The idea is easy enough. Take a conventional rotary servo motor and unwrap it. So now what was the stator is now a forcer and the rotor can be 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 more and more 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 keeping the step motor benefit 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 Hall-effect 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 because of improved magnet materials.
Linear Motor Benefits:
Choosing the right linear motor for an application is not a simple task. Selecting the right technology for the application, force calculations, thermal considerations, bearing loading, commutation methods, etc., must be considered. Within this article, technology will be discussed, not sizing solutions. However, knowing the basic types and the associated advantages and disadvantages will assist in the end solution. Three technologies of brushless motors are discussed. They are; ironcore, aircore (ironless), and slotless.
We offer a variety of NEMA stepper motors and servo motors especially for use with linear actuators. Through our Your Motor Here program we can supply the correct mounting for any motor you specify. We also have stepper drivers/stepper controllers and ac servo motor designed for use with electric linear actuators.
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 Hall-effect 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 because of improved magnet materials.
Linear Motor Benefits:
- 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.
Choosing the right linear motor for an application is not a simple task. Selecting the right technology for the application, force calculations, thermal considerations, bearing loading, commutation methods, etc., must be considered. Within this article, technology will be discussed, not sizing solutions. However, knowing the basic types and the associated advantages and disadvantages will assist in the end solution. Three technologies of brushless motors are discussed. They are; ironcore, aircore (ironless), and slotless.
We offer a variety of NEMA stepper motors and servo motors especially for use with linear actuators. Through our Your Motor Here program we can supply the correct mounting for any motor you specify. We also have stepper drivers/stepper controllers and ac servo motor designed for use with electric linear actuators.
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