one channel microstepping motor driver board - Inter

The 6th edition of the
Interdisciplinarity in Engineering International Conference
“Petru Maior” University of Tîrgu Mureş, Romania, 2012
ONE CHANNEL MICROSTEPPING MOTOR DRIVER BOARD
Alexandru MORAR#1, Zsolt Albert BARABAS*2
#
Department of Electrical Engineering and Computer Science, “Petru Maior” University of
Tg. Mureş, No.1 N.Iorga St., Tg. Mureş Romania
1
[email protected]
*
Abbott Laboratories S A , Bucuresti, Romania
2
[email protected]
ABSTRACT
This paper presents a 1 channel stepping motor driver can drive bipolar stepping motors
with up to 2.25 amperes(peak) in full-, half-, fourth- or eighth-step mode. The board is
based on the A3977 specialized chips, that supports all features of microstep signal
generation and includes the amplifier.The step, direction, sleep, and enable inputs will be
passed out of the board to go the main controller or from PC parallel port. There wont
be anything fancy in the design, but there will be jumpers to set the step mode and the
current set resistors will be addressed. The board will use a standard two-layer process
and attempt to use all commodity cables for interconnects.
Keywords: stepper motor, microsteppimg,PWM, chopper, dedicated IC, positioning system.
1. Introduction
The most remarkable effect of the integrated
circuits increasing complexity and functions number
is represents by, as it is widely accepted, its
“intelligence”. There is almost no applications
domain in which the microelectronic devices
“intelligence” shouldn’t have played a major role, one
of the fields enjoying its advantages being the low
power electric drives [1][14]. By introducing the
“intelligence” in the drives command, this one will
take over some complex functions usually
accomplished by the human factor. In the automatic
regulation systems, the electric stepper motors are
utilized as execution elements. Stepper motor is the
most utilized motor in low power adjustable electrical
drives due to relatively simple methods of speed
control. The stepper motors are used in many
applications because of their advantages[2][13]. Thus,
they move in quantified increments (steps) which
lands them easy to digital control motion systems in
open-loop mode. In addition, their drive signals are
square waves which are easily generated by the
digital circuits with relatively high efficiency. But
stepper motors are not free of problems. The most
typical application for these drives is represented by
the precision positioning systems. These ones must
satisfy relatively exacting dynamic conditions,
generally difficult to be fulfilled, sometimes even
contradictory, fact that partially explains why is
necessary that the command devices must be
“intelligent”. Taking into consideration the above
mentioned aspects, the authors presents in this paper
1 channel stepping motor driver board based on the
A3977[10] specialized integrat circuit, that can drive
motors with up to 2.25 amperes(peak) in full, half,
fourh or eighth stepe mode.
2. Functional description
The A3977[10] is a complete microstepping
motor driver with built in translator for easy operation
with minimal control lines. It is designed to operate
bipolar stepper motor in full, half, quarter and eighth
step modes. The functional block diagram is shown in
figure 1. The current in each of the two output Hbridges, all n-channel DMOS, is regulated with fixed
off time pulse-width modulated (PWM) control
circuitry. The H-bridge current at each step is set by
the value of an external current sense resistor (RS), a
reference voltage (VREF), and the DACs output
voltage controlled by the output for the translator. At
power up, or reset, the translator sets the DACs and
phase current polarity to initial home state (see
figures for home-state conditions), and sets the
current regulator for both phases to mixed-decay
mode. When a step command signal occurs on the
STEP input the translator automatically sequences the
DACs to the next level(see table 2 for the current
level sequence and current polarity).
The microstep resolution is set by inputs MS1 and
MS2 as shown in table 1. If the new DAC output
level is lower than the previous level the decay mode
for that H-bridge will be set by the PFD input(fast,
slow or mixed decay. If the new DAC level is higher
or eqal to the previous level then the decay mode for
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that H-bridge will be slow decay. This automatic
current-decay selection will improve microsteping
performance by reducing the distortion of the current
waveform due to the motor BEMF. The electrical
schematic of the realized board is shown in figure 2.
The RESET input (active low) sets the translator to a
predefined home state (see figures for home
conditions) and turns off all of the DMOS outputs.
The HOME output goes low and all STEP inputs are
ignored until the RESET input goes high. The HOME
output is a logic output indicator of the initial state of
the translator. At power up the translator is reset to
the home state (see figures for home state conditions).
A low-to-high transmition on the STEP input
sequences the translator and advances the motor one
increment. Thetranslator controls the input to the
DACs and the direction of current flow in each
winding. The size of the increment is determined by
the state of inputs MS1 and MS2 (see table 1). The
state of the DIRECTION input will determine the
direction of rotation of the motor. Each H-bridge is
controlled by a fixed off time PWM current-control
circuit that limits the load current to a desired value
(ITRIP). Initialy, a diagonal pair of source and sink
DMOS outputs are enabled and current flows through
the motor winding and RS. When the voltage across
the current-sense rezistor eqals the DAC output
voltage, the current-sense comparator reset the PWM
lath, which turns off the source driver (slow-decay
mode) or the sink and source drivers (fast or mixeddecay modes). The maximum value of current
limiting is set by the selection of RS and the voltage at
the VREF input with a transconductance function
approximated by: ITRIPmax = VREF/8RS. The DAC
output reduces the VREF output to the current-sense
comparator in precise steps (see table 2 for %
ITRIPmax at each step).
ITRIP = ( %ITRIPmax/100) x ITRIPmax. It is critical
to ensure that the maximum rating (0.5V) on the
SENSE terminal is not exceeded. For full step mode,
VREF can be applied up to the maximum rating of
VDD, because the peak sense value is 0.707 x VREF/8.
In all other modes VREF should no exceed 4V. The
internal PWM current-control circuitry uses a one
shot to control the time the driver(s) remain(s) off.
The one shot off-time, toff, is determined by the
selection of an external resistor (RS) and capacitor
(CT) connected from the RC timing terminal to
ground. The off time,over a range of values of CT =
470 pF to 1500 pF and RT = 12 kΩ to 100 kΩ is
approximated by:
toff = RT x CT. In addition to the fixed off time of the
PWM control circuit, the CT component sets the
comparator blanking time. This function blanks the
output of the current-sense comparator when the
outputsare switched by the internal current-control
circitry. The comparator output is blanked to prevent
false over-current detection due to reverse recovery
currentsof the clamp diodes, and/or switching
transients related to the capacitance of the load. The
blank time tBLANK can be approximated by: tBLANK =
1400CT. The full-step, half-step, quarter-step and
8microstep/step operation is shown in figure 3 to
figure 6.
Fig. 1 – Functional block diagram[10]
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Fig. 2
– The electrical schematic of the realized board
Table 1. Microstep Resolution
Truth Table
MS1 MS2 Resolution
L
L Full step
H
L Half step
L
H Quarter step
H
H Eighth step
ENABLE (active-low) input, enables aii of the
DMOS output. When logic high the outputs are
disabled. Inputs to the translator (STEP,
DIRECTION, MS1, MS2) are all active independent
of the ENABLE input state.
SLEEP mode: an active-low control input used to
minimize power consumption when not in use.
This disables much of the internal circuitry
including the outup DMOS, regulator, and charge
pump. A logic high allows normal operatio and
startup of the device in the home position. When
coming out of sleep mode, wait 1ms before issuing a
STEP command to allow the charge pump (gate
drive) to stabilize.
Percent Fast Decay Input (PFD). When a STEP
input signal commands a lower output current from
the previous step,it switches the output current decay
to either slow, fast, or mixed decay depending on the
voltage level at the PDF input. If the voltage at the
PDF input is greater than 0.6VDD then slow-decay
mode is selected. If the voltage on the PDF input is
less than 0.21VDD then fast-decay mode is selected.
Mixed decay is between these two levels.
Active mode. When the SR input is logic low,
active mode is enabled and synchronous rectification
will occur. When the SR input is logic high,
synchronous rectification is disabled. This mode is
typically used when external diodes are required to
transfer power dissipation from the A3977 package to
the external diodes.
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Table 2. Step Sequencing[10] 1:1/8 th-Step subdivision current ratios
Home state = 45o step angle, DIR = H
Fig. 3 – Full Step Operation[10]
MS1=MS2=,,0”( L ); DIR =,,1”( H )
Fig. 4 – Half Step Operation[10]
MS1= =,,1”(H), MS2=,,0”( L ), DIR =,,1”( H )
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Fig. 5 – Quarter Step Operation[10]
MS1=,,0”( L ), MS2=,,1”( H ), DIR =,,1”( H )
Fig. 6 – 8 Microstep/Step Operation[10]
MS1=MS2=,,1”( H ); DIR =,,1”( H )
Fig. 7 – Layout diagram of the realized microstepping board
Fig. 8 – General view of the realized microstepping board
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Fig. 9 – General view of the realized experimental precision positioning system
3. Experimental results
The experimental research was performed in the
Electrical Drives Laboratory of the Electrical
Engineering Faculty, “Petru Maior” University of
Târgu-Mureş, where it has been realized an high
performance microstepping system for stepper motor
control[3] [4][6]. Figure 7 shows layout diagram of
the realized microstepping board.
Figure 8 shows the general view of realized
microstepping board. In Figure 9 is shows the general
view of the realized experimental precision
positioning system.
In order to measure the phase currents, two hall
sensors (LEM modules – LA25NP) were used, and a
data acquisition numerical system dedicated to the
electric drives as in [3] [4][6].
As experimental results, the phase currents
of a two-phase bipolar stepper motor (1, 2, 4, 8
microstep/step) are shown in Figure 10.
4. Conclusions
The last progress both in control in motor drive
domain impose on the researchers a continuous
reorientation in order to solve the design problems
with the newest technical means. The modern
solution involve new power semiconductor devices
with high performances, dedicated command circuits
with multiple specific function and new control
techniques. In this sense the authors have developed
an original microstepping system for the open-loop
control one stepper motors for precision positioning
systems. Among the facilities offered by this system
we mention:
• resonance’s are significantly reduced
• reduced audibile motor noise
• increased step accuraty
• reduced power dissipation
• very high step resolution
•
•
•
•
•
•
•
•
dramatically simplified stepper motor
driving small-and medium-sized motors
the A3977 interface is an ideal fit for
applications where a complex PC or µP
flexibility in selecting constant-speed runnig
frequencies
low cos assembly
precise one axe position control
precise rotation control
robotics and assembly equipment
other stepping motor application
References
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Their
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implementate pe
calculatoare pesonale
(Electronic systems for stepping motor control
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[7] Baluta, Gh., Actionari electrice de mica putereAplicatii, Editura POLITEHNIUM, Iasi, 2004.
302
Full Step, 20 Hz
Full Step, 200 Hz
Half Step, 20 Hz
Half Step, 200 Hz
Quarter Step, 20 Hz
Quarter Step, 200 Hz
8 Microstep/Step, 20 Hz
8 Microstep/Step, 1000 Hz
Fig. 10 – Experimental results, the phase currents of bipolar stepper motor (1,2,4,8 microstep/step)
303
[8] Palaghita, N, - Electronica de putere-partea IDispozitive semiconductoare de putere, Editura
MEDIAMIRA, Cluj-Napoca, 2002.
[9] Palaghita, N, - Electronica de putere-partea IICircuite electronice de putere, Editura
MEDIAMIRA, Cluj-Napoca, 2004.
[10] http://allegromicro.com/datafile/3977.pdf
[11] ***
SGS-THOMSON,
Motion
Control
Application Manual, 2010.
[12] *** SGS-THOMSON, “Microelectronics”, Data
on disc, 2011.
[13] *** “LEM Module”, Data Book, Geneve,
Switzerland, 2010
[14]
*** Portescap: “Motion Systems”, 2011.
304