www.parkermotion.com
Parker Hannifi n Corporation
Electromechanical Automation Division
6
Fig 16(b): Servo Motor Control (position sensor located at load)
Fig 16(a): Servo Motor Control (position sensor located at motor)
4.0 Servo Control and Feedback
Linear motors can offer the ultimate in high precision and motion
dynamics. However, overall system performance is dependent on other
components – particularly the servo controller and feedback mechanism
used. In this section, we will examine how linear motors are commutated,
how their position is sensed, and how it is important to have an adequate
controller to optimize system performance.
Figure 16 shows the traditional cascaded structure of servo motor control.
The same structure can be applied to linear motors. One advantage is that
the position sensor can typically be located right at or closer to the load,
thus improving the overall accuracy of the system.
One drawback is that the lack of a traditional mechanical transmission
results in the effects of external forces being signifi cantly greater. For this
reason, the quality of the position signal (resolution and accuracy) and
the performance of the servo controller (sampling time, trajectory update,
and control algorithms used) are of prime importance in determining the
degree of “positional stiffness” that can be achieved.
Servo Controller
Amplifier
Motor
Load
Commutation
Sensor
Position Sensor
Servo Controller
Amplifier
Motor
Load
Commutation
Sensor
Position Sensor
Servo Controller
Amplifier
Motor
Load
Commutation
Sensor
Position Sensor
Servo Controller
Amplifier
Motor
Load
Commutation
Sensor
Position Sensor
3.0 Guide Systems
Even though a linear motor system lacks the rotary transmission com-
ponents of traditional positioning systems, the user is still required to
provide some sort of linear guide / bearing. Typically, a linear bearing must
be selected based on high speed and acceleration capability, long service
life, high accuracy, low maintenance costs, high stiffness, and low noise.
Other considerations may include, for example, the site space available,
the mounting accuracy (fl atness, parallelism, inclination), and the thermal
expansion.
Different guide systems are available to fulfi ll these requirements:
• Slide bearings (dry running or hydrodynamic)
• Hydrostatic bearings
• Aerostatic bearings (air bearings)
• Track rollers (steel or plastic roller wheels)
• Rolling-contact bearings (square rail, cross roller, or round rail)
• Magnetic bearings
In practice, slide bearings, rolling-contact bearings, and air bearings
are the most popular. For applications with low demands on precision
and load-bearing capacity, dry-running slide bearings may be a suitable
option. Guide systems based on rolling-contact like square rail and cross
roller bearings exhibit good stiffness and excellent load-bearing capability.
In addition, they offer excellent straightness and fl atness over the length
of travel. Air bearings offer the ultimate in performance. With practically no
limits to max speed and acceleration and virtually no breakaway forces,
air bearings are the best solution for ultra-high precision applications.
Parker offers slotless motors in both component kits and precision
positioning systems. Please refer to this catalog under 400LXR Positioners
or to the “SL Series” linear motor section on www.parkermotion.com
(Fig. 14 and 15).
Fig. 15: 404LXR Slotless
Linear Motor Positioning System
Fig. 14: SL Series Slotless Linear Motors
