A direct-drive motor is a rotary or linear motor whose load is directly connected to the motor without mechanical transmission components such as gearboxes or belt and pulley systems. In other words, the motor directly drives the load.
Direct drive rotary motor ▲▲▲
Rotary direct-drive motors are often called torque motors because they can generate high torque at low speeds, even when stalled. Torque motors are mostly brushless permanent magnet synchronous motors, which are similar to traditional servo motors but have many magnetic poles. They are typically designed with a frameless structure, without a housing, bearings, or feedback devices, and these components are purchased and integrated separately by the user.
▲ Rotary direct drive motors, also known as torque motors, are typically designed with a frameless structure, without a housing, bearings, or feedback devices.
Another type of rotary direct-drive motor is the pancake motor, also known as a Lorentz force motor or printed armature motor. These motors belong to the brushed DC motor family, with the armature windings printed on a disk of non-magnetic insulating material. The armature disk is placed between two stator disks, which have permanent magnets with alternating north and south poles. Magnetic flux extends axially along the length of the motor, and the current flows radially (rather than axially as in conventional motors). This generates torque on the motor shaft due to the Lorentz force.
▲ A flat motor consists of a printed armature (rotor) between two permanent magnet stators. This gives them a very thin profile and a large diameter, hence the name "pancake motor".
Although piezoelectric motors and voice coil motors (actuators) are highly specialized types of motors, they are also classified as direct-drive motors due to the direct coupling between their load and the piezoelectric or voice coil mechanism.
Direct-drive linear motor. ▲▲▲
Direct-drive linear motors are often simply referred to as "linear motors." These motors include both coreless and cored types, depending on the structure of the primary components (including the windings). Coreless motors have windings embedded in epoxy resin, while cored motors have windings mounted in a stack of laminations. Another significant characteristic of linear direct-drive motors is whether they have a flat or tubular structure.
Flat direct drive motor ▲▲▲
▲ Flat linear direct drive motors can have coreless (top), slotted core (middle), or slotless core (bottom) structures.
Coreless flat direct-drive motors have flat magnets (secondary components) and a primary component or pressure head consisting of coils mounted on an aluminum plate. These motors offer excellent speed control but produce less force than other types of motors. Another structural variation of the coreless motor uses two magnetic tracks (sometimes called U-channel or hollow linear motors), with the secondary component or pressure head located between the magnetic tracks. Because these motors lack cogging effect, they can produce very high acceleration and deceleration.
Direct-drive motors with iron cores can be slotted or slotless, with the slotted iron core design being a more common variation. The secondary portion of a slotted iron core linear motor consists of a base and iron teeth or laminations, with coils wound around these laminations. They offer the highest output capacity but suffer from significant cogging effect.
Slotless designs are considered a hybrid between coreless and traditional slotted core designs because their coils are wound without a core layer and mounted on the back of an iron plate. Their secondary components are typically housed in an aluminum casing. Compared to slotted core designs, these motors have less cogging effect and lower inertia, but they also have relatively lower output force.
Another form of flat direct-drive motor is the linear stepper motor.
Tubular linear direct drive motor ▲▲▲
Another structural variation of direct-drive linear motors involves housing the magnets within a cylindrical tube and the windings within a press or thrust block surrounding the tube. Similar to their flatter counterparts,
Tubular linear motors can be constructed with or without a core in a secondary section (i.e., with or without a core). The main advantage of tubular linear motors is that their symmetrical design allows all magnetic flux to be used to generate thrust.
▲Tube-driven direct-drive motors can be cored or coreless. They offer an alternative to pneumatic and ball screw actuators, providing high speed and high thrust capabilities.
Advantages and applications of direct drive motors
▲ Rotary torque motors are commonly used in robotic applications.
Regardless of their design—rotary or linear, flat or tubular, iron-core or coreless—direct-drive motors offer the advantage of eliminating the reduction in positioning accuracy and repeatability caused by backlash from certain mechanical components. Eliminating mechanical connections also reduces load inertia and allows for greater dynamic response—that is, greater acceleration and deceleration under heavier loads—with less overshoot and oscillation. Direct-drive motors are also quieter than conventional motors, which is crucial for noise-sensitive applications such as the medical and laboratory industries.
Without additional drive components, direct drive motors tend to be more compact than conventional motors, making them easier to integrate into machines and systems with limited space. Furthermore, due to fewer mechanical parts (typically, the only wear component is the direct drive rail), maintenance is reduced, and mean time between failures (MTBF) is increased.
Rotary torque motors are used to drive goniometers, gimbals, rotary tables, SCARA systems, and 6-axis robot arms. Many designs have a center hole to allow cables and pneumatic lines to pass through the center of the motor.
▲ A tubular (as shown here) direct drive motor is used for gantry configuration.
Linear torque motors are used in a wide range of automation applications, including packaging machines that require continuous and rapid strokes, machine tools that require extremely high positioning accuracy and high load capacity, and semiconductor manufacturing equipment that requires ultra-smooth and precise motion.
