Linear motion positioning stages are found in many
applications from material handling and assembly to inspection and testing and
employ many different technologies. Just looking at the different drive types
one finds linear stages based on flexible belts, ballscrews or leadscrews, and
linear motors. Here we will evaluate the use of linear motors in a precision
positioning stage, examine the factors that affect its performance and suggest
ways to optimize designs
Linear motors have several benefits relative to the other
drive types: high speeds, high accelerations, long travels and the advantages
associated with their non-contact configuration such as no wear, cleanliness and
little to no friction. These benefits make linear motor-based stages very
attractive for use in electronics manufacturing and medical or biological
applications, but must be balanced against higher costs and increased
complexity. The challenge of using linear motors, therefore, is to understand
and account for this increased complexity to fully exploit their
benefits.
For this discussion we will consider an ironless type linear
motor composed of two parts: the forcer and the magnet track. The forcer is
analogous to the electro-magnet stator of a common brushless rotary motor, and
the magnet track is like the rotor with permanent magnets (see Figure 1 for a
cutaway view of a linear stage). Unlike a rotary motor that includes rotary
bearings to hold the rotor in place, the designer must mount a linear motor in a
linear bearing system to allow linear motion while maintaining proper alignment
of the magnet track and forcer.
In the design of a linear positioning stage, the designer can
select which part will be fixed in the base of the stage and which part will be
moving. For long travel applications, the magnet track is fixed and the forcer
is attached to the carriage or moving portion of the mechanism. The carriage is
connected to the base with a linear bearing system. Long travel can be achieved
by mounting shorter sections of magnet track end-to-end to form long magnet
tracks. This configuration is simple in concept but complex in practice because
of five main factors: maintaining forcer-to-magnet track alignment; managing
stiffness in the linear bearing system to reduce off-axis errors; integrating
the linear encoder; managing electrical connections to the moving forcer; and
accommodating for external forces such as
gravity.
Breaking down the
complexity
Maintaining alignment and managing
stiffness
For precision applications, maintaining the correct alignment
between the forcer and magnet track is critical. This is true not only for
proper operation of the motor, but also to reduce the torque ripple that can
occur if the forcer moves toward and away from the fixed magnets in the tracks
as the forcer moves along them. This alignment must be held in three dimensions.
When multiple magnet tracks are used for long travel, the alignment of the
magnet tracks must also be held to tight tolerance. The choice of linear bearing
system and the configuration of its components ultimately determine the
tolerance of this alignment. For a precision linear motor stage such as the
Primatics PLG series (see Figure 2), two linear guides are placed in parallel on
a precision machined base. The base includes features to accurately locate and
align multiple magnet tracks. A carriage is attached to the guides in four
places with recirculating ball type trucks. The forcer is attached to the
carriage. In addition to maintaining proper alignment between the forcer and
magnet tracks, this configuration also provides high stiffness to reduce roll,
pitch and yaw errors as the carriage moves.
Integrating the linear
encoder
Most linear motor positioning stages also use linear
encoders. Care must be taken to locate the encoder to minimize errors due to
geometry. If the encoder is placed outside of the linear guides in the example
above, yaw angle movement of the carriage will result in encoder position
errors. These errors can be minimized by locating the encoder between the linear
guides. Similar to the linear motor, linear encoders are composed of two parts:
the encoder scale and read head. Like the magnet track, the length of the
encoder scale is matched to the length of travel of the stage. And like the
forcer, the read head is attached to the carriage of the stage.
Managing electrical
connections
Now that the linear motor, linear bearing system and encoder
are properly placed, reliable electrical connections must be made to the forcer
and read head. The most common method to make these connections is with a cable
track attached to the outside of the stage. The cable track routes the
electrical signals through cables installed in the track. These cables and the
track are fixed to the base at one end and attached to the moving carriage at
the other. The designer must select a cable track and cables to maximize cable
life and minimize size. Often overlooked, however, is the importance of
minimizing the drag force imparted on the carriage from the cable track system.
A cable track with cables acts as a spring that wants to relax. When attached to
the carriage of the stage, it imparts a force in the direction of the curve of
the cable track. This force may be small in most cases, but for some
applications, this force can be significant - particularly when it comes to
tuning the servo loop. As an alternative, the Primatics PLG stage in our example
uses flat flexible cables inside the stage (see Figure 1). These offer the
advantage of small size, remove the need for a cable carrier and impart very
small forces to the carriage. As a side benefit, this method is better suited to
cleanroom environments, takes up less space than an external cable track, and is
protected by the stage itself.
Accommodating for external
forces
The last factor to consider is the effect of external forces
on a linear motor stage. Linear motor stages have inherently lower friction and
drag than a similar ballscrew stage because of the non-contact feature of the
linear motor. This can create a problem if an external force is applied to the
stage carriage. A simple example is using a linear motor stage in a vertical
orientation where gravity is applying a constant downward force. The problems
are two fold: the external force (gravity) must be compensated for in the servo
loop tuning, and allowance must be made for the loss of servo control and
possible uncontrolled motion of the carriage (it falls). The most common
solution for this situation is a counterbalance. In most cases a counterbalance
is implemented by the user, but factory solutions are available from some
manufacturers. For example, the Primatics PCR43 series of linear motor stages
can be ordered with a pneumatic assisted counterbalance system for vertical
applications. This system includes a vertical mounting bracket, a pneumatic
cylinder providing the counterbalancing force matched to the payload and a
small-profile pulley and cable system that keeps the overall size to a minimum
(see Figure 3).
Payoff is better performance for precision
systems
The ultimate payoff with proper consideration of these
factors is improved performance of the resulting system. This is particularly
true for precision systems. Motion control systems have three major parts:
motion controller, motor drive and positioning stage. For precision
applications, high performance digital motion controllers are typically used.
These controllers have very fast servo update rates, high encoder bandwidth,
advanced servo algorithms and many features for the system programmer to ease
application development. They are often paired with digital servo drives. These
drives offer many performance and convenience advantages over their analog
cousins: easily configurable operating parameters (current limits, gain, current
loop characteristics); encoder-based sinusoidal commutation; and safety features
(encoder feedback loss detection, commutation error detection, etc.) Many
digital drives also allow operation in either torque or velocity modes.
Most digital motion controllers offer tuning tools that
include plotting frequency response of the system to aid in servo loop tuning.
Here is where the importance of a good stiff mechanical layout is realized.
Off-axis compliance between the forcer and magnet track can allow the forcer to
wiggle side-to-side or up-and-down as it moves in the magnet track, which can
result in torque ripple. Similarly, lack of stiffness that results in relative
movement between the encoder read head and forcer can adversely affect the
frequency response.
Both the servo controller and the digital drives rely on
encoder feedback to operate the servo loop and commutation timing. This
emphasizes the importance of a well designed encoder system. Errors in the
encoder output feedback represent misinformation to the motion controller and
drive that adversely affect their performance.
Commonly the drive is operated in torque mode where the
digital motion controller commands torque (current) to the drive and on to the
linear motor forcer. The controller closes both the velocity and position loops
based on the linear encoder feedback. Care should be given to reduce unnecessary
force perturbations from the mechanics. Bias and feed-forward terms in many
motion controllers can be used to counteract the effects of external forces and
friction, but it is difficult to tune a servo loop under changing conditions
that could be present with poorly implemented cable management.
Precision linear positioning stages using linear motors offer
many benefits relative to other technologies for certain applications. However
care must be used in the design or evaluation of these products to assure that
these benefits can be achieved.
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Figure 1. Cutaway view of linear
positioning stage with linear motor. Use of an internal flexible flat
cable to connect to the motor electronics saves space and minimizes drag
force compared to external
cabling.
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Fig. 2 Primatics PLG 110 Linear Motor
Stage
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Fig. 3 Pneumatic Counterbalance on
Primatics PCR XZ S
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