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 m/s with straightness, load capacity and stiffness. Today there are many linear bearings with these attributes and they are fairly cost effective. Today’s linear encoders and other devices are also able to meet this challenge, are less noise susceptible and cost less.

So, with two of the three primary linear motion elements rising to the high-speed, high-accuracy challenge, the limiting factor is the drive train.

Improvements in linear, mechanical drives have also moved forward. Ball screws with higher accuracy and faster leads result in higher throughput. Timing belts have high repeatability and speeds, well over 5 m/s. Both of these technologies have historically solved motion control applications, and will continue to do so. However, neither of these provides the speed and accuracy combination required by an increasing number of today’s motion applications.

The linear motor concept

The idea is simple enough. Take a conventional rotary servo motor and unwrap it. So, 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 motors had the coils in the linear rail, while 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, which limited high-speed operation.

Linear step motors have both windings and permanent magnets within the forcer. It travels along a rail having an etched tooth structure, which results in 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 commutation. Hall effect sensors located within the forcer are activated by the magnets on the rail. The amplifier translates these signals into appropriate phase currents. Sinusoidal 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.

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 m/s with 1 micron resolution and over 5 m/s with coarser resolution.
• High precision — The accuracy, resolution and repeatability of a linear motor driven device is controlled by the feedback 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.
• 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.
• Zero backlash — Without mechanical transmission components, there is no backlash. Resolution considerations do exist. That is, the linear motor must be displaced by one 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.

What are the down sides?

• Cost — Linear motors are expensive. This is due to the relative low volume produced, and the price of the rare earth magnets that are mounted to the length of the rail. However, as the popularity of linear motors continues, volume will rise and cost will decline.
• Higher bandwidth drives and controls — Since there is no mechanical reduction between the motor and the load, servo response must be faster. This means higher encoder bandwidth and servo update rates.
• Force per package size — Linear motors are not compact force generators compared to a rotary motor with a transmission offering mechanical advantage. For example, to produce even 65 N of continuous force, a linear motor’s cross section is approximately 50 x 40 mm. Compare this to the cross section of a 10 mm diameter ball screw which produces over 400 N of thrust.
• Heating — In most linear motor applications, the forcer is attached to the load. Any I²R losses are then directly coupled to the load. If an application is sensitive to heat, thermal management techniques need to be applied. Air and water cooling options are popular and common.
• Minimal friction — This may not sound like a problem, but it certainly can be. For instance, a linear motor is travelling at 3 m/s and loses power. Without some resistance in the system, it does not take long before the motor reaches the end of stroke and mechanical stops.

Choosing a linear motor

Choosing the right linear motor for an application is not a simple task, but knowing the basic types and the associated advantages and disadvantages will assist in the end solution.

Iron core linear motor

This motor takes its design straight from a brushless rotary motor.

As shown in Figure 1, the motor consists of a flat iron rail to which rare earth permanent magnets are bonded. The forcer is constructed of laminations and coils wound around the ‘teeth’ of the laminations. Thermal sensors are mounted internal to the windings, sensing temperature. Hall effect sensors are either mounted in the coil area or mounted on the edge of the motor. These sensors are activated by the magnets on the rail and used for commutation of the windings.

Figure 1: Iron core linear motor

The iron core linear motor has a number of advantages:

• Highest force per size — Utilising laminations to concentrate the flux field, this type of motor produces the greatest force per package size.
• Lower cost and lower weight rail — Using a single magnet rail reduces the cost and weight compared with other designs.
• Good heat dissipation — Since the windings are wound around an iron lamination, heat dissipation is excellent.

There are also disadvantages:

• Normal attractive force — Since the forcer is primarily made of iron, and the gap between the forcer and the winding is normally only 0.8 mm, there is a very strong attractive force between the forcer and the rail. This force can be as much as 10 times the continuous thrust force of the motor.
• Cogging — Since the forcer is made of iron and it passes over magnets, there is a variation in the thrust force as it passes each magnet. This is referred to as cogging and affects low speed smoothness (velocity ripple).

Air core linear motor

Two magnet rails oppose each other, north and south, as shown in Figure 2. A spacer bar between them keeps the two sides from closing together.

Figure 2: Air core linear motor

The forcer is constructed of coils wound and held together with epoxy. This winding assembly is then topped off with an aluminium bar. This bar is used for mounting the load and also for heat removal. The winding itself has no iron in it, but as with the iron core motor, thermal sensors and Hall effect sensors are mounted on the forcer.

The advantages of air core linear motors are:

• No attractive forces — Because the forcer contains no iron, there are no attractive forces between the forcer and the rails. This means no additional forces on the bearings.
• No cogging — With its ironless forcer, this style of motor has no cogging. This is ideal in applications requiring extreme velocity control. This type of motor is normally used in conjunction with air bearings due to the air bearing’s ‘frictionless’ characteristics.
• Low weight forcer — These forcers have low weight. In applications that have very light payloads, this can be a benefit, because higher acceleration and deceleration may be possible.

Disadvantages of the air core motor are:

• Heat dissipation — Since the forcer is made of wound coils and held together with epoxy, the heat must leave the coils by travelling up the coil to the aluminium mount plate and out to a heat sink. Heat also passes through the air gap and into the magnet rail. Both of these paths have high thermal resistances and thus make thermal management of the motor difficult.
• Structural stiffness — Because the force is generated at the coil, all of the exerted force is on the windings and epoxy. This weaker structure compared to the iron core structure limits the maximum sizes and forces which these types of motors can withstand without additional structural members being added.
• Force per package size — Due to the thermal and structural limitations, the force per package size of this type of motor is low. In addition, the double rail design also takes up additional space.

Slotless linear motor

The rail is the same as those used for the iron core design — simply a flat iron plate with magnets bonded to it.

Figure 3: Slotless linear motor

The forcer has a coil similar to those used in the air core motor. A ‘back-iron’ plate is placed behind the coil. This assembly is placed inside an aluminium housing with an open bottom. The housing is then filled with epoxy, securing the winding and back-iron into the housing.

Compared with iron core and air core motors, slotless designs offer:

• Lower cost and weight magnet rail — Since the same type of magnet rail is used as the iron core (single row), this design also has a low-cost magnet rail compared to the air core and the lower weight means higher throughput in multi-axis systems.
• Stronger, lighter forcer — With the body of the forcer being made of aluminium and the windings being bonded to this housing, the strength of the forcer is much greater than that of the air core motor, and the lighter weight results in higher throughput in low load applications.
• Lower attractive forces — The back-iron causes an attractive force between the forcer and the rail that is significantly less than that of the iron core — in the order of five or seven times the continuous force.
• Lower cogging — With the larger magnetic gap between the magnets and forcer back-iron, the slotless design has lower cogging, enabling the slotless design to operate in applications requiring good velocity control.
• Heat dissipation — With the coil resting across the back-iron in direct contact with the aluminium housing, there is very good heat transfer that is easy to manage.
• Force per package size — The force per package size of this design is between that of the air core and the iron core motors. Since the slotless design has a very good thermal path, it is capable of handling higher currents than the air core design and thus generates higher forces.

Summary Chart

Below is a summary of linear motor attributes and how each type of motor compares to the others.

Attribute	Iron core	Air core	Slotless
Cost	Low	High	Lowest
Attractive force	Highest	None	Moderate
Cogging	Highest	None	Moderate
Force/size	Best	Moderate	Good
Thermal characteristics	Best	Worst	Good
Forcer weight	Heaviest	Lightest	Moderate
Forcer strength	Best	Worst	Good

Table 1: Linear motor attributes

 

Parker Hannifin
www.parker.com