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Motion control without breaking the bankMotion control without breaking the bank

DN Staff

June 19, 2000

9 Min Read
Motion control without breaking the bank

Mention motion control, and the instant reaction of many engineers used to be, "It's too expensive." Today, however, more and more engineers are discovering that they can get servo- and near-servo performance at an economical price. For one thing, the cost of servo motors and controllers has come down significantly in recent years. Lower-cost alternatives that deliver servo-like performance, such as ac vector and variable frequency drives, are also gaining in popularity. Moreover, when factored over the entire life cycle of a product, an investment in motion control can pay off big by delivering benefits such as increased throughput, easy changeovers on-the-fly, and lower maintenance costs.

In this article, we'll examine a number of different ways to obtain superior operating performance through motion control-all without breaking the bank.

DeviceNet servo control cuts cost in half

A leading packaging machinery supplier for high-volume bottling applications recently cut servo control costs in half, increased throughput over 80%, and maintained process accuracy on its next-generation machines using a DeviceNet-based distributed control architecture from Whedco (Ann Arbor, MI), a subsidiary of GE Fanuc.

With little prospect for improving the machine's mechanics, the engineering team concluded that the only way to meet its goals was to replace the existing standalone motion controller and add-on boards for encoder feedback and programmable limit switch (PLS), with a distributed control scheme. They predetermined DeviceNet as the network for implementation of the new architecture, handling both I/O and peer-to-peer data communication.

Upgrading controls on this bottling machine to a DeviceNet-based distributed system cuts cost in half, increases throughput more than 80%, and maintains label-placement accuracy.

Because DeviceNet handles peer-to-peer, multi-master, and master-slave architectures with a single network, engineers felt that if they couldn't find a DeviceNet servo control for their packaging machine, their distributed control architecture would fail. "Most of our motion control competitors do networking on a master-slave, or remote I/O basis," explains Katherine Voss, director of sales and marketing at Whedco. "But because our standard product supports DeviceNet, we can do motion control networking on a peer-to-peer basis."

Because this application was supremely motion intensive, says Voss, Whedco responded by assigning a high-level application engineer to demonstrate the DspMotion(R)product. The dual processor, drive-controller product met initial application requirements for increasing machine throughput and accuracy. Then, Whedco's design engineering team made a few firmware and hardware modifications to handle the customer's peripheral devices, which would become feature enhancements to the standard product.

The application was so demanding that every command line in the program had to be optimized, but the end result was approximately a half-microsecond servo response time for the position trigger outputs.

For more information about DspMotion from Whedco: Circle 552

Switch to ac induction motor cuts costs, improves stability

A motor-control module designed for use with both dc brushless and ac induction motors enabled GE Medical Systems to make an easy design change on a dual-head nuclear imaging system while still in the prototype stage, reducing problems with oscillations while cutting costs.

The imaging system consists of a rotating gantry structure and patient positioning table. Two heads, each holding a nuclear detector enclosed in an 800-lb lead box, mount to the gantry and rotate about its curved beams. They can be positioned up to 180 degrees from each other. To control the gantry's dc brushless servo motor, GE engineers selected ACS-Tech80's (Minneapolis, MN) SB1091DE universal control module, which includes an integrated digital drive. The motor also drives the two heads via a 1,000:1 gear ratio.

Unfortunately, the first prototype suffered from oscillations due to the high load-to-motor inertia ratio. It was also impossible to tune the system for stability at all head positions.

To solve the problem, engineers replaced the low-inertia dc brushless motor with a high-inertia ac induction motor. This change eliminated the oscillation problem, since the change in the reflected inertia of the load caused by repositioning of the heads is relatively small compared to the inertia of the rotor.

What made this simple change possible is the fact that the control module is capable of controlling induction motors using vector control technique by quickly modifying a few programmable software parameters. While theoretically the position and velocity frequency response and acceleration is lower due to higher inertia of the motor, in this case the large load was the limiting factor. So the solution solved the stability problem with no sacrifice in performance, while providing a significant side benefit: cost savings. The ac induction motor was less than $300 vs. more than $1,000 for the dc brushless motor.

For more information about SB1091DE universal control module from ACS-Tech80: Circle 553

Variable frequency drive, controls provide precision handling

Long a workhorse of overhead lifting applications, electric chain hoists have always been limited by the inability to control the motor speed. Now, Mannesmann Dematic's DK variable-speed hoist offers precise, finger-tip control over lifting and lowering speeds, thanks to a low-cost variable frequency drive, ac induction motor, and a patented push-button technology.

"What we've done is give the operator fine positioning control throughout the entire speed range-including the high end-using a basic, single-speed ac motor," says John Paxton, hoist and components product manager, Mannesmann Dematic. A 550-lb capacity unit has a speed range of 0.39 to 26.2 ft/min, while the 1,100-lb capacity unit's speed range is 0.65 to 41 ft/min (at less than the full rated load, max speeds increase by up to 40%). Exceptionally fast single-speed chain hoists operate at about 100 ft/min.

Engineers were able to design it cost-effectively because the price of variable frequency drives, which control the speed of a motor by regulating input current or voltage, has come down in recent years. But while the technology has been applied to chain hoists before, Paxton says no one up until now has been able to provide stepless control of the hoist.

Dematic engineers got around that limitation through a patented analog switching element in the control pendant. Using a magnetic field to sense the distance that the operator depresses the control switch, the speed-control electronics send a signal to the inverter to slow down or speed up the motor.

Web-unwinding goes high-tech

Engineers with a web unwinding application found a cost-effective way to get servo-like performance for under $500, using a single-axis motor controller on a master/follower variable speed drive system.

A single axis motion controller delivered servo-like control on this unwind application at a lower cost while providing a simpler setup.

When the system is running, the lead process pulls the webbing out of the accumulator in "spurts." The lead process always pulls the same amount of web per pull, and in the same amount of time (the cycle is repetitive). The purpose of the follower unwind process is to keep filling the accumulator with webbing at a fairly constant speed, so as to hold the accumulator near the middle of the capacity. Average line speed is approximately 65 feet/min.

The MS320 motor controller from Drive Control Systems (Minnetonka, MN) adds digital PID velocity control by monitoring a signal emanating from a feedback sensor and fine-tuning the speed reference. Although this application uses dc regenerative drives and 1/2-hp dc motors, the control is also compatible with ac motors.

An encoder traction wheel riding along the surface of the webbing on the unwind spool provides surface line speed feedback to the MS320 so it can close the loop, and run the web at the programmed setpoint speed. A programmable variable maintains constant line speed as the core builds or unwinds.

In closed loop mode, the controller achieves 0.02% speed regulation in both master and follower mode. To aid in running the unwind spool as smooth as possible and filter out any "step-response" action, the PID variables are set fairly soft.

For more information about MS320 motor controller from Drive Control Systems: Circle 555

Servos keep cookies from crumbling

As President of Machine Builders and Design (Shelby, NC), Bud Mims has made a career out of cookies. Actually, building machines that find, orient, flip, fill, cap, and pack cookies such as Little Debbie, Oreo, and Moonpie products.

10 Challenges for the decadeEngineers in the coming decade will face an array of challenges and opportunities as they design the products that will define our way of life. This report on economical motion control is the sixth in a series analyzing those challenges and opportunities.

Twenty-five years ago, when Mims started the business, its scope was very broad. "You have a problem? We'll solve it, tell you how much it will cost, and guarantee the solution," says Mims. But as fate would have it, the focus of the business quickly narrowed. "It wasn't long before we became a leader in the cookie processing niche," says Mims. "And thanks to servo technology, we are still the leader."

"We did try out some other technologies," says Mims, "but we found the most reliable to be from PacSci." In fact, Pacific Scientific (Rockford, IL) supplies the servo drive and motor that allows Machine Builders and Design's cookie processing equipment to handle five to six thousand cookies per minute. The PC832 digital brushless servo drive has three power levels and multiple command interfaces.

"Servos simplify our designs, and increase the machines' precision, versatility, and throughput," says Mims. "One of our machines produces 33 different products, making it very popular to smaller businesses, allowing us to expand our market share."

A typical capper machine for sandwich cookies takes individual cookies from a conveyor, flips every other one, applies filling, and then puts the top on the bottom. The process requires that devices above the cookies are synchronized with devices below. For example, changing from a spot deposit of filling on the cookie to a line deposit requires changing the relative speed. "We use servos to synchronize up to 13 axes, phase locked. The push of a button changes ratios, profiles, and speeds for quick changeovers between different products," says Mims. "Only servo technology can give us this kind of versatility."

For more information about Servos from Pacific Scientific: Circle 556

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