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Two Controls Engineers Discuss Servo Motors

July 15, 2010
Servo Motors Are Good for a Change: Machine Builders Discuss the Pluses and Minuses of Using Servos
By Mike Bacidore, Managing Editor

In a recent survey on motion control, we asked Control Design readers about their use of motors and reported the findings in three videos (www.ControlDesign.com/motionmir). Respondents were pretty evenly spread between 65% who use standard motors and 64% who use servo motors, while only 28% said they're using stepper motors.

I invited Danny Vujovic, senior controls engineer at Tekkra Systems (www.tekkra.com), a developer of shrink wrapping/bundling and integrated packaging systems in Romeoville, Illinois, and Craig Schmeiser, program manager for Wheelift (www.wheelift.com), a heavy transporter built by Doerfer in Waverly, Iowa, to give a more hands-on explanation of how their companies are using servo motors in real-world equipment design.

Tekkra utilizes servos on practically every system it manufactures, explains Vujovic. "The applications include, but are not limited to, conveyors, transfer assemblies, robotics, and collation systems," he says. "We tend to shy away from pneumatics for a few reasons. Servos are much more accurate and easily adjustable, while pneumatics are extremely inconsistent."

With the use of literally hundreds of servos yearly on nearly as many totally different machines Doerfer sets out to attach servos on anything that requires precision, speed and flexibility, says Schmeiser. "When you only have one chance to get it right, and the choice is down to a servo or a fixed displacement device such as a cylinder, we lean toward the servo to allow for adaptation during the commissioning and debug phase," he explains.

"The limitation of mechanical cams is that once the cam has been fabricated and machined, the cam profile cannot be changed unless the cam is replaced," says Vujovic. "Many of the systems Tekkra manufactures are custom systems, and the mechanical cams are determined based off of theory and experimentation. Utilizing electronic cams gives us the ability to rapidly and easily modify the profile of the cam through the PLC program. We can also utilize different cam profiles for different product running on the machine."

Cams do offer their own benefits, but changeover is a key consideration, explains Schmeiser. "They have a strong place in machinery automation and, when used appropriately, can't be beat," he says. "However, a servo can give machinery the ability to alter or adjust on the fly with nearly zero changeover time to a new product or cycle. When a customer is not able to get funds approved for multiple machines or maybe they don't want changeover downtime, then a digital servo in place of a cam is a great solution."

When compared to pneumatic actuators or stepper motors, servos have their place. "Pneumatic actuators make up a big portion of the active devices we employ on our equipment, but they all typically have a couple of sizable drawbacks: lack of motion profiles and a fixed displacement," explains Schmeiser. "When an application requires a variable speed, torque or position profile, then Doerfer engineers think servo. Steppers have their place, but lately the cost of digital servos has been steadily falling, making servos a better choice often even over a stepper motor-drive combination."

The highest priority in choosing between servo motors and pneumatic actuators is functionality, says Vujovic. "Although it must be financially beneficial, the system must work properly," he explains. "The cost of servos has drastically been reduced over the past 5-10 years. Air cylinders work in many applications; however, if precise positioning is required, then a servo is the better fit. Many of Tekkra's customers request that their machine contain little or no pneumatics. In our experience, servo motors are a much better fit than stepper motors. The systems we design typically require a high degree of precision and speed. We have found that servos provide much better performance capabilities."

As far as wish lists go, Schmeiser and Vujovic each have some suggestions.

"Two things come to mind," says Schmeiser. "Ethernet-based servo control and higher speed/high torque motor and drive packages. Beckhoff utilizes EtherCat, and Allen-Bradley is releasing CIP motion. Both allow for high-speed Ethernet-based motion control which, for us, is the way of the future. What we've seen is a constant market trend toward more output with less hardware, both in physical space and as a pricing point. Physics defines what we can expect out of motors on the market today, regardless of the manufacturer. What we need is for the next big leap in technology to allow a soup-can-sized motor to perform equally to the current varieties that are three times that size and twice the price."

Vujovic is more than happy with the capabilities of servo motors. "Rockwell is our standard for control components, but sometimes we'll get a request for a different controls manufacturer such as Siemens, GE or Elau," he explains. "It would be nice if some of these could play well together. When my customers request that I design their system with a non-standard manufacturer, it would be helpful if the integration between the two systems would be more seamless."

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