the leads 24, 26, 28, and 40 of the encoder to 
provide a signal indicating rotation of the armature even though the speed 
of rotation may be very slow. 
The leads also include a lead 46 which carries an armature current signal. 
There is also a lead 48, which is a returning lead, and which lead may 
have therein an output of a fault signal level. 
Referring now to FIG. 2, it will be seen that there is illustrated the 
details of one form of stall shutdown circuit which is the subject of this 
invention, the circuit being generally identified by the numeral 50 and 
being coupled to the drive control 32 through the aforementioned leads 38, 
40, 42, 44, 46, and 48. 
Before going into the details of the stall shutdown circuit 50, it is noted 
that the speed signal provided by the lead 44 is a composite of pulse 
signals from the motor encoder. While the individual encoder elements 
typically provide two cycles per motor revolution, the composite signal is 
three times this or six cycles per revolution. 
With respect to the armature current signal, this is an amplified signal 
(see the drive control 32) which is proportional to the current flow of 
the DC power supply. The level of this signal is an indication of the 
torque demand of the motor. Typically, this signal will be +2.5 volts at 
the rated motor current. 
The fault signal is an input to the drive control circuit and is a portion 
of the reference signal against which the fault circuit will respond to 
excessive motor currents. 
The supply power through the leads 38, 40 and 42 is provided for excitation 
of the stall shutdown circuit and typically includes a +7 volt DC and a -7 
volt DC supply. 
The speed signal enters the circuit 50 through lead 44 and feeds through a 
resistor R1 and a capacitor C1 arranged in series and develops a 
differentiated signal across resistor R2 which is connected between the 
speed signal lead 44 and the common lead 40. 
There is also coupled between the speed signal lead 44 and the common lead 
40 parallel to the resistor R2 a diode D1. 
It is also noted that the differentiated signal across the resistor R2 is 
an important feature of the stall shutdown circuit 50 for it is only in 
this manner that a sense of motor rotation is transmitted to the remainder 
of the circuit. 
The differentiated signal feeds into the base of a transistor Q1. The 
transistor Q1 is also coupled to the common 40 by a lead 52 and through a 
lead 54 to a lead 56 which extends between the negative voltage lead 42 
and the common 40. The lead 56 has coupled therein in series a capacitor 
C2 and a resistor R4. A further lead 58 extends from the lead 56 
intermediate the capacitor C2 and the resistor R4 to one pin of an 
amplifier A. 
A terminal portion of the common 40, identified by the numeral 60, is 
coupled to the negative lead 42 and has incorporated therein a resistor R3 
and a resistor R5. A lead 62 extends between the portion 60 and the common 
between the resistors R3 and R5, and is coupled to the amplifier as shown 
in FIG. 2. 
Further, the positive voltage lead 38 and the negative voltage lead 42 are 
also coupled to the amplifier A in the manner shown. 
It is to be understood that the negative differentiated pulses from the 
speed signal momentarily turn on the transistor Q1 discharging the 
capacitor C2 through the transistor Q1 collector to emitter. 
It is further noted that the amplifier A is biased by a negative voltage 
input into pin 5, this bias being developed by the resistors R3 and R5. 
The negative voltage bias will cause the amplifier output to swing 
negative or "LO". 
As previously described, the other input to the amplifier A is to pin 6 and 
is obtained from the capacitor C2 and the resistor R4 junction. If 
capacitor C2 is allowed to charge to full voltage from the negative 
voltage supply through the resistor R4, the input to pin 6 would exceed 
the level at pin 5 and cause the amplifier A output to swing positive or 
"HI". However, the speed signal is repeatedly turning on the transistor Q1 
and discharging the capacitor C2, and it is only when the motor is at rest 
or zero speed that the capacitor C2 is permitted to charge. 
From the foregoing it will be apparent that the positive or "HI" output of 
the amplifier A is, therefore, a basic indication that the motor is at 
rest or stall. The time constant of resistor R4 and capacitor C2 is 
selected so that a very low speed of operation will indicate a rotating 
condition for the motor. 
It will be seen that the output of the amplifier A is through the lead 48 
by way of a diode D2 and a resistor R6 back to the drive control circuit 
32 so that when there is a positive fault signal level, a power supply to 
the motor will open and automatic fault shutdown will occur. 
It will be apparent from the foregoing that the so described portion of the 
stall shutdown circuit 50 will assure fault shutdown in the event of a 
motor stall. However, the motor may not be startable in a normal manner 
from an "at rest" condition with only this portion of the stall shutdown 
circuit 50. It is only when the motor current and, therefore, transistor 
currents reach excessive levels that the logic information that the motor 
is stalled need be permitted to interact with the drive control circuit 
32. 
Accordingly, there is provided an amplifier B to enable or dis-able the 
stall signal amplifier A. With a signal input from the armature current 
signal through lead 46 at terminal 2, the amplifier B will be normally 
positive or "HI" and will swing to negative or "LO" when the armature 
current exceeds normal level. This action is obtained by a positive bias 
signal into pin 3 of the amplifier B. 
At this time it is noted that the armature current signal passes into the 
pin 2 of the amplifier B from the lead 46 through resistor R7. It is also 
pointed out that a lead 64 which is coupled to the pin 3 of amplifier B is 
coupled to a lead 66 between the positive voltage lead 38 and ground 
between resistors R8 and R9 incorporated in the lead 66 in series. 
Amplifier B has an output lead 67 which is coupled to the pin 6 amplifier A 
through lead 58. The output of amplifier B passes through a diode D3 and a 
resistor R10. 
When amplifier B is positive under low current conditions, the stall 
indicating circuit is dis-abled by forcing amplifier A to a "LO" condition 
or negative condition. When the armature current level exceeds the normal 
threshold, amplifier B output swings to "LO" and diode D3 blocks its 
effect upon the circuit of amplifier A and the stall indication is 
enabled. 
Returning once again to the output of amplifier A, it is to be understood 
that diode D2 will only pass a signal of a positive level which is 
indicative of stall condition. This signal, when applied to the drive 
control circuit 32, modifies the calibration of the reference level of the 
fault shutdown circuit. By reducing the level of "FAULT TRIP," the circuit 
is caused to shut down before damage occurs to the power transistors 
1Q-6Q. 
It is to be understood that other circuit configurations may be utilized to 
provide the same function. For example, a CMOS NAND circuit element may be 
utilized to perform the same function as amplifier A as is shown in FIG. 
3. This control circuit is generally identified by the numeral 70 and 
includes the transistor Q1 to which the speed signal lead 44 is connected 
in the manner of the circuit 50 through the resistor R1 and the capacitor 
C1. On the other hand, in lieu of the connections being primarily with the 
common 40, resistor R2 is in a lead 72 between the positive voltage lead 
38 and the speed signal lead 44. In a like manner, the diode D1 is in a 
lead 74 which is also between the positive voltage lead 38 and the speed 
signal lead 44. In addition, in lieu of the lead 52, a lead 76 connects 
the transistor Q1 to the positive voltage lead 38. 
Finally, and most particularly, in lieu of the common being connected by a 
lead portion to the negative voltage lead, a lead portion 78 couples the 
positive voltage lead 38 to the negative voltage lead 42, and the 
capacitor C2 and the resistor R4 are incorporated therein. A lead 80 
couples the lead 78 at the juncture of the capacitor C2 and the resistor 
R4 to the transistor Q1 as shown. A further lead 82 extends from the 
juncture of the capacitor C2 and the resistor R4 to the NAND circuit 
element, identified by the numeral 84. The lead 48 is connected to the 
output of the NAND circuit element 84 and has incorporated therein the 
diode D2 and the resistor R6. The lead from the output of the amplifier B 
is coupled to the lead 82 so that the output of the transistor B may 
override the input from the speed signal. 
It is to be understood that the control circuit 70 operates in generally 
the same manner as the aforedescribed controller circuit 50. 
It is also pointed out that while in the present control circuits a 
positive signal acts to reduce the level of "FAULT TRIP," future drive 
control circuits may be such that a negative signal will be required to 
reduce the trip level. In this event, the control circuit will be 
configurated to produce a negative output signal for a stall condition. 
As will be apparent to one skilled in the art, various modifications can be 
made within the scope of the aforesaid description. Such modifications 
being within the ability of one skilled in the art form a part of the 
present invention and are embraced by the appended claims.