Making The Most Of Motors And Drives
by Jack Browne
, Contributing Editor
Robotic Industries Association Posted 04/07/2008
Electric motors and drives improve year after year more as an evolutionary process than by revolutionary developments. Since these smaller motors and drives are integrated into robotics systems, they are helping to bring new levels of efficiency and productivity to more compact robotic solutions. Sorting through the choices in electric motors can be challenging, considering the many different types of commercially available standard motor types and hundreds of manufacturers for those motors and related drive electronics. In addition, a large number of manufacturers offer motors tailored to a customer's specific requirements.
As a brief summary, the different types of electric motors include alternating current (AC), direct-current (DC), servo, and stepper motors. AC motors can be further subdivided into asynchronous and synchronous types. For example, an induction AC motor is an asynchronous type unit that is essentially comprised of a wire-wound stator and a rotor. Power is connected to the wire and AC current flowing through it induces an electromagnetic (EM) field in the coiled wire, with a strong-enough field providing the force for rotor motion. Synchronous motors are constant-speed motors that operate in synchronism with AC line frequency and are commonly used where precise constant speed is required.
Many industrial applications, including robotics, make widespread use of DC motors because of the ease of controlling speed and direction. They are capable of an infinite speed range, from full speed down to zero, with a wide range of loads. They can be used in many motion-control applications, for repetitive motion applications, such as sanding, material removal, and polishing, and are commonly applied for vehicular dynamic-braking applications where the energy from DC motors is used to replace or reduce the size of mechanical brakes.
Because DC motors feature a high ratio of torque to inertia, they can respond quickly to changes in control signals. A DC motor can be smoothly controlled to zero motion and instantly accelerated in the opposite direction without the need for complex power-switching circuitry. DC motors can be designed for continuous or intermittent duty and with or without brushes. Permanent-magnet brushless DC motors are usually more expensive than brush types, although they can provide advantages in power consumption and reliability. Brushless motors have a wound stator that surrounds a permanent-magnet rotor in contrast to the inverse arrangement of a brush-type DC motor. Without a commuter, brushless motors can operate more efficiently and at higher speeds than conventional DC motors. Most brushless DC motors run on a trapezoidal AC waveform, but some of the motors operate with sine waves. Sine wave-driven brushless motors can achieve smooth operation a lower speeds with low torque ripple, making them ideal for grinding, coating, and other applications requiring surface finishing.
Servo motors are used in closed-loop systems with a digital controller. The controller sends velocity commands to a driver amplifier, which in turn feeds the servo motor. Some form of feedback device, such as a resolver or encoder, provides information on the servo motor's position and speed. The resolver or encoder may be integrated with the motor or located remotely. Because of the closed-loop system, a servo motor can operate with a specific motion profile that is programmed into the controller. Types of servo motors include AC brush-type, DC, and AC brushless servo motors.
Stepper motors can operate with or without feedback, with the rotation of the motor broken up into small angular steps. It is controlled by pulsed command signals, and can stop precisely at a commanded point without need for brakes or clutch assemblies. When power is removed, a permanent-magnet stepper motor generally remains in its last position. Multiple stepper motors can be maintained in synchronization by driving them from a common source. The price that is paid for this positional control, however, is lower efficiency than most other motor types, with much of the input energy dissipated as heat.
As industrial customers ask for faster, more compact robots, manufacturers of those robots must seek out improvements at the component level, including in motors, drives, and other parts of the system. Michael Ferrara, Director of EPSON Robots (www.epson.com), points out that enhancements in his company's latest line of SCARA robots ‘‘represents significant performance improvements that will result in more parts processed and more profits for our customers.’‘
In general, robotic systems suppliers such as ABB, Inc. ( www.abb.com), Motoman Inc. (www.motoman.com), Nachi Robotic Systems, Inc. ( www.nachirobotics.com), Rimrock Corp. (www.rimrockcorp.com), Stäubli Robotics (www.staubli.com), and KUKA Robotics (www.kukarobotics.com) are fielding more demands for faster, lower-cost industrial robots that will deliver higher production yields, be it in materials handling, palletizing, or semiconductor processing. Higher-speed operation, of course, places increased design requirements on motion-control components, such as motors, drives, and encoders.
In some cases, even older systems can benefit from newer motor technology. For example, in a Case Study article written by Charles Boyer of Wolf Robotics (www.wolfrobotics.com), a division of Rimrock and a strategic partner of ABB for arc welding and cutting systems, and originally appearing in the late and lamented Robotics World magazine, a host of upgrades were performed on an ABB welding system. Rather than replace the system, it was more cost-effective to replace parts of the system, including replacing older DC motors with new AC brushless servo motors for improved positioning accuracy. Copies of the article are available for free download at the Rimrock Website at (www.rimrockcorp.com/News/Case-Study.pdf).
Applications for motors and drives extend far beyond robotics, although more than a few users are seeking higher performance from smaller packages. In response, a number of motor/drive suppliers have developed extremely small robots with performance that belies their size.
For example, Baldor Electric Co. (www.baldor.com) has developed brushless AC servo motors built into miniature 1.6- and 2.4-in. (40- or 60-mm) frame sizes (Fig. 1). Capable of substituting for DC servo or stepper motors, these miniature motors can deliver peak torques of to 33.63 lb.-in. (3.8 N-m) with fast acceleration. They weigh as little as 0.88 lbs. (0.4 g) but can provide full-size service in materials handling, semiconductors, and coil winding applications. Power and feedback connections to the motors are made by long flying leads. As an option, the motors can be specified with a resolver or encoder for positional feedback; an additional option is a +24-VDC brake.
Integrating motors with the drive electronics can also save valuable space. Companies such as Nippon Pulse America (www.nipponpulse.com) have developed integrated solutions in which hybrid stepper motors and their drive electronics are packed into a common size 17 National Electrical Manufacturers Association (NEMA) housing. Integrating two of the functions of a robotic system within the common housing saves space and also can lower the bill of materials (BOM) for the system design by eliminating the need to run power-management cables.
Danaher Motion (www.danahermotion.com) has developed direct-drive-rotary (DDR) motors that do not require mechanical intermediate elements, such as gearboxes, clutches or belts. By eliminating parts and using high bandwidths, the DDR motors can achieve improved accuracy and reliability compared to conventional drive-motor combinations.
Danaher Motion's CT series stepper motors have been designed to provide more torque with less power consumption than comparable standard hybrid stepper motors. At the systems level, this translates into higher operating efficiency and increased machine throughput with reduced machine size and operating cost.
As Jeff Arnold, Technology Specialist for Danaher Motion, explains, the company's DDR motors provide improvements ‘‘by eliminating all mechanical transmission parts to provide maintenance-free operation in just minutes. Users will also benefit from increased machine uptime, a noticeable 20-dB noise reduction, and up to a fifty percent increase in accuracy and torque density compared with traditional servo systems.’‘ In contrast, he notes that ‘‘conventional servo systems typically include up to 15 mechanical transmission components per axis of motion that can limit performance and reliability while increasing operational costs.’‘
Portescap (www.portescap.com), a Danaher Motion company, has developed a line of miniature flat brushless motors that measure just 32 mm in diameter and 11.7 mm in length (Fig. 2). The high-speed slotless design significantly reduces vibration to maintain stable speed compared with other miniature flat motors. By drawing less current than conventional flat motors, the new design minimizes generation of heat, but still achieves speeds to 50,000 rpm and output power to 30 W.
Maxon Precision Motors (www.maxonmotorusa.com) offers a variety of precision motors, including its electronically commuted (EC) motors which are available in a flat design for critical positioning and start/stop operations. The motors feature neodymium permanent magnets that turn within a stationary winding. For applications that really require savings in space, the drive electronics can be integrated into the motor assembly.
Stepper motors from Oriental Motor USA Corp. (www.orientalmotor.com) integrate two-phase hybrid stepper motors with a compact microstep driver in a compact package. The +24-VDC board-level driver can divide the fundamental 1.8-deg.-per-step motor motion by a maximum of 16 microsteps per step to achieve 0.1125 deg. per step without the use of a reduction mechanism or other mechanical elements. The driver is 62% lighter and requires 41% installation space than the company's conventional drivers.
A trend in motor controllers is growing flexibility of interface options, with more systems offering combinations of CANbus, RS-422, and RS-485 communications interfaces. In terms of further motor and drive education, Newport Corporation (www.newport.com) offers an excellent series of white papers on their Website, including ‘‘Motion Basics and Standards’‘ and ‘‘Motion Control Metrology Primer’‘ for system developers tasked with precision measurements.
This article has been reviewed by members of the RIA Editorial Advisory Group.
For more information, you may contact any of the experts listed in this article or visit Robotics Online, Tech Papers.
Michael Ferrara, Director, Factory Automation Robotics, EPSON Robots, (562) 290-5944, Michael_Ferrara@ea.epson.com
Jeff Arnold, Technology Specialist, Danaher Motion, (540) 633-3400, Jeff.Arnold@DanaherMotion.com
Chuck Boyer, Marketing Coordinator, Wolf Robotics, (970) 225-7600, firstname.lastname@example.org