For linear motion applications, ball screw, rack & pinion, or belts may not meet application needs, especially if the application needs multiple performance characteristics, such as high accuracy and high speed. The answer may be a linear motor
Linear motors generate force rather than torque. Many linear motors are based on their rotary counterpart, except their movement is linear, Figure 1.
The linear motors are for open-loop positioning applications, servo applications that require high forces, and applications needing long linear movement. For example, a linear motor can operate in a milling and drilling machine tool, glue dispensing, probing, mechanical assembly, packaging, and measurement and inspection machines, Figure 2.
Force to inertia ratio and stiffness — depending on the type of linear motor — can be as high as 30:1, and to 0.9 million N/mm or 5 million lb/in. respectively. Some versions offer smooth motion to within a fraction of a percent of their nominal velocity, which can range between 1 micron/sec to over 5 m/sec. Others can operate continuously, provide 5 g (or higher) acceleration, and offer a low settling time, in some cases, less than 50 msec. With few parts to break down and minimal maintenance needs (because of its non-contact characteristics) this type of motor is considered reliable.
Linear motors all have the same basic structure. Imagine a rotary ac or dc motor that has been sliced along its axis and opened up flat, Figure 3, resulting in two sections. One section, known as the primary, is a set of electrical coils embedded in a core (typically of steel, epoxy, or aluminum). The structure of the second section (the secondary) depends on the type of linear motor. A typical air gap of 0.024 in. (0.6 mm) separates the two sections for non-contact force transmission.
Types of linear motors
As mentioned earlier, there are different types of linear motor classifications. The most common are:
• Voice coil
• Step motor
• Induction motor (asynchronous)
• Synchronous motor
Voice coil. These motors usually are single phase with a stationary coil and moving magnet, with a maximum travel of less than 1 in. They generate high acceleration, to 100 g in some versions, and can oscillate at high frequencies.
Step motor. These versions have the same basic advantages as rotary step motors. They are discrete actuating devices that, in most applications, are used without positioning feedback for control. Thus, step motor systems are the least expensive of all linear motor types. Depending on the type of step linear motor system, speeds can range from 1 to 100 ips (25 to 2,500 mm/sec); force ranges from 2 to 50 lb (9 to 220 N); and positioning accuracy ranges from 2 to 250 μm. A microstep system with a driver capable of 125 microsteps/ full step can offer positioning accuracy to 1 or 2 μm per step. Linear step motors main advantages are low cost. The main disadvantage is low stiffness.
Induction motor. These motors operate by generating currents with two vector components. One component is for the coil currents (primary), the other induces currents in the secondary section, which encapsulates short circuit rods. The induced currents generate a magnetic flux, which interacts with the coil current to generate force.
The primary advantage of this type of linear motor is the low cost of its secondary. Its limitations are its large size (compared to an equivalent permanent magnet linear motor) and, for precision motion applications, it requires a complicated control algorithm. It functions best in positioning applications that require low-duty cycle, where high heat is not a problem, and where travel lengths are long.
Permanent-magnet motor. These motors, also known as synchronous motors, are the most commonly used linear motor in high performance applications. Several versions are available:
Brush type motors are the least expensive. Operating from direct current, each motor has a stationary coil assembly and moving magnets. The motor cable does not move in this configuration. Limitations include velocity, up to 1 m/sec; and acceleration, up to 0.5 g. Above these values there is excessive arcing and rapid deterioration of the brushes. These motors are excluded from clean room and vacuum applications.
Brushless, aluminum-core linear motor operates from three phase power, with a moving coil and cable. In applications requiring short travel lengths, though, the coil can be stationary and the magnets moving. The core of the primary encloses the windings in aluminum. There is no magnetic attraction between the two motor parts. Therefore, it may require a double-sided magnet assembly to close the magnetic circuit effectively. This type of linear motor generates smooth motion.
Depending on the moving load, it can accelerate to 4 g’s, but can also develop eddy currents at speeds over 1 m/sec. This motor is ideal for vacuum applications.
Brushless, epoxy-core linear motor is also non-ferrous with its epoxy-based core, Figure 4. It too, provides smooth motion. It has an advantage over aluminum in that it does not experience eddy currents at high speed. It has low stiffness (typically 35,000 N/mm) at high coil temperatures (to 125 C) and it tends to give off gases in high vacuum environments.
Brushless, steel-core linear motor uses steel lamination, in the main body of the primary, to enclose the copper windings. This motor uses a single-sided magnet assembly and is air or water cooled for high duty cycle applications. It functions well in applications that require such duty cycles and velocities up to 200 ips (5 m/sec). But this motor is subject to cogging. It can generate strong magnetic attraction between the two motor parts, which must be accounted for in the load carrying capacity of the bearing system. Typical applications are machine tools where high force, 5,000 lb (23,000 N), may be required, or in general automation where speed of several m/sec is needed 120 to 200 ips (3 to 5 m/sec).
Various controls can drive linear motors. In the machine tool industry, CNCs are the usual control. Several linear motors use a PC-based control connected to a servo amplifier. A linear stepmotor can use a microstep drive. And several manufacturers are investigating the use of programmable controls as control devices.
Servo linear motors, for example, often use a PC-based, PID control that commands the servo amplifier through +10 V analog signature. Currents from the amplifier go into the coils of the primary. There, they interact with the magnetic flux density in the secondary to generate force. The force, which moves the slide, is proportional to the product of the current in the primary and the flux density in the secondary. Hall effect signals or software commutation control the timing of the current into the coils. Encoder signals are fed back to the control to generate an error signal for the PID control loop. Software commutation also uses encoder signals to time the current.
Cost and performance
If you need more than one high performance characteristic in a positioning system, a linear motor can be a cost effective solution, Tablee 1. For example, if an application requires only long travel, rack and pinion are effective and economical. If only high speed is required, belts can be used. If mostly high accuracy is needed, a ball screw is a good choice.
However, for any combination of performance requirements, such as high accuracy and high speed, or high speed and high smoothness, or low settling time and high accuracy, then a linear motor is a good choice.
Aside from the semiconductor, general automation, and machine tool markets already using linear motors for the combination of performance requirements, here are a few other reasons an engineer might select a linear motor over a rotary motor in positioning applications. The application requires:
• High mean time between failure (MTBF) or low mean time to repair (MTTR).
• Vacuum or clean room compatibility.
• Low maintenance.
• Very high closed-loop bandwidth for either position ( over 100 Hz), velocity (over 300 Hz), or acceleration (over 2,000 Hz) as may be needed in master-slave applications.
Once a type of linear motor is chosen, use the following steps to size the motor and amplifier. (First, refer to the manufacturer’s brochures).
1. For required peak force, calculate all the forces that are acting on the moving slide at all positioning phases, i.e. acceleration, constant velocity, and deceleration. Find the resultant required force in each phase. The linear motor must be able to provide the maximum required force plus a recommended 30% safety margin to account for unknown forces such as seal resistance, eddy current damping, cable drag, and so on.
2. For required continuous force, calculate the sum of all resultant forces squared (found in step for each motion phase, where each squared quantity is multiplied times its phase duration). Find the square root of this quantity, divided by the cycle time.
Where: Lrms = RMS value of the load which can be in any unit, hp, amp, etc. L1 = Load during time of period 1. L2 = Load during time of period 2, etc. t1 = Duration of time for period 1. t2 = Duration of time for period 2, etc.
3. For required peak amplifier current, calculate it using the required peak force divided by the motor force constant (pound or Newtons per ampere).
4. Required continuous amplifier current equals the continuous motor force divided by the motor force constant.
5. Required amplifer voltage equals the back EMF constant (obtained from motor manufacturer’s cataloges) times maximum velocity plus peak motor current times coil electrical resistance (at hot temperatures) plus a recommended 20% safety margin (for cable resistance and inductive requirements).
Dr. Boaz Eidelberg is the product manager of linear motors for Anorad Corp., Hauppauge, N.Y.