Direct Drive Rotary An Increasingly Attractive Servo Choice DDR systems are available in frameless, housed and the newly developed Cartridge motor format. While many engineers are familiar with the basics of selecting conventional servo systems, they may not be as knowledgeable in how to select a DDR system. This article looks at the different types of DDR systems, their pros and cons, and when to select which type of system for a given application. Chris Radley, CMfgE Senior Product Manager Danaher Motion Wood Dale, IL www.danahermotion.com ContactUs@DanaherMotion.com 866-993-2624 Most servo systems use a conventional rotary servomotor that is coupled to the driven load through a gearbox or belt drive. While conventional servo systems in and of themselves are maintenance-free, performance issues such as position inaccuracy or difficult tuning problems can be introduced by the backlash or compliance from the gearbox or belt drive. In fact, even systems that do not employ gearboxes or belt drives can have excessive compliance issues from the couplings used to connect the servomotor to the load. These coupling methods will take a maintenance-free servo motor and introduce new maintenance requirements, such as gearbox lubrication and belt adjustment/replacement. Direct drive rotary (DDR) systems provide the benefits of a brushless servomotor solution, while eliminating the drawbacks created by compliant couplings, gearbox
backlash and gearbox/belt maintenance. By coupling the motor directly to the load, all mechanical transmission components are eliminated. In addition to solving the maintenance headaches, DDR systems do not require inertia matching, position and velocity accuracies can be increased up to 50 times, and audible noise is reduced by up to 20 db. For designers who need to reduce machine size, the elimination of 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 Frameless DDR motors are best suited to high performance applications where space is limited or weight is critical. Frameless DDR systems are comprised of a separate rotor and stator without bearings. These components are designed into the machine and become a direct part of the machine itself. The feedback device (such as a resolver, encoder, hall sensors, etc.) must also be designed into the machine. An electronic drive amplifier that runs the motor and manages the feedback device is also required. The design engineer may obtain all of the components from a single supplier, or may select the drive amplifier and feedback device separately from the motor. Frameless DDR systems are the original DDR technology. Most frameless DDR motors are custom designs, which make them good candidates for applications where space is limited or weight is critical. There are standard catalog versions of frameless DDR motors available. These are less common but offer the benefits of ready availability, UL approved design and preengineered feedback designs. Frameless DDR Motors 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 selected feedback device More complex to service or repair
Frameless DDR motors are ideal for aerospace/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 being good examples. When the lowest size and weight must also deliver optimum performance, and cost is less of a factor, frameless DDR systems are an excellent solution. Housed DDR systems Housed DDR motors are best suited for use on machines that do not include existing bearings, such as indexing and rate tables. Housed DDR systems, also referred to as full frame systems, integrate the rotor, stator, and factory aligned feedback within a housing that includes precision bearings. While such systems still require the selection of a compatible drive amplifier, they represent a standard solution that is much easier to select, integrate and service than a frameless solution. Housed DDR systems are best suited to applications where the load is designed to ride 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 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 Housed DDR motors are generally used in applications without existing machine bearings, such as indexing and rate tables. For applications suited to the housed DDR design, they offer fast and easy installation, a pre-aligned feedback device, built-in bearings, and significantly lower cost than a frameless design.
Cartridge DDR systems Cartridge DDR motors represent a completely new category of direct drive technology that combines all the performance benefits of frameless DDR motors with the ease of installation of a full frame motor, and at a significantly reduced price point than conventional DDR technology. Their unique bearing-less design facilitates mount-and-run operation in less than 30 minutes. Cartridge DDR systems represent a new approach to direct drive rotary motors. They provide all the benefits of housed designs incorporated into a unique bearing-less housing. This makes it possible to easily use direct drive technology on machinery that already has bearings. The innovative compression coupling that connects the rotor to the shaft, and the distinctive shipping clamp design, facilitate mount and run operation in less than 30 minutes. Cartridge DDR Motors 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 Cartridge DDR motors can be used in a wide variety of applications in the converting, factory automation, packaging, printing, and semiconductor industries. The cartridge technology can be adapted 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 a common set of benefits that include zero maintenance, reduced downtime, zero backlash, improved repeatability, zero compliance, improved accuracy, more compact mechanical design, quiet operation, and improved velocity regulation. Many DDR designs are also available in hollow shaft configurations that permit electrical lines, pneumatic lines or other machine components to pass through the motor for optimized space utilization. Not all direct drive systems are available from all manufacturers. Moreover, different electromagnetic designs will result in different levels of performance, different cost points (dollars of cost for the torque available), and different top speed capabilities.
Variable reluctance motor designs do not offer the performance, torque or speed capabilities available from brushless, high-energy permanent magnetic motors. Also, variable reluctance motors often require that the drive amplifier be specifically matched to the motor. This limits flexibility in drive selection and makes service more difficult. Some machines can benefit from the use of different DDR designs on different machine axes. Ideally the motors should come from a single manufacturer that can provide a common drive amplifier design to run all the different motors. This provides a common platform and eliminates the need to learn and support multiple designs. Direct drive systems will have higher initial costs than conventional servo systems. The degree of difference can vary from as little 30% more for a cartridge system, to as much as four times more for a frameless system (total cost including design and integration). This cost picture changes quickly when the lifecycle costs are examined. Over a typical five-year life the conventional system will cost from three to ten times the direct drive system costs due to maintenance and downtime. On a final note, the benefits of direct drive technology are not limited to just rotary motion applications. Direct drive linear (DDL) systems eliminate mechanical translation devices such as ball/lead screws or rack & pinions, offering the 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 zero maintenance operation. As before, it is possible to find manufacturers that offer a single drive amplifier platform that can support all of the direct drive axes and even conventional rotary servomotor axes; this makes it possible to easily design high performance systems that are simple to setup and operate.