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Linear servomotors are a drive technology for high-performance motion-control applications. From automation to semiconductor and electronics industries, increasing requirements for throughput, flexibility, and dynamics have led manufacturers to opt for direct-driving. The two predominant types of brushless linear motors include ironless, or U-channel motors, and ironcore or single-sided magnet types.
While popular knowledge of these motors has advanced, their subtle differences are not always understood. In effect, a linear motor is an unrolled version of a brushless dc rotary motor. The permanent magnet rotor becomes a flat linear magnetic way (also called a secondary) that typically serves as the fixed motor part. The stator becomes a flat coiled part (also called a forcer, glider, or primary) that acts as the moving part.
The magnetic way design and the forcer material together determine the linear motor type. As its name implies, an ironless linear motor (also called an air-core motor) has no iron inside. The glider is a plate made of epoxy, into which copper coils are inserted. The forcer slides between two rows of magnets facing each other, linked on one side by a spacer. Ironless motor peak power is a few thousand Newtons. Thier lower-weight gliders allow for higher acceleration. For these reasons, ironless motors are best used for light, delicate motion. The peak-to-continuous force ratio is high (a factor of four or higher is typical) —- another reason why these motors are suitable for dynamically demanding applications with light payloads. In applications where very smooth motion is required, ironless motors are preferred due to the absence of detent force.
Ironcore motors are composed of a slotted lamination stack made of steel. Laminations are insulated from each other as they are in a rotary motor to reduce Eddy currents that cause excessive iron losses. Ironcore peak force range can go up to several tens of thousands of Newtons. Still, though traditionally they’ve been used for heavier jobs, specially designed features allow these motors to be used in dynamically demanding applications reserved in the past for ironless motors. With cooling, the peak-tocontinuous force ratio of ironcore motors is usually in the range of 2.5.
The first parameter to consider when deciding on a linear motor is available force density. Continuous force per active surface unit — basically, a given level of continuous force — can be up to two times greater from an ironcore motor than an ironless. Still, ironless motors deliver on smooth forces when ironcore motors need special magnetic designs or servocontrollers to achieve the same performance.
Ironless motors have zero cogging because the moving forcer has no iron. Force ripple is the result of this cogging and commutation effects. In an ironcore motor, the forces between magnets and lamination stacks cause not only an attraction force but a force in the direction of motion as well. This force depends on the relative position of the laminated teeth to the magnetic poles. Always present in ironcore motors, cogging is independent of current flow in the motor. Skewed lamination stacks or magnets can reduce this effect. Choosing the appropriate combination of teeth and magnetic pitches has the same effect as skewing one part. The right angle of clearance at both ends of a motor also helps reduce cogging.
As stage mapping can help improve the accuracy of a mechanical slide by detecting mechanical defaults and compensating for them, an electronic compensation can reduce force ripple. Actual force ripple is measured at different positions (depending on the application) and stored in a table. The current loop then integrates a corrective current value — associated with a given position of the motor — along the magnetic period. Electronic compensation commonly reduces force ripple by a factor of five or even ten. The commutation effect can be generated by both motor and electronics. A motor can generate force oscillations if the back emf voltage is not a perfect sinus. Electronics induce force oscillations as well, due to current ripple.
When iron concentrates the magnetic flux in an ironcore motor, iron is placed in front of magnets. This generates high attraction forces, which can be up to six times greater than motor peak force ratings. Depending on the guiding system used (mechanical, aerostatic, or hydrostatic), attraction forces can be a help or a hindrance. All available mechanical ball bearings have large load capacities that overcome attraction forces without problem — the tricky part is choosing the right rails size to ensure a reasonable lifetime. As opposed to mechanical bearings (with high stiffness), air bearings have no or very low stiffness. To generate the film of air required between moving and static parts, a preload is required. Then the attraction forces of ironcore motors can be used as an efficient preloading mechanism.
For the ultimate in smooth operation, ironless linear motors are paired with air bearings. This is because ironcore motors display attraction forces of inconstant intensity. (If attraction force is defined at zero current, its value at peak current vary 10%.) On these highly smooth nonmechanical bearing systems, ironcore motors are inappropriate. Ironless motors are a better option because (with the absence of attraction force and related fluctuation) they maintain the smooth motion of the system.
Heat and efficiency
Like all motors, linear motors generate heat during operation. But because linear motors are linked directly to their payload and are embedded in machinery mechanics, preventing and removing heat is especially important to avoid machine thermal drift. The motor constant Km expresses a given motor’s heat generation behavior. It is defined as the ratio of force that a motor can produce to the square root of its power dissipation at this force. Km is typically expressed in the units N/√W; a higher constant indicates a more efficient motor.
Where la. = motor width
Bd. = the magnetic field
Senc = section of the slot
Kcu = copper filling factor
Lm = average length of one coil turn
What determines variable values in this equation? The rare earth materials of motor magnets fix the magnetic field level Bd. (The most commonly chosen material — Neodymium-Iron-Boron — is a good compromise between power capability and price.) The magnetic design of the motor determines the slot section Senc, while the remaining two parameters are determined by the magnetic design and the motor’s coil style. In some open-slot designs, coils are made compact and dense for a copper filling factor of up to 60%, or 80% in special designs. This results in a high Km efficiency value.
Just as important as a linear motor’s efficiency is its ability to dissipate heat. The motor type intrinsically characterizes the most efficient heat removal method. As we shall see, this is an area where ironcore motors excel.
• Conduction. In this mode, heat transfer is dependent on the surface of attachment of the motor to system mechanics. The thermal conductivity of the motor’s structural material is also influential. The typical thermal conductivity of iron is 50 W/(m.K); in effect, the lamination stack on an ironcore motor acts as a heat sink. Because the thermal conductivity of an ironless motor’s epoxy is about 1.02 W/(m.K) or so, the structure acts more as insulation and is harder to cool.
• Radiation. For ironless motors, up to two-thirds of total heat transfer is done through radiation. For ironcore motors, radiation isn’t a significant heat transfer mechanism. This phenomenon heats motor magnets a lot. The latter has great effect on machine design because the higher the temperature, the lower magnet power dips. In ironless systems, the motor radiates heat on each side to both rows of magnets. In an ironcore solution, the motor radiates on only one row of magnets for cooler operation.
• Convection. Convection is heat transfer through a fluid by circulation from one region to another. The impact of convection on motor cooling is important, especially in applications where movement amplitudes are wide and speed is high. One-third of heat transfer occurs through this mechanism. Sometimes generated convection is used to combat heat. Ironcore motors are especially easy to equip with cooling options. Water cooling ironcore motors is suitable in heavy-duty applications where payloads are high (such as in machine tool applications.) Until recently, air cooling linear motors was not very effective because most designs simply blew air instead of water into cooling channels. But newer designs built specifically for air cooling are more efficient; when blown at the right location, air can increase continuous power ratings to almost 20% beyond non-cooled ratings. Air cooling on ironless motors can be used too; air is forced on the motor frame instead of copper. Since the thermal conductivity of the epoxy frame is lower, continuous ratings of an ironless motor increases by only 10 to 15%.
The alternative to cooling motors is letting them heat up. To do this safely, motors must be well insulated from mechanics. One drawback is unavoidable: A decrease in motor performance does result because continuous ratings are based on assumed amounts of heat dissipation through mechanics.
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