The Advantages of Linear Motors:
The main advantage of any linear motor is that it totally eliminates
the need, cost and limitations of mechanical rotation-to-translation
mechanisms such as racks and pinions or belts and pulley, sources
of elasticity and backlash. This way the complexity of the mechanical
system is drastically reduced.
High Speeds: The maximum speed of a linear motor is limited only by the bus voltage and the speed of the control electronics. Typical speeds for linear motors are 3 meters per second with 1 micron resolution and over 5 meters per second, 200ips, with coarser resolution.
High Precision: The accuracy, resolution, and repeatability of a linear motor driven device is controlled by the feed back device. With the wide range of linear feedback devices available, resolution and accuracy are primarily limited to budget and control system bandwidth.
Fast Response: The response rate of a linear motor driven device can be over 100 times that of a mechanical transmission. This means faster accelerations and settling times, thus more throughput.
Stiffness: Because there is no mechanical linkage, increasing the stiffness is simply a matter of gain and current. The spring rate of a linear motor driven system can be many times that of a ball screw driven device. However it must be noted that this is limited by the motors peak force, the current available and the resolution of the feedback.
Zero Backlash: Without mechanical transmission components, there is no backlash. Resolution considerations do exist. That is, the linear motor must be displaced by 1 feedback count before it will begin to correct its position.
Maintenance Free Operation: Because the linear motors of today have no contacting parts there is no wear.
Linear Motors Complement Today’s Linear Motion Technologies
Today’s linear motion applications are more demanding than ever before. Faster throughput, more exact positioning, longer life, less maintenance, fewer moving parts, the list is never ending. Motion control companies strive to meet and exceed these requirements by continual technological advancement. Less than a decade ago, it was a difficult task to find a commercially available linear bearing that could travel 5 meters per second with straightness, load capacity and stiffness. Today there are many linear bearings with these attributes and they are fairly cost effective.
Advancements in linear encoder technology allow higher speed operation too. Today’s linear encoders and other devices are able to meet this challenge, are less noise susceptible, and cost less.
The Linear Motor Concept
The idea is simple enough. Take a conventional rotary servo motor and unwrap it. So now what was the stator is now a forcer and the rotor becomes a coil or magnet rail. With this design, the load is connected directly to the motor. Direct linear motion is achieved without any rotary to linear transmission devices. Linear motor technology is not new. Step motor and brushed linear motor products have been available for quite some time. Brushless technology is becoming increasingly popular as applications take advantage of its technology. Brushed linear had the coils in the linear rail and the magnets were in the forcer. Commutation was accomplished by a linear commutation bar that ran the length of the motor with brushes in the forcer. This method was both expensive and limited. The cost of winding feet after feet of linear motor rail was time and material intensive. High speed operation was limited due to commutation bar and brushes. Linear step motors have both windings and permanent magnets within the forcer. It travels along a rail having an etched tooth structure. While maintaining the step motor advantage of open loop operation, the technology does have some limitation in speed and available force.
With brushless servo motor technology and the supporting electronics to drive them, the above limitations have been eliminated. The forcer is now a set of windings while the stator is a rail of magnets. Commutation is done electronically either by Halleffect sensors or sinusoidal. Hall effect sensors located within the forcer are activated by the magnets on the rail. The amplifier translates these signals into appropriate phase currents. Sine commutation is accomplished using the linear encoder signals back to the controller. A common technique is the use of Hall-effect initially and then switching to sinusoidal commutation. In any case, the speed of commutation is not the limiting factor.
The force generated by the same size motor is greater than brush motor technology due to improved magnet materials.
Reference:
Jack Barret, Tim Hamed, Jim Monnich. Linear Motion Basics. Parker Hannifin Corporation. 1 July 2009. <http://www.parkermotion.com/whitepages/linearmotorarticle.pdf>.
David Kaiser. Fundamentals of Servo Motion Control. Parker Compumotor. 1 July 2009 <http://www.parkermotion.com/whitepages/ServoFundamentals.pdf>.
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