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Electric
motors
are everywhere. I counted over 100 of them in my home, and
the number keeps growing. If I were to buy a new PC, for example,
I’d have to count many more. Two or three motors drive different
cooling fans.
The
DVD/ROM drive and DVD burner each contain four motors: tray
control, spindle drive, pickup sledge drive, and laser focus
servo. The hard drive has two motors, and even though floppy
drives are no longer included, common peripherals such as
force feedback joysticks, printers, and scanners more than
make up for their loss.
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Outside
the home, trains, cranes, and electric cars contain motors
that can output anything from 100 to several thousand horsepower.
Stationary applications contain big motors as well. Industrial
compressors, pumps, and fans are just a few examples.
Different
applications also require different types of motors. A small
stepper motor can position an inkjet printer head with incredible
precision, but it takes a monstrous three-phase AC induction
motor to drive a city water supply pump.
For
almost every motor, an electronic motor drive circuit controls
its speed and torque. The electronics that control the print
head motor and the water pump motor are as different as the
motors themselves. A $.50 8-bit microcontroller can drive
a stepper motor, while the current drive comprises a few surface-mounted
metal-oxide semiconductor field-effect transistors (MOSFETs).
On
a water pump motor, the fat copper rails connecting it to
the power stage hint at large electric currents. The power
stage is based around gigantic insulated gate bipolar transistor
(IGBT) transistors that alternate the direction of the current
through the motor’s three-phase windings.
Serious
Computing Power Needed
The
computing horsepower required to efficiently control pump
motors is almost as impressive as the motor itself – at least
to an electrical engineer. At the heart of the motor drive
sits a high-performance CPU, and a busy one at that.
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Among
its many tasks is generating the three 120-degree out-of-phase
sine waves on which AC induction motors thrive. It varies
the duty cycle of the drive currents at high frequency, and
the inductive load “sees” the waveform as a smooth sine wave.
This is pulse width modulation (PWM).
The
sine waves, amplified many times by the power stage, make
the motor spin. The CPU controls the speed and torque of the
motor by varying the frequency and amplitude of the sine wave,
respectively. However, as in an automobile, holding the accelerator
in a fixed position doesn’t guarantee travel at a constant
speed. This is why some clever engineers invented the cruise
control, and also why equally clever engineers came up with
electronic motor controls.
Throw
Some Math in There
In
simple terms, the CPU controls the speed and torque of the
motor this way:
Read
speed and torque values
Compare
with desired settings
Calculate
increments or decrements
Adjust
speed and torque using values from #3
Repeat
series from #1
The
most commonly used method to calculate the adjustments in
step #3 is known as PID – proportional integration and derivation.
PID comprises a series of computations that take the differences
between desired and current operation as inputs and spit out
suitable adjustments proportional to those differences – small
adjustments for small deviations, bigger changes if the motor
is really off-kilter.
You
can’t just add or subtract the entire difference, because
it takes several control loop iterations for the motor to
react to a change in input. Such an approach would lead to
instability, with wildly oscillating values.
The
motor drive measures the speed of the motor by decoding signals
from an optical or magnetic encoder on the motor’s drive shaft.
A state machine known as quadrature encoder interface (QEI)
decodes these signals. Measuring phase currents at certain
points in the PWM cycle gives you the torque.
You
can then feed other parameters to the control algorithm, such
as voltage and current waveform profiles. If you take the
latter and throw complex math at it, such as an algorithm
bearing the “Star Trek”-like name of flux vector control,
you can predict the motor’s behavior very accurately without
an opto-mechanical or magnetic interface.
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schematic
This
is useful when controlling smaller motors, where gadgets like
shaft encoders take up too much space and cost more than the
incremental CPU power to eliminate them.
Most
industrial motor drives are also connected to some kind of
network, so when you add the computational powers required
to run a TCP/IP or DeviceNET stack on top of everything, performance
requirements can reach as much as several hundred MIPS – if
you solve your embedded computing needs with brute force.
Like hunting sparrows with a rocket launcher, it works, but
there are more elegant approaches to meeting performance requirements.
Look
Ma, No Brushes
The
AC induction motor is generally considered the workhorse of
all industrial motor types. Another popular motor for high-performance
applications is the brushless DC motor (BLDC), also known
as the permanent magnet AC motor.
The
apparent confusion arises from the fact that it is built on
the principles of a DC motor but operates from an alternating
current. The BLDC motor doesn’t need a sine wave, but the
motor drive must know its exact rotational position at any
time, thus calling for a different position encoding/decoding
mechanism than AC induction motors. When it comes to controlling
currents and voltages, PWMs are still the name of the game.
Doing
It My Way
Because
you cannot build a high-performance motor drive without PWMs,
many embedded processors and microcontrollers come with PWMs
built-in. These PWMs are the result of long discussions between
the chip vendor’s marketing and engineering departments. It’s
a battle between customer requirements on one side and die
size estimates, design time, and testability on the other.
The resulting design is a compromise between what the “average”
customer wants and what can be done within the confines of
cost and time-to-market requirements.
A
Square PWM in a Square Hole
PWMs
come with different features and settings: varying resolution
and speed, edge-aligned versus center-aligned, dead time generation,
and symmetrical outputs. Most on-chip modules offer software
configurations of these parameters, but more often than not,
off-the-shelf modules will not meet your exact requirements.
Even if they did, that technology would also be available
to your competitors. Using a standard module often requires
limiting adaptations to the power stage and control software.
A
clever motor drive design requires that software, power stage,
and PWMs work together in perfect unison. In other words,
you need full freedom in designing all three, which effectively
rules out on-chip PWMs.
FPGAs
to the Rescue
Custom
PWMs = custom logic = FPGAs. With an FPGA, you can also do
custom QEI interfaces, an ADC interface, safety circuitry,
and timer arrays, all designed exactly the way you want them.
The
Next Step: Embedded Integration
The
traditional high-performance AC induction motor drive comprises
an FPGA and an embedded processor, as shown in Figure 1. This
configuration works well, except that the control loop traverses
the bus between the two components twice. Because this bus
is often shared with other system functions, performance is
indeterministic and easily becomes an unnecessary bottleneck
in loop response time.
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Xilinx®
embedded technology allows you to bring the embedded processor
onto the FPGA. The benefits of this approach go beyond fewer
components, smaller size, and fewer suppliers. It also improves
both performance and design time.
Co-Processor
Interface
It
does not matter whether you choose the 32-bit MicroBlaze™
soft-core processor or hard-coded PowerPC™ 405 processor.
Both offer some unique benefits through circuitry dedicated
to interfacing with on-chip peripherals in the FPGA fabric.
Figure 2 shows some design examples in which all control loop
data travels to and from the CPU over a dedicated, deterministic
link that is not shared with any other resources.
Eliminating
bottlenecks in a design is like peeling away the layers of
an onion. You design around one hurdle, and the next one reveals
itself immediately. Now that an important hardware pipe has
been effectively unclogged, you should not be surprised to
see a potential for improvements in your software. Figure
3 shows the software analyzer that comes packaged with the
Xilinx Embedded Development Kit (EDK). This tool could prove
exceptionally helpful in identifying resource-consuming functions.
Motor control software, like anything else, is likely to have
its share.
Intelligent
Versus Brute Force
I
have already mentioned the brute-force approach to designing
embedded systems. FPGA processors with a co-processing interface
effectively put an end to throwing raw MIPS at any performance
challenge. A carefully designed hardware/software mix can
reduce CPU performance requirements by several orders of magnitude.
Typical
software bottlenecks in a drive are floating-point math functions,
DSP functions, and of course, the PID function. As the designer,
you should define the optimum mix between hardware and software
modules. The FPGA gives you full freedom to balance this any
way you want.
A
New World
of Debugging
The
traditional design split of an embedded processor plugged
onto an FPGA can be a nightmare to debug. With two different
sets of debugging tools probing two different chips, visibility
of interaction between the two is extremely limited. The Xilinx
ChipScope™ Pro analyzer lets you debug software and hardware
interaction like never before. You can set the debugger to
trigger when the base counter inside the PWM reaches 0x3FF
and observe what code lines were executed within a 50 ìs window
around that point – without halting execution. Or set the
debugger to trigger on each Phase X zero crossing in the sine
table and observe the resulting PWM waveform over the next
5 ms.
With
full visibility of any chain of events within the system,
you can track down even the most elusive bugs in minimum time.
Motor
Control Components Ready to Go
As
much as we at Xilinx recognize the fact that you are the drive
design experts, we do provide a few bits and pieces to give
you some ideas of what’s possible with embedded processing
on FPGAs.
Our
reference design, “Spinning Wheels,” contains complete implementations
of one BLCD and one AC induction motor drive. The IP of the
solution is openly available as VHDL source code:
BLDC
drive unit
Hall
effect BLCD decoder
AC
induction drive unit
QEI
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The
solution comes packaged with reference designs that include
MicroBlaze integration with C code examples, tutorials, application
notes, and simulation test benches if you don’t have a power
stage and a motor handy at all times.
Take
a look at the blocks and play with them to get a feeling for
what’s possible. Once you have pieced them together, feel
free to sprinkle the design with an FPU and a CAN2.0B interface,
or whatever it takes to make it the perfect drive.
Motor
control design is tough work. The quest for smarter, faster,
and more power-efficient designs imposes an enormous strain
on software and hardware engineers. By adopting FPGAs with
on-chip embedded processors, you are free to implement the
creative design ideas needed to pull ahead of your competitors.
With full flexibility in PWM design and control over the hardware/software
split in the control functions, the possibilities are endless.
Carmakers
are taking increasingly complicated measures to protect the
vehicle owner's investment. Understanding antitheft technology
can save service time on vehicles.
If
ever there was a work in progress, its automotive security
systems. GM is already on its fourth-generation system since
it first introduced VATS (Vehicle Anti-Theft System) in 1986.
New antitheft systems from all of the major brands in the
U.S., Japan
and Europe seem to come under attack as soon as they're introduced.
Last
year, almost $8 billion worth of cars were stolen in the U.S.,
and only 65% of them were recovered. Canada
alone thinks that some of its 20,000 stolen vehicles were
exported last year.
In
the meantime, these systems have an enormous effect on the
automotive service industry. You don't even have to have your
vehicle stolen to be affected. Lose your keys? Recently, a
Ford-owning friend forked over $108 to have a key and fob
replaced. Own an expensive European make? Replacement of the
key and the security computer could cost you thousands of
dollars. Now there's yet a new generation of these systems
on your doorstep.
Let's
take a look at where we are and what's coming while we guess
how long it will take the bad guys to circumvent this latest
generation technology.
GMs
antitheft systems have been out there the longest and are
fairly representative of the types of antitheft systems used
on other makes. Originally named VATS, this is a good example
of a first-generation system. The key uses an embedded resistor.
When the key is inserted into the lock, two fine wires contact
the resistor. The VATS module measures this resistance and
compares it to the stored value. If they match, the vehicle
is allowed to start. If they don't, a delay of up to 10 minutes
is required before another attempt can be made to start the
car.
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In 1996, VATS was superceded in favor of the Passlock system.
This system works similarly to VATS, except the resistor has
been moved into the ignition switch. The Passlock II system
actually had two variants, but essentially was still a resistorbased
system.
Passlock
III, Passlock III+ and Ford's securilok system, among others,
use transponder-based radio frequency identification (RFID)
technology. At its most basic level, this is a wireless radio
link used to identify objects or people.
When
a transponder enters a read zone, its data is captured by
the reader and then passed along to a computer or other logic
controller for action. It's much the same as the technology
that identifies a runner in a marathon, a vehicle passing
through a tollbooth without having to slow down or the plastic
card that lets you through the security gate at work.
For
automotive applications, key-based immobilizer systems consist
of four main components. The heart of the system is the transponder,
which is a batteryless chip. To operate the chip, it has to
be supplied power from an external source. The transceiver
generates a high-frequency (134.2kHz) magnetic field, which
radiates from an antenna coil. The energy activates the transponder,
which sends a data stream in the form of an RF-modulated signal.
This signal is demodulated by the transceiver and then passed
to the controller for processing.
There
are two different types of transponder systems-full-duplex
and half-duplex. In a fullduplex system, tire energy for the
transponder and the data signal generated by the transponder
are transmitted at the same time. In a half-duplex system,
they're sent separately. The transponder stores the energy
it receives in a capacitor. As soon as the transmitter is
turned off, the transponder uses that stored energy to transmit
the data. The two types of systems have different effective
ranges and cost factors, but the degree of security is die
same.
There
are some evildoers out there with the motivation to bypass
both resistor-based and transponder-based systems. But among
the legitimate bypassers are the makers of remote engine starting
systems. A driver would be foolish to leave the key in the
ignition. So at least for the amount of time needed to start
a vehicle, the modules diese companies sell, along with the
remote starter, bypass the antitheft systems.
The
next generation of systems coming to market offer much higher
levels of security. They use cryptographic techniques to ensure
the codes cannot be stolen or bypassed.
The
need for this type of security got its start with the garage
door industry. Early garage door openers basically transmitted
a unique ID code to make sure it was your garage door you
opened and not your neighbors. As with any signal being broadcast,
those people with the right equipment could receive the signal
and record it. Later, when you're away, the signal could be
re-sent, opening your home to would-be intruders. How much
of this really went on is unknown, of course, but these "code
grabbers" were widely talked about as a threat to home
security.
The
technology developed to defeat this possibility is called
rotting code technology. The remote and the receiver keep
internal counters that begin by being synchronized with each
other. Each time the remote is used, the stored code number
is increased by a specific amount. When the user pushes the
button on the remote, the current value of the counter is
transmitted, along with the fixed ID number. The door opens
only if both numbers match.
AC
motor
AC
motor drives interface controllers to AC motors. They match
the control signals (voltage and power levels) as well as
the signal type (analog or digital). They also provide power
conversion, amplification, and the sequencing of waveform
signals. AC motor drives are used with many types of AC motors.
Induction motors induce current into the rotor windings without
any physical connection to the stator windings.
They
are suitable for many different environments and are capable
of providing considerable power as well as variable speed
control. Synchronous motors are no-slip devices that operate
at constant speed up to full load. Subcategories include reluctance
motors and permanent magnet devices such as AC servomotors.
Many AC servomotors use brushless commutation with feedback
provided by Hall effect sensors.
Pole
changing motors use pole number control, a method for changing
the number of poles on the primary winding. Vector drive motors
provide independent control of both the voltage and frequency,
resulting in low-speed torque outputs that approach those
of DC motors. In some designs, encoders or resolvers are used
to provide feedback about position and speed. Linear motors
generate force in only the direction of travel. Common technologies
include moving coil, moving magnet, and switched reluctance
designs.
Selecting
AC motor drives requires an analysis of application categories.
Multi-axis controllers are used to control and monitor multiple,
independent axes of motion. Motor speed controllers are application-specific
and used to control machines such as conveyors. Robotic motion
controllers use digital motion control hardware and software
for the coordinated multi-axis control of industrial robots
and robotic systems.
Servo
amplifiers are electronic modules that convert low level analog
command signals to high power voltages and currents. Inverter
drives convert AC power inputs to DC outputs. High frequency
drives supply power to AC motors at frequencies that are considerably
higher than those used in standard-power applications. Regenerative
drives support motor braking. Variable speed drives support
speed control and adjustment. AC motor drives that use microcontrollers,
silicon controlled rectifiers (SCR), digital signal processors
(DSP), and pulse width modulation (PWM) are also available.
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AC
motor drives differ in terms of electrical ratings, operating
parameters, configurations, and features. Electrical ratings
include maximum output voltage, rated power, continuous output
current, peak output current, AC supply voltage, and DC supply
voltage. AC motor drives use either single-phase or three-phase
inputs at 50, 60, or 400 Hz. Operating parameters include
specifications for setup and control. Some AC motor drives
have manual controls such as knobs, DIP switches, jumpers
or potentiometers. Others include a joystick, digital control
panel, computer interface, or slots for PCMCIA cards. Control
programs can be stored on removeable, nonvolatile storage
media. Hand held devices are designed to be programmed remotely.
Wireless and web-enabled controls are also available. Configurations
for AC motor drives include several mounting styles. Most
devices mount on a chassis, DIN rail, panel, rack, wall, or
printed circuit board (PCB). Standalone devices and integrated
circuit (IC) chips that mount on PCBs are also available.
Features for AC motor drives include soft starting; dynamic,
injection, or regenerative braking; brake outputs or auxiliary
inputs/outputs (I/O); auto-tuning, self-diagnostics, and status
monitoring; and alarms for conditions such as overvoltage.
Computer-based
AC motor drives use many different types of buses and communication
standards. Bus types include advanced technology attachment
(ATA), peripheral component interconnect (PCI), integrated
drive electronics (IDE), industry standard architecture (ISA),
general-purpose interface bus (GPIB), universal serial bus
(USB), and VersaModule Eurocard bus (VMEbus). Communications
standards include ARCNET, AS-i, Beckhoff I/O, CANbus, CANopen,
DeviceNet, Ethernet, small computer systems interface (SCSI),
and smart distributed system (SDS). Many serial and parallel
interfaces are also available
Design of Electric Motors, Generators and Drive Systems
This
course focuses on the analysis and design of electric motors,
generators, and drive systems. Special emphasis will be placed
on the design of machines for electric drives. This course
will focus on fundamentals by using commercially available
software for mathematical analysis (MATLAB) in the context
of design. Extensive "hands-on" exposure will be
provided through computer based laboratory exercises and through
the opportunity to construct and test an actual power electronic
drive for a test motor in our laboratory.
The
construction of aggressive, high-performance motor drives
requires a detailed understanding of machine characteristics
and associated interactions with power electronic drives.
The successful application of modern control techniques, such
as field-oriented control, depends critically on an intimate
knowledge of machine parameters and characteristics. Lower
shaft horsepower drives, for example, may exhibit a relatively
speedy decay of electrical transients in comparision to mechanical
transient settling times. In very large drives, the situation
can be reversed. Even for drives employing machines of the
same general type, appropriate analytical approximations,
thermal management and modeling, control techniques, and transient
performance and disturbance rejection for low and for high
power drives may therefore be very different. Computer-based
tools for estimating machine parameters and performance can
remarkably speed a designer's understanding of when different
control and machine design assumptions are applicable, and
how gracefully these assumptions fail as performance limits
are approached. Hands-on experiments will give the opportunity
to compare analytical results with real motor/drive systems
in the laboratory.
In
this course, fundamental principles of energy conversion,
applicable to all types of electric machinery, are first reviewed
to provide analytical foundations for understanding all types
of drives. The specific principals of the basic machine types,
including synchronous, induction and variable reluctance machines,
are then introduced. Extensive use of computer-based analysis
tools will be made as the major classes of machines and their
physical basis for operation are reviewed. Next, control strategies
for the different machine types will be discussed, all with
extensive use of computer-based simulation tools. Power electronic
circuits required to drive the machines will be considered
and a real drive circuit will be constructed, de-bugged and
tested by each participant. Throughout the course, performance
considerations, trade-offs, and different design approaches
will be presented. Access to computer facilities, analysis
routines and laboratory hardware facilities will be provided
for practice machine analysis design and test.
Required
Background
A
basic knowledge of electric circuit analysis and working familiarity
with principles of electromagnetism is assumed.
Program
Description
Lectures
will be given in the morning of each day, Monday through Friday,
of the program. In the afternoons students will work with
the instructors in a computer facility to explore and develop
design routines for electric drives. Registrants will receive
course notes, reprints of references and a suite of programs
written in MATLAB for assisting in the design of electric
machines.
Learning
Objectives
Describe
fundamental principles of energy conversion which are the
analytical foundations for understanding all types of drives.
Identify
the principals of the basic machine types, including synchronous,
induction and variable reluctance machines.
Evaluate
the use of computer-based analysis tools to review the major
classes of machines and their physical basis for operation.
Describe
control strategies for the different machine types, following
the use of computer-based simulation tools.
Examine
very high performance machine designs, such as extremely high
speed drives.
Analyze
performance considerations, trade-offs, and different design
approaches.
Evaluate
the hands-on use of machine analysis and design, using computer
facilities and analysis routines.
Topics
Elements
of energy conversion: energy, co-energy, force and torque
as derivatives of energy, field- based force calculations.
Energy
conversion in electric machines: force and shear density,
machine power density and efficiency.
Review
of the principles of the basic machines types: synchronous,
induction, variable reluctance.
Introduction
to and exercises in the use of MATLAB.
Induction
machines in some depth: reduction to an equivalent circuit
and calculation of the elements of the circuit.
Performance
evaluation of induction machines.
Field-oriented
control of induction machines.
Permanent
magnet machines: review of basics, principles of energy conversion
and design fundamentals.
Control
strategies for PM machines: torque/speed limitations, taking
advantage of negative saliency, elements of field oriented
control.
Unusual
machine designs
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