Machinedesign 1220 Iron Core Brushless Servomotors 0 0

How linear motors measure up

April 1, 2001
Understanding how linear motor manufacturers define their specifications is an essential first step to choosing the right motor

Anyone who uses engineered products is probably aware of “specmanship.” Simply put, specmanship is the art of manipulating theoretical and experimental data to cast a product in a better light. It happens all the time, especially in areas like linear motors where there are few standards to keep everyone honest.

All you need to do is go through a couple of linear motor catalogs and you’ll see how units are manipulated, or specifications left out, to minimize or gloss over performance limitations. And instead of solid numbers, you’ll see a lot of footnotes, small print, and disclaimers.

Understanding how each manufacturer measures and presents linear motor performance is essential before you begin a search. It also helps to know how motor construction—not all motors are alike—affects performance and application. Otherwise you may end up with a design that works on paper, but bombs in the field.

The Kth lie

When developing a linear motor a manufacturer has to make an important decision. What value will it use for the thermal dissipation constant? This number usually appears in the specifications as Ktor Kth, having units of W/°C or °C/W.

The thermal dissipation constant tells you the maximum amount of heat, in Watts, that the motor is capable of dissipating at its rated peak temperature. Of course, the amount of heat that can be removed from a surface (ultimately the motor coils) depends on many things; the size of the heat transfer surface, the material it is constructed from, the type of flow in the medium surrounding the surface, and the temperature and material surrounding the enclosure.

There are no standards today for testing a linear motor’s thermal performance. The most conservative manufacturers specify motors in a stall condition with no added thermal sinking. Other motor makers assume that users will be adding large aluminum heat sinks (up to 12-in. square and 1-in. thick), and/or running the motor at high speeds, greatly inflating the Kth number with forced convection. Naturally, any motor specified under these assumptions will overheat if it lacks a sufficiently large sink or moves slower than anticipated.

When choosing between different sized motors rated for the same continuous force, look closely at the Kth values. The smaller motor will undoubtedly have a higher (probably inflated) W/°C number; inflated because all linear motors share similar physical qualities—copper, thermally conductive epoxy, aluminum mounts, and in some cases steel laminations—so it is inconceivable that any one motor is significantly more thermally conductive than another. The difference in the numbers actually comes from the difference in testing methods.

No matter the application, a conscientious designer should always take the method of deriving Kth into account. Many applications, such as semiconductor processing and handling, require accuracies and repeatabilities that are compromised if motor heat is not properly managed. An improper, or misunderstood, use of a motor’s Kth is apt to cause thermal expansions that severely degrade system accuracy.

Constant comment

The true indication of any motor’s worth is the motor constant Km, the ratio of output force to the square root of input power. Although important, it is a theoretically calculated value and should not be considered an indication of Kth. However, some manufacturers do just that when they use experimentally determined continuous force to rate the motor. Any motor constant determined this way is itself a function of Kth, so it can’t be used to quantify it.

Motor constant, expressed in terms of lbf/sqrt (W) or N/sqrt (W), is based on the physical qualities of the motor itself; magnetic field, the windings, winding resistance, conductor length, number of turns, and so on. In essence, Km is a number that indicates efficiency; the higher the number, the better the motor. Given two motors of identical size and construction, the one with the higher Km will develop more force at a lower temperature. If Km differs between two seemingly identical motors, it’s probably because of stronger magnets, better winding efficiencies, or better thermal epoxies.

Don’t drop in

Linear motor manufacturers tend to follow their own guidelines for size, shape, and performance, so there are many lengths and cross sections available. The reason for all the different sizes stems from the nature of linear motors.

A rotary motor’s important functional parts are inside, with the external case simply providing protection and a thermal sink. Linear motors, on the other hand, are open frame devices, leaving their functional parts exposed. This, combined with the fact that linear motor primaries are usually proprietary, makes it difficult to find “drop-in” replacements.

A linear motor consists of two main pieces, the magnet track or secondary, and the coil or primary. Magnet tracks consist of steel and magnets. If not for the prohibitive cost of magnets in low quantities, it would be common to see OEMs building their own tracks and simply buying the coil assemblies.

Basic designs

There are three basic types of linear motors; iron core, cog free “ironless,” and linear stepper motors. Each type has applications where it excels over the other.

Iron core motors are characterized by steel laminations in the moving primary. The laminations, by channeling magnetic flux, make the motors more efficient. Typically, iron core motors have higher force ratings and are more efficient than comparably sized cog free motors. They also have a greater thermal mass and thermal time constant, making them better at high-force intermittent duty cycle operations; and are better suited to external cooling methods because of their wide, flat cross section.

Laminations have drawbacks, however. Steel in the core reacts with the magnets in the secondary, often developing a great deal of attractive force. The amount varies from five to 15 times the output force the motor can develop. An iron core motor with a rated force of 50 lbf could have an attractive force of up to 750 lbf. Making matters worse, this force often fluctuates as the motor moves through its electrical cycle.

Iron core motors also exhibit cogging as a result of the motor moving from one magnetic field to the next. This force, small in comparison to the attractive force, causes the linear motive force to “ripple” as the coil moves forward, resulting in velocity variations.

Another consideration with iron core motors is weight. These motors typically weigh three to five times more than similarly rated cog free coils. In addition, due to the attractive force, the support members and bearings used with iron core motors also must be heavier than normal.

Iron core motors are typically used for applications where high force is required, but at lower speeds, such as machine tool and transfer line operation. They can also be used for general-purpose actuators requiring mainly point-topoint positioning.

Cog free motors are characterized by a coil assembly without the steel found in their iron core counterparts. The moving coil consists of wire, epoxy, and perhaps some nonmagnetic support structures. Such coils have none of the attractive force or cogging issues of an iron core coil.

Due to the lack of steel, cog free motors are lighter than iron core types. They’re also less efficient, necessitating additional magnets. The additional magnets are seen in the use of a “U” shaped track instead of the flat plate used in iron core motors.

On the positive side, cog free motors do not suffer from velocity ripple like iron core motors. There is a slight amount of ripple, however, that varies with construction, magnetic flux density, and irregularities in the shape and position of the motor coil. Some motor designs are specifically configured for very low force ripple while others are not.

The drawbacks to the cog free designs are few, but meaningful. They are inherently more expensive and their magnets add material costs that are not offset by their simpler primaries. Cog free motors also are less accepting to external cooling, so there is less benefit in adding air or liquid coolers.

Cog free motors are best suited to applications where velocity control is important, such as scanning or inspection equipment. They also excel in higher acceleration applications due to their lower mass. Applications such as semiconductor pickand- place, chip sorting, as well as solder and adhesive dispensing have seen the benefits of cog free technology.

Linear step motors have been available longer than linear servos, and the pricing shows that. On a per axis basis, linear steppers sell for a half to a fourth of the price of a servo.

The mature market for stepper drives, controls, and linear step motors is still growing, fueled by the pursuit of productivity and reduced part count. The fact that linear steppers offer a complete motion system (actuator, bearings, and base plate) for a lower price than servos also helps.

Typical linear step motors produce 2 to 50 lb of force, while achieving velocities to 80 in./sec. They also offer open-loop step resolutions down to 2 μm.

Another advantage is that step motors always produce force and, when in position, they do not dither as servomotors do. What’s more, advances in drives and controllers are letting steppers run in closed-loop servo mode. With the addition of a linear encoder, these servo-steppers can achieve resolutions similar to servomotors.

John Heilig is the engineering manager for linear products at Baldor Electric Co., Linear Motor Division, Santa Clarita, Calif.

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