'Smart', Constant-Torque, Microstepping Drive Circuitry for Under $10

Technical Paper
STP 99-11B
APPLICATIONS
INFORMATION
‘SMART’, CONSTANT-TORQUE,
MICROSTEPPING DRIVE CIRCUITRY FOR UNDER $10
by Paul Emerald and Ken Kadrmas
FOREWORD
Initially, (nearly) all step motor systems utilized
simple, unipolar, constant-voltage drives; bipolar and/or
PWM, constant-current circuitry was then deemed as too
expensive to implement (power transistors were then
large, expensive ‘metal cans’). Later, the advent of
integrated ‘glue’ logic (gates, flip-flops, etc.), and the
continuing, relentless decline in semiconductor costs,
precipitated an inexorable conversion from the venerable,
low performance unipolar to two-phase, bipolar activation
with its higher torque (at low-to-modest stepping rates).
Subsequently, new, higher performance designs began
exploiting PWM (pulse-width-modulated), constantcurrent drives; and, also, stimulated the escalating utilization of fractional stepping modes. Even today, many
microstepping drives are large, ‘boxes’ or ‘boards’ commanding prices that greatly exceed the cost of (even) the
larger step motors; thus, many designers (especially
‘nonelectronic’ engineers) are often faced with very
formidable decisions between drive system performance
and the cost objectives. Step motors have become a lowcost, commodity item (particularly the small ‘canstack’
types); therefore, justifying ‘hundreds’ of dollars for
microstepping electronics represents an extremely contrary circumstance for many of today’s motion control
systems designers.
INTRODUCTION
Recently, the emergence of microstepping power ICs
has (greatly) alleviated the cost dilemma for many highvolume OEM applications. However, many other (usually
smaller) organizations do not possess the expertise to
develop the microcontroller, microprocessor, or computerbased control that is essential to utilizing these
microstepping power ICs. Therefore, this set of users
often face the tough decision of whether to purchase
expensive ‘boxes’ or ‘boards’ vs the prolonged, costly
effort involved in developing the electronics (this often
precipitates retaining consultants to define or create the
system hardware and software).
This paper discusses the very practical application of
bipolar and unipolar microstepping driver ICs and multichip modules and the related controller circuitry that
makes designing with such devices easy, inexpensive, and
productive. A ‘reference’ design technique is integral to
the implementation of a system that includes the power
circuitry, a low-cost, 8-bit, preprogrammed microcontroller and the other components needed to complete the
control ‘hardware’. The preprogrammed, 8-bit, microcontroller eliminates any need for software development;
and, thereby, expedites the product creation and hastens
the time to market.
The reference design can be utilized directly or
integrated into a larger PCB. A further benefit is the
compactness of the circuit layout. The power ratings
available are: 50 V and ±1.5 A (bipolar) and 46 V (unipolar) and either 1.25 A or 3 A. All of the reference designs
support stepping formats that include full-step, half-step,
quarter-step, and 1/8th (microstepping) increments.
One example that illustrates this reference design
concept is a 50 V, ±1.5 A bipolar drive. This detailed
design includes: two microstepping ICs, a serial-toparallel logic IC, an 8-bit microcontroller, crystal, eight
diodes, and the various passive discrete components
(about 25) to complete the entire drive electronics for a
two-phase stepping motor.
To ease and simplify the design effort, the user only
provides the following signals: (a) direction, (b) stepping
clock (8x the full-step frequency), (c) mode logic (three
inputs determine the operation from full-, half-, quarter-,
or 1/8th stepping), (d) reset input (initiates a ‘detent’
position), and (e) recirculation control (this allows establishing the percent of fast- vs slow-decay in the phase
winding). However, an optional microcontroller program
offers ‘automatic’ recirculation control. This eliminates
‘SMART’, CONSTANT-TORQUE,
MICROSTEPPING DRIVE
CIRCUITRY FOR UNDER $10
the need for evaluating the impact of stepping rate vs the
sinusoidal current profile.
PHASE A
Although recirculation control may provide slight
improvements (i.e., lower current ripple, reduced motor
heating [a few degrees], and diminish audible noise levels
[minimal differences]); this entails an evaluation of the
motor (and step frequencies) to determine the proper ratio
of fast- and slow-decay. The benefits of ‘tuning’ the
recirculation ratios are small, and the time and effort
required can be considerable. Hence, the uninitiated user
should opt for the automatic recirculation control, and
avoid the (essentially) unnecessary activity.
PHASE B
Dwg. WK-004-7
PRELUDE: ‘INTEGRATED’ MICROSTEPPING
The ‘missing-link’ for ‘turn-key’ stepper systems has
been an ‘off-the-shelf’ controller/sequencer. However,
this design issue has been resolved via programming an
8-bit microcontroller to deliver the logic signals needed to
operate two low-cost, microstepping ICs. The PWM
output currents are regulated (in this example) by
sequencing the 3-bit DAC (digital-to-analog) inputs that
control the current ratios for the two motor phases. The
motor phases are driven by ratioed PWM currents that
(digitally) emulate a cosine/sine relationship as illustrated
in Figure 1. The winding currents are 90° out-of-phase
(i.e., in quadrature), and the motors’ rotational direction is
determined by the phase relationship of the A vs B
currents.
The cosine/sine relationships of the phase winding
currents induce intermediate rotor positions which are
between the usual full-step alignments. With the 1.8° PM
hybrid stepper there are 200 full-step positions per shaft
revolution, but this can be further subdivided via
microstepping. The 360° electrical cycle of four full-steps
is illustrated in Figure 2A; half-stepping in Figure 2B;
quarter-stepping in Figure 2C; and microstepping in
Figure 2D.
Figure 1: Current waveforms (1/8th step)
A
A
AB
B
B
2
B
B
AB
AB
A
A
Dwg. OP-005-11
Figure 2A: Single-phase, full step
A
AB
Figure 2B: Half step
AB
AB
B
B
AB
B
AB
A
Dwg. OP-005-14
A
AB
The microstepping power ICs used as an example are
controlled by three digital-to-analog-conversion (DAC)
inputs. Table 1 lists the ratioed currents that induce
fractional subdivisions of one full step. Note: the correct
mathematical calculations for microsteps #1 and #7 are:
Phase A = 98.1% and Phase B = 19.5% (#1), and the
AB
Dwg. OP-005-15
Figure 2C: 1/4 Step
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
Copyright © 1999, Intertec International Inc.
B
AB
AB
A
Dwg. OP-005-16
Figure 2D: 1/8th Step
‘SMART’, CONSTANT-TORQUE,
MICROSTEPPING DRIVE
CIRCUITRY FOR UNDER $10
reverse for #7 (19.5%; 98.1%). However, the ‘non-linear’
DAC tolerance is ±3%; hence, this ‘9th’ current level
(98.1%) was deemed as unnecessary since it is within the
±3% window. Further, the step motor positional accuracies are also ≤±3% (for most PM hybrid motors, while
most ‘canstack’ steppers are >±5%); therefore, the 98.1%
current ratio is basically unnecessary for 1/8th-step
operation, and this ‘9th’ current value is circumvented.
Table 1: 1/8th-Step subdivision current ratios
Step
Series
Current
Phase A
Current
Phase B
Step
Position
0
1
2
3
4
5
6
7
8
x†
100.0%
100.0%*
92.4%
83.1%
70.7%
55.5%
38.2%
19.5%
0.0%
100.0%
0.0%
19.5%
38.2%
55.5%
70.7%
83.1%
92.4%
100.0%
100.0%
100.0%
full step
1/8 step
1/4 step
3/8 step
1/2 step
5/8 step
3/4 step
7/8 step
full step
1/2 step
* Calculated (cosine) value ≈ 98.1%
† 2-Phase drive at 100% ≈ 141% torque.
Similarly, the first subdivision with a 1/16th step
format calculates to a 99.5% ratio for Phase A. Based
upon the ±3% DAC tolerance and the step motor accuracy
(≈±3%), this 17th current ratio is also deemed as needless
for low-cost, simple, open-loop, microstepping designs.
The non-linear DAC inputs determine the current
ratios in each phase winding, and the direction of current
is controlled by the logic level on another input (Phase).
The magnitude of winding current is regulated by a
current-sensing resistor, voltage reference, and a comparator in each power IC.
Another integrated function that can be utilized in
microstepping applications is recirculation control. This
provides ‘mixed-mode’ decay of the winding current and
allows ‘tuning’ the percentage of fast decay in the coil
with the stepping rates, etc. to enhance motor performance. However, this adds (yet) another level of com-
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plexity (plus the added time and cost) to the development
of the system software. The proper application of recirculation control allows the decaying current to effectively
track the sinusoidal PWM waveform of Figure 1.
Without some measure of fast-decay during each
descending half-cycle, the motor current is very susceptible to (serious) distortion as the winding current is
diminishing toward zero. The effects of improper decay
are: added heating (of the motor and circuitry), heightened
audible noise levels, plus increased velocity ripple of the
rotating shaft. A further complication of using mixed
mode (fast- and slow-decay) is its directional dependency.
Having looked inward from the motor and driver
circuitry, the various facets of a preprogrammed
microcontroller can now be addressed. Clearly, an
inexpensive, uncomplicated answer (small and a minimal
parts count) is very beneficial to most systems designers.
Additionally, many of today’s motion-control designs are
being attempted by a growing, new constituency (i.e.,
‘nonelectronic’ designers. This may include mechanical
engineers, software programmers, etc.). Hence, a mounting interest in very basic, ‘turn-key’ solutions that escape
the rising sophistication of today’s costly, high-performance motion systems.
A very low-cost, serial, 8-bit, ‘preprogrammed’
microcontroller (Figure 3) is a pivotal component used in
a new design alternative that exploits inexpensive,
microstepping power ICs over the costly ‘boards’ and
‘boxes’ frequently used in step-motor systems applications. The projected hardware costs for this (reference)
design are <$10 for components (5 k to 10 k systems
annually).
MICROCONTROLLER OPERATION
Although ‘hardware’ control of the microstepping ICs
is feasible, without a specific (ASIC), monolithic IC
controller the prime solution becomes a ‘software’ option.
From the user’s perspective, a ‘preprogrammed’
microcontroller appears little, or no, different than a
‘dedicated’ controller and sequencer IC expressly created
for microstepping applications of the two power ICs.
Further, the flexibility of a software-based drive is cer-
3
‘SMART’, CONSTANT-TORQUE,
MICROSTEPPING DRIVE
CIRCUITRY FOR UNDER $10
tainly a basic benefit (high-volume production of 8-bit
microcontrollers transposes to low-cost circuitry).
MODE SELECT1
1
18
MODE SELECT0
MODE SELECT2
2
17
DIRECTION
CONTROL IN
STEP IN
3
16
OSC/CLOCK IN
RESET
4
15
OSC/CLOCK OUT
GROUND
5
14
SUPPLY
MONITOR OUT
6
13
PFDA
SHIFT CLOCK
7
12
PHASEA
SERIAL DATA OUT
8
11
PFDB
STROBE OUT
9
10
PHASEB
VDD
Dwg. PP-071A
Figure 3: Microcontroller IC connections
As an indicator of the logic signals needed to control
the power ICs, Table 2 lists the A3955 inputs to the 3-bit
DAC for 1/8th-step operation. These I/O signals are serial
data from the microcontroller, then converted to a parallel
mode by a 74HC595 as the ‘interface’ between the
microcontroller and the two microstepping power ICs.
The complexities of control and sequencing should be
very evident from Table 3. This only involves four fullsteps (the 360° electrical cycle shown in Figure 1), but
should corroborate the value of the microcontroller
operation of: six DAC input lines, two direction signals
(θ), and two lookup tables, which control the PFD
(percent-fast-decay) for both phase windings. Note: Table
3 corresponds with one direction (only); two ‘lookup’
tables are required to control the phase relationship of the
PFD inputs (i.e., reversing direction involves transposing
the A0 and A1 signals in Table 3; this inverts the slowand fast-decay with direction).
4
Table 2: Digital-to-analog truth table
D2
D1
D0
DAC%
1
1
1
1
0
0
0
0
1
1
0
0
1
1
0
0
1
0
1
0
1
0
1
0
100.0
92.4
83.1
70.7
55.5
38.2
19.5
0
Table 3 incorporates an ‘automatic’ recirculation
control concept into the ‘lookup’ tables. The user can
disregard the necessity to: evaluate the motor current
waveforms, eliminate adjusting the PFD ratio, and delete
six passive components from the bill-of-material and PCB
layout. This reference design is also suitable for utilizing
the A3957 (and 1/16th stepping with very little modifications.
The versatility offered by software control allows the
operating modes listed in Table 4. This table itemizes the
various logic inputs that determine direction, stepping
format, reset, 1/8th vs 1/16th sub-steps, etc. Note: during
power-up, Pin #7 is sampled for a pull-up or pull-down
resistor to establish the fractional step limit. A pull up sets
up a 1/8th-step format (A3955) and pull down sets up
1/16th-operation for the A3957.1
Table 3 incorporates an ‘automatic’ recirculation
control concept into the ‘lookup’ tables. The user can
disregard the necessity to: evaluate the motor’s current
waveforms, eliminate adjusting the PFD ratio, and delete
six passive components from the bill-of-material and PCB
layout. This reference design is also suitable for utilizing
the A3957 (and 1/16th stepping with minimal modification). The longer truth table of the A3957 is not included.
Table 5 lists the microcontroller lead connections and
provides the essence of the circuit operation (a schematic
illustrating a typical stepper design follows in Figure 4).
A brief description of the microcontroller I/O should
clarify the connections of the various elements of the
drive electronics.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
‘SMART’, CONSTANT-TORQUE,
MICROSTEPPING DRIVE
CIRCUITRY FOR UNDER $10
Table 3: Input logic sequence and operating modes for two-phase step motor
Driver IC #1
(Phase A)
Driver IC #2
(Phase B)
Step Motor
Torque
θ
D2
D1
D0
PFD
Current
MO
θ
D2
D1
D0
PFD
Current
MO
Value
Angle
∆∠
1
1
1
1
X
100.0%
X
0
0
0
0
X
0.0%
D
1.000
0.00
11.03
1
1
1
1
1
1
1
0
1
1
1
1
0
0
0
0
1
1
0
0
1
1
0
0
1
0
1
0
1
0
1
0
A0°
A0°
A0°
A0°
A0°
A0°
A0°
X
100.0%
92.4%
83.1%
70.7%
55.5%
38.2%
19.5%
0.0%
F
F
F
F
F
F
F
D
1
1
1
1
1
1
1
1
0
0
0
1
1
1
1
1
0
1
1
0
0
1
1
1
1
0
1
0
1
0
1
1
A1
A1
A1
A1
A1
A1
A1
X
19.5%
38.2%
55.5%
70.7%
83.1%
92.4%
100.0%
100.0%
S
S
S
S
S
S
S
X
1.019
1.000
0.999
1.000
0.999
1.000
1.019
1.000
11.03
22.46
33.74
45.00
56.26
67.54
78.97
90.00
11.03
11.43
11.28
11.26
11.26
11.28
11.43
11.03
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
1
0
1
1
0
0
1
1
1
1
0
1
0
1
0
1
1
A1
A1
A1
A1
A1
A1
A1
X
-19.5%
-38.2%
-55.5%
-70.7%
-83.1%
-92.4%
-100.0%
-100.0%
S
S
S
S
S
S
S
X
1
1
1
1
1
1
1
0
1
1
1
1
0
0
0
0
1
1
0
0
1
1
0
0
1
0
1
0
1
0
1
0
A0°
A0°
A0°
A0°
A0°
A0°
A0°
X
100.0%
92.4%
83.1%
70.7%
55.5%
38.2%
19.5%
0.0%
F
F
F
F
F
F
F
D
1.019
1.000
0.999
1.000
0.999
1.000
1.019
1.000
101.03
112.46
123.74
135.00
146.26
157.54
168.97
180.00
11.03
11.43
11.28
11.26
11.26
11.28
11.43
11.03
0
0
0
0
0
0
0
1
1
1
1
1
0
0
0
0
1
1
0
0
1
1
0
0
1
0
1
0
1
0
1
0
A0°
A0°
A0°
A0°
A0°
A0°
A0°
X
-100.0%
-92.4%
-83.1%
-70.7%
-55.5%
-38.2%
-19.5%
0.0%
F
F
F
F
F
F
F
D
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
1
0
1
1
0
0
1
1
1
1
0
1
0
1
0
1
1
A1
A1
A1
A1
A1
A1
A1
X
-19.5%
-38.2%
-55.5%
-70.7%
-83.1%
-92.4%
-100.0%
-100.0%
S
S
S
S
S
S
S
X
1.019
1.000
0.999
1.000
0.999
1.000
1.019
1.000
191.03
202.46
213.74
225.00
236.26
247.54
258.97
270.00
11.03
11.43
11.28
11.26
11.26
11.28
11.43
11.03
1
1
1
1
1
1
1
1
0
0
0
1
1
1
1
1
0
1
1
0
0
1
1
1
1
0
1
0
1
0
1
1
A1
A1
A1
A1
A1
A1
A1
X
19.5%
38.2%
55.5%
70.7%
83.1%
92.4%
100.0%
100.0%
S
S
S
S
S
S
S
X
0
0
0
0
0
0
0
1
1
1
1
1
0
0
0
0
1
1
0
0
1
1
0
0
1
0
1
0
1
0
1
0
A0°
A0°
A0°
A0°
A0°
A0°
A0°
X
-100.0%
-92.4%
-83.1%
-70.7%
-55.5%
-38.3%
-19.5%
0.0%
F
F
F
F
F
F
F
D
1.019
1.000
0.999
1.000
0.999
1.000
1.019
1.000
281.03
292.46
303.74
315.00
326.26
337.54
348.97
360.00
11.03
11.43
11.28
11.26
11.26
11.28
11.43
11.03
Input codes:
θ = Phase input (direction), PFD = Percent fast decay, MO = Operating mode.
Operating modes:
D = Disabled (output off), F = Fast decay, S = Slow decay,
X = Irrelevant, A0° = Automatic fast decay, A1 = Automatic slow decay.
Motor codes:
Value = Torque vector magnitude, Angle = Rotor position, ∆∠ = Delta angle
NOTES:
Logic sequencing and operating modes correspond to FOUR full steps.
Preprogramming of microcontroller provides automatic recirculation control.
www.allegromicro.com
5
CLK
MS2
MS1
MS0
DIR
MON
10 kΩ
A3957
A3955
+5 V
10 kΩ (5)
+5 V
20 MHz
Q'H
QH
QG
QF
QE
QD
QC
QB
QA
+5 V
22 pF
D3B A3957
D3A A3957
SDO
+5 V
10 kΩ
50 kΩ
4.99 kΩ
470 pF
39 kΩ
+5 V
470 pF
39 kΩ
10 kΩ
A3955
0.56 Ω
VCC
VBB
REF
D0
OUTA
D1
D2
OUTB
PFD
PHASE SENSE
RC
GND
GND
GND
GND
A3955
VCC
VBB
REF
OUTA
D2
D1
OUTB
D0
PFD
PHASE SENSE
RC
GND
GND
GND
GND
+5 V
10 kΩ
Figure 4: Low-cost microstepping drive circuitry
STEP OSC IN
MS2 OSC OUT
MS1
PFDA
MS0 PHASEA
DIR
R
MON
VDD
SCLK
GND
SDO
PFDB
ST
PHASEB
PIC16C621A
22 pF
RCK
SER
SCK
G
GND
VCC
SCLR
100 kΩ
0.1 µF
33 kΩ
10 kΩ
50 kΩ
74HC595
33 kΩ
+5 V
+
LOAD
SUPPLY
0.56 Ω
+
LOAD
SUPPLY
47 µF
Dwg. EP-068A
!N4934 (4)
6
47 µF
STEP MOTOR
PHASE B
STEP MOTOR
PHASE A
!N4934 (4)
+5 V
‘SMART’, CONSTANT-TORQUE,
MICROSTEPPING DRIVE
CIRCUITRY FOR UNDER $10
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
‘SMART’, CONSTANT-TORQUE,
MICROSTEPPING DRIVE
CIRCUITRY FOR UNDER $10
Table 4: Controller/sequencer IC operational logic
Binary inputs
DIR MS2 MS1 MS0
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
Operating mode
(Command executed on L → H of CLK)
Comments
(Applicable power ICs)
CW, Full step (single-phase)
CW, Half step (constant torque)
CW, 1/4 step (constant torque)
CW, 1/8th step (constant torque)
CW, 1/16th step (constant torque)
Disable A3955/57 holding torque2
Enable A3955/57 holding torque
Reset PIC16C621A sequencer IC
CCW, full step (single-phase)
CCW, half step (constant torque)
CCW, 1/4 step (constant torque)
CCW, 1/8th step (constant torque)
CCW, 1/16th step (constant torque)
Disable A3955/57 holding torque2
Enable A3955/57 holding torque
Reset PIC16C621A sequencer IC
A3955/57
A3955/57
A3955/57
A3955/57
A3957 only
At present position
From present position
A3955/57
A3955/57
A3955/57
A3955/57
A3955/57
A3957 only
At present position
From present position
A3955/57
Mode-select inputs (pins 1, 2, and 18)
Step (clock) input (pin 3)
These three inputs (MS2, MS1, and MS0) determine
the stepping format, disable/enable motor power, and reset
the controller/sequencer. In conjunction with the direction
input, the mode inputs control the sixteen operating states
listed in Table 4. Note: deactivating stepper power in any
position except ‘detent’ (i.e., a single phase activated)
results in the motor rotor advancing, or retracting, from its
intermediate position and alignment with a natural (i.e.,
minimum-reluctance flux field) orientation. The absolute
position may be affected by inertia, load, fractional
position, ringing, etc. and cannot be determined without
feedback. Phase currents must be maintained to immobilize the rotor/load in any intermediate position. 2
The controller stepping-clock frequency must be a
multiple of the actual stepping rate. The A3955 requires a
stepping-clock input frequency equal to eight times the
intended stepping rate for the motor; the A3957 requires a
stepping clock rate that is sixteen times the actual stepping rate of the motor. However, neither design necessitates that the step frequency be varied should the operating mode(s) be switched during operation. Shifting from
one stepping format does not mandate a simultaneous
(and equivalent) change in the clock frequency.
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Using a 20 MHz crystal (maximum limit for the
PIC16C621A) allows a 50 kHz stepping clock for the
A3955, and this equates to 6 250 full steps per second (50
kHz/8). For the A3957 this 50 kHz stepping clock
translates to 3 125 full steps per second (50 kHz/16).
These frequencies represent the attainable limits with the
PIC16C621A .
7
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Table 5: Controller/sequencer pin descriptions
Pin
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Function
Input
Input
Input
Input
Power
Output
I/O
Output
Output
Output
Output
Output
Output
Power
Output
Input
Input
Input
Description
Mode select 1
Mode select 2
Step in
Reset
Ground
Monitor out
÷8, ÷16/Shift CLK
Serial data out
Strobe out
Phase B
PFD B
Phase A
PFD A
Supply
Osc/clock out
Osc/clock in
Direction control in
Mode select 0
Comments, connections, etc.
Static and/or dynamic control of stepping mode
Static and/or dynamic control of stepping mode
Governs full-step rate (÷8 for A3955; ÷16 for A3957)
Resets DIR., MS2, MS1, and MS0 to 0000 (detent)
Power return
Signals full-step rotor alignment (active low)
Pull up for A3955; pull down for A3957/shift CLK out
Shifts 8-bit serial data to 74HC595 serial input
Latches 8-bit data into 74HC595 (latch clock in)
Controls current direction in phase B (A3955/57)
Phase B recirculation control (A3955/57)
Controls current direction in phase A (A3955/57)
Phase A recirculation control (A3955/57)
(VDD) Recommended range: 4.5 V to 5.5 V
Crystal oscillator connection
Crystal oscillator connection/ext clock input
Determines direction of step motor rotation
Static and/or dynamic control of stepping mode
Although not a necessity, a stepping clock with a 50%
duty cycle represents the simplest technique for providing
an appropriate (≤50 kHz) stepping clock rate. The step
clock varies depending upon the start, acceleration,
slewing, deceleration, and stop trajectories mandated by
the motion system control and ‘point-to-point’ timing
objectives.
Reset input (pin 4)
Per Table 4, the preprogrammed microcontroller
incorporates two ‘software’ reset states that are serially
loaded with MS2, MS1, and MS0 all ‘high’. However,
the direct ‘hardware’ reset is actuated with a logic ‘0’ (i.e.,
‘active low’) on this input An input low level ‘defaults’ to
the 0000 binary state in Table 4, and sets the rotor to its
natural (or ‘detent’) position with one-phase energized.
8
Monitor output (pin 6)
An output ‘low’ signal indicates a rotor alignment
corresponding to a single phase on position. Any changes
in the operating mode (microstepping to full-step, etc.)
should coincide with the interval that the monitor output
is in the ‘low’ state’. This alleviates noise problems,
excessive ringing, etc. that may result from changing the
stepping modes ‘on-the-fly’. ‘Nonlinear’ (such as ‘Scurve’) acceleration profiles should exploit this signal to
achieve very smooth, quiet stepper operation.
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Shift clock output (pin 7)
This I/O pin serves a dual purpose. On power up, the
microcontroller samples this terminal as an input. Per
Table 5, connecting a pull-up resistor results in 1/8th-step
format; while a pull-down resistor configures the controller for its 1/16th-step mode (A3957 only). Clearly, this
provides versatility, simplicity, and cost-effectiveness for
most users.
Operating in its output mode, this I/O constitutes the
shift clock signal for the 74HC595. Data is transferred
from the microcontroller serial-data output (pin 8) to the
serial-data input of the 74HC595. This 8-bit serial format
is converted into parallel signals controlling the 3-bit (or
4-bit) DAC input lines to the two microstepping ICs.
Serial data from the serial data output is ‘valid’ on the
low-to-high clock transitions and eight clock pulses shift
serial control signals into the 74HC595. A basic timing
diagram (showing serial data, shift clock, and strobe) is
depicted in Figure 5. Signal timing is controlled by the
preprogrammed microcontroller; data entered into the
74HC595 shift register is then latched by the low-to-high
transition of the strobe input.
SDO
D7
D6
D5
D4
D3
D2
D1
D0
SCLK
µcontroller utilizes ‘look-up’ tables to provide overall
control of direction, stepping format, and recirculation
mode (PFD). The µcontroller ‘reads’ inputs and (then)
outputs time-based signals to control both microstepping
ICs.
Strobe output (pin 9)
After the 8-bit serial data has been loaded into the
shift register, a low-to-high transition on the strobe output
transfers the serial data from the shift register into the
eight ‘D’ flip-flops that compose the parallel-data outputs.
This ‘latched’ data controls microstepping current ratios
for both power ICs, and is ‘updated’ after eight step
clocks.
Phase control outputs (pins 10 and 12)
Two of the microcontroller outputs are devoted to
direction control; per Table 5, pin #10 furnishes signals
for phase B, while pin #12 provides logic control of phase
A. The phase signals follow the sequencing itemized in
Table 3 to provide the 1/8th-step ‘sinusoidal’ waveforms
as illustrated in Figure 1. Phase outputs (θ in Table 3) are
parallel, and connected directly to the power IC inputs
(A3955 in the schematic). Obviously, both phase signals
must coincide with the 8-bit serial-to-parallel conversion
signals that control the phase current ratios (D2, D1, and
D0).
PFD control outputs (pins 11 and 13)
ST
Dwg. WP-040A
Figure 5: Serial data, shift clock, strobe
Serial data output (pin 8)
Table 4 enumerates the binary signal instructions that
control each of the microstepping power ICs. The first
3-bits (or 4-bits) control the digital-to-analog conversion
in one power IC, while the next 3 (or 4-bits) ratio the
second driver current. The microcontroller monitors all
the various static inputs (i.e., Direction, Mode Select(s),
Reset), and by exploiting the Stepping Clock for its input
frequency, transfers the 8-bit data commands to the driver
ICs via the serial-to-parallel interface IC. Per Table 3, the
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Another pair of microcontroller outputs constitute
parallel, direct control of the recirculation paths during
each PWM current cycle. The percentage of fast-decay
(or four-quadrant) to slow-decay (or two-quadrant)
recirculation determines how effectively the output
current tracks a sinusoidal waveform. In Table 3, ‘mixedmode’ operation was utilized for those portions of each
microstepping cycle in which the current is decreasing
(denoted by A0° in Table 3). The application of ‘mixedmode’ decay allows the PWM current to properly decay
during each fixed-off-time interval. An illustration of
‘improper’ (i.e., insufficient) decay is presented in Figure
6A, and a more effective ‘sinusoidal’ waveform is shown
in Figure 6B.
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Another, more recent evaluation revealed that it is
very feasible for designers to exploit automatic recirculation control. Tuning two potentiometers results in minimal benefits; thus, six components can be deleted from
the circuit schematic which applies ‘mixed-mode’ recirculation. Although the cost impact is insignificant, omitting
six passives (R1, R4, R5, R15, R16, R18) proves favorable.
Figure 6A: 100 Hz, 1/8th step, ‘slow decay’
Figure 6B: 100 Hz, 1/8th step, ‘mixed decay’
The ‘mystique’ of recirculation control prompted an
evaluation that revealed only a small (less than 5°C)
motor temperature differential after realizing the thermal
equilibrium (≈60 minutes of operation). This (and the
waveform contrasts of Figures 7A, 7B, 7C, and 7D)
establishes the validity of applying fully digital (automatic) recirculation control.
Figure 7A: 200 Hz Waveform (slow/mixed)
The circuit schematic (Figure 4) includes a pair of
49.9 kΩ variables and allows setting the ratios of fast- and
slow-decay to match the individual characteristics of the
step motor, step frequency, supply voltage, etc. As the
stepping frequency is increased, the waveform anomalies
become more pronounced, and the benefit of ‘mixed
mode’ (or fast decay) also becomes very noticeable.
A comprehensive discussion of the ‘mixed-mode’
operation is included in Allegro Technical Paper STP 975A.
10
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Figure 7B: 800 Hz Waveform (slow/mixed)
Figure 7D: 800 Hz Waveform (slow/fast)
resistor from pin 15) may be required if AT-cut crystals
are utilized.
Also, this terminal is used if an external clock is
utilized in lieu of a crystal or ceramic resonator. The
20 MHz frequency limit applies to using any external
clock source (microprocessor, etc.).
Direction control input (pin 17)
Figure 7C: 200 Hz Waveform (slow/fast)
Oscillatoroutput/clock output (pin 15)
This is one of the device connections used for the
crystal oscillator (or ceramic resonator) that supplies the
microcontroller with an oscillator/time base (≤20 MHz).
The controller/sequencer may (also) use an external clock
input (from a system computer, etc.). If so, this connection is left open and only the oscillator input is utilized.
Oscillator input/clock input (pin 16)
This device connection serves as one connection for a
parallel-cut crystal (pin 15 is the other). In the schematic
in Figure 4, two 20 pF capacitors are advised; and a series
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This input controls the sequencing of the winding
currents. Per Table 4, a ‘low’ induces clockwise motor
rotation, while a ‘high’ results in counter-clockwise
rotation of the shaft. Reversing direction may necessitate
deceleration (perhaps full stop) to avoid problems associated with the load inertia causing the motor to overshoot
the proposed stop/reverse position.
This closes the description of the microcontroller; but,
because the PIC16C621A does not control the actual
PWM phase currents, leaves an unresolved issue. The 3or 4-bit data inputs determine the output current ratios, but
do not set the phase currents. This is a separate aspect of
the PWM control.
CONTROL of PHASE CURRENTS
Prior to establishing the technique for setting the
PWM output current, a very consequential factor cannot
be neglected. Customarily, the (nominal) motor currents
are specified for two phases on. This refers to the 70.7%
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ratios listed in Table 1 and the (1/8th-step) constant torque
curve shown in Figure 8. The single-phase on current, to
induce the same torque, requires the peak value to be set
to 1.4 times that of two-phases on (both on constitutes the
rms value). Therefore, unless a motor is specified for
unipolar drive, the PWM output current must correspond
to 140% of rated current for two-phase operation. This
results in the constant-torque curve illustrated in Figure 8,
and induces very smooth, quiet motion, and rated torque.
Disregarding this peak (vs rms) current setting results in
significant torque loss.
A
MAXIMUM FULL-STEP
TORQUE (141%)
100
92.4
P
1/8 ST
EP
T
ST
AN
1/4
ST
N
O
STE
C
TO
2
E
ST
U
EP
Q
R
3/8
70.7
55.5
1/
CURRENT IN PER CENT
0%
EP
10
83.1
EP
5/8
ST
38.2
3/4
19.5
EP
FULL STEP
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 (1/8th step)
The driver ICs regulate the winding currents, and the
user establishes the design value via a stable voltage
reference and a current-sensing resistor for each winding
(R8 and R19, Figure 4). The single-phase on value (100%
in Figure 8) is calculated based upon the following
formula:
ITRIP = VREF/(3 x RS)
12
As mentioned, this value of phase current equates to
two-phase operation at ≈710 mA/phase, and corresponds
to the constant torque half-step loci in Figure 8. The
integrated non-linear 3-bit (or 4-bit) DACs control the
sine/cosine currents that induce the fractional step positions. An extension of the (peak) current formula provides the ratios.
ITRIP = SRCR x VREF/(3 x RS)
The internal digital-to-analog converter reduces the
reference input voltage to the current sensing comparator
in increments as itemized in Tables 1 and 2. The eight
step reference current ratios (SRCRs) induce the intermediate rotor loci which constitute fractional (or
microstepping) increments.
UNIPOLAR MICROSTEPPING
P
STE
7/8 ST
B
Selecting standard resistor values to fit the PWM
formula entails adjusting the voltage reference to the
sensing resistor, vice versa, or iterations of both. Per the
schematic, using the R2/R3 divider, with two 0.56 Ω
current-sensing resistors; the single-phase on current
value is approximately ±1.0 A per phase) and corresponds
to a 710 mA two-phase drive.
Another low-cost microstepping alternative is the
combination of a preprogrammed microcontroller and two
power multi-chip modules created for microstepping. The
unipolar drivers are specified for operation to 46 V
(maximum); the MOSFET output current ratings are:
1.25 A (SLA7042M) and 3.0 A (SLA7044M). These
power modules provide step formats that include: full,
half, quarter, and 1/8th stepping. The 3.0 A rating satisfies
many of the larger, higher-power motor applications that
are (often) beyond other driver ratings. Both power
devices offer programmable current control for unipolar
designs; with NMOS outputs internal power (heating) is
minimized and system cooling or heatsinking needs are
alleviated.
The bipolar and unipolar driver reference designs are
eminently suitable for the very swift creation of stepmotor-based motion-control designs within the stepping
cost solution to the control and sequencing aspects of
applying integrated microstepping ICs. With driver
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capabilities to ≤50 V and ±1.5 A, a design can be implemented with only four ICs and various discrete components. Currents exceeding ≈1.0 A should incorporate
external free-wheeling diodes (1N4934s in the schematic)
to reduce the power IC dissipation and heating via shunting the recirculation currents through the outboard diodes.
The preprogrammed microcontroller relieves designers of the necessity to develop both the hardware and
software to implement expensive, complex motion
control. Of special interest for small-to-modest volume
systems applications, a ‘turn-key’ design can save considerable development time and cost. Some volume users
may (also) opt for exploiting this hardware design to
alleviate the necessity of burdening staff and schedules to
develop a system that might (virtually) become a duplicate.
Motion-systems designers are now offered (yet)
another alternative between developing the system
(themselves) from the component level and high-cost,
packaged, ‘board-level’ hardware that may embody
performance and technology ‘overkill’ for today’s stepmotor applications. The relative cost of utilizing this
technique could easily translate to an ‘order-of-magnitude’
difference (i.e., lower) in the ‘cost-per-axis’ for motion
control systems.
Hence, the dominant advantages of utilizing these
reference designs for stepper control are cost and the
simplicity of and speed of implementation.
REFERENCES
1Emerald,
P.; The Merits, the Mystique, and the Myths of
Microstepping (Part 2); PCIM ‘99 Europe; Nürnberg,
Germany; June 22-24, 1999
Emerald, P. and Kadrmas, K.; The Merits, the Mystique,
and the of Myths of Microstepping; AMD&C;
PowerSystems World ‘98; Santa Clara, CA; pp 45-57
Emerald, P., Pickett, R., and Rowan, T.; Integrated
‘Mixed-Mode’ Microstepping Drivers Afford Smoother,
Quieter Operation and Reduce Step Motor Heating; PCIM
Europe ‘98; Nürnberg, Germany; May 1998
www.allegromicro.com
Emerald, P., Peppiette, R., and Seliverstov, A.; Monolithic, Programmable, Full-Bridge Motor Driver Integrates
PWM Current Control & ‘Mixed-Mode’ Microstepping;
PCIM ‘97 Hong Kong; October 1997; Allegro Paper STP
97-5A
Emerald, P.; Design Regimen for Resolving the Selection
of Integrated Power Electronics for Step Motors; Rocky
Mountain Technology Expo; Denver, CO; April 1997
2Emerald, P., Sasaki, M., and Takahashi, H.; CMOS Step
Motor IC and Power Multi-Chip Module Combine to
Integrate Versatile, Multi-Mode PWM Operation and
Microstepping; PowerSystems World ‘96; Las Vegas, NV;
September 7-13, 1996
Freeman, J.; Stepping Motor Specsmanship; Incremental
Motion Control Systems and Devices Proceedings; June
1987; pp. 297-304
Leenhouts, A.; Step Motor System Design Handbook, 2nd
Edition; Published by Litchfield Engineering; Kingman,
Arizona; Copyright 1997
McCarthy, K.; Getting More Out of Microstepping;
Motion Control; March 1991; pp. 12-16
Mazzola, T., and Floyd, R.; Intelligent Microsteppers for
Stepper Motor Controls; Motion Control; September
1991; pp. 16-21
Raskin, C.; Designing with Motion Handbook, Chapter 7,
Stepper Motion; pp 83-98
Viegnat, N.; Microstep Operation of Stepper Motors;
Motion Control; March 1991; pp. 48-50
Step Motor and Servo Motor Systems and Controls;
Compumotor Division of Parker Hannifin; 1996-1997
Edition, p. A29
Thinkescap® 5; Stepper Motor Drives, Technical Publication from Portescap; publication date unknown.
This paper was presented at PowerSystems World,
Chicago, IL on November 10, 1999. Reprinted by permission.
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GENERAL INFORMATION
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