Machine and design engineers are being required to use servo drives and
motion technology more often than ever before. In this two part feature,
motion expert Andy Sumner discusses the fundamentals of the technology
that will need to be considered for every application. Part One: a
glossary of terms
The stepper motor Strictly speaking, stepper motors are not servo
devices; however, they offer a form of precision motion and have the key
advantages of being low cost and simple to use. They use a ‘chopper’
drive, so actually rotate in a precise number of steps (typically of 1.8
degrees, although microsteppers have a step angle of 0.1 degree or less).
Their motion profiles are programmed in a series of step counts, and
their performance is adequate for many applications. They are usually
used in simple open loop configuration, so cannot self-correct if an
error occurs or if there is any lost motion in the system. The loop can
be closed by a position feedback device but this adds expense and
complexity. A servo might be more appropriate in this case.
The dc brushed servomotor dc brushed servos were the original type of
servomotor, being in common use from the early 1970s but now increasingly
constrained to cost-sensitive applications. They have a permanent magnet
stator, and voltage control of the rotor varies the speed and torque.
Nowadays they are rarely used in sizes above 5Nm (minimum size being
about 0.1Nm), but an advantage is their low cogging torque (‘cogging’ is
step-like output, rather than smooth), so they are favoured in many
applications such as instrument drives that require very smooth output.
These motors need to be carefully selected for any given application and,
like all servomotors, require feedback of position in order to work.
The dc brushless servomotor Brushless motors are currently the most
popular servo configuration and have been since their design was
perfected about 15 years ago. They are cost effective due to their simple
commutation electronics, but at low speeds net efficiency can be poor and
cogging pronounced. The cogging is due to the intrinsic square wave or
trapezoidal output produced by the commutation technique and can produce
a torque ‘ripple’ effect. The dc brushless version tends to be the
preferred option for applications from 0.1Nm up to 10Nm where cost is not
an overriding constraint, and is frequently favoured up to 20Nm. it is
important to note that the output of a dc brushless motor is ac! The ac
brushless servomotor Today, the ac brushless servo is in the ascendancy
and is gradually taking over from the other types, which will eventually
serve particular niche applications only.
The ac brushless servomotor represents the final and mature version of
servomotor design; it will not be surpassed, so will be in use for many
decades to come. The price premium over dc is only about 20% and this
buys some very important advantages and benefits.
The output from an ac motor is sinusoidal, and with the phases (which are
typically three, but can also be two, four or eight) overlapping, a
smooth, effectively ripple- and cog-free output results. The lack of
brushes reduces maintenance to practically zero, while auto-tuning
software makes installation a simple ‘plug-and-ply’ operation that can be
accomplished by most engineers. Starting from 0.05Nm or even lower, the
size range of ac brushless motors is practically limitless. They are
often used on winders, rotary knives, carriage drives and indexers
requiring power of up to 55kW and far larger units are available for
specific applications.
Feedback
In every servo system a feedback device, usually fixed to the motor’s
output shaft, constantly tells the motor the current position of the load
so that errors can be corrected instantly.
Encoders The traditional feedback device is an optical encoder. These
comprise a rotating glass disc etched with radial lines that serve to
chop alight beam passing through it. This creates a digital or pulse
signal indicative of the motor’s speed and duration of operation. A
second light beam positioned 90 degrees from the first can be used to
create a phase shifted second signal so that direction can be ascertained.
An absolute encoder uses multiple channels to generate a unique signal
for every position increment. One of the most popular and useful types of
absolute encoder is the sine-cosine device. this uses two channels, one
generating a sine wave voltage output, the other a cosine signal. These
are processed to produce and inverse tangent value that can uniquely
identify every rotary position. Encoders are low cost and simple, but are
rather too fragile for many industrial applications. They produce a
quadrature output, which limits the resolution to which they can
effectively measure - typically 1,000 lines per revolution.
Resolvers Originally expensive, resolvers are increasingly taking over
from encoders as their prices are coming down. They are effectively a
rotating transformer with one rotary primary coil and two perpendicular
stationary coils forming the stator. The motor’s position is determined
by measuring the instantaneous voltage in each stator coil and their
phase position relative to the primary. This gives a sine wave output,
which can be resolved to great accuracy. A 17-bit resolver, for example,
provides 64,000 counts per revolution.
Servo amplifiers
A servomotor is controlled by a servo amplifier or drive, which is
basically the electronics to control the commutation. The feedback device
sends its signal to the amplifier to provide the closed loop constant
self-checking characteristic of servo systems.
The amplifier’s core function is to supply the +/-10V signal that
controls the motor’s output speed and direction. This signal is initially
generated by the motion profile programmed into the amplifier, but before
it is sent to the motor it is error checked against the incoming position
signal from the feedback device. Error checking ensures that the
servomotor always provides perfect output - it nips error in the bud,
often within a tiny fraction of a degree of the error occurring.
A servo amplifier is software programmed to produce the desired motion
profile, and this can be interactive with other elements of a control
system. For instance, a vision system could be used to locate the precise
position and orientation of a product within a target area; the motion
system could then move a nozzle head to a ‘start point’ on the product so
that sealing mastic can be applied in a two dimensional path or pattern.
Motion control
Servo technology really comes into its own in multi-axis applications.
Here several axes need to be controlled in perfect synchronism with each
other. Traditionally, this is done in a series of moves where axes are
linked together either by simple interpolated moves or more complex
algorithms, the benefits being faster interpretation time and therefore
greater productivity. Multi-axis servo or motion control exploits the
accuracy, resolution, self-correcting and programmable characteristics of
servo technology to automate the generation of complex motions. It has
virtually limitless applications potential in industrial automation and
in a wide range of other application fields such as special effects,
research and therapy and exercise machines.
Motion control is basically a software user interface that allows
engineers to describe the motions they require in an intuitively human
way. The programme then analyses this and converts it into the necessary
individual axis commands. The founding principle of motion control is
that it should simplify what was previously complex; the technology has
developed to the point where it is now meeting these criteria - as more
and more engineers are discovering!
Andy Sumner is with Omron Electronics