Ah yes, the servo loop. It sounds so simple. And when it comes right down to it, there are only two issues involved — how close you come to hitting the final point (accuracy) and whether or not you can hit it again and again and again (repeatability).

The trouble begins with the forces of nature and physics. Foes such as friction and external torque impact things like rate feedback, current feedback, and position feedback. Then there’s gain and damping to be considered and manipulated in order to achieve positional accuracy. The good news is that many components have been designed to help fight against the enemies of precise motion and enable your system to work in a reliable and repeatable manner.

The following journey through words and images starts with a look at the three main sources of position errors and winds through a forest of solutions and examples that may just spark the ideal design idea in your quest for positional perfection.

Motion’s holy grail — precision

To discuss system precision in motion control systems, an understanding of the various sources of position errors is a must. These include three main elements: errors due to instability, errors in response to external disturbances, and errors due to mechanical transmission. In the following discussion these elements are considered and addressed by Jacob Tal, president, Galil Motion Control, Rocklin, Calif.

Instability errors

When a closed loop motion control system becomes unstable, it produces a cyclic motion, known as a limit cycle, superimposed on top of the motion. In extreme cases, the back-and-forth motion can overheat the motor and even damage it. Fortunately, this type of error is easy to detect and may be eliminated with something as simple as a digital filter. Furthermore, most controllers have the ability to do automatic tuning that selects the filter parameters such that the resulting system remains responsive and stable over time.

The degree of stability can be determined by a test called the step response. Here, the system is required to move back and forth, and the actual response is noted. An overshoot indicates less damping and a less stable system.

The following figures, Figures 1 & 2, show how an overshoot is corrected by increasing the D (damping) term in a PID filter.

Disturbance errors

To a motion control system, all outside effects are considered disturbances. These include mechanical friction, noise on input signals, and torque ripples in the motor. All of these inputs create disturbances that take the motor away from its nominal path, creating position errors.

To handle disturbances, the designer basically has two choices. One is to make the control system “stiff” and responsive. This means the control loop must have high gain that makes it reject the disturbances. However, as loop gain increases so too does the risk of the system going unstable, forcing the designer to walk the fine line between high gain on one hand, and system stability on the other. This is the area where automatic tuning software programs can be very useful.

A system’s ability to reject disturbances is related to its bandwidth — the range of frequencies it can follow. If the system can follow command signals at high frequencies, it can also reject disturbances at those frequencies.

Another way to deal with disturbance error is to reduce the disturbances themselves. This approach, however, is quite costly because it requires high quality elements. For example, friction disturbances can be eliminated by using air bearings, which are normally very expensive. Likewise, lead screw errors can be reduced with precision parts, and motor torque ripple by purchasing “smooth” motors. Given the cost associated with this approach, it is often used as a last resort.

Transmission errors

The most common transmission errors are caused by lead screws and include backlash and lead screw non-linearity. However, transmission errors exist in all mechanical systems. The common situation is that even if the motor is driven correctly to its proper position, the load may be off due to mechanical errors.

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