Chris Radley
Senior Product Line
Manager Danaher Motion Wood Dale, Ill.
www.danahermotion.com

Frameless DDR motors are best suited to high-performance applications where space is limited or weight is critical.

Direct-drive rotary (DDR) systems are available in frameless, housed, and the newly developed cartridge motor format. Although many engineers are familiar with the basics of selecting conventional servo systems, the rules are a bit different when selecting a DDR system.

Most servosystems use a conventional rotary servomotor coupled through a gearbox or belt drive to the load. Even though conventional servosystems are largely maintenance-free, backlash and compliance from the gearbox or belt drive can introduce performance issues such as position inaccuracy or tuning problems. In fact, even systems without gearboxes or belt drives can have excessive compliance from the couplings that connect the servomotor and load.

Housed DDR motors are best suited for use on machines that do not include existing bearings, such as indexing and rate tables.

DDR systems provide the benefits of a brushless servomotor while eliminating the drawbacks created by compliant couplings and gearbox backlash. And they eliminate maintenance requirements, such as gearbox lubrication and belt adjustment or replacement. Because DDR systems do not require inertia matching, position and velocity accuracies can increase up to 50 times and reduce audible noise by up to 20 dB. For designers who need to reduce machine size, eliminating the mechanical transmission components results in a smaller machine that boasts higher performance, quieter operation, and zero maintenance of the motion-control system.

FRAMELESS SYSTEMS

Frameless DDR systems are comprised of a separate rotor and stator without bearings. The components are designed into the machine and become a direct part of the machine itself. The feedback device (such as a resolver, encoder, or Hall sensor) must also be designed into the machine. An electronic drive amplifier that runs the motor and manages the feedback device is also required. Designers can obtain all of the components from a single supplier, or can select the drive amplifier and feedback device separately from the motor.

Frameless systems are the original DDR technology. Most are custom designs, which make them good candidates for limited-space or weight-critical applications. Standard catalog versions of frameless DDR motors are available. These are less common but offer the benefits of ready availability, UL approval, and preengineered feedback designs.

Cartridge DDR motors are a new category of directdrive technology. They combine the performance benefits of frameless DDR motors with the ease of installation of a full-frame motor, and cost less than conventional DDR systems. Their unique bearingless design lets systems mount-and-run in less than 30 min.

Frameless DDR motors are ideal for aerospace and defense applications such as aircraft or ground-vehicle radar and weapons systems. They also make sense in high-performance industrial applications, robotics, and precision grinding. When the lowest size and weight must combine with optimum performance, and cost is less of a factor, frameless DDR systems are especially attractive.

HOUSED SYSTEMS

Housed or fullframe DDR systems integrate the rotor, stator, and factory-aligned feedback within a housing that includes precision bearings. Though such systems still require a compatible drive amplifier, they are easier to select, integrate, and service than a frameless system.