Servos Move Light Manufacturing
POSTED 11/13/2007 | By: Kristin Lewotsky, Contributing Editor
Courtesy of robust servo motor designs, motion control is penetrating new applications.
Manufacturing has long been the province of automation, with motion control establishing a strong foothold in packaging. Motion control isn’t just about packaging any more, though. Thanks to today’s robust servos, and networks offering intelligent components and system-wide communication, motion control is moving into a variety of non-traditional applications.
“Servos in manufacturing processing have really taken some quantum leaps forward in the last few years with regard to ease of use and reliability,” says Rosser Pryor, president of system integrator Factory Automation Systems Inc. (Atlanta, Georgia). “A lot of people who wouldn’t use servos in the past are finding that all of their systems are becoming more technologically sophisticated.”
Of course, the manufacturing environment is a harsh one. Achieving speed and precision goals in motion control is a challenge at the best of times, but in factories, system integrators have to hit demanding performance targets while operating amid electronic noise, contamination, heat, and other hazards.
On a typical factory floor, with its range of machinery and cables snaking everywhere, electronic noise is an omnipresent barrier to the communication of precision electrical signals. “You have to take into consideration the initial design, such as running the feedback cable away from the power cable, for example,” says Madison Steadman, technical leader in the C&SI Department of machine builder and system integrator ABCO Automation Inc. (Brown Summit, North Carolina). “If you have to cross them, cross them at 90 degrees.”
Following manufacturer installation guidelines generally leads to good results, he says. Of course, it’s not always so cut and dry. Often, cables will need connectors attached; strip too much shielding off the cable in the process and you’ve got a problem. “Noise suppression and grounding is both a science and an art,” says Pryor. “An electrician might install ten servos and wire them exactly the same way. Nine of them will work perfectly and the tenth will have a noise issue.”
One option is to use fiberoptic cabling to transmit data like encoder feedback. Optical fiber isn’t a universal solution -- it can’t be used to drive motors, for example -- but where appropriate, the optical signal provides noise immunity. The cabling itself can be fragile, though, as the fine glass fiber can fracture when subjected to repeated flexing or bends around tight corners.
There are alternatives, such as plastic optical fiber says Paul Derstine, motion products manager at GE Fanuc Automation (Charlottesville, Virginia). It tends to have higher loss than the glass versions, but depending on the application, it can be a good option. “Over shorter distances the plastic fiber might work just fine and be more durable relative to the handling,” he notes.
Durability is important because cabling tends to be the most common point of failure in motion control systems. Cables are rarely protected in conduits all the way to the motor, which means that if a part falls off a belt or molten glass or metal spatters, the cabling may suffer damage.
Heat and Dust
Such examples only serve to underscore the fact that electronic noise is far from the only environmental concern in a manufacturing environment. Heat, humidity, even conductive dust can be an issue.
In extreme environments such as those used for molten metal or glass processing, the components are subject to extreme temperatures. While heat itself may not be a problem for motors, thermal cycling can be, especially if there’s humidity involved. If water vapor gets into the motor and the temperature then drops, it can create a vacuum in the motor, causing it to suck in contaminants.
This is not necessarily a concern for the motors, which are fairly robust, but it can be a problem for electronic components like controllers and drivers. The choice then becomes whether to try to seal those components in with the motors or to put them all in a cabinet and run cabling to the motor. The issue is that cabling is generally specified for a finite distance -- run the cables too far and you compromise performance.
“There’s always a tradeoff,” says Pryor. “Do you put the controller near the line, do you put it with the motor or do you isolate it in its own little cocoon up in a control room? There’s no one good answer for that -- you have to consider your environment, the brand of servos, the size, the cost of the cable versus the cost of building a panel that will go near the line, and so on.”
If the environment is too hot or too humid, problems can arise with controller and driver electronics. “We’re always on the edge of being outside the specs for temperature or moisture,” Pryor says. “In our experience, because we often work with environments that are hot or wet, there are often issues with the smart motors.”
That said, he’s a fan of integrated motors, which he says can minimize wiring headaches. The downside is that they tend to be larger, which means they’re not suitable for applications with limited space. “There’s also a more limited range of options,” he adds. “There tends to not be 25 different size, torque and speed variations available.”
The Customer-Eye View
ABCO has built motion control systems for everything from packaging boards for hardwood flooring to handling dishwashers, Steadman says. Customers have become steadily more accustomed to servos and to new trends, such as networking and distributed control. “People recognize the need to keep up with technology,” he says. “Servos become a lot more available to them when they start using that technology and seeing the benefits. Instead of saying, ‘We’re not going to do that because that requires servos,’ they’re saying, ‘We’ve got a servo on this system. Maybe we can put another one on to take care of some flexibility or speed or precision issues we’re concerned about.”
Robustness and ease of operation and maintenance have facilitated this transition, he notes. Customers want servo motors in their systems, but they don’t want to haul the system integrators or machine builders out every time a problem crops up. They want to be able to call their local parts suppliers and replace the components themselves or make modifications. User-friendliness is key.
Pryor agrees. “In the past, servo systems were typically out on the extremes of what a manufacturing plant was capable of dealing with. We’ve found that by using the brands of servos that integrate well with PLCs [programmable logic controllers] and then designing the system with troubleshooting diagnostics, built-in displays and preprogramming methods for setting up positions or adjustments, we can make it so that the servo is only slightly more difficult to deal with than the PLC or whatever the customer is used to. That’s one of our goals: to make servos friendly and easy to use with plant operations.”
Servos In the Real World
Pryer and his group have used servo motors to replace a hydraulic linear actuator that indexes a continuous wood process press. Making the switch to a 50 kW servo motor eliminated the downtime typical of the cleanout cycle of a hydraulic machine, although the process wasn’t trivial. “There were considerable amounts of heat and particulate contaminants in the environment,” he says. “We used a hollow-shaft gearbox right on the press head shaft that we were actually turning. The advantage was that we got rid of the hydraulic system which always required maintenance. There’s an energy savings and the new system is more reliable.”
The flexibility offered by motion control is another advantage that is fueling the switch from pneumatic/hydraulic actuators to servo motors. Being able to adjust positioning electronically is a far cry from manually adjusting hard stops, and a necessary flexibility in today’s market.
“It’s all part of just-in-time manufacturing and reduced inventory,” Pryor says. “Manufacturers want to make much smaller lot sizes and servo applications allow faster setup.” He points to a recent system built to process and package truck brake pads. The parts are sold in a single carton that contains both front and back brake pads. Using servo motors, the machine first produces then packages the product, one carton at a time. “The machine will grind five of one part, then grind five of another. We’re actually changing part numbers every five parts. That wasn’t possible before the servo system was applied to that grinder.”
The availability of explosion-proof servo motors has allowed ABCO to take a filling machine designed for a washdown environment and move it to a new market. “It’s the same filling application, but we’ve taken it from a washdown environment to an explosion-proof environment,” says Steadman. “We were able to take it to a new application and a new industry.”
Going forward, Steadman expects to see increasing levels of integration between motion and vision systems. “Robotics, vision and motion have blossomed from art forms to products,” he says. “When the vision system finds what it’s looking for, you typically want to do something about it. You can marry servos up with other technologies like robotics and vision to allow us to solve some applications in a way that we would not have done traditionally before.”
Thanks go to Matt Dahlen of Kim Automation (Austin, Texas) for additional input.