Digital acoustic noise reduction in electric motors driven by switching power amplifiers

A system and method is provided for controlling a brushless DC motor (100), the motor being of the type having a plurality of coils (25, 26, 27) and a switching amplifier coil driving circuit (25A, 26A, 27A) including a plurality of transistors (21, 22, 29, 30, 33, 34). Current application to the plurality of transistors is controlled to obtain, near a commutation point of the motor, a simultaneous rise in current applied to a first of the transistors and a fall in current applied to a second of the transistors. Controlling application of current to the plurality of transistors involves, for each of the transistors, generating a PWM gate drive signal by selectively switching between a nominal PWM signal and a constant signal. The selective switching is in response to a synthesized state signal, the synthesized state signal being generated to alternate variably between the two states in accordance with a desired ramping of current to the first and second transistors. In one embodiment, the control system is employed for a motor used to rotate a reel of a helical scan tape drive.

BACKGROUND 
1. Field of Invention 
This invention pertains to operation of electric motors driven by switching 
power amplifiers, and in particular to noise reduction for such motors. 
2. Related Art and Other Considerations 
Axial gap brushless-DC motors typically comprise a magnetic disk axially 
separated from a set of electrically driven coils. The magnetic disk 
essentially lies in a plane and is connected to or otherwise forms a rotor 
or the like of the motor. The rotor turns as electrical signals are 
applied to the coils in order to influence rotation of the magnetic disk 
about its central axis. In particular, in order to keep the motor turning, 
the motors' coils must be energized or de-energized when the magnetic disk 
is at fixed angular positions (called commutation points) about its axis. 
When this occurs, one set of coils is turned on, and another set is turned 
off, resulting in a new set of force vectors acting on the magnetic disk. 
The new force vectors act primarily to pull or push the rotor of the motor 
in the desired angular direction in the plane of the disk. However, the 
new force vectors also cause a small amount of "out of plane" movement. 
While the motor bearings are designed to constrain such undesirable 
motion, a small amount nevertheless does occur. The "out of plane" 
movement causes an audible noise that increases in frequency and volume 
with motor speed. Noise also occurs to a lesser extent in other types of 
electric motors, but their mechanical designs are typically less sensitive 
to this effect. 
Axial gap brushless-DC motors produce significant acoustic noise under high 
speed, high load conditions. This occurs (as explained above) because the 
motor torque acts not only to rotate the magnet disk, but also to tilt it 
in an undesired direction. 
All electric motors require specialized drive circuitry, of which can be 
divided into two broad classes, "linear" and "switching" power amplifiers. 
A linear power amplifier applies an essentially continuous voltage to each 
motor coil as long as the motor is between two commutation points. This 
range of motion is called the commutation interval or angle. The resulting 
motor current is a function of this applied voltage, coil resistance and 
back-emf. 
In contrast, a switching power amplifier applies one of only two voltages 
(V- and V+), rapidly switching between the two many times during each 
commutation angle. The current in each motor coil responds relatively 
slowly to the applied voltage such that it is the percentage of time that 
V+ is applied (the "duty cycle") which determines the average current 
level in the motor. In some cases the switching frequency is high enough 
so that the resulting motions are beyond the range of hearing. But, 
unfortunately, the commutation frequency normally cannot be selected to be 
in a particularly desired range for avoiding acoustic noise, since the 
frequency of commutation depends on the motor speed and the number of 
commutation points per revolution. As a result, acoustic noise is prone to 
develop in some applications, such as for reel motors utilized in magnetic 
tape drives, for example. 
If a motor is driven by a linear control circuit, commutation noise can be 
reduced by the use of snubber circuits. Snubber circuits slow the rise and 
fall of current in each motor coil at the commutation points. While this 
decreases efficiency, it also reduces acoustic noise because the 
transition from one coil set to another occurs more gradually. 
Those skilled in the art will appreciate that the use of snubbers in a 
switching power supply is limited by the switching speed required, which 
in turn depends on the inductance of the motor, the maximum voltage and 
other factors. This limitation precludes the use of a snubber with a time 
constant long enough to have a significant effect on acoustic noise. 
Hence, another technique is required to address acoustic noise in an 
electric motor when it is driven by a switching type power supply. 
SUMMARY 
A system and method is provided for controlling a brushless DC motor, the 
motor being of the type having a plurality of coils and a switching 
amplifier coil driving circuit including a plurality of transistors. 
Current application to the plurality of transistors is controlled to 
obtain, at a commutation point of the motor, a simultaneous rise in 
current applied to a first set of the transistors and a fall in current 
applied to a second set of the transistors. Controlling application of 
current to the plurality of transistors involves, for each of the 
transistors, generating a PWM gate drive signal by selectively switching 
between a nominal PWM signal and a constant signal. These sets of 
transistors may, and typically do, partially overlap. The selective 
switching is in response to a synthesized state signal, the synthesized 
state signal being generated to alternate variably between the two states 
in accordance with a desired ramping of current to the first and second 
sets transistors. In one embodiment, the control system is employed for a 
motor used to rotate a reel of a helical scan tape drive.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 illustrates a brushless DC motor control system 110 which controls 
brushless DC motor 100. Brushless DC motor 100 which is controlled by 
control system 110 is collectively illustrated in FIG. 1 and FIG. 2. FIG. 
1 shows a coil-based portion 14 of brushless DC motor 100. FIG. 2 shows an 
axial alignment of coil-based portion 14 with a magnetic rotor 102. FIG. 2 
further shows magnetic rotor 102, particularly illustrating rotor 102 as a 
flat disk having eight equal pie shaped sections 102A-102H, with each 
section comprising a north-south magnet. 
Coil-based portion 14 of motor 100 includes three stationary coils spaced 
120 mechanical degrees apart, particularly coil 25, coil 26, and coil 27. 
First ends of each of coils 25, 26, and 27 are connected together as shown 
in FIG. 1. 
Each coil 25, 26, and 27 has a coil driving circuit. Coil 25 has coil 
driving circuit 25A, coil 26 has coil driving circuit 26A, and coil 27 has 
coil driving circuit 27A. Each coil driving circuit has a coil interface 
node which is connected to a second end of the coil which it drives. In 
each coil driving circuit an "upper" transistor (FET) and an "upper" 
flyback diode are connected in parallel between +V and the coil interface 
node; a "lower" transistor and a "lower" flyback diode are connected in 
parallel between the coil interface node and ground. 
As an example of the foregoing, coil driving circuit 25A has a coil 
interface node 25N connected to the second end of coil 25. Upper 
transistor 21 and upper flyback diode 23 are connected in parallel between 
+V and coil interface node 25N; lower transistor 22 and lower flyback 
diode 24 are connected in parallel between coil interface node 25N and 
ground. Similarly, with respect to coil driving circuit 26A, upper 
transistor 29 and upper flyback diode 31 are connected in parallel between 
+V and coil interface node 26N; lower transistor 30 and lower flyback 
diode 32 are connected in parallel between coil interface node 26N and 
ground. Likewise, with respect to coil driving circuit 27A, upper 
transistor 33 and upper flyback diode 36 are connected in parallel between 
+V and coil interface node 27N; lower transistor 34 and lower flyback 
diode 35 are connected in parallel between coil interface node 27N and 
ground. 
In addition to the coils and coil driving circuit, coil-based portion 14 of 
motor 100 has three Hall Effect sensors 28 which sense the angular 
position of magnetic rotor 102. In conventional manner, Hall Effect 
sensors generate on Hall line 5 a three-bit signal indicative of angular 
position of magnetic rotor 102. Although Hall line 5 in FIG. 1 is shown 
generally as being connected only to coil-based portion 14 of motor 100, 
it should be understood that Hall line 5 is appropriately connected to the 
three Hall Effect sensors 28. 
Motor control system 110 which controls motor 100 includes processor 1; 
state machine 20 (also known as the state signal generator); mode switch 
6; motor tachometer 15; and, a plurality of Commutation Look-Up Table and 
PWM (Pulse Width Modulation) Control circuits collectively referred to as 
machine 8. A Commutation Look-Up-Table and PWM Control circuit is provided 
for each of the transistors in coil driving circuit 110, specifically 
transistor control circuits 8.sub.21, 8.sub.22, 8.sub.29, 8.sub.30, 
8.sub.33, and 8.sub.34, each transistor control circuit 8 being 
subscripted with a corresponding number for a respective one of the 
transistors 21, 22, 29, 30, 33, and 34 with which it is associated. 
Processor 1, which can be a microprocessor, for example, has mode select 
output line 2 connected to a mode select control terminal of mode switch 6 
and PWM output line 10 connected to each transistor control circuit 
8.sub.21, 8.sub.22, 8.sub.29, 8.sub.30, 8.sub.33, and 8.sub.34, in machine 
8. The PWM signal carried on line 10 is known herein as a processor PWM 
signal or a nominal PWM signal. 
Mode switch 6 is, in the illustrated embodiment, a multiplexer having a 
first set of input terminals connected to Hall Effect sensors 28 via Hall 
line 5 and a second set of input terminals connected to state machine 20 
via line 4. Mode switch 6 has a mode select output line 7 connected to 
each transistor control circuit 8.sub.21, 8.sub.22, 8.sub.29, 8.sub.30, 
8.sub.33, and 8.sub.34, in machine 8 as hereinafter discussed. It should 
be understood that mode switch 6 can take forms other than a multiplexer 
in other embodiments. 
State machine 20, as shown in FIG. 1, comprises a serial connection of tach 
decoder 17; position counter 18; and Hall translation device 19. In the 
preferred embodiment, state machine 20 is an integrated circuit. 
Tachometer 15 outputs a two-bit signal on tachometer output line 16 to 
state machine 20. Tach decoder 17 receives the tachometer output signal on 
tachometer output line 16; Hall translation device 19 generates a noise 
reduction input signal applied on line 4 to mode switch 6. 
As indicated earlier, Commutation Look-Up Table and PWM Control Machine 8 
comprises six sets of circuits 8.sub.21, 8.sub.22, 8.sub.29, 8.sub.30, 
8.sub.33, and 8.sub.34, one circuit for each of the six transistors 21, 
22, 29, 30, 33, and 34 of motor 100. Although FIG. 1 illustrates the 
specifics of only circuits 8.sub.21 and 8.sub.34 for machine 8, it should 
be understood that six such circuits are actually provided, each 
transistor control circuit 8.sub.21, 8.sub.22, 8.sub.29, 8.sub.30, 
8.sub.33, and 8.sub.34 being connected to receive PWM output line 10. 
Accordingly, machine 8 outputs six PWM output signals, i.e., outputs 
signals on each of gate feeding lines 13.sub.21, 13.sub.22, 13.sub.29, 
13.sub.30, 13.sub.33, and 13.sub.34, to gates of respective transistors 
21, 22, 29, 30, 33, and 34 of motor 100. 
Each of the six circuits comprising machine 8 includes a commutation table 
11 and signal source select switch 12. In the illustrated embodiment, 
commutation table 11 is unique for each circuit in machine 8. In essence, 
commutation table 11 is a look-up table which, for the particular one of 
the motor transistors with which it is associated, uniquely determines a 
state of a select control signal to signal source select switch 12 based 
on the mode select output signal on line 7 from mode switch 6. It should 
be understood, however, that another embodment, information for the six 
commutation tables 11 can be combined into a single commutation table 
which is indexed so as to be referenced appropriately for each transistor. 
In the illustrated embodiment, signal source select switch 12 is a 
multiplexer having a control terminal connected to receive its select 
control signal from its associated commutation table 11. For other 
embodiments, structure other than a mulitplexer can be employed so long as 
a similar function is accomplished. 
Signal source select switch 12 has three sets of input terminals--a "1" 
input terminal; a PWM input terminal which is connected to line 10 to 
receive the PWM signal output by processor 1; and a "0" input terminal. 
Signal source select switch 12 connects its output line 13 to one of these 
three terminals, the connection being in accordance with the state of its 
select control signal from its associated commutation table 11. Thus, 
signal source select switch 12 can select either the PWM signal on line 
10, or 0 or 1 to be applied on line 13 to the gate of its associated 
transistor. For purposes of this discussion, a "0" turns the transistor 
off (meaning no current can flow in it), while a "1" turns it on. The 
"PWM" signal is simply a rapid (compared to the commutation frequency) 
switching between 1 and 0. 
OPERATION 
As used herein, a "commutation state" refers to which transistors are on 
(for the duration of the commutation state), which transistors are off, 
and which transistors are being switched rapidly (using the PWM signal on 
line 10). This in turn defines which coils 25, 26, an 27 have current 
flowing therethrough and the polarity of that current. In the illustrated 
embodiment, there are six commutation states for every ninety degrees of 
rotation of rotor 102. In each state, current is flowing in only two of 
the three coils. Commutation states are associated with a particular three 
bit value of Hall sensors 28. The transition from one commutation state to 
another is called a "commutation point", and is associated with a 
particular angular position of rotor 102. 
Between ramps of a given commutation cycle, four of the six transistors 21, 
22, 29, 30, 33 and 34 will be off ("0" applied to their gate inputs) at 
any given time, while another transistor will be on ("1" applied to it's 
gate), and the last transistor will rapidly switch on and off (having a 
gate drive PWM signal applied on its respective line 13 to it's gate). The 
switching frequency of the PWM signal is understood to be much faster than 
the commutation frequency. As motor 100 rotates, different transistors 
will be turned on and off, although there will always be four transistors 
off, one transistor on, and one transistor will be switching (PWM'd). 
Two modes of operation are discussed herein--a "normal" mode and a "noise 
reduction" mode. As explained in more detail below, a difference between 
these modes is a technique for controlling the switching of transistors 
being switched, and particularly for controlling the timing (i.e., 
inception and termination) of the switching and the duty cycle PWM signal 
applied to the switched transistors. As used herein, "duty cycle" is the 
ratio of how long a PWM signal is "1" divided by the PWM cycle time ("1" 
plus "0" time). 
In the first mode or "normal" mode, acoustic noise reduction techniques are 
not implemented. In the normal mode, processor 1 applies on line 2 a 
signal which causes mode select switch 6 to apply consistently the Hall 
output values on line 5 to machine 8. Thus, in the normal or non-noise 
reduction mode of operation, the signal on line 7 is selected to come from 
the Hall sensors 28. Hall sensors 28 encode the angular position of rotor 
102 to a resolution of 24 parts per revolution (15 degrees) in the 
preferred embodiment. In the normal mode, state machine 20 and synthesized 
Hall signals on line 3 are not used. As rotor 102 rotates, the values of 
the Hall sensor signals on Hall line 5 change, and this in turn controls 
which signals are applied to the transistors (21, 22, 29, 30, 33 and 34) 
through the action of table 11 and switch 12. 
FIG. 3 shows the normal mode of operation. At the commutation points, one 
coil is turned off abruptly as another is turned on. As indicated 
previously, this sudden change can result in acoustic noise, especially in 
"axial gap" type motors. In the illustrated example of a three phase, 
eight pole brushless dc motor, commutation takes place every 15 degrees of 
motor rotation. The illustrated embodiment shows the transistor switching 
sequence of a "two quadrant" power amplifier. In this embodiment, the 
current in the motor is approximately proportional to the duty cycle. The 
voltage applied to one of the motor coils is either zero (when the PWM 
signal is zero) or "+V" (when the PWM signal is 1). In the preferred 
embodiment, "+V" is twelve volts. 
In the noise reduction mode, discussed in more detail below, processor 1 
selects (by changing the state of mode select line 2) the state signal on 
line 4 to be applied via switch 6 to machine 8. The state signal on line 4 
is derived by state machine 20 using position tachometer information on 
line 16. The angular resolution of the tach signal on line 16 must be much 
greater than that of the Hall sensors 28. In the preferred embodiment, 
there are 2880 distinct angular positions per motor revolution resolved by 
tachometer 15 compared to only 24 resolved by Hall sensors 28. 
Thus, reviewing the two modes of operation, the position of the switches 12 
in each of the circuits comprising machine 8 depends on how its associated 
commutation table 11 responds to the value of the signal on line 7. In 
accordance with the value of the mode select signal applied on line 2 from 
processor 1 to mode select switch 6, the value on line 7 may come either 
from the magnetic Hall position sensors 28 in motor 100 (in the normal 
mode) or be a state value applied on line 4 from state machine 20 (in the 
noise reduction mode). In essence, the state value applied on line 4 is a 
"synthesized" signal which is derived from the tach sensor 15 via machine 
20. 
Turning now in more detail to the noise reduction mode of operation, 
attention is directed to FIG. 4 as graphically showing two consecutive 
steps of a noise reduction commutation sequence. FIG. 4 shows how control 
system 110 of the invention controls the average duty cycle applied (on an 
appropriate one of lines 13) to each coil. Before motor 100 rotates to the 
normal commutation point, the transistor gate PWM signal applied on line 
13.sub.30 to transistor 30 (transistor Qb2) is switched between "0" and 
the PWM signal on line 10. Such switching is effected by signal source 
select switch 12, in response to its select control signal as output from 
its associated table 11, which in turn is based on the value of 
synthesized Hall signal on line 4 as applied on line 7 via switch 6. 
Since the processor or nominal PWM signal on line 10 is also changing 
between 0 and 1, it is important that the switching action of signal 
source select switch 12 be slower than that of the processor PWM signal on 
line 10. The resulting gate drive signal on line 13 will therefore consist 
of at least one on/off pwm cycle, followed by some time at 0. The motor 
current is reduced to the extent that the gate drive signal on line 13 
spends more time at 0 than at 1. This results in a reduced average duty 
cycle being applied on line 13, and consequently a reduced average current 
in that coil. 
The processor PWM signal on line 10 can have any duty cycle from 0 to 100%, 
but is chosen to be constant in the illustrated embodiment. However, due 
to the action of signal source select switch 12, the average duty cycle of 
the gate drive signal on line 13 is changing. 
FIG. 4 shows that the gate drive PWM signal output on line 13 spends some 
time at a maximum duty cycle. This maximum can be anything between 0 and 
100%, and the time spent at the maximum can as short as desired in order 
to tune the system to the characteristics of motor 100. 
As motor 100 rotates closer to a commutation point, the gate drive signal 
on line 13 spends increasingly more time connected to a zero than it does 
connected to the processor or nominal PWM signal on line 10 (which is 
itself rapidly switching between zero and one). The current in coil 26 is 
therefore ramped down. At the same time, the current in coil 27 is ramped 
up by a similar action. The result is an overlap of current flow in coils 
26 and 27 which greatly reduces acoustic noise. By contrast, in the prior 
art, there is no overlap and one coil is always off. 
FIGS. 5-8 show which transistors are turned on (indicated by a "1" next to 
their gate input) and off ("0") as motor 100 commutates from coils 25-26 
to coils 25-27. Note that only one of the upper transistors 21 is on, 
while lower transistors 30 and 34 are on at various times depending on the 
value of the PWM signal and the duty cycle ramp. In FIG. 5, transistors 21 
and 30 are turned on and current flows though them and through coils 25 
and 26. Even before the duty cycle ramp begins, the PWM signal applied to 
transistor 30 switches on and off. FIG. 6 shows (in broken lines) where 
the current flows when PWM=0 through the "flyback" diode 31 (due to an 
effect called "inductive kick"), but still from transistor 21 to coils 25 
and 26. 
Current builds up and decays in the coils at a much slower rate than the 
PWM switching action. In FIG. 5, the current in both coils 25 and 26 is 
increasing while in FIG. 6 it is decaying. By allowing more time (greater 
duty cycle) for current to build up than for it to decay, the average 
current level increases. Conversely, if the duty cycle is decreased, the 
average current level drops because it has more time to decay than it has 
to increase. 
FIGS. 7 and 8 show the same sequence of events for current flow in coils 25 
and 27, which corresponds to the next commutation state. The lower 
transistor 34 is switched on and off and the current flows alternately in 
transistor 34 and diode 36, but always in upper transistor 21 and coils 25 
and 27. The other four transistors (i.e., transistors 22, 29, 30, and 33) 
are all off. 
FIGS. 5-8 thus correspond to what happens when the "Average PWM duty cycle" 
of FIG. 5 has reached its maximum. It is this average PWM duty cycle that 
is the gate drive PWM signal applied on line 13. During the ramp, motor 
100 and its power transistors are switched between commutation states to 
achieve an overlap in the current flow. 
As the ramp begins (see FIG. 5), commutation step 1 directs the gate drive 
(average) PWM signal exclusively to transistor 30. The gate drive to 
transistor 30 (on line 13.sub.30) switches between 0 and 1 and the current 
flows alternately in transistor 30 and flyback diode 31. Then the gate 
drive (average) PWM signal begins to be reduced to transistor 30 by 
occasionally applying zero (on line 13.sub.30 via signal source select 
switch 12) instead of the nominal PWM signal. At the same time, controller 
110 occasionally applies the nominal PWM signal to transistor 30. 
Thus, in the noise reduction mode of operation, there are three duty cycles 
to consider: the duty cycle of the nominal or processor PWM applied on 
line 10 (which is fixed in the illustrated embodiment); the duty cycle of 
the synthesized Hall signal applied on line 4 from state machine 20 to 
machine 8; and, the duty cycle of the resulting gate drive PWM or 
"average" duty cycle applied on line 13. 
During the overlap (shown for example in FIG. 4), both coils 26 and 27 
conduct current at the same time as if commutation steps 1 and 2 were 
mixed together. Since coil 25 remains on, all three coils 25, 26, and 27 
conduct simultaneously for a period of time. At the end of the ramp the 
gate drive PWM signal is being applied exclusively to transistor 34 while 
transistor 30 if turned off. 
FIG. 9 shows a flow diagram useful for summarizing operation of motor 
control system 110. At step 900, processor 1 determines whether it has 
been programmed to be in the normal mode or the noise reduction mode. If 
the normal mode has been selected, steps 902 and 904 are executed. If the 
noise reduction mode has been selected, steps 906-911 are executed. 
In the normal mode, at step 902 processor 1 applies its mode select signal 
on line 2 to mode select switch 6 so that actual Hall signals from Hall 
sensors 28 (from line 5) are applied on line 7 to the six circuits 
comprising machine 8. The changing of state of the actual Hall signals 
every 15 degrees of rotation of rotor 102 accordingly causes each table 11 
to change its commutation state every 15 degrees in the manner illustrated 
in FIG. 3. In the normal mode of operation, the synthesized Hall signals 
generated by state machine 20 are not utilized. 
In the noise reduction mode, at step 906 processor 1 applies a signal on 
mode select line 2 so that mode select switch 6 transmits the synthesized 
Hall state signal on line 4 for use as the signal to be applied on line 4 
to the six circuits in machine 8. The synthesized Hall state signal on 
line 4 can alternate between two digital values, a first of the digital 
values being known as the "old" state and a second of the digital values 
being known as the "new" state. As indicated below, the Hall equivalent 
translator 19 controls switching of the value on line 4 between the old 
and new states. 
As indicated by step 907, state machine 20, particularly Hall equivalent 
translator 19, has been programmed or downloaded (e.g., on line 3 from 
processor 1) with various synthesizing input parameters. These 
synthesizing input parameters include a ramp starting point input 
parameter, a ramp slope input parameter, and a ramp termination input 
parameter. In accordance with other operating considerations such as motor 
speed, and in the manner described below, Hall equivalent translator 19 
uses the synthesizing input parameters to determine a ramp starting 
differential, a ramp termination differential, and timing of state changes 
of the synthesized Hall signal applied on line 4 in order to affect the 
desired ramping technique as shown in FIG. 4. The ramp starting point and 
ramp termination point are in terms of angular position of rotor 102, and 
the differentials are in terms of degrees (e.g., both differentials being 
shown as the same "X" degrees in the particular embodiment shown in FIG. 4 
and FIG. 9). FIG. 4 shows how the ramp starting differential and the ramp 
termination differential are utilized to ascertain an exemplary ramp 
starting point RSP and ramp termination point RTP. 
Step 908 reflects monitoring by state machine 20 of the tach signal on line 
16 from tachometer 15. Decoder 17 constantly decodes the tach signal on 
line 16, so that position counter 18 can determine at any moment the 
rotational position of rotor 102. Step 908 shows the tach count being 
read, and comparison of the thusly known rotational position of rotor 102 
with the commutation points. Then, as indicated at step 909, Hall 
equivalent translator 19 determines if the current rotational position of 
the rotor 102 is within the ramp starting differential of the commutation 
point (i.e., if the ramp starting point RSP has been reached). If the ramp 
starting point has been reached, step 910 is executed. Otherwise, as 
indicated by step 911, the value of the synthesized Hall signal on line 3 
is maintained at a constant, since no ramping is then to occur and only 
one set of transistors is energized (i.e., current flows in two of the 
three coils). 
At step 910, Hall equivalent translator 9 causes the signal on line 4 to 
alternate between its "old" and "new" states. Upon reaching a ramp 
starting point RSP, the signal on line 4 initially spends most of its time 
in the old state, but increasingly more time is spent in the new state (as 
indicated by the ramping up in FIG. 4) until the signal reflects only the 
new state. The signal on line 4 will not be steady with the new state 
until "X" degrees after the commutation point. 
It should be understood that while the synthesized signal on line 4 is 
being interpreted by machine 8 to ramp up one transistor (e.g., 
interpreted by transistor control circuit 8.sub.34 to ramp up transistor 
Qc2=transistor 34 in FIG. 4), the same signal is being interpreted by 
machine 8 to ramp down another transistor (e.g., interpreted by transistor 
control circuit 8.sub.30 to ramp down transistor Qb2=transistor 30 in FIG. 
4). 
Similarly understood from FIG. 9, a similar positional monitoring technique 
is utilized so that Hall equivalent translator 19 knows when to start 
ramping down, e.g., to start spending increasingly more time in the old 
state than the new state. 
Thus, restated differently, in the noise reduction mode, state machine 20 
constantly monitors the position of motor 100 as indicated by the 
tachometer signal 16. This position is compared to the known commutation 
points and a programmable value "X" which represents the starting point of 
the ramp, located some number of degrees before the nominal commutation 
points. A second programmable value represents the slope of the ramp. A 
third programmable value can be used to adjust the phase of the ramp, 
relative to motor angular position. 
If the motor position is more than "X" degrees from the nominal commutation 
point, say between 3 and 12 degrees position (with the nominal commutation 
points at 15, 30, 45 . . . degrees), the synthesized hall signal on line 4 
and the nominal hall signals are the same. The six outputs of machine 8 
likewise are static (except in the sense that the gate drive PWM signal is 
switching between 0 and 1, but it is applied to the same one or two 
transitors during this time). 
At 12 degrees of position, a commutation state ramp begins: the value of 
the signal on line 4 is switched between the old state (used exclusively 
between 3 and 12 degrees) and the new state (which will be the exclusive 
state between positions 18 to 27 degrees). At first, the mix is mostly old 
with a little new, but as motor 100 rotates further, the mix is changed to 
include more of the new state and less of the old until, at position 18 
degrees, the mix is 100% new state. The rate at which machine 20 switches 
between these two states is slower than the PWM frequency, but much faster 
than the time between nominal commutation points. 
For maximum flexibilty, the input parameters are programmable, e.g., the 
starting position of the ramp, the slope, and perhaps the phase (so the 
ramp might start and end early or late). 
It will be understood by the man skilled in the art, in connection with 
conventional commutation table construction, how to prepare commutation 
tables 11 of machine 8 in order to implement the invention as described 
herein. 
In one embodiment, illustrated in FIG. 10, motor control system 110 as 
described herein as application in a helical scan magnetic tape drive 120, 
particularly for controlling operation of reel motors such as a take up 
reel 126. FIG. 10 shows a capstanless a helical scan recording system or 
drive. FIG. 10 illustrates generally with reference numeral 120 a tape 
path. In particular, FIG. 10 shows a magnetic tape 122 (such as an 8 mm 
magnetic tape, for example) having a first end wound around a supply reel 
124 and a second end wound around a take-up reel 126. The path traversed 
by tape 122 is defined at least in part by a series of tape guides 
128A-128G and a rotating scanner or drum 130. Tape guides 128 and drum 130 
are ultimately mounted on a deck floor. In all operations excepting a 
rewind operation, tape 122 travels from supply reel 124 to take-up reel 
126 in the direction depicted by arrow 131. 
As shown in FIG. 10, drum 130 has read heads R1 and R2 as well as write 
heads W1 and W2 mounted on the circumference thereof. Drum 130 rotates in 
the direction depicted by arrow 132. In addition, drum 130 has a servo 
head S mounted circumferentially thereon. As drum 130 rotates, at any 
moment a portion of its circumference is in contact with travelling tape. 
During a recording or write operation, write heads W1 and W2 are 
periodically positioned to record "stripes" or "tracks" as heads W1 and W2 
move in a direction of head travel across tape 122. FIG. 10 also shows a 
tensioning arm 144. 
Details regarding the positioning of the heads W1, W2, R1, R2 and S, as 
well as the particular track recording scheme achieved by drum 130, are 
disclosed in United States Patent Application Ser. No. 08/150,726 (filed 
Nov. 12, 1993) of Georgis and Zweighaft entitled "Method And Apparatus For 
Controlling Media Linear Speed In A Helical Scan Recorder" (incorporated 
herein by reference). 
Take-up reel 126 includes a geared take-up motor 100. The motor control 
system 10 of the present invention is utilized, in the embodiment of FIG. 
10, to control take-up reel motor 100 in the manner aforedescribed. 
Application of motor control system 110 to a helical scan magnetic tape 
drive can be understood in further detail by reference, for example, to 
U.S. patent application Ser. No. 08/150,730 entitled CAPSTANLESS HELICAL 
DRIVE SYSTEM; U.S. patent application Ser. No. 08/150,731 entitled HIGH 
PERFORMANCE POWER AMPLIFIER; and U.S. patent application Ser. No. 
08/150,727 entitled POWER-OFF MOTOR DECELERATION CONTROL SYSTEM, all of 
which are incorporated herein by reference. In such embodiment, the ramp 
starting point RSP and ramp termination point RTP as shown in FIG. 4 may 
be equidistant between commutation points (e.g., 7 degrees before and 
after a commutation point, respectively). 
Advantageously, motor control circuit 110 of the present invention 
digitally emulates the effect of a snubber. According to the invention, 
the gate drive duty cycle of the applied voltage is ramped down with 
respect to a nominal gate drive value to some chosen minimum value 
corresponding to one of the commutation points. As the commutation point 
is passed, the gate drive duty cycle is ramped back up to the nominal gate 
drive value and held there until the next commutation point approaches. 
This process is repeated at every commutation point. Because the motor 
current is controlled by the gate drive duty cycle, it will be minimized 
at the commutation points where one set of coils are energized while a 
second set are de-energized. This greatly reduces the abruptness of the 
coil changeover that is the primary cause of acoustic noise, resulting in 
nearly silent operation. 
It is thus possible to overlap the rise and fall of current in some subset 
of the coils as the motor passes through a commutation point. In this case 
the coil changeover does not occur all at once at some angular position, 
but rather occurs gradually over some finite distance. This technique 
results in improved noise reduction. 
While the invention has been particularly shown and described with 
reference to the preferred embodiments thereof, it will be understood by 
those skilled in the art that various alterations in form and detail may 
be made therein without departing from the spirit and scope of the 
invention. For example, it should be understood that the nominal PWM 
signal applied on line 10 can be generated by means other than a 
processor, as is conventionally known. Moreover, rather than having an 
essentially constant processor or nominal PWM value applied on line 10, 
the PWM value on line 10 can be varied in accordance with desired average 
torque in the motor. Further, while the embodiment described herein 
corresponds to a "2 quadrant" switching scheme, it should be understood 
that the invention applies equally well to other schemes, such as "4 
quadrant" switching.