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4697
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> Amplifier
and Comparator Circuits > APP 4697
Maxim > Design support >
App notes
> Interface
Circuits > APP 4697

May 10, 2010

Keywords: motor control, industrial control, hall
effect sensor, DC Motor, brushless DC, AC induction,
electrical components, block diagram,
Related
parts

Download,
PDF format (189kB)

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APPLICATION NOTE 4697

Overview of industrial motor control systems

Abstract: This
overview of industrial motor controls highlights the
differences and subsystems for DC motor, brushless DC, and AC
induction motors. An in-depth analysis of critical subsystems
focuses on monitoring and measuring current; sensing
temperature; sensing motor speed, position, and movement;
monitoring and controlling multichannel currents and voltages;
and high-accuracy motor control with encoder data
interfaces.

Electric
motors consume almost 50% of the world's electricity. With the
cost of energy rising steadily, industry is focused on
replacing inefficient constant-speed motors and drives with
microprocessor-based, variable-speed drives. This new
motor-control technology will reduce energy consumption by
more than 30% compared to the older drives. While these
variable-speed controllers add cost to a motor, the forecasted
energy savings and increased motor functionality should easily
offset those initial expenses within a few years.

Popular motor designs
The DC motor, brushless DC, and
AC induction motor are the popular motor designs used in
today's industrial applications. Each of these motor types has
its own unique characteristics, but they all operate on the
same basic electromagnetic principle: when a conductor
carrying current, such as a wire winding, is located in an
external magnetic field perpendicular to the conductor, then
the conductor will experience a force perpendicular to itself
and to the external magnetic field.

DC motors: low cost and accurate drive performance
A
DC motor was among the first motor types put to practical use,
and it is still popular where low initial cost and excellent
drive performance are required. In its simplest form, the
stator (i.e., the stationary part of the motor) is a permanent
magnet, and the rotor (i.e., the rotating part of the motor)
carries an armature winding connected to a mechanical
commutator which switches current on and off to the winding.
The magnet establishes the field flux which interacts with the
armature current to produce the electromagnetic torque,
thereby enabling the motor to perform work. The motor's speed
is controlled by adjusting the DC voltage applied to the
armature winding.

Depending on the application, a
full-bridge, half-bridge, or just a step-down converter is
used to drive the armature winding. The switches in these
converters are pulse-width modulated (PWMed) to achieve the
desired voltage. Maxim's high-side
or bridge-driver ICs like the MAX15024/MAX15025
can be used to drive the FETs in the full- or half-bridge
circuit.

DC motors are also widely used in servo
applications where speed and accuracy are important. To meet
speed and accuracy requirements, microprocessor-based
closed-loop control and information about rotor position are
essential. Maxim's MAX9641 Hall-effect
sensor provides information about rotor position.

Block
diagram of a typical industrial motor control. For a list of
Maxim's recommended motor-drive solutions, please go to: www.maxim-ic.com/motordrive.

AC induction motors: simplicity and ruggedness
An AC
induction motor is popular in industry because of its
simplicity and ruggedness. In its simplest form, this motor is
a transformer
with the primary-side voltage connected to the
AC-power-voltage source
and the secondary side shorted to carry the induced secondary
current. The name "induction" motor derives from this induced
secondary current. The stator carries a three-phase winding
and the rotor is a simple design, commonly called a "squirrel
cage" in which the copper or aluminum bars are short circuited
at both the ends by cast-aluminum end rings. The absence of
rotor windings and brushes makes this motor design especially
reliable.

Rotor
and stator of an induction motor.

When operated
from the 60Hz voltage, the induction motor operates at a
constant speed. However, when power electronics and a
microprocessor-based system are used, the motor's speed can be
varied. The variable-speed drive consists of an inverter,
signal
conditioner, and microprocessor-based control. The
inverter uses three half bridges in which the top and the
bottom switch are controlled in a complementary
fashion. Maxim offers multiple half-bridge drivers like the
MAX15024/MAX15025 which control the top and bottom FETs
independently.

Precise measurement of three-phase motor
current, rotor position, and rotor speed are necessary for
efficient closed-loop control of an induction motor. Maxim
offers many high-side and low-side
current amplifiers, Hall-effect sensors, and
simultaneous-sampling analog-to-digital converters (ADCs) to
accurately measure these parameters in the harshest
environments.

A microprocessor uses data on the current
and position to generate logic signals for the three-phase
bridge. A popular closed-loop control technique called vector
control decouples the vectors of field current from the stator
flux so that it can be controlled independently to provide a
fast transient response.

Brushless DC motors: high reliability and high-output
power
A brushless DC (BLDC) motor has neither commutator
nor brushes, so it requires less maintenance than a DC motor.
It also offers more output power per frame size compared to
induction and DC motors.

The stator of the BLDC motor
is quite similar to that of the induction motor. The BLDC
motor's rotor, however, can take different forms but all are
permanent magnets. Air-gap flux is fixed by the magnet and is
unaffected by the stator current. The BLDC motor also requires
some form of rotor position sensing. A Hall-effect device
embedded in the stator is commonly used to sense the rotor's
position. When the rotor's magnetic pole passes near the
Hall-effect sensors, a signal indicates whether the north or
the south pole passed. Maxim offers several Halleffect sensors
like the MAX9641, which simplifies designs and reduces system
costs by integrating two Hall-effect sensors and digital logic
to provide both positional and directional outputs of the
magnet.

The importance of sensors, signal conversion, and data
interfaces
Several types of sensors provide feedback
information in the motor-control loop. These sensors also
improve reliability by detecting fault conditions that can
damage the motor. The following sections examine the role of
sensors in motor control in greater detail. Specific attention
will be given to current-sense amplifiers, Hall-effect
sensors, and variable-reluctance (VR) sensors. Other important
topics include monitoring and controlling multichannel
currents and voltages with high-speed analog-to-digital signal
conversion (ADCs), and the encoder data interfaces needed for
high-accuracy motor control.

Monitoring and measuring current for optimal motor
control

Current monitoring
Current is a common signal to be
sensed, monitored, and fed back to the motor-control loop.
Current-sense amplifiers make it easier to monitor the current
into and out of the system with a high level of precision. If
current-sense amplifiers are used, no transducer is needed, as
the electrical signal itself is being measured. Current-sense
amplifiers detect shorts and transients, and they monitor
power and reverse-battery conditions.

Current measurement
There is a variety of techniques
to measure current, but by far the most popular uses a
current-sense resistor. In this technique the voltage drop
across the current-sense resistor is first amplified by an op
amp set up in a differential gain stage, and then
measured. Traditionally, this approach has been implemented
with discrete components. However, discrete solutions also
introduce some disadvantages such as the requirement for
matched resistors, poor drift, and a larger solution area.
Fortunately, these multiple and varied disadvantages can be
overcome by integrating current-sense amplifiers into the
design. Not only do the amplifiers measure the current, but
they also sense the direction of current, accommodate wide
common-mode ranges, and provide more precise
measurement.

Current measurement employs either the
low-side principle in which the sense
resistor connects in series with the ground path, or the
high-side principle in which the sense resistor connects in
series with the hot wire. In low-side measurement, the circuit
has a low-input common-mode voltage and the output voltage is
ground referenced. The low-side resistor adds undesirable
extraneous resistance
in the ground path. In high-side measurement, the load is
grounded but the high-side resistor must cope with relatively
large common-mode
signals. High-side sensing also allows detection of fault
conditions such as the motor case or winding that shorts to
ground.

High-side
current-sense amplifiers like the MAX4080/MAX4081
employ a current-sensing resistor placed between the positive
terminal of the power supply and the supply input of the
monitored circuit. This arrangement avoids extraneous
resistance in the ground plane, greatly simplifies the layout,
and generally improves the overall circuit performance.
Maxim's unidirectional and bidirectional
current-sense ICs like the MAX9918/MAX9919/MAX9920
are available with or without internal sense resistors. This
variety of parts adds considerable flexibility to designs and
simplifies part selection for a wide variety of ADCs and
applications.

Sensing motor speed, position, and
movement
Hall-effect sensors are used to sense the speed,
position, and direction of motors. With integrated device
logic, the sensors then communicate that data to the system
for real-time feedback. The sensor also detects and reports
any interruption to the motor so corrective action can be
taken. Typically, to detect the direction of movement two
Hall-effect sensors are used.

Commutation can be
synchronized to Hall edges if the system has the same number
of Hall-effect devices as motor phases and if the mechanical
geometry of the Hall-effect devices is correlated with the
electrical geometry of the motor phases. Maxim's MAX9641 combines
two Hall-effect sensors and sensor signal conditioning to
provide both positional and directional
outputs.

Hall-effect sensors can also be used with
special Hall-effect sensor interface products like the MAX9621. The
interface devices provide several functions: protect against
supply transients, sense and filter the current drawn by the
Hall-effect sensors, and diagnose and protect against
faults.

Hall-effect sensors improve robustness and
repeatability, compared to mechanical photointerrupter-based
systems which are compromised in environments with dust and
humidity. Since Hall-effect sensors detect the magnetic field
produced by a magnet or current, they can operate continuously
in such harsh environmental conditions.

In some
applications vibration, dust, and high temperature
cause active sensors to operate improperly. In these
situations passive elements can be used to sense the motor's
operation and feed that data to the system with an interface
IC. Alternatively, variable-reluctance (VR) sensors can be
used in these extreme operating conditions.

VR sensors
like the MAX9924—MAX9927
have a coil to sense the speed and rotation of motors. When
the toothed wheel of the shaft attached to a motor passes by
the face of the magnet, the amount of magnetic flux passing
through the magnet and, consequently, the coil varies. When
the tooth is close to the sensor, the flux is at a maximum.
When the tooth is further away, the flux drops off. The
rotating toothed wheel results in a time-varying flux that
induces a proportional voltage in the coil. Subsequent
electronics then process this signal to get a digital waveform
that can be counted and timed more readily. Integrated
VR-sensor interface solutions possess many advantages over
other solutions, including enhanced noise immunity and
accurate phase information.

Monitoring and controlling multichannel currents and
voltages
To monitor and control a motor, multiple currents
and voltages need to be measured and the phase integrity
between the channels preserved. Designers are faced with two
choices for the ADC
architecture: use multiple single-channel ADCs in parallel,
a design that makes it very difficult to synch up the
conversion timing; or use a simultaneous-sampling ADC. The
simultaneous-sampling architecture uses either multiple ADCs
in a single package, all with a single conversion trigger or
with multiple sample-and-hold amplifiers (also referred to as
track-and-hold amplifiers) on the analog
inputs. In the case of multiple sample-and-hold amplifiers, a
multiplexer
is still used between the multiple analog inputs and the
single ADC. Simultaneous sampling eliminates the need for
complicated digital-signal-processing
algorithms.

Sampling speeds of 100ksps or more are
common for motor-control applications. At these speeds the ADC
continuously monitors the motor for any indication of errors
or potential damage. At the first sign of trouble, the system
can correct itself or shut
down when necessary. If the ADC does not sample fast
enough, an error condition might not be identified early
enough to be addressed.

The amount of dynamic
measurement range varies for each motor-control application.
In some cases 12 bits of resolution are sufficient. For the
more precise motor-control applications, however, 16 bits of
resolution are a more common standard. A high-performance
16-bit ADC like the MAX11044 or MAX11049 allows a
system to achieve better than 90dB of dynamic
range.

Maxim offers a broad portfolio of
simultaneous-sampling ADCs designed for motor control. Devices
have both serial
and parallel interfaces, and 12-, 14-, or 16-bit
operation.

High-accuracy motor control with encoder data
interfaces
The accuracy with which a motor needs to be
controlled depends on the system requirements. In some
applications the accuracy requirements are very high, as in
industrial robotics or in bottling. A welding robot, for
example, is expected to operate with high speed and high
precision. Similarly, the motors in a bottling factory must be
controlled accurately so that bottles are stopped at the right
position for filling, capping, and labeling. To control a
motor precisely, the rotor's speed, direction, and position
have to be determined. These can be monitored with analog
sensors like resolvers, synchros, RVDTs, or rotary
potentiometers. High accuracy is obtained with the use of
encoders like optical encoders and Hall-effect sensors.
Encoders provide the controller
with incremental and/or absolute shaft-angle
information.

A motor controller, commonly implemented
algorithmically by a digital signal processor (DSP),
calculates the rotor's present speed and angle. It adjusts the
actuating power stages to achieve the desired response
efficiently and optimally. This feedback control loop requires
robust and reliable information from the sensor, typically
communicated over long cables from the encoder to the
controller.

Incremental information is typically
transmitted to the controller by quadrature
signals, i.e., two signals phase shifted by 90°. These signals
can be in analog form (sine + cosine) or in binary form.
Absolute position information, in contrast, is only
communicated by a serialized binary data stream through RS-482
or RS-422.

As
the working environments are harsh, the data paths need to be
robust and reliable. EMI
levels are high, which explains the use of differential
signaling. High temperatures are commonly encountered due
to the proximity to the motor.

Maxim's extensive range
of RS-485/RS-422 and PROFIBUS interface devices are targeted
for these motor-control applications. Interface devices like
the MAX14840E
high-speed RS-485 transceiver
exhibit the high-signal integrity and robustness expected for
stringent safety control and for sustaining the up-time of
large capital investments.

Related parts

MAX11044

4-/6-/8-Channel,
16-/14-Bit, Simultaneous-Sampling ADCs

Free
samples

MAX11045

4-/6-/8-Channel,
16-/14-Bit, Simultaneous-Sampling ADCs

Free
samples

MAX11046

4-/6-/8-Channel,
16-/14-Bit, Simultaneous-Sampling ADCs

Free
samples

MAX11047

4-/6-/8-Channel,
16-/14-Bit, Simultaneous-Sampling ADCs

Free
samples

MAX11048

4-/6-/8-Channel,
16-/14-Bit, Simultaneous-Sampling ADCs

Free
samples

MAX11049

4-/6-/8-Channel,
16-/14-Bit, Simultaneous-Sampling ADCs

Free
samples

MAX13442E

±80V Fault Protected
RS-485 Half-Duplex Transceiver with Foldback
Current Limit

Free
samples

MAX13443E

±80V Fault Protected
RS-485 Half-Duplex Transceiver with Foldback
Current Limit

Free
samples

MAX13448E

±80V Fault-Protected,
Full-Duplex RS-485 Transceiver

Free
samples

MAX1377

Dual, 12-Bit, 1.25Msps,
Simultaneous-Sampling ADCs with Serial
Interface

Free
samples

MAX1379

Dual, 12-Bit, 1.25Msps,
Simultaneous-Sampling ADCs with Serial
Interface

Free
samples

MAX1383

Dual, 12-Bit, 1.25Msps,
Simultaneous-Sampling ADCs with Serial
Interface

Free
samples

MAX14770E

High-ESD PROFIBUS
RS-485 Transceiver

Free
samples

MAX14840E

40Mbps, +3.3V, RS-485
Half-Duplex Transceivers

Free
samples

MAX14841E

40Mbps, +3.3V, RS-485
Half-Duplex Transceivers

Free
samples

MAX3430

±80V Fault-Protected,
Fail-Safe, 1/4-Unit Load, +3.3V RS-485
Transceiver

Free
samples

MAX3440E

±15kV ESD-Protected,
±60V Fault-Protected, 10Mbps, Fail-Safe
RS-485/J1708 Transceivers

Free
samples

MAX3444E

±15kV ESD-Protected,
±60V Fault-Protected, 10Mbps, Fail-Safe
RS-485/J1708 Transceivers

Free
samples

MAX9621

Dual, 2-Wire
Hall-Effect Sensor Interface with Analog and
Digital Outputs

Free
samples

MAX9641

Low-Power, Dual
Hall-Effect Sensor

MAX9918

-20V to +75V Input
Range, Precision Uni-/Bidirectional, Current-Sense
Amplifiers

Free
samples

MAX9919

-20V to +75V Input
Range, Precision Uni-/Bidirectional, Current-Sense
Amplifiers

Free
samples

MAX9920

-20V to +75V Input
Range, Precision Uni-/Bidirectional, Current-Sense
Amplifiers

Free
samples

MAX9924

Variable-Reluctance
Sensor Interface with Differential Input and
Adaptive Peak Threshold

Free
samples

MAX9925

Variable-Reluctance
Sensor Interface with Differential Input and
Adaptive Peak Threshold

Free
samples

MAX9926

Variable-Reluctance
Sensor Interface with Differential Input and
Adaptive Peak Threshold

Free
samples

MAX9927

Variable-Reluctance
Sensor Interface with Differential Input and
Adaptive Peak Threshold

Free
samples

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Download,
PDF format (189kB)
APP 4697: May 10, 2010

AN4697, AN 4697, APP4697,
Appnote4697, Appnote 4697

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