Using
Stepper
Motors
Part 5 | by Paul Cooper | technobotsonline.com
Bipolar stepper motor driver modules
I
n this final part of our mini series
on using stepper motors we will focus
on the features of commercially available stepper driver modules suitable for
home constructor use. The stepper drivers in this article are all manufactured
from the US company pololu.com and
are available in Europe from UK company Technobotsonline.com
Before we delve into the more advanced
stepper drivers available, we need to be
sure we know how to select a hybrid
stepper motor, as its characteristics may
dictate the stepper driver required.
Hybrid stepper motor
characteristics
In part 1, we briefly introduced the basics
of choosing a stepper motor in terms of
frame size (NEMA), shaft type, holding
torque, speed and step size. However, the
range of stepper motors available can be
quite daunting and their differences may
appear subtle. Nevertheless, understanding
the features will help in finding the ideal
match for your application.
Let’s assume, as it is most likely to be
the case, each stepper motor you look at
has a step angle of 1.8° and microstepping
will be used to get the resolution needed.
The remaining two vital considerations
are holding torque and angular velocity.
It’s straightforward to work out the angular
velocity or rotational speed (RPM) that
your project needs, but holding torque
is not always so easy to find. Holding
torque is certainly of interest for building
a steady cam for example, but for
applications where torque is needed at
high rotational speeds, beware that the
torque available from a stepper motor
decreases as its rotational speed increases.
Table 8 compares the specification for four
similarly sized NEMA 17 (42 × 42mm)
hybrid motors.
Table 8 highlights variations in motor
performance, which is primarily due to
the winding conductor size and which
in turn relates to the number of turns on
the winding. Motors with many turns
of fine wire, such as for types 1 and 3
offer a reasonable holding torque (32
to 36 Ncm) for a phase current of 0.4 to
0.8A. However, many turns of fine wire
results in a higher inductance; and a
higher inductance translates into a lower
peak rotational speed. Note how motor
type 1 has about half the torque of type
2 at 450 RPM, even though their holding
torques are similar. Type 3 cannot even
reach 450 RPM; it will stall before it gets
to that speed.
If our application needs good torque at
high speeds then we need a motor with
a low inductance, such as types 2 and 4.
Low-inductance motors have fewer turns of
larger-gauge winding wire, but this comes
at a cost of needing a higher phase current.
The resulting lower operating voltage
is a benefit in that we can easily overdrive the motor to further counteract the
inductance, as described in the previous
part of this series. Out of these four motors,
you would choose type 4 for applications
where you needed high torque at high
speed and type 3 where low speed and
low operating current were specified.
Specialist suppliers can offer many
hundreds of variants of stepper motors
for optimum matching, but if your
application is not so critical then many
other electronics retailers offer a smaller
range of motors.
Case study
Recently, I was involved in selecting hybrid
motors for a CNC router. The x and y axes
needed to move at 9m/min with a step
size of 15µm, but the force needed was
uncertain. Theory suggested a suitable
NEMA 23 motor specification for the
speed, but reality was quite different for
the forces involved. It took three attempts
to get the right motor specification. Each
motor was tested by accelerating the y-axis
with the gantry restrained by a spring
balance on a tether. A point would be
reached when the motor stalled and the
corresponding maximum force was read
off the spring balance. While each motor
would work in terms of the 9m/min speed
needed, two of the motors failed to provide
adequate torque. The third was successful
and was adopted for production.
Terminology
In describing stepper motors, we have
used some engineering terms that may not
be familiar to all readers, so let’s expand
on a few of the more fundamental terms:
torque, hysteresis and angular velocity.
Table 8: Comparison of four NEMA 17 hybrid stepper motors
54
Type
Motor depth
(mm)
Holding torque
(Ncm)
Voltage
(V)
Phase current
(A)
Phase
resistance (Ω)
Inductance
(mH)
Torque at 450
RPM (Ncm)
1
39
36
5.4
0.85
6.3
10
15
2
40
45
2.2
2
1.1
2.6
28
3
48
32
12
0.4
30
25
Stall
4
48
59
2.4
2
1.2
3
39
Practical Electronics | February | 2020
D
T
r
1 kg
F
Fig.35. Relationship between torque, force
and distance.
Torque
First, torque, which is a turning force
that causes rotation around an axis and
has the SI unit of newton metres (Nm),
although for the size of stepper motors we
generally use, newton centimetres (Ncm,
where 1Nm = 100Ncm) is more commonly
used. A frequently asked question from
customers at Technobotsonline.com is,
‘What size motor do I need to lift a small
load?’. (This question equally applies to
brushed DC motors as well as stepper
motors) This enquiry is often missing
any supporting customer data to allow
a calculation to take place. Ultimately,
it is torque at a particular RPM that we
need to derive.
For example, we require a stepper motor
fitted with a 50mm-diameter pulley fixed
to its shaft to lift a mass of 1kg a distance
of 1m in 1s (see Fig.35).
The force F due to the 1kg mass is
given by: F = ma, where m is the mass
and a is the acceleration due to gravity
(g, so here, a = g = 9.8m/s2). Thus, we
have: F = 1 × 9.8 = 9.8N (newtons).
The torque (T) is the force (F) multiplied
by the perpendicular distance from the
point where the force acts to the axis of
rotation, which for a circular pulley is
simply its radius (r), or half its diameter
(D). In other words, T = F × r = F × D/2.
For the 50mm (0.05m) diameter pulley,
T = F × D/2 = 9.8 × (0.05/2) = 0.245Nm
(or 24.5Ncm).
That’s fine for the holding torque but
we need that torque at a certain RPM to
lift the mass in the specified time period.
The pulley circumference (C) is given
by: C = D = 3.14 × 0.05 = 0.157mm.
Therefore, we need 1/0.157 = 6.37
rotations of the pulley in one second or
6.37 × 60 rotations per minute, which
is 382 RPM.
It looks very much like only motor
types 2 and 4 in Table 8 will manage the
24.5Ncm at 382 RPM, but there is another
important consideration, and that is the
acceleration required. The quicker you
wish to accelerate the load, the more
force and thus more torque required.
Even maintaining a constant velocity will
require more motor torque.
Let’s see what effect this has on the
stepper motor torque when we accelerate
our 1kg mass to full speed in just 0.05s.
The required acceleration from rest
is calculated from: final velocity / time,
where final velocity (m/s) = 1m/s and time
= 0.05s, giving 1/0.05 = 20m/s2.
The additional force due to accelerating
the mass is again F = ma, which supplies
1 × 20 = 20N. Adding this additional force
of 20N to the steady state force of 24.5N
gives 44.5N, and thus a revised torque of
T = 44.5 × (0.05/2) = 111Ncm.
It is now becoming obvious that
none of the motors in Table 8 will have
sufficient torque to meet this criteria. If
the application allows, rather than choose
a different motor, reducing the rate of
acceleration significantly could allow
motor type 4 to be used.
This demonstrates the importance of
knowing the forces involved when sizing
a stepper motor. The theoretical example
above ignores any other losses such as
friction, so as with most engineering
designs, it would be prudent to add a
safety margin to the calculated motor size.
Hysteresis
In the last article we discussed the problem
of too small a step size resulting in the motor
rotor not responding to a step change due
to losses in the motor and the relatively
small step change in the magnetic field.
Thw result is a positional error caused by
the magnetic hysteresis and/or mechanical
friction. Put simply, if you push something
and it moves, when you release it, does
Table 9: Pololu.com motor stepper motor drivers comparison
Driver
chip
Min
operating
voltage
(V)
Max
operating
voltage
(V)
Max
continuous
current per
phase1 (A)
Peak
current per
phase2
(A)
Microstepping
down to
A4988
8
35
1
2
1/16
DRV8825
8.2
45
1.5
2.2
1/32
DRV8834
2.5
10.8
1.5
2
1/32
DRV8880
6.5
45
1
1.6
1/16
4.5
35
1.5
TB67S279 FTG
10
47
1.1
2
1/32
Auto Gain Control
ADMD
TB67S249 FTG
10
47
1.6
4.5
1/32
Auto Gain Control
ADMD
STSPIN 820
7
45
0.9
1.5
1/256
STSPIN 220
1.8
10
1.1
1.3
1/256
TB67S279 FTG
10
47
1.2
2
1/32
Auto Gain Control
ADMD
TB67S249 FTG
10
47
1.7
4.5
1/32
Auto Gain Control
ADMD
AMIS-30543
6
30
1.8
3
1/128
TB67S128 FTG
6.5
44
2.1
5
1/128
DRV8711
8
50
4
6
1/256
MP6500 Pot
MP6500 Digital
2.5
2
Special
feature
AutoTune
1/8
Digital current control
Digital current control
SPI interface
Low-EMI PWM
Auto Gain Control
SPI interface
Back EMF fb
ADMD
Back EMF feedback
1. On Pololu carrier board, at room temperature, and without additional cooling.
2. Maximum theoretical current based on components on the board (additional cooling required).
Practical Electronics | February | 2020
55
V 3 P 3 (o u t)
adjusts the current decay time for the
winding current to reduce ripple and
switch to a fast decay in order to reach
the next step quickly. It is also possible to
configure the driver to have fixed slow, fast
or mixed decays. Other features include
over-temperature thermal shutdown, overcurrent shutdown, short-circuit protection
and under-voltage lockout.
A number of the drivers in Table 9 have
auto gain control and ADMD like the
TB67S279FTG (Fig.37). ADMD is the term
Toshiba use to describe their Advanced
Dynamic Mixed Decay architecture.
ADMD is similar to the Texas mixed
decay described above, although Toshiba
claim theirs is able track input currents
more closely than ‘conventional’ mixeddecay modes.
D R V 8 8 0 0
E N A B L E
V M
M 1
G N D
M 0
B 1
T R Q 1
B 2
T R Q 0
A 2
S L E E P
A 1
S T E P
F A U L T
D IR
G N D
+
M o to r p o w e r
su p p l y ( 6 . 5 – 4 5 V )
1 0 0 µ F
V D D
M i cr o co
n tr o lle r
G N D
T O F F
Auto gain control (AGC)
L o g ic p o w e r
su p p l y ( 1 . 8 – 5 . 3 V )
Fig.36. The Texas DVR8880 driver breakout board. (Image courtesy of Pololu.com).
it return to the starting point? If not, it’s
exhibiting hysteresis.
Angular velocity
Angular velocity is defined as the rate of
change of angular position of a rotating
body and is measured in angle per unit of
time; for example, degrees or radians per
second, or revolutions per minute (RPM).
While RPM is not really a recognised way
of describing angular velocity, RPM and
angular velocity are used interchangeably,
especially where motors are concerned.
Advanced stepper motor drivers
and current per phase for example. Let’s
now look at some of the special features
on selected drivers.
DRV8800
Take the DRV8800 (Fig.36) for example.
It has some rather nice extra features. The
on-board potentiometer sets the maximum
current just like on the A4988 from last
month, but the digital inputs TRQ1 and
TRQ2 can be used to scale the current
limit to 25%, 50%, 75% and 100%. This
allows you to reduce the winding current
when full speed or torque is not required;
for example, when the motors are idle.
The second new feature to describe
is the default mode of ‘Autotune’ (a
trademark of Texas Instruments). Autotune
automatically tunes stepper motors for
optimal current regulation performance
and compensates for motor variation and
aging effects. Autotune automatically
Auto gain control automatically
optimises the motor current by sensing
the load torque applied to the motor and
dynamically reducing the current below
the full amount. This allows it to minimise
power consumption and heat generation
when the motor is lightly loaded, but if
the driver senses an increased load, it
will quickly ramp the current back up
to the full amount to try to prevent a
stall. Stepper motors driven at their rated
maximum current will most likely run
hot, especially when idle, and they may
even become be too hot to touch. AGC
can help reduce the heat rise in the motor.
Another feature of the Toshiba driver
is yet another acronym, ACDS or
Advanced Current Detection System.
This can monitor the motor winding
current without the need for external
current-sense resistors. Over-temperature
shutdown, over-current detection, motor
load open are also standard in the driver
chip but additionally, Pololu.com has
added reverse-voltage protection up to
40V to the breakout board.
L T H
B O O S T
F L IM
C L IM 1
C L IM 0
56
A G C 1
A G C 0
In the last article, the A4988 driver was
described because it offered the basic
requirements of a bipolar stepper motor
driver in terms of step, direction and
microstepping. From this point on, we
will differentiate between stepper motor
drivers and stepper motor controllers.
Drivers like the A4988 interface
via their step and direction
T B 6 7 S 2 x9 F T G
V M
lines, but generally need a
G N D
microcontroller for all but
L o g ic p o w e r
(
5
V
o
u
t
)
V C C
u
s
p
p
l
y
(
5
V
)
the most basic of applications
V IN
V R E F A – V R E F B
that do not require positional
M o to r p o w e r
G N D
D M O D E 0
su p p l y ( 1 0 – 4 7 V )
control. Stepper controllers
G N D
D M O D E 1
contain a microcontroller
5 V
O U T B +
D M O D E 2
on-board, allowing for other
G N D
O U T B –
(D IR ) C W /C C W
means of interfacing that do not
5 V M i cr o co n t r o l l e r
O U T A –
necessarily require an external
(S T E P ) C L K
microcontroller.
O U T A +
E N A B L E
Table 9 details 15 different
R E S E T
stepper motor driver chips
M O
A t l e a st o n e D M O D E ( st e p
that are all available as a
r e so l u t i o n p i n m u st b e
L O 1
co n n e ct e d t o l o g i c h i g h
Pololu.com breakout board
L O 2
ready for use in your project.
Some of the differences
between the drivers are selfexplanatory; minimum and
maximum operating voltage Fig.37. The Toshiba TB67S2x9FTG series of driver breakout boards. (Image courtesy of Pololu.com).
Practical Electronics | February | 2020
AMIS-30543 driver
A M IS -3 0 5 4 3
V D D (5 V o u t)
The AMIS-30543 (Fig.38) is a driver
I
O
R
E
F
we rather like at Technobotsonline.
L o g ic p o w e r
G N D
su p p l y ( 5 V )
com because compared to others we
V M O T
N X T (S T E P )
have tried, motors run cooler and
M o to r p o w e r
G N D
D IR
quieter. It does have one significant
su p p l y ( 6 – 3 0 V )
D O
complication – before it can be used it
5 V M i cr o co n t r o l l e r
M O T X P
D I
has to be initialised with an external
M O T X N
microcontroller over its SPI bus to
C L K
set its various operating parameters.
M O T Y N
C S
Therefore, this controller cannot be
M O T Y P
C L R
V D D
used in a standalone application
E R R
G N D
even though it does have step and
P O R /W D
direction pins; you will need a PIC
S L A ( filte r e d )
or Arduino microcontroller.
Pololu.com have provided an
G N D
V B B (o u t)
Arduino library that takes care of
the basic SPI interfacing as well
as some of the more advanced Fig.38. The ON Semiconductor AMIS-30543 driver breakout board. (Image courtesy of Pololu.com).
features. Setting the current limit,
microstepping and enabling the driver
TB67S279FTG driver described earlier.
The driver chips are very limited in
over SPI are required before you can
Also on the controller breakout board is
this regard, but with some additional
use the driver, and these values are not
a PIC18F45K50 microcontroller to take
circuitry it is quite possible. Pololu.
retained when power is cycled. In a
care of the additional interfacing options
com have a range of stepper controllers,
forthcoming article we shall be using this
and new features.
which they call their ‘Tic family’. In
driver for a 3-axis CNC controller, so we’ll
In addition to the inherent features of
addition to step/direction control, each
provide a more in-depth explanation of
the TBS67249FTG, with the Tic T249
of the controllers offers six additional
this driver then.
you have:
means of interfacing:
Features of the AMIS-30543 include
n Adjustable acceleration and
n USB (direct connection to a computer)
thermal warning and shutdown, overdeceleration periods
n TTL serial
current detection, open-coil warning,
n Maximum stepper speed: 50,000 steps
n I2C serial for microcontrollers
charge-pump failure and SLA. SLA (speed
/ sec; minimum speed: 1 step / 200 secs
n RC servo pulses
/ load angle) is an output pin that provides
n Digitally adjustable current limit
n Analogue voltage (potentiometer /
an output voltage that indicates the level
n Input calibration and adjustable scaling
joystick)
of the motor back-EMF (BEMF) voltage.
degree for analogue and RC signals
n Quadrature encoder
The SLA pin can be used for stall detection
n Optional limit-switch inputs with
or closed-loop control of the torque and
homing capabilities
Many of the settings in the Tic controllers
speed based on the load angle. This is a
n Optional kill switch input
can be configured using a free utility
more advanced feature, but does give an
(for Windows, Linux and Mac) which
indication of what is now being included
I2C, USB and TTL are beyond the scope
simplifies initial setup and allows for
in modern stepper driver chips.
testing and monitoring of the controller
of an introductory article, but we hope to
using a micro-B USB cable.
cover them later; however, the other three
options are certainly worth examining here.
Stepper motor controllers
All of the drivers described so far use
Tic T249
the step and direction interface, but it
One example of these stepper drivers is the
Radio control Servo
could be useful to have other interface
Tic T249 (Fig.39) based on a TB67S249FTG,
Many of us have dabbled with radio control
options available for certain applications.
which is a higher-rated version of the
(RC) models and are likely to be familiar
with servos connected to the RC receiver.
Theses servos use an industry-standard
S T E P
D i r e ct d r i ve r
signal with a pulse width in the range of
a ce
s
D IR
1-2ms, where 1ms is 100% transmitter
G N D
U S B M i cr o - B
R e ve r se - p r o t e ct e d
stick down, 2ms is 100% stick up and
co n n e ct i o n
V I N a ce
s
V M
1.5ms is in the neutral or middle position.
Traditionally, this pulse is usually updated
E R R
every 20ms. Connect your RC receiver to
R S T
the Tic (Fig.40) and the Tic behaves as
S C L
though it is a servo. Unlike a traditional
I2 C
S D A /A N
V IN (1 0 V – 4 7 V )
servo, there is no positional feedback;
G N D
G N D
it is open loop, although you can use a
T X
homing switch input.
T T L se r i a l
R X
A 2
Using the configuration utility, you can
M
R C
A 1
set the number of steps taken in either
( r e g u la te d o u tp u t) 5 V
B 1
direction relative to the RC pulse width.
G N D
B 2
Another RC mode is available where the
Tic acts like an electronic speed controller
B i p o l a r st e p p e r m o t o r
(ESC) but with a stepper motor rather than
T IC T 2 4 9
a DC motor. This time the RC pulse width
relates to the stepper motor speed, where
Fig.39. Tic T249 stepper controller. (Image courtesy of Pololu.com)
Practical Electronics | February | 2020
57
T ic
T ic
P o te n tio m e te r /
a n a l o g u e j o yst i ck
5 V
+
S
R C
4
r e ce
i ve
3
2
–
r
1
S
+
–
R C
5 V (o u t)
G N D
Fig.40. Connecting an RC radio receiver to
a Tic controller to act as a servo. (Image
courtesy of Pololu.com).
1.5ms is stopped, 1ms is full reverse speed
and 2ms is full forward speed.
Analogue voltage
In this mode, instead of the RC pulse input,
you can use a voltage or potentiometer
(Fig.41) to control the position of the
stepper motor or the motor speed, just
like the RC option.
Encoder position
Stepper motors are usually driven in open
loop, which means there is no physical
feedback of its actual angular position
(which you would have in closed loop).
CNC-type machines will have a home
switch, the controller would on request
58
SCL
S D A /A N
G N D
Fig.41. Connecting a potentiometer to the Tic
controller. (Image courtesy of Pololu.com).
a rotary encoder handwheel (Fig.42);
for example this item: http://bit.ly/
pe-feb20-hand
Like the RC servo and analogue modes,
the encoder mode can be set up for either
positional or motor speed control.
Over these five articles, we have covered
the various types of stepper motors and
stepper drivers – from a basic controller
you can build with simple components
to advanced drivers and controllers. I
hope some of the mysteries surrounding
the use of stepper motors have now gone
and you are already looking at how you
can incorporate steppers into your next
project. We shall be covering the build of
a 3-axis CNC controller next, producing a
system that is ideal for making a desktop
router or similar.
drive until the home position switch is
reached, and this would reset the internal
counters to its origin position. From there,
the controller would keep track of how
many steps it moves so it believes it knows
where it is. If the motor is stalled
T ic
or loses steps then you will have a
positional error. The trick here is to
5 V
5 V
100kΩ
engineer the system so that the motor
Q u a d r a t u r e e n co d e r
(both)
is never stalled or driven so quickly
G N D /C O M /C
GND
that steps are lost. Alternatively,
B
T X
encoders are available in both linear
A
R X
and angle (rotary) varieties to give
5 V (o u t)
(V C C )
feedback of mechanical position and
hence enable closed-loop control.
The Tic T249 is not capable of
providing closed-loop motor control Fig.42. Connecting a quadrature encoder to a
with encoder feedback but it can use Tic controller. (Image courtesy of Pololu.com).
Practical Electronics | February | 2020
