Avoiding the runaround with direct-drive rotary motion

June 3, 2004
Here's a look at the pros and cons of various direct-drive rotary systems.

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

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

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

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.

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.

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 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.

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 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.

Most compact design
More expensive than other DDR systems
Becomes integral part of machine
Longer design time
Virtually any custom design possible
Longer, more complex integration
Easily use customer-elected feedback device
More complex to service or repair
Easiest installation
Less flexible, standard design
Integrated feedback device
Not suited for systems with existing bearings
Single part number solution
Larger, heavier than frameless designs
Generally less expensive than frameless  
Fast and easy installation
Larger, heavier than frameless designs
Bearing-less design with integrated feedback
Not for systems that need motor bearings
Single part number solution  
Least-expensive solution  

Housed-DDR systems are best suited to applications where the load rides on the motor's bearings. Where a machine already has bearings, the motor must be coupled to the load or multiple sets of bearings must be aligned, a complex and time-consuming task.

Housed-DDR motors are generally used in applications without existing machine bearings, such as indexing and rate tables. In such cases, they offer fast and easy installation, a prealigned feedback device, built-in bearings, and significantly lower cost than a frameless design.

Cartridge DDR systems represent a new approach to directdrive rotary motors. They provide all the benefits of housed designs in a unique bearingless housing, permitting use on machinery that already has bearings. A compression coupling connects the rotor to the shaft, and a shipping clamp facilitates mounting so a unit can be up and running in less than 30 min.

Cartridge DDR motors can be used in converting, factory automation, packaging, printing, and semiconductor industries. The cartridge technology adapts to nearly any application that has existing bearings, providing all of the benefits of direct drive at the lowest cost, and with fast, easy installation.

All DDR systems provide benefits that include:

  • Zero maintenance
  • Zero backlash
  • Improved repeatability
  • Zero compliance
  • Improved accuracy
  • More compact mechanical design
  • Quiet operation
  • Improved velocity regulation

Many DDR designs are also available in hollow-shaft configurations that permit electrical and pneumatic lines or other machine components to pass through the motor and optimize space.

However, different electromagnetic designs will have different performance levels, cost points (cost for the torque available), and top-speed capabilities.

For instance, variable-reluctance motors do not offer the performance, torque, or speed available from brushless, high-energy permanent-magnetic motors. Also, variable-reluctance systems often must specifically match the drive amplifier to the motor. This limits flexibility in drive selection and makes servicing more difficult.

Some machines can benefit from the use of different DDR designs on different machine axes. Ideally, a single manufacturer that can provide the motors and a common drive amplifier to run them all. This common platform eliminates the need to learn and support multiple designs.

Direct-drive systems have higher initial costs than conventional servosystems. This can vary from as little as 30% more for cartridge systems to as much as four times more for frameless systems (total cost including design and integration). This cost picture changes quickly when life-cycle costs are examined. Over a typical five-year life, conventional systems will cost from three to 10 times more than direct-drive systems due to maintenance and downtime.

The benefits of direct-drive systems are not limited to just rotary-motion applications. Direct-drive linear (DDL) systems eliminate mechanical translation devices such as ball/lead screws or rack and pinions, offering equivalent performance and maintenance-free operation improvements to linear systems that DDR systems offer to rotary systems. Machines that combine DDR and DDL systems provide the ultimate in performance and zeromaintenance operation.

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