Monolithic, Programmable, Full-Bridge Motor Driver Integrates PWM Current Control and 'Mixed-Mode' Microstepping

Technical Paper
STP 97-5A
PRODUCT
DESCRIPTION
MONOLITHIC, PROGRAMMABLE, FULL-BRIDGE MOTOR DRIVER
INTEGRATES PWM CURRENT CONTROL AND ‘MIXED-MODE’
MICROSTEPPING
Paul Emerald, Roger Peppiette, and Anatol Seliverstov
INTRODUCTION to ‘MIXED-MODE’ IC
Specifically created and intended for bidirectional current
control of two-phase step motors, this IC merges pulsewidth-modulation (PWM) current regulation with innovative mode control circuitry. Although singularly targeted
for bipolar-drive of ‘series connected’ step motors, the
digital current control techniques incorporated in the
device may prove useful in other inductive load applications.
The user selectable choices include: programming (regulating) the load current; setting the duration of the fixed
OFF-time interval; mode control of the (decaying) recirculation current; and ‘tuning’ (or adapting) the ‘mixedmode’ current decay to optimize microstepping operation.
Both logic and linear current control circuitry is included,
and a 3-bit digital-to-analog converter (DAC) is used to
control and ratio the output currents that are very essential
to smooth, resonance-free motion.
Two distinctive attributes are incorporated in this fullbridge motor IC: a 3-bit ‘non-linear’ DAC, and the
‘mixed-decay’ PWM operation. Both are very crucial for
‘tuning’ of microstepping designs.
The full-bridge, PWM motor IC is rated to 50 V and ±1.5
A maximums; includes integral ‘free-wheeling’ diodes for
clamping inductive transients generated during switching;
incorporates thermal shutdown and under-voltage protective circuitry; and internal timing prevents ‘cross-over’
currents associated with simultaneous (output) conduction.
MIXED DECAY
SLOW DECAY
Such benchmark functions are now very standard; however, the capability to provide ‘mixed-mode’ current
decay during operation and its impact on tuning the
recirculating current to best emulate a sinewave are not
commonplace circuit functions.
Typically, with any ‘mixed-mode’ operation, the two
quadrant, slow-decay recirculation mode is exploited
during the ascending ‘half-cycle’ of the phase current.
Four quadrant, fast-decay is used on the descending ‘halfcycle’ of the step motor winding current. The ratio (e.g.,
percentage) of fast-decay to slow-decay during this
descending portion of the sinewave can be controlled to
tune both drive and motor to the actual system needs.
A voltage applied to the Percent Fast-Decay (PFD) input
controls and ratios the time spent in fast-decay, regenerative operation during each of the discrete PWM cycles.
The outcome is much lower motor current ripple,
smoother and quieter rotor movement, and reduced motor
heating; but without sacrificing motor current regulation.
Per figure 1, the emulation of a sinewave using only eight
(8) current ratios and a ‘mixed-mode’ drive is portrayed.
In the PFD mode, switching is regulated by the internal
circuitry, and a user is able to adjust or adapt the ratios of
slow-decay and fast-decay. This allows ‘fine-tuning’ the
driver and motor combination to match specific systems
requirements, motor characteristics, etc.
MIXED DECAY
SLOW DECAY
Dwg. WK-004-3
Figure 1: ‘Mixed-mode’ microstepping emulation of sine wave
1
FULL-BRIDGE MOTOR DRIVER
with PWM CURRENT CONTROL and
‘MIXED-MODE’ MICROSTEPPING
INCREMENTAL MOTION TECHNOLOGY
Various control methods and circuit configurations offer
contrasts in complexity, cost, performance, etc. The
essential applications of step motors are open-loop systems that may apply various types of operating modes and
drive configurations; and those that pertain to this
microstepping IC include:
Full-step Incremental motion
’Wave’ (single-phase) drive
Two-phase drive
Fractional step incremental motion
Half-stepping operation
Quarter-stepping operation
Microstepping (1/8th step increments)
sacrifice in motion system performance; and, also, very
often affords lower systems costs, less complexity, and
diminished power and heating.
Full-Step Incremental Motion
‘Wave’ (Single-Phase) Drive
Although today seldom used in full-step designs, ‘wave’
drive is an integral part of microstepping. Only one
winding is activated (at rated current), and a four-step
sequence is depicted in figure 2. All the energized states
correspond to the ‘detent’ (unenergized) rotor positions
and are the ‘natural’ full-step alignments of rotor and
stator.
Two-Phase Drive
Power, torque, and positional correlations
’Wave’ vs. two-phase drive
Unipolar vs. bipolar drive
Constant torque drive
Operational, usable torque
Torque vs. rotor displacement
The basic relationships of step motor connections and
drive configurations are itemized in table 1; and A3955
current ratios listed in table 2. The bipolar-series drive is
the most preferred scheme for the A3955 microstepping
IC. The bipolar-parallel connection demands twice the
current of bipolar-series; this raises internal dissipation
and heating, and reduces the availability of integrated
drivers.
Two-phase operation affords the increased torque associated with activating both coils. Compared to ‘wave’
(single-phase) drive the torque vector becomes 141% and
the rotor alignment is 45° as shown in figure 3. However,
an unenergized step motor cannot retain this ‘half-step’
position, and must be powered to maintain this alignment.
Table 2: Digital-to-analog truth table
D2
1
1
1
1
0
0
0
0
Generally, the intent is to select a cost-effective unification of the step motor and drive circuitry. Frequently, this
translates to a rationale to select a step motor with a lower
current/higher voltage ratings combination. Such a
determination offers definite benefits without any tangible
D1
1
1
0
0
1
1
0
0
D0
1
0
1
0
1
0
1
0
DAC%
100%
92.4%
83.1%
70.7%
55.5%
38.2%
19.5%
0%
Table 1: Step motor rating relationships
Mode
1Ø, ‘Wave Drive’
2Ø, Unipolar
Bipolar, Parallel
Bipolar, Series
Power
Current
Voltage
Torque*
Time constant
0.5†
1.0
1.0
1.0
1.0†
1.0
1.4
0.7
1.0
1.0
0.7
1.4
≈0.7
1.0
≈1.4
≈1.4
1
1
2
2
* ‘Holding’ (or static) torque; step motor is energized, but not rotating.
† Per table 1, ‘wave drive’ operation involves switching only one winding.
2
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
Copyright © 1997, ZM Communications GmbH
FULL-BRIDGE MOTOR DRIVER
with PWM CURRENT CONTROL and
‘MIXED-MODE’ MICROSTEPPING
Fractional Step Increments
Quarter-Stepping Operation
Half-Stepping Operation
Current ratioing and the resultant constant torque are
imperative for any fractional step increments beyond the
2-1-2 half-step technique associated with figure 6. Per
figure 6, quarter-stepping is incremental motion using
22.5° sub-steps. This involves combinations of 92.4%
with 38.2%, and 38.2% with 92.4% to produce the rotor
positions and step subdivisions represented in figure 6.
Half-stepping necessitates driving both windings, and the
simplest, most universal technique uses a 2-1-2 ON
activation sequence that combines two-phase and singlephase drive. However, as illustrated in figure 4, the torque
varies from the 100% value of ‘wave’ drive (A, B, A, and
B) to the 141% level of two-phase drive (AB, AB, AB,
and AB). Whereas the 2-1-2 drive mode induces intermediate rotor positions and smoothes rotation, it is not the
best technique for half-step operation (the torque ripple is
substantial).
Microstepping (1/8th Step) Operation
Further subdividing one full-step into 1/8th steps is
usually designated “microstepping” (figure 7). The
angular increment diminishes and corresponds to 11.25°,
and nine current levels are required to obtain the perfect
current ratios. However, this necessitates a 4-bit DAC,
and a very viable approximation is attained using the 3-bit
DAC with 100% current in PHASE A ratioed with 19.5%
in PHASE B. The ‘ideal’ phase currents are 98.1% and
19.5%, but this inconsistency is insignificant. The calculation for this 1/8th step is predicated upon decreasing coil
current per: cosine of (90 ÷ 8) or 98.1%.
Constant torque half-stepping follows the vectors illustrated in figure 5; and offers smoother and quieter operation than that of figure 4: output currents are ratioed
(70.7%) for the two-phase increments of the cycle (AB,
AB, AB, and AB). The two-phase currents that induce the
half-step (45°, 135°, 225°, 315°) rotor positions, and
create constant torque involve the sine and cosine vector
of output A and output B (i.e., 0.707).
A
A
AB
AB
B
AB
AB
B
B
B
AB
AB
AB
A
AB
A
Figure 2: ‘Wave’ drive (1-phase)
Figure 3: Two-phase, full-step
Figure 4: Half-step (2-1-2)
A
A
A
AB
AB
B
AB
B
AB
AB
A
Figure 5: Half-step (constant torque)
AB
AB
B
B
AB
AB
A
Figure 6: Quarter-step
AB
B
B
AB
AB
A
Figure 7: Microstepping
3
FULL-BRIDGE MOTOR DRIVER
with PWM CURRENT CONTROL and
‘MIXED-MODE’ MICROSTEPPING
TORQUE (141%)
P
STE
EP
AN
T
1/4
ST
N
TO
2
E
ST
U
EP
Q
R
3/8
70.7
ST
83.1
O
This 90° phase current differential is illustrated in figure
9; and the diagram illustrates the phase currents in the
PWM microstepping mode. Note that these 1/8th-step
ratioed currents emulate the sinewave they overlay. As
92.4
C
Two-phase motor operation requires the winding currents
differ by 90°. It should be noted that step motors can
operate as synchronous motors if ac power is applied and
a 90° phase difference is maintained between the winding
currents.
MAXIMUM FULL-STEP
0%
55.5
1/
MICROSTEPPING A TWO-PHASE MOTOR
A
100
10
The constant torque vectors depicted in figure 8 epitomize
control of the phase currents over the full-step from A ON
(100%) to B ON (100%) and illustrates a 90° arc partitioned into 11.25° increments that are 1/8th step subdivisions. With a 1.8° motor (200 steps per revolution) a
1/8th-step increment equates to a 0.225° movement of the
motor rotor and shaft.
The phase currents in figure 9 actually represent six fullsteps, and four full-steps correspond to the sequence
starting with PHASE A = 100% and PHASE B = 0%. The
full series is enumerated in table 3; and the input logic
sequence and motor operation correlates with the PWM
microstepping waveforms illustrated in figure 9. Table 3
EP
Implementing quarter-stepping and microstepping is
predicated upon developing the unequal, ratioed currents
listed in table 2. The rotor is deflected toward the stronger
pole, and the magnetic field strength is directly related to
ampere turns. The equal currents (70.7%) of half stepping
produce rotor positions of 45°, etc. with constant torque.
1/8 ST
CONSTANT TORQUE OPERATION
will be shown later, with ‘mixed-decay’ operation the
filtering effects of the motor windings produce output
waveforms that very closely approximates a sinewave.
This affords the smooth, quiet, resonance-free motion that
is the primary attribute of microstepping.
CURRENT IN PER CENT
Because the cosine error in current (<2%) is quite negligible, and the 3-bit DAC accuracy is ±3% with a reference
input ≥1.0 V (±4% ≤1.0 V), eight current ratios can satisfy
most requirements.
EP
5/8
ST
38.2
P
3/4
19.5
STE
EP
7/8 ST
FULL STEP
B
B
19.5
A
38.2
55.5
70.7
CURRENT IN PER CENT
83.1 92.4
100
Dwg. GK-020-1
Figure 8: Constant-torque vectors
PHASE A
Figure 9: Phase current waveforms (microstepping mode)
PHASE B
Dwg. WK-004-4
4
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
FULL-BRIDGE MOTOR DRIVER
with PWM CURRENT CONTROL and
‘MIXED-MODE’ MICROSTEPPING
Table 3: Input logic sequence and operating modes for two-phase step motor
Driver IC #1
(Phase A)
Ø D2 D1 D0 PFD
1 1
1
1
1
1 1
1
1
0
1 1
1
0
0
1 1
0
1
0
1 1
0
0
0
1 0
1
1
0
1 0
1
0
0
1 0
0
1
0
0 0
0
0
0
0 0
0
1
1
0 0
1
0
1
0 0
1
1
1
0 1
0
0
1
0 1
0
1
1
0 1
1
0
1
0 1
1
1
1
0 1
1
1
1
0 1
1
1
0
0 1
1
0
0
0 1
0
1
0
0 1
0
0
0
0 0
1
1
0
0 0
1
0
0
0 0
0
1
0
1 0
0
0
0
1 0
0
1
1
1 0
1
0
1
1 0
1
1
1
1 1
0
0
1
1 1
0
1
1
1 1
1
0
1
1 1
1
1
1
1 1
1
1
1
Driver IC #2
Current
100.0%
100.0%
92.4%
83.1%
70.7%
55.5%
38.2%
19.5%
0.0%
-19.5%
-38.2%
-55.5%
-70.7%
-83.1%
-92.4%
-100.0%
-100.0%
-100.0%
-92.4%
-83.1%
-70.7%
-55.5%
-38.2%
-19.5%
0.0%
19.5%
38.2%
55.5%
70.7%
83.1%
92.4%
100.0%
100.0%
MO
S
M
M
M
M
M
M
M
D
S
S
S
S
S
S
S
S
M
M
M
M
M
M
M
D
S
S
S
S
S
S
S
S
Ø
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
(Phase B)
D2 D1 D0 PFD
0
0
0
0
0
0
1
1
0
1
0
1
0
1
1
1
1
0
0
1
1
0
1
1
1
1
0
1
1
1
1
1
1
1
1
1
1
1
1
0
1
1
0
0
1
0
1
0
1
0
0
0
0
1
1
0
0
1
0
0
0
0
1
0
0
0
0
0
0
0
1
1
0
1
0
1
0
1
1
1
1
0
0
1
1
0
1
1
1
1
0
1
1
1
1
1
1
1
1
1
1
1
1
0
1
1
0
0
1
0
1
0
1
0
0
0
0
1
1
0
0
1
0
0
0
0
1
0
0
0
0
0
Current MO
0.0%
D
19.5%
S
38.2%
S
55.5%
S
70.7%
S
83.1%
S
92.4%
S
100.0%
S
100.0%
S
100.0% M
92.4%
M
83.1%
M
70.7%
M
55.5%
M
38.2%
M
19.5%
M
0.0%
D
-19.5%
S
-38.2%
S
-55.5%
S
-70.7%
S
-83.1%
S
-92.4%
S
-100.0% S
-100.0% S
-100.0% M
-92.4%
M
-83.1%
M
-70.7%
M
-55.5%
M
-38.2%
M
-19.5%
M
0.0%
D
Step Motor
Torque
Value Angle
1.000
0.00
1.019 11.03
1.000 22.46
0.999 33.74
1.000 45.00
0.999 56.26
1.000 67.54
1.019 78.97
1.000 90.00
1.019 101.03
1.000 112.46
0.999 123.74
1.000 135.00
0.999 146.26
1.000 157.54
1.019 168.97
1.000 180.00
1.019 191.03
1.000 202.46
0.999 213.74
1.000 225.00
0.999 236.26
1.000 247.54
1.019 258.97
1.000 270.0
1.019 281.03
1.000 292.46
0.999 303.74
1.000 315.00
0.999 326.26
1.000 337.54
1.019 348.97
1.000 360.00
∆∠
11.03
11.03
11.43
11.28
11.26
11.26
11.28
11.43
11.03
11.03
11.43
11.28
11.26
11.26
11.28
11.43
11.03
11.03
11.43
11.28
11.26
11.26
11.28
11.43
11.03
11.03
11.43
11.28
11.26
11.26
11.28
11.43
11.03
Input codes: Ø = PHASE input (direction); PFD = percent fast-decay; MO = operating mode
Operating modes: D = disabled (OUTPUT OFF); S = slow-decay; M = ‘mixed-mode’ decay
Motor codes: Value = torque vector magnitude; Angle = rotor position; ∆∠ = delta angle
NOTE: The logic sequencing and operating modes pertain to four full steps of figure 9.
5
FULL-BRIDGE MOTOR DRIVER
with PWM CURRENT CONTROL and
‘MIXED-MODE’ MICROSTEPPING
Whereas microstepping can provide an increased positional resolution, its primary benefit is quiet, smooth and
resonance-free motion (especially at the lower step rates).
The reduction in overshoot and ringing (the system
oscillation that follows an abrupt change in either velocity
or position) is depicted in figure 10. Also, the linearity of
the time and position relationship is very evident, and this
characteristic supports decreasing the elapsed time for the
system movements.
at very low step frequencies) to circumvent resonance and
audible vibration, and to reduce settling times (decreased
ringing and overshoot compared to full-step). In most
instances, especially designs exploiting very small ‘canstack’ motors, subdividing a full-step cannot (readily)
assure that a precise, repeatable incremental rotor displacement is the result.
Microstep
POSITION
lists the complete 32 fractional (1/8th) steps required to
rotate the motor four (4) full-steps (i.e., 7.2° with a 1.8°,
(200 steps-per-revolution motor), and corresponds with
the vector diagram denoted in figure 7. Note that both
torque magnitude and incremental angles (∆∠) are very
consistent; this effectively illustrates the constant torque
vectors and the sine/cosine phase current relationships.
Full step (exhibits
overshoot and ringing)
OPEN-LOOP MICROSTEPPING
Most step motors operate in an ‘open-loop’ mode (no
positional or velocity feedback signals); this (essentially)
affords the benefits of lower system complexity and costs.
However, the capability to attain improved positional
accuracy and resolution is another prospect for
microstepping. Typically, the most critical element to
realizing intermediate positions is the motor itself, and
motors with low ‘detent’ torque (also known as ‘residual’
or ‘idle’ torque) consistently surpass standard versions
that have higher detent torque specifications.
Designers intending to increase system positional resolution should evaluate the drive circuitry and motor response
if microstepping is used to extend incremental motion by
subdividing the steps. The rotor may (or may not) follow
the current ratios; thus, the resulting angular displacement
can often be quite non-linear. The torque vs. displacement
characteristics depicted in figure 11A are clearly not
suitable for accurately subdividing a step; but the torque/
displacement properties of Motor #2 in figure 11B are
very linear and uniform and well suited for increasing
positional resolution.
Matching/mating the drive circuitry with the step motor
can surmount such non-linearity; however, using step
motors designed and manufactured for microstepping is
often the most reliable and best solution to accurate,
uniform fractional steps. Practically, open-loop
microstepping is exploited for smooth, quiet motion (even
6
TIME
Figure 10: Step motor settling time
(Courtesy of Compumotor Division of Parker Hannifin Corp.)
Figure 11A: Torque vs. displacement (motor #1)
Figure 11B: Rotor vs. displacement (motor #2)
(Courtesy of Litchfield Engineering; Kingman, AZ)
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
FULL-BRIDGE MOTOR DRIVER
with PWM CURRENT CONTROL and
‘MIXED-MODE’ MICROSTEPPING
STEP MOTOR CONTROL/DRIVE SYSTEM
switches both OUTPUTA high and OUTPUTB low; this
corresponds to a positive (left-to-right) current that is
associated with the upper portion of the sinewave shown
in figure 1, and the drive current path shown in figure 13.
Designed for small, low-cost step-motor drives, two
A3955s, plus the external, passive discretes, are required
to power both coils of a two-phase step motor. The
functional block diagram shown in figure 12 represents
the circuitry designed for directly controlling and driving
each of the step motor windings (two per motor are
required).
An internally developed ‘dead-time’ (≈1.5 µs) interval
when switching the PHASE input (changing current
direction) precludes damaging/destructive ‘cross-over’
currents associated with overlapping (i.e., simultaneous)
conduction of upper and lower outputs. Exploiting this
‘deadtime’ interval avoids dynamic (switching) mismatches that can result in a momentary ‘shorting’ of the
supply to ground through an overlapping ON state of the
upper and lower outputs. Obviously, ‘cross-over’ currents
can damage or destroy the IC and must be averted.
Typically, only three external passive components (per
coil) are needed; a current-sensing resistor, RS, plus the
RC network (RT, CT) required to set the fixed OFF-time
interval for PWM operation.
ELEMENTS of MOTOR CURRENT CONTROL
Directional Control of Motor Current
These ‘shoot-through’ currents are related to the upper
(sourcing) outputs and their (much) slower turn-OFF
characteristics (vs. the sinking outputs).
10
6
VCC
PHASE
15
Figure 12: Functional block
diagram of ‘mixed-mode’
driver IC (A3955)
LOAD
SUPPLY
OUTB
OUTA
LOGIC
SUPPLY
The PHASE input controls the direction of current flow to
the motor. A logic-high level applied to the PHASE input
16
7
VBB
GROUND
4
5
UVLO
& TSD
12
13
MIXED-DECAY
COMPARATOR
CURRENT-SENSE
COMPARATOR
SENSE
11
+
–
R
+
–
Q
S
÷3
BLANKING
DISABLE
RS
+ –
RT
V TH
2
8
9
14
D0
RC
3
D1
VCC
D/A
D2
1
BLANKING
GATE
REF
PFD
PWM LATCH
CT
Dwg. FP-042
7
FULL-BRIDGE MOTOR DRIVER
with PWM CURRENT CONTROL and
‘MIXED-MODE’ MICROSTEPPING
Power Output Operation and Truth Table
In addition to directional control, the operation of the
motor IC is governed by: three logic inputs to the digitalto-analog converter (DAC); a stable, fixed reference
voltage (although this could also entail a 0.5 V to 2.5 V
range to adjust the PWM current); plus the circuitry that
controls the PFD (percent fast-decay) input. The device
operates per the conditions listed in table 4 (Output Truth
Table/Recirculation Modes for the microstepping IC).
Per table 4, the outputs are completely disabled when the
digital signals (D , D , D ) are LOW. As mentioned, the
0
1
2
current direction is determined by the PHASE input, and
the recirculating current decay mode is governed by the
voltage applied to the PFD input. Properly ‘tuning’ the
ratio of fast and slow decay allows minimizing current
ripple, and with suitable control the PFD voltage can be
dynamically varied for optimal performance over a broad
range of step frequencies.
The specified winding current (100%) is derived from a
formula consisting of a suitable reference voltage and
current-sensing resistor. Per tables 2 and 4, the 100%
value is delivered when the DAC inputs (D2, D1, and D0)
are all high (1); and the calculations are predicated upon:
V
BB
ITRIP = VREF/(3•RS)
RS = VREF/(3•ITRIP)
DRIVE CURRENT
RECIRCULATION
(SLOW-DECAY MODE)
RECIRCULATION
(FAST-DECAY MODE)
The phase currents are determined by the 3-bit DAC
inputs and the formula above; per table 2, the PWM
current supplied to each motor winding involves correctly
ratioing phase currents to attain constant torque. Hence,
Step Reference Current Ratio (SRCR) is another factor in
calculating the actual current applied to develop constant
torque; and the phase current formula becomes:
ITRIP = VREF•SRCR/(3•RS)
RS
Dwg. EP-006-2A
Figure 13: Load and recirculation currents
The phase current ratios (SRCR) associated with the 3-bit
DAC are enumerated in table 2, and it should be noted that
the logic sequence follows the binary format used in updown counters, etc. Thus, ‘hardware’ motion control is
very viable.
Table 4: Output truth table and recirculation modes
8
D2
D1
D0
Phase
PFD
OutA
OutB
Description
0
0
0
X
X
OFF
OFF
Outputs disabled
1
>0.6•VCC
H
L
Slow current decay
All
1
(0.22 to 0.6)VCC
H
L
Mixed current decay
other
1
<0.22•VCC
H
L
Fast current decay
input
0
>0.6•VCC
L
H
Slow current decay
states
0
(0.22 to 0.6)VCC
L
H
Mixed current decay
0
<0.22•VCC
L
H
Fast current decay
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
FULL-BRIDGE MOTOR DRIVER
with PWM CURRENT CONTROL and
‘MIXED-MODE’ MICROSTEPPING
‘MIXED-MODE’ MICROSTEPPING
Following on the essentials of incremental motion technology is a discussion of the operation and of the benefits
of ‘mixed-mode’ microstepping. At the outset, it was
pointed out that the circuitry is utilized to emulate a
sinewave drive to the motor windings. Figure 9 illustrated
the coil currents, and table 3 enumerated the input control
signals, etc. required to complete a four-step cycle, which
then repeats to microstep the motor.
Frequently, with step motor applications there are instances when Slow Decay recirculation fails to properly
control (e.g., regulate) the phase current. ‘Mixed-Mode’
Decay fragments the PWM fixed-OFF time into intervals
of both Fast- and Slow Decay, and solves those situations
where only the slowly decaying (i.e., two-quadrant)
recirculation proves incapable of following the descending
half of a sinewave current. This is especially evident in
microstepping designs and transpires as current decay is
impeded by the back EMF and inductive properties of the
motor and decays too slowly.
Utilizing fast-decay during a portion of the fixed OFF
time offers improved current regulation, but 100% fastdecay can induce excessive ripple in the load current.
ITRIP
IAVG(SLOW DECAY)
toff(FAST) ≈ 0.3 toff(SLOW)
IAVG(FAST DECAY)
FIXED toff
Dwg. WP-032
Figure 14: Current waveforms (three modes)
I PEAK
I TRIP
PFD
t OFF
Dwg. WP-031
Figure 15: ‘Mixed decay’ current waveform
‘Mixed-Decay’ offers a mode that allows ‘tuning’ the
proper ratio of fast- and slow-decay to optimize motor
current regulation without creating undue, undesirable
heating. Per figure 14, the fast-decay current excursions
are much greater than either the slow-decay (dotted) or
‘mixed-mode’ [toff(FAST) ≈ 0.3 toff(SLOW)].
Although the current ripple in ‘mixed-mode’ has heightened, it is much lower than the fast-decay mode. The
‘mixed-mode’ operation is controlled by comparing the
(fixed-OFF time) RC voltage to the PFD voltage determined by the user. The output of the mixed-decay comparator determines the portion of each PWM cycle that the
driver IC spends in fast- or slow-decay. Per table 4, for
mixed-decay operation the PFD voltage must be within
the range of 0.22•VCC and 0.6•VCC.
A resistor divider can establish this PFD voltage; thus, as
a PWM decay period commences, the IC starts in the fastdecay mode. When the voltage on the RC network
declines to a value below the PFD input, the device
switches into slow decay. The percentage of the PWM
OFF-time spent in fast decay is derived via the divider R1
and R2:
% Fast-Decay = 100•ln 0.6•(R1/R2 + 1)
An illustration of ‘mixed-decay’ PWM appears in figure
15; and, commencing at the peak current (ITRIP), fast
decay (PFD) is initiated. The fixed OFF time is divided
into approximately one third fast-decay and the remaining
portion operating in the slow-decay mode. An RC network fixes this OFF-time interval, and a parallel resistor
(RT) and capacitor (CT) establish the tOFF as follows:
tOFF ≈ RT•CT
Per figure 15, when the RC voltage has dropped to its
lower limit (≈0.22•VCC), the PWM latch is again set and
the driver(s) switched ON. This restores conduction to the
motor winding and the current ramps to the design trip
value, and PWM operation continues until the control
logic changes the current value or direction.
The RC network capacitor (CT) also establishes the
comparator blanking time. Functionally, this blanks the
comparator output during any switching involving the
internal control circuitry (change of direction or enabling
DAC inputs); and precludes erroneous current detection
during switching.
9
FULL-BRIDGE MOTOR DRIVER
with PWM CURRENT CONTROL and
‘MIXED-MODE’ MICROSTEPPING
The benefits of ‘mixed-mode’ decay can readily be
illustrated via a number of oscilloscope plots. While the
focus is upon microstepping, the other modes of operation
are included for comparative purposes. Because the
primary thrust is emulating a sinewave, only a few token
illustrations of half- and quarter-stepping are included.
All examples illustrate operation at ±1.0 A, and each
figure is labeled with its step rate and recirculation data.
Shifting to quarter-stepping displays (again) that 100%
fast-decay offers the preferred waveform (figure 19), but
100% slow-decay (figure 20) is a poor ‘third’ to the 50%/
50% of figure 21, and very similar to the 50%/15%/35%
‘mixed-decay’ represented in figure 22.
For comparison purposes, 50 steps per second, half-step
operation is depicted in figures 16, 17, and 18. Per
notations, figure 16 illustrates 100% slow-decay; figure 17
portrays operation with 100% fast-decay, and figure 18
depicts 50% ratios of fast- and slow-decay. At this
stepping rate, the fast-decay mode offers the most desirable result as portrayed by the ‘clean’ half-step waveform.
Figure 19: 1/4 Step,
50 steps per second,
100% fast
Figure 16: 1/2 Step,
50 steps per second, 100% slow
Figure 20: 1/4 Step,
50 steps per second,
100% slow
Figure 17: 1/2 Step,
50 steps per second, 100% fast
Figure 21: 1/4 Step,
50 steps per second,
50%/50% mixed
Figure 18: 1/2 Step,
50 steps per second,
50%/50% mixed
Figure 22: 1/4 Step,
50 steps per second,
50%/15%/35% mixed
10
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
FULL-BRIDGE MOTOR DRIVER
with PWM CURRENT CONTROL and
‘MIXED-MODE’ MICROSTEPPING
Continuing with 50 Hz plots, figure 23 displays a 1/8thstep waveform that exhibits considerable ‘distortion’ of
the desired sinewave. Aberrations on both ascending and
descending portions of the waveform are very obvious,
and this operation is neither desirable nor very acceptable.
Figure 24 follows the sine curve much better, but registers
much larger current ripple at each subdivision of stepping.
Figure 25 indicates some improvement, but figure 26
represents lower current ripple at this step rate with
microstepping (1/8th steps).
Slow decay performs very poorly as the stepping rate
increases, and the 100% slow recirculation shown in
figure 27 does not emulate a sinusoidal waveform.
Hereon, only the microstepping mode (1/8th steps) is
illustrated and contrasted. Because microstepping is most
advantageous at the lower to mid-range stepping frequen-
cies, only the most pertinent plots are included. Figures
28 and 30 portray the optimal waveforms at the rates
noted, and figure 29 makes it apparent that the fast-to
slow-decay (PFD) is linked to the stepping rate. 50%/
15%/35% serves >100 Hz, but not 200 Hz.
Figure 26: 1/8th
Step,
50 steps/second,
50%/15%/35% mixed
Figure 23: 1/8th Step,
50 steps/second,
100% slow
Figure 27: 1/8th Step,
100 steps/second,
100% slow
Figure 24: 1/8th
Step, 50 steps/
second,
100% fast
Figure 28: 1/8th
Step, 100 steps/
second,
50%/15%/35% mixed
Figure 25: 1/8th
Step, 50 steps/
second,
50%/50% mixed
Figure 29: 1/8th
Step, 200 steps/
second,
50%/15%/35% mixed
11
FULL-BRIDGE MOTOR DRIVER
with PWM CURRENT CONTROL and
‘MIXED-MODE’ MICROSTEPPING
Once beyond the resonance range (usually from ≈50-150
Hz), the motor can be operated in the full-step mode for
slewing, and then be switched to microstepping for
deceleration. However, for a broad stepping range with
fixed PFD, the 50% plots shown in figures 25, 30, 31, 32,
and 33 depict the preferred waveforms (many other plots
omitted due to space limitations).
Figure 30: 1/8th
Step, 200 steps/
second,
50%/50% mixed
For optimal performance, ‘dynamically’ switching the
PFD ratio in correlation to the stepping rate, and the
specific motor characteristics, is feasible. Obviously, the
control logic (perhaps a dedicated microcontroller) and
software are more complex than a ‘fixed’ 50%/50% PFD
ratio solution such as the easy voltage divider, broadrange solution represented in figures 25, 30, 31, 32, and
33.
Figure 31: 1/8th Step,
333 steps/second,
50%/50% mixed
SUMMARY and CONCLUSION
The A3955 is an innovative, full-bridge, PWM step motor
driver IC created for modest power, cost-sensitive
microstepping applications. The IC is capable of constant
current (PWM) drive of one phase (one winding) of a twophase motor, and rated to maximums of 50 V and ±1.5 A.
Ratioed currents provide constant-torque operation and are
controlled via three logic inputs and a 3-bit DAC with
±3% accuracy. PWM current control involves one
current-sensing resistor, an RC network for fixed OFF
time, and a reference voltage.
Figure 32: 1/8th Step,
500 steps/second,
50%/50% mixed
The fundamentals of incremental motion control from
‘wave-drive’ through microstepping (1/8th steps) were
summarized for their applicability to this ‘mixed-mode’
step motor drive IC. The IC is best suited for the bipolarseries configuration; and, as illustrated in various figures,
the ‘mixed-mode’, 1/8th-step operation delivers a waveform that quite effectively emulates a sinewave.
Either a fixed PFD voltage, which determines the ratio of
slow- and fast-decay, or operation with dynamic ‘tuning’
(i.e., adjusting the PFD potential based upon the step rate)
is feasible. Operation with a fixed PFD input voltage can
(as shown) deliver smooth microstepping over a rather
broad range of stepping speeds. However, each system
should be evaluated for its specific characteristics as the
step motor is the foremost factor affecting overall performance. Subdividing each full step to extend the positional
resolution is also directly and distinctly related to the
motor characteristics.
12
Figure 33: 1/8th Step,
666 steps/second,
50%/50% mixed
The products described here are the A3955SB and A3955SLB fullbridge PWM microstepping motor drivers. This paper was originally presented at PCIM’97 in Hong Kong, October 14-17, 1997.
Reprinted by permission.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000