Servos adapt to their loads

Oct. 20, 2005
Nonlinear motor control is a novel way to handle production lines that demand quick changeovers

Mark Bourgeois
YET Inc.
Manchester, N.H.

An example of NCT-based servodrives is the XtraDrive line of servoamplifiers by YET Inc. in Manchester, N.H. YET servos include standard rotary, direct-drive, and linear versions. The XtraDrive servoamplifiers cover a power range from 30 to 5,000 W in 115, 230, and 480-Vac input power versions. Control compatibility of the XtraDrive includes inputs for step, direction, an analog velocity reference, and an analog torque reference.

The XtraDrive comes standard with an embedded 1.5-axis controller to facilitate stand-alone or distributed-control structures. Network connectivity supports Profibus, DeviceNet, Mechatrolink, and Sercos. The system supports standard A Quad B incremental encoders, Yaskawa serial encoders (Sigma I and II), resolvers, and sine-encoder feedback devices.

Nonlinear-control technology (NCT) for YET's motor control is said to provide tight command tracking, near zero settling times, and quick servotuning. Here the motor undergoes a load change, forcing a power increase to maintain speed at 2,400 rpm. The regular servomotor's feed-forward branch creates a large tracking error between actual and target motor speed before settling back to its optimal rpm. Nonlinear control produces just a small tracking error at the application of the load, and practically no settling time for return to normal speed.

The XtraDrive divides the feedback loop into four distinct paths of proportional, integral, derivative, and instantaneous velocity. Each path contains an independent variable-gain stage, the G function block, which automatically adjusts the gain of each feedback path based on operating conditions. The variable G functions provide gain increases over 10 that of ordinary feedback circuits.

A manufacturer of turn-key material-delivery systems for the packaging industry uses a special kind of "nonlinear-controlled" servomotors to boost throughput and extend machine flexibility.

The servodrives are XtraDrive servos with nonlinear control technology (NCT) developed by YET Inc. Unlike traditional PID controls, NCT uses nonlinear control algorithms to boost servosystem performance. A key trait is that NCT provides robust system control regardless of the motor manufacturer or mechanical configuration of the machine.

Position and velocity loops within NCT are integrated into a single loop divided into four branches: proportional, integral, derivative, and instantaneous velocity. Unique to each branch is a variable gain or G function that changes according to system conditions. The G functions apply branch-gain multiplication factors 10 times or more higher than those with conventional servocontrol. Moreover, gain changes as load conditions change to preserve system stability. The internal system adapts automatically without outside user intervention.

This adaptive technology fits industries where flexible production lines require quick changeovers. Changing production requirements place widely varying loads on servocontrol systems. Production floor technicians fail at times to properly retune servo gains, slowing new production startup as the system hunts for optimum performance. NCT-based drives inherently adjust to changing machine dynamics quickly reasserting operational stability. The result is a system that controls parameters tightly right from the start without risking system stability.

The adaptive nature of NCT control ensures the system corrects for equipment variations such as tighter bearings or looser belts. The unique control structure of NCT-based drives lends itself to a global gain adjustment. This function modifies shaft stiffness without altering system tuning. Global gain lets inexperienced users change system performance on the fly, helping them get servosystems up and running quickly. Special software supplied with NCTbased drives provides an intuitive Windows interface to set up, tune, and program the system.

One company using XtraDrive servos is Smart Belt Corp. of America. Smart Belt specializes in food-related packaging lines. Its equipment is found on the production lines of many well-known food companies in North America.

Smart Belt faces challenges typical of all equipment manufacturers in adapting core machine designs to specific application requirements. Each system is tailored to handle a specific product and, in some cases, to handle many different products.

System configurations meet the specific mechanical requirements of each production line and to integrate seamlessly into production-line control systems.

Smart Belt typically incorporates a central PLC to manage machine sequencing. Typical systems use from one to 32 axes per machine with each axis handling jobs of varying complexity. Servos in such systems operate in one of two ways. In the first method a reference signal from a multiaxis controller drives the servos. The second uses the PLC to control each axis individually.

Before going with XtraDrive servocontrols, Smart Belt used servosystem products from various manufacturers and tried to match products to performance requirements. Implementation and support difficulties arose because each application became a whole new integration effort. Idiosyncrasies of different products complicated the process. The task of staying up to speed on each product for the different machines became unmanageable.

To further complicate matters, Smart Belt became intrigued by an innovative motor and gearbox package. But this innovation created additional issues. The gearmotor supplier didn't have acceptable drives and driving the gearmotor with third-party amplifiers proved problematic.

Smart Belt searched for a servodrive system that would become its standard. Explains Tim McCarthy, Smart Belt president, "We wanted to choose a standard servo platform that is on the cutting edge of technology. Once we decided on a system, we knew we would be using it for many years to come and we did not want to get old technology that was in the declining phase of the life cycle. We wanted to learn to use it and be able to use that expertise for years to come."

, (866) 938-8080,

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