Servo trade-offs

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With the abundance of servo products available today, it's important to choose an overall system design that best matches machine performance requirements. This task isn't easy, given all of the associated trade-offs associated with the various elements of a servo system (see diagram). And it is all the more critical when attempting to increase machine performance. To help design engineers better understand the pros and cons of various servo solutions, this article addresses some of the key trade-offs associated with: Load-to-motor inertia ratio; resolver and encoder feedback devices; and analog and digital servo amplifiers.

Inertia mismatch. Determining an acceptable load to motor inertia ratio, J_L/J_M, is a topic of confusion and concern for many engineers. That's because servo vendors offer a wide range of rules-of-thumb for this ratio, and the result of misapplied system components can be unsatisfactory or unpredictable performance.

First, consider that acceleration of a system is maximized by minimizing system inertia. Low motor inertia helps, but can result in a high inertia mismatch (J_L/J_M). A high mismatch can lead to machine resonance if too much compliance exists in the system.

A compliant servo system is one that has lost motion or springiness between the motor and load, which can result from backlash in gears, or undersized shafts and belts. Sometimes the structure supporting these components can be compliant. Compliance results in resonance, which is oscillation or ringing of the system. (Although all mechanisms have some degree of compliance, compliance that results in resonant frequencies in the hundreds of Hertz is not normally problematic for the servo system.)

To offset this problem, suppliers offer low- and medium-inertia motors that have equivalent torque and speed ratings. Using such a motor decreases the inertia mismatch, thereby providing greater stability. The trade-off, though, is reduced acceleration rate because the total system inertia (J_L + J_M ) has increased.

Load couplings, belts, gears, and long shafts like ballscrews have varying amounts of compliance, which determines the machine stiffness and tendency to resonate. The table at left lists typical inertia mismatches that can be tolerated without exciting resonance. The variability is largely due to expectation of the system response. To avoid instability, a highly dynamic response requiring high velocity and position loop gains requires that the inertia mismatch be less. Always consult the manufacturer of your drive components for specific recommendations. One alternative to increasing motor inertia is to stiffen the mechanical system. More rigid couplings, wider belts or idlers, beefier shafts, and larger gearboxes will help reduce compliance. Another option is to choose a direct drive motor.

Direct drive motors provide torque or linear force directly to the load. A direct drive rotary motor provides high torque at speeds less than 500 rpm, much like what a gear reducer produces in combination with a shafted motor, without any compliance. Similarly, a direct drive linear motor provides linear force, directly replacing a ballscrew, rack-and-pinion, or other rotary-to-linear transducers. An added advantage is the linear motor's ability to achieve higher speeds, thereby delivering stability without sacrificing dynamic performance. Additional benefits of direct drive motors include a maintenance-free mechanical system, quiet operation, and a simplified mechanical design.

Considering all these benefits, why don't we see more direct drive motors on machines? One reason is education. These motors are still new to many machine designers, although increased availability and application information is helping make them a considered choice. The other reason has been cost. Initially considered for highly specialized, cost-insensitive applications, linear motors are finding their way into more and more applications, particularly as industry gains experience costs continue to come down.

Feedback options. Regardless of the chosen motor technology, a good mechanical design is only the first step to a well-performing servo system. The next consideration is system feedback, which affects the controllability of the load and accuracy to position. Even the best of mechanical designs will not meet expectations if an inappropriate feedback device controls the system.

A position feedback device is essential to a brushless servo system, pro- viding both position information for commutation (the switching of windings based on relative position of armature and stator magnetic fields), and its derivative, velocity feedback. This device typically also provides position feedback to the position controller.

For many years, servo providers focused on one type of feedback or another for their systems. Some suppliers now offer a choice. Although many types and variations of feedback devices exist, the resolver and incremental encoder are dominant in the marketplace and are the subject of this comparison. The table on the preceding page summarizes some of the differences.

Resolvers are used because they are rugged, provide adequate accuracy, and are relatively inexpensive. In addition, they provide absolute positioning so that a motor can be started easily upon system power-up, thus minimizing the complexity of the amplifier control scheme.

In contrast, the incremental encoder is more sensitive to temperature and shock, and provides incremental feedback. The need for absolute positioning is accomplished with the addition of absolute position feedback signals like Hall effect devices or commutation tracks on the encoder. The key advantage of encoders, however, is increased accuracy compared to the resolver.

Amplifier options. The final consideration is the servo amplifier. In addition to differences in the way an amplifier processes feedback from the motor, other attributes are important in determining the right technology.

An important consideration in the selection of brushless motor amplifiers is analog versus digital technology (The table above compares some of the key characteristics of each type).

Analog technology has been the standard for brush and brushless motor amplifiers for decades. With analog amplifiers, a voltage reference command results in speed or torque (depending on the mode of operation) that is proportional to the command. Compensation (tuning) of the amplifier is done by analog circuitry. Potentiometers, switches and/or a compensation "comp" board for component changes are required for velocity mode systems. Torque-mode compensation is considerably simpler and doesn't need tweaking.

Analog systems have the advantage of having continuous control responding "instantaneously" to changes in command or feedback and are less expensive, especially at low current ratings. An inherent disadvantage, however, is drift due to thermal variations.

Digital electronics offer enhanced drive features and minimize set-up concerns. The command reference for digital drives can be either digital or analog via internal A-to-D converters. Digital input options are typically via a serial pulse format or through a RS-232 or 485 interface. Typically, the digital command format achieves better long-term regulation of the system. Digital drives also allow tuning through a digital interface. This feature benefits field compensation relative to inertia, friction and compliant factors that are often difficult or impossible to simulate under lab conditions.

Many digital amplifiers have an advanced self-tuning algorithm further easing the tuning effort. Another benefit is the diagnostic enhancements digital technology offers. Monitoring of internal functions of the drive such as following error or current can be easily done. Fault diagnostics are typically more detailed than with analog drives and fault histories are maintained through power loss. A disadvantage of digital systems it is that they have delays resulting from "sample and hold" processes, resulting in phase lag that contributes to instability.

As is the case with any design, any improperly chosen component will undermine the performance of the entire system. Considering both the individual and interactive aspects of your component selection will put you on the path to success.

For those who want a greater understanding of the many factors regarding system response, simulation software is available. Kollmorgen offers a seminar titled "How to Improve Servo Systems", and a free software package called Model Q. Motor and amplifier selection software is also available. MOTIONEERING^TM, provides a convenient way to calculate static and dynamic load conditions. For more information visit Kollmorgen's website at www.motionvilliage.com.

Regarding inertia mismatch

Suppliers of servo components offer a wide range of rules regarding the desired inertia ratio between load and motor, (J_L/J_M⁾. It's all a matter of perspective. Sometimes the number is 1:1 based on optimizing a load reducer ratio for maximum acceleration. This approach is not always necessary, practical, or cost effective. An alternative way to determine the desired ratio involves considering the known or assumed characteristics of system components. In order to verify component compatibility, engineers should always check with specific vendors.