Choose Drives for Speed, Accuracy

Servo Motors and Drives Provide Advantages in Appropriate Applications

Phil BurgertBy Phil Burgert

Electronic drive options for discrete manufacturing present a number of choices, with speed, accuracy and control of multiple motors among factors that should be considered.

“Standard Volts-per-Hertz inverters and vector drives are primarily shaft turners, although vector drives provide an added level of control and performance,” says Paul Kling, North American sales manager for Intelligent Motion Systems. “Servo drives take control to the next level of precision for speed regulation and ultimately position control.”

The difference between inverter and vector drives, which are also known as variable frequency drives (VFD) or standard drives, says Murray, is that the vector drives perform more complicated functions such as motor modeling and compensation for current-times-resistance losses that are load-dependent. They also compensate for voltage drop across the stator. “With these capabilities you get better control at the lower end of the speed range,” says Matt Murray, electronics trainer with SEW-Eurodrive.

Servo motors and drives, on the other hand, provide advantages in appropriate applications, notes Murray. “One is just flat-out speed,” he says. “Servos have a lower inertia in their rotor so they achieve much higher acceleration and deceleration rates,” he notes. “You can start and stop very quickly and get tremendously higher cycle times since some servos are fast enough to do cycles per second.”

Another advantage of servo systems is torque density. “Look at how much torque a motor can put out relative to the space it takes up,” says Murray. “A servo beats an induction motor hands-down.

“Today most inverters are as powerful as servo drives for loop execution rates,” says Corey Morton, product manager for B&R Industrial Automation. “So the lines blur a bit there. Probably the real determining factor for choice goes to the mechanics and to the demands of the application.”

Before, a lot of wiring was involved, but now networked drives or electronic lineshafting are available, says Greg Dieck, motion solutions engineer with Omron Electronics. “Most servo networks are still proprietary, but some are starting to open up,” he explains.

“Microprocessors today are so much more powerful that the drives contain the subroutines to run the different modes, be it vector, servo or a Volts-per-Hertz mode,” says Craig Nelson, drives product manager for Sinamics S120 at Siemens E&A. “Basically, this drive could run any motor you’d put in a factory or machine.”

Modern drives, notes Karl Rapp, automation and machine tool branch manager for Bosch-Rexroth, also provide detailed diagnostic information via networks, a built-in display, logbooks or other means for error logging, preventative maintenance and intelligent monitoring.

To determine which drive technology to use, Paul Whitney, product marketing manager for servo drives at Rockwell Automation, says users should start by determining whether the application requires coordinated motion across multiple axes. Highly synchronized applications, such as pick-and-place robotics and vertical form, fill and seal, call for the coordinated movement provided by servo drives, he says. “On the other hand, pumps and applications need to speed up quickly and run continuously use a VFD,” says Whitney.

The choice also might come down to linear vs. rotary drive/motor configurations. Linear drives provide extremely high accuracy and higher speeds compared to the rotary approach, explains John Mazurkiewicz, product and market manager for Baldor Electric. While an application using a rotary motor and drive can attain accuracy of 0.02 mm, linear drives can attain 10 to 20 times better accuracy, he says.

Much higher speeds are also possible with linear drives. While a rotary drive can reach speeds of 1 m/sec, linear drives can do 10 m/sec. These speeds are limited only by the encoder data rate and bearing system, says Mazurkiewicz.

Phil Burgert is a freelance writer, specializing in technical trade media.