Faster, smaller, and smarter is the mantra of today's servo-controlled machines. As a result, end users have come to expect more and better performance, forcing designers to continuously improve equipment. That said, expanding a machine's servo performance can be challenging because so many factors come into play, including servomotors, feedback sensors, and servo drives, not to mention the mechanical transmission.

Servo performance directly affects the quality of parts that a machine makes and the time it takes to produce them. For example, positional inaccuracy in a servo often translates into dimensional variation of parts produced in cut-to-length applications, and registration inaccuracy in printing applications. System smoothness — or lack thereof — affects the variation of coating thickness in coating machines and part finish in polishing applications. Response time affects the rate of production: The fastest servos cut more plastic bags, print more labels, test more blood samples, and assemble more printer cartridges in a given amount of time. Yet while servos clearly affect machine performance, it's not always easy to translate “what the servo does” to “how the machine operates.” Three key measures bridge those concepts:

Accuracy — how close moving parts settle versus commanded position or velocity

Response — how fast the motion tracks the command

Robust stability — how reliably the motion tracks the command under various operating conditions

Accuracy

Accuracy is usually quantified in two ways — settled-position error and cyclical error. Settled accuracy, the positional accuracy of the servo when it's settled to a commanded position, is straightforward: Errors in servo system position translate to dimensional tolerance buildup. For example, if the cut-to-length servo in a bag machine has a position error of ±0.01 in., it will probably contribute a variation of 0.01 in. to the bag length. The feedback device largely determines settled accuracy. Sine encoders are the most accurate feedback devices with errors measured in arc-seconds, but they can be expensive. In contrast, resolvers and digital encoders are less costly, but have position errors an order of magnitude larger.

Kollmorgen's AKM servomotor family supports a wide range of feedback devices.

The second type of accuracy — cyclical error — is more complicated. When a motor turns at constant speed, position errors translate into an apparent velocity ripple. This ripple repeats during every revolution of the motor, hence the name cyclical. The apparent velocity ripple on the feedback signal feeds the velocity loop, which creates current to compensate for that ripple. Unfortunately, that current creates actual velocity ripple. The result is often a loss of smoothness at speed and an increase in audible noise and motor heat. The fix: Higher-accuracy feedback devices cure cyclical error. Sine encoders have so little cyclical error that they often produce no measureable effects; the same cannot always be said of resolvers and digital encoders. The key for machine designers is to select the right feedback device for each axis of motion.

Mechanical transmissions can also contribute to accuracy problems, because most machines rely on motor feedback as the primary position signal. If a motor connects to the load through a gearbox, the positional error of a gearbox causes the motor feedback signal to vary from the load position.

Transmission components such as lead screws, gearboxes, and belts and pulleys all contribute to error between motor and load. Many of these problems can be adequately addressed by selecting high-quality transmission components. However, for machines that demand the highest accuracy, designers can consider two other solutions.

First, a secondary feedback device can be placed on the load side of the transmission. For example, a linear glass scale can be added to a screw-driven gantry to eliminate accuracy problems in the screw. The motor feedback device can still be used to improve performance if the servo drive supports dual loop — a servo configuration where both motor and load feedback are used simultaneously. The need for dual loop is created because the mechanical compliance between motor and load can severely limit the servo performance when only a load-side position sensor is used.

While dual-loop solves many problems caused by load inaccuracy, the best solution is a direct-drive system, which eliminates the transmission altogether. In direct-drive systems, the motor is directly coupled to the load, significantly improving accuracy — to 10 times better than that of traditional systems, and 40 dB quieter. Other measures, such as servo response, acceleration rates, and reliability also can improve dramatically. For the most demanding servo applications, direct drive is the final step of evolution for the mechanical design.

Many of the alternatives discussed here have implications for the servo drive. When solving accuracy issues, engineers must consider several components, including feedback device families, motor type (standard or direct drive), and the servo loop (support of dual loop).