B OTH S I D ES OF T HE LI N E
In modern motion control applications, "power" means "motor", and a developer's
choices are limited. One chooses either a traditional rotary motor or a newer linear
model. Since the goal in most applications is the transformation of rotary motion to
linear motion, one might conclude that using a linear motor would give the developer an
innate advantage.
There are many reasons why one might want to use a direct-drive, permanent-magnet,
brushless linear motor:
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High speed capability, typically 3 to 5 m/s (up to 10 m/s)
High acceleration, typically 5g and up
Very low settling time
Very high accuracy and/or repeatability
Very long travel (several meters)
Very quiet operation
Reliable operation for your global operation
Cost is approaching traditional methods that use precision ball screws
Attaining market differentiation from
your competitors
After reviewing these benefits and having decided to use
a direct-drive, permanent-magnet linear motor for the
linear application, the next step is to choose among the
available technologies. Essentially, this breaks down to
selecting either a zero-cogging ironless motor or an iron
core motor. I'll outline the benefits of each and show
why, in many cases, the iron core is a better solution in
terms of both cost and performance. Let's first review
the construction differences.
Figure 1 - Thermally conductive epoxy
encapsulates an ironless motor's windings.
Ironless linear motors
Ironless refers to the construction of the coil assembly (often called the glider or forcer).
Using copper windings that are arranged in an epoxy or other nonmagnetic core and
encapsulated with a thermally conductive epoxy. A metallic stiffener, secured to the
edge, serves as a mounting surface. This technology is often called an epoxy, or slotless,
motor since there is absolutely no iron in the magnet field and hence no slots (Figure 1).
The magnet ways, or channels, for an ironless
motor are composed of two plates. Each plate has
magnets of opposing polarity mounted to it,
arranged in a U shape (Figure 2). Magnet ways
can be used in unity or butted together to achieve
any desired length.
Figure 2 -The magnet ways of an ironless
motor are arranged in a U shape.
Iron-core linear motors
Iron core refers to coil assembly construction in which copper coil windings are inserted
in a low-less steel lamination that serves as the coil assembly structure. The entire
assembly is encapsulated in a thermally conductive epoxy, which stiffens the structure
and seals the assembly. We can picture this type of linear motor as a rotary brushless
motor cut lengthwise and laid flat (Figure 3).
The magnet ways of an iron-core motor are made
of an iron plate carrying a single row of magnets
with alternating polarities (Figure 4). In most
cases, this plate is mounted directly on the
machine bed and several plates of standard length
are butted together to make a desired length.
An iron-core linear
motor.
Figure 3 - Rotary (l) and linear ( r ) brushless
motors.
There are five key parameters that affect the
selection, equipment design, and implementation
of a linear-motor-driven
machine: cogging,
attractive force, peak
force, acceleration, and
accuracy.
Figure 4 -A single row of alternating -polarity magnets
forms an iron-core.
Cogging
Cogging, a disturbance present only in ironcore motors, is defined as the force variation
when the coil is pushed along the magnet
ways, and results from the variable reluctance
path of the magnetic circuit; note that the
cogging measurement is made with no power
to the motor. Just like in a rotary brushless
motor, skewing the magnets, the lamination
stack, or both can minimize the cogging,
which is typically only 3-5% of rated force.
A significant advantage of using unskewed
magnets and lamination stacks is
eliminating the cosine effect, which allows
for the most efficient use of the available
magnet flux, maximizing available force.
In any case, because the frequency of the
cogging is very low due to the relativity
high motor pitch (typically more than 1
inch between two magnets of the same
polarity), it is very easy to make an
electronic compensation with the amplifier.
Cogging should not be confused with the
force ripple observed when the motor is
energized and driven with a perfect
sinusoidal current. This force ripple is due
to the manufacturing tolerance of not only
the coil and magnet placement, but also the
variation of flux from magnet to magnet.
Note that the force ripple is present in both
ironless and iron-core
motors and is
a function of both the motor and, more
significantly, the drive electronics.
Figure 5 - Newton's Second Law affects coil costs.
Coil 1
Magnet Way 1
net Way 1
Magnet Way 2
Figure 6 -A balanced load
design configuration.
Coil 2
Attractive Force
There is a fixed relationship between the
peak force of the linear motor and the
attractive force. With commonly available,
high performance neodymium iron boron
magnets, the ratio of attractive force to peak
force is approximately 4:1. This means, for
example, that for a motor with a rated peak
force of 800 lbf,, the attraction force is
approximately 3,200 lbf.
Figure 7 - Both base and slide must be thick
enough to prevent deflections; a rigid structure
improves system performance.
This corresponds to the weight of an
average car, but due to the high efficiency
of the linear bearings available today (with
a typical friction coefficient of 0.002 to
0.003), the additional moving force is only
3,200 lbf, times a friction coefficient of
0.003, or 9.6 lbf.
Thus, the attractive force represents a low
percentage of the peak motor force (in this
case, only 1.2%). Many linear bearings are
available to handle high loads in both
perpendicular and side-loaded directions.
Therefore, while the machine structure does
have to be designed correctly to adequately
handle the attractive force,the penalty on the
coil sizing is very minor; simply oversizing the
motor by a few percentage points will correct it.
Figure 8 - Mounting linear bearings close to the
motor (a) reduces thickness requirements and lowers
weight; wider spacing (b) may result in considerable
slide deflection.
Peak Force
Because of the large air gap in an
ironless motor and the absence of iron
to concentrate the flux, the peak and
continuous forces are lower than in an
iron-core design with the same surface
area. The iron less design has a limited
thermal conduction path between the
coils and the mounting bracket (small
contact area), and, in many cases, it is
recommended that you thermally isolate
your machine from the coil to avoid
heat transfer. So the net result is low
forces produced; ironless motors are
Figure 9 -Mounting the linear encoder close to the
limited to a few hundred pounds of
bearing (a) minimizes yaw errors and improves settling
times; distant mounting (b) can cause signal errors.
available force only. In an iron-core
motor, the coils have an effective
thermal path to the steel lamination stack (whose mass is several times that of the
coils), and the small air gap between the magnets and coil assembly produces a very large
motor force. The coil is usually mounted on a large plate that acts as a heat sink to
improve the thermal resistance of the system, resulting in higher attainable peak and
continuing higher attainable peak and continuous forces. A single coil assembly can
achieve 3,000 to 5,000 lbf of peak force.
If we compare the same coil surface area of the two technologies, an iron-core coil
creates 2.5 times the force of an ironless design. This is achieved with half the volume of
magnets since the iron-core motor uses only a single row.
One drawback of the iron-core motor is the saturation effect of the magnetic circuit. In
applications with very low duty cycle, the ratio of peak force to continuous force can
reach up to 5:1 or 6:1 for an ironless motor and approximately 3:1 for an iron-core motor.
A comparison of the peak force/peak current ratio to continuous force/continuous current
ratio will yield similar numbers if the ratings are derived at or below the saturation point.
If the ratios are very different, this means the published peak force is obtained at a higher
demagnetization level. Some suppliers are more conservative than others, but
realistically, very few application have a very low duty cycle.
Acceleration
An ironless motor has a low mass due to its lightweight construction, which results in a
much higher acceleration than seen in an iron-core motor. Typically, the theoretical
acceleration (peak force is divided by the coil weight) is four times higher than for an
iron-core motor with comparable force.
The key word here is theoretical, since mounting the coil ion a structure reduces the
practical acceleration due to the actual moving mass. For example, if the mass to carry is
nine times the weight of the coil, the practical acceleration will be only one-tenth the
theoretical acceleration.
Also keep in mind that a low coil mass means a low thermal time constant; an ironless
coil will heat faster than an iron-core coil.
Now an easy thing to remember: if you double your acceleration requirement (let's say
going from 2 to 4 g) you will also double the cost of your coil (Figure 5). The reason for
this is that force, proportional to the acceleration, is also proportional to the surface area
of the coil. If you cannot grow in coil length you will have to grow in coil width, and, in
that case you double the cost of your magnet ways. If you cannot grow in length or in
width, you will have to either use a water-cooled coil or place a second coil somewhere
in the design.
In summary: establish realistic goals for your machine, study the overall machine cycle
time (including all the other parts), and do not over specify your acceleration
requirement. If you need to double the available power, try first to double the speed; this
will involve having to double the available current (or voltage) of the amplifier, but this is
more cost-effective than doubling the size of the linear motor.
Accuracy
Conventional wisdom holds that an ironless motor with an air-bearing structure will
allow the best possible accuracy. Experience shows that a good iron-core motor,
combined with a very good positioner and quality linear bearings, can achieve both
higher stiffness and position with repeatability within a few nanometers. Table 1
summarizes the features of both iron-core and ironless motors.
Machine Structure
There is no freebie here: if you want to boost the dynamic performance of your machine
(increase acceleration, speed or both), you can't simply replace a leadscrew, rack-andpinion, or timing belt arrangement with a high-performance linear motor. The old timers
in motion control certainly remember that replacing a brushless motor with higher speed
and acceleration was useless if no change was made to the mechanics. This has never
been truer than with linear motors.
Some practical design guidelines can help you focus on the right approach. The success
of your project depends on good implementation of the machine by your best mechanical
engineer-one experienced in structural design, mechanics, dynamics, and kinematics. I
also recommend that you work closely with your motion control supplier, who preferably
has long experience in motion control and makes his own motors and drives. Depending
on your needs and requirements, you may want to seek recommendations on
configurations that feature a moving coil, a moving magnet, or a special design to cancel
out or balance the attractive force on your bearing structure (Figure 6). Do not hesitate to
seek guidance and advice on your machine design at an early stage.
Good iron-core design practices
As shown in Figure 7, the thickness of the coil support (e1) should be the same as that of
the base support (e2) and should be such that the calculated vertical deflection is very
small. This will increase the frequency resonance of the system and provide a very low
settling time with good electronics.
The bearing supports should be located as close as possible to the coil. This will
minimize the vertical resonance of the linear encoder's reading head (Figure 8).
The encoder should be located as close as possible to the linear bearing, either inside or
outside (Figure 9).
The encoder's reading head should be mounted very rigidly and should allow for both an
air-gap height adjustment and a lateral adjustment to centre the head on the linear encoder
tape.
Table 2 lists some typical applications for ironless and iron-core linear motors. For
functional or technical reasons, some are clearly limited to a single technology, but a
large percentage can use either technology. By using the correct design approach, many
ironless application could be switched to iron-core motors, thus improving performance
while lowering cost.
Table 1- Ironless vs. iron-core linear motors
Ironless Motor
Zero Cogging
Iron-Core Motor
Low Cogging
Notes
Typcially less than a
Small % OK for
>95% applications
Low coil mass
High coil mass
N/A
High coil +
Magnet ways mass
Low coil +
magnet ways mass
Important only if
there is more than one
Axis (the second axis
Has to carry the first)
High peak-to-RMS
Force/current
Peak current limited to
3x's continuous current
Many applications
need high force
High acceleration
Medium acceleration
N/A
Low inductance
Medium inductance
N/A
Low electrical time
Constant
High electrical time constant
N/A
High thermal resistance
Low thermal resistance
N/A
Low electrical time
Constant
Medium electrical time constant
N/A
Low forces
(few hundred lbs)
High forces (few thousand lbs)
More than one order
of magnitude; more
force with iron-core
motors
Table 2-Typical linear-motor applications
Low moving weight
<5 lbs
Medium moving weight
5-50 lb
High moving weight
50-20,000 lb
Ironless Motors
Semiconductor applications
Iron-Core Motors
Small Conveyor
Packaging, Conveyor
Robotics
Machine-tool
applications