I. Linear Motor
A linear motor is a transmission device that directly converts electrical energy into linear motion mechanical energy without any intermediate conversion mechanism. It can be viewed as a rotary motor cut radially and unfolded into a plane.
Linear motors, also known as linear actuators, linear motors, or linear push rod motors, are most commonly used in the flat, U-slot, and tubular types. The coils typically consist of three phases, with brushless commutation achieved using Hall effect sensors.
Linear motors are often simply described as flattened rotary motors, but their operating principle is the same. The mover (or rotor) is made by compressing coils together with epoxy material; the magnetic track is where magnets (usually high-energy rare-earth magnets) are fixed to steel. The mover of the motor includes coil windings, Hall element circuit boards, a thermostat (temperature sensor to monitor temperature), and electronic interfaces. In rotary motors, the mover and stator require rotary bearings to support the mover and maintain the air gap between the moving parts. Similarly, linear motors require linear guides to maintain the position of the mover in the magnetic field generated by the magnetic track. Just as the encoder of a rotary servo motor is mounted on the shaft to provide position feedback, a linear motor requires a linear encoder to provide linear position feedback. This encoder can directly measure the position of the load, thereby improving the positional accuracy of the load.
The control of linear motors is similar to that of rotary motors. Unlike brushless rotary motors, where the mover and stator have no mechanical connection (brushless), and the mover rotates while the stator remains fixed, linear motor systems can be driven by either magnetic tracks or thrust coils (most positioning systems use fixed magnetic tracks and moving thrust coils). Motors using thrust coils have a very small weight-to-load ratio for the thrust coils. However, they require highly flexible cables and their management system. Motors using magnetic tracks must bear both the load and the mass of the magnetic tracks, but do not require a cable management system.
Similar electromechanical principles are used in both linear and rotary motors. The same electromagnetic force that produces torque in a rotary motor produces linear thrust in a linear motor. Therefore, linear motors use the same control and programmable configurations as rotary motors. Linear motors can be flat, U-shaped, or tubular. The most suitable configuration depends on the specifications and operating environment of the specific application.
II. Characteristics of Linear Motors
Before the advent of practical and affordable linear motors, all linear motion had to be derived from rotating machinery using ball or roller screws, belts, or pulleys. For many applications, such as those involving heavy loads and where the drive shaft is vertical, these methods remain the best. However, linear motors offer many unique advantages over mechanical systems, such as very high and very low speeds, high acceleration, virtually zero maintenance (no contact parts), high precision, and no backlash. Achieving linear motion requires only a motor, eliminating the need for gears, couplings, or pulleys, which is significant for many applications by removing unnecessary parts that reduce performance and shorten mechanical life.
(1) Simple structure. The tubular linear motor can generate linear motion directly without intermediate conversion mechanism, which greatly simplifies the structure, reduces the moment of inertia, and greatly improves the dynamic response performance and positioning accuracy; at the same time, it also improves reliability, saves costs, and makes manufacturing and maintenance easier. Its primary and secondary can be directly integrated into the mechanism, and this unique combination further demonstrates this advantage.
(2) Suitable for high-speed linear motion. Because there is no centrifugal force constraint, ordinary materials can also achieve high speeds. Moreover, if an air cushion or magnetic pad is used to maintain the gap between the primary and secondary, there is no mechanical contact during movement, and therefore no friction or noise in the moving parts. In this way, there is no wear on the transmission components, which can greatly reduce mechanical losses and avoid noise caused by tow cables, steel cables, gears and pulleys, thereby improving overall efficiency.
(3) High utilization rate of primary winding. In tubular linear induction motors, the primary winding is disc-shaped and has no end windings, thus the winding utilization rate is high.
(4) No lateral edge effect. The lateral effect refers to the weakening of the magnetic field at the boundary caused by the lateral break. However, the cylindrical linear motor has no lateral break, so the magnetic field is uniformly distributed along the circumference.
(5) It is easy to overcome the problem of unilateral magnetic pull. The radial pulls cancel each other out, so there is basically no problem of unilateral magnetic pull.
(6) Easy to adjust and control. Different speeds and electromagnetic thrusts can be obtained by adjusting the voltage or frequency, or by changing the secondary materials, making it suitable for low-speed reciprocating operation.
(7) High adaptability. The primary iron core of the linear motor can be sealed with epoxy resin as a whole, which has good anti-corrosion and moisture-proof properties, making it easy to use in humid, dusty and harmful gas environments; moreover, it can be designed into a variety of structures to meet the needs of different situations.
(8) High acceleration. This is a significant advantage of linear motor drives compared to other lead screw, timing belt and rack and pinion drives.
