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Frameless, housed & cartridge types provide options
By Chris Radley and David Jones
There was a time when servomotor control systems were considered a very niche and costly technology. During the 1970s, they were mainly used in applications that required a high degree of precision, such as machine tools and CNC based machining centres.
As costs continue to come down, we have seen a "conversion" take place, where servomotors have replaced what have been considered lower level technologies and components, such as pneumatic and hydraulic actuators and cylinders, as well as clutch-brake systems. Latest generation servo motor designs have also replaced dc motors in non-industrial products such as exercise equipment.
 The most exciting news to come out of servo motor design of late has surrounded the Direct Drive concept, a concept that focuses more on the motor and the method of transmitting mechanical power.
Direct drive rotary (DDR) systems are available in frameless, housed and the newly developed "cartridge" motor format. 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, allow position and velocity accuracies to be increased up to 50x, and reduce audible noise by up to 20 db.
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 or hall sensors) 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 motors are suitable for applications such as aircraft or ground vehicle radar and weapons systems, but also make sense for robotics and precision grinding. 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, 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 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.
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. An innovative compression coupling connects the rotor to the shaft and a distinctive shipping clamp design facilitates "mount and run" operation in <30 min.
 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.
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 4x 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 3x to 10x the direct drive system costs due to maintenance and downtime.
Chris Radley, CmfgE, is Senior Product Manager, Danaher Motion, Wood Dale, IL, (danahermotion.com) and David Jones, is Corporate Manager - Motion Control, Vickers Warnick (vickers-warnick.com), Mississauga, ON. |
