Machine Design


Master-slave systems are common in web presses where one or more axes must follow the speed and acceleration of a master axis. This synchronization is needed to keep the web from stretching, tearing, or bunching as it winds through the machinery.

The conventional way of synchronizing axes in these machines is with mechanical drive shafts. A master axis turns all other axes on the machine through connections such as gearboxes and universal joints. A problem with some applications is that the drivetrain may twist appreciably, introducing position error in rolls from the master axis. Such torque buildup causes axes located farther away to increasingly lag the master axis. A result is that jerks and oscillations can take place as the web comes up to speed, so that remote roller stands do not come up to speed at the same time as the master. Web material produced during such intervals may be unusable.

For these and other reasons, electronic control systems are increasingly used to provide perfect synchronization between the master motor and remote rollers. Feed-forward control is a well-known means of providing such behavior. Here, a preprogrammed feed-forward signal is applied to the master motor during acceleration intervals. This signal is added to the motor command generated by the position feedback. The feed-forward signal reduces the amount of position error needed to generate a given output voltage to the motor.

In control systems that use this method, one axis typically serves as a master. An encoder monitors the position of this axis. The signal from this encoder is used as a master position signal. The signal is sent both to the processor controlling the master axis and to the processors controlling other slaved axes in the system.

Each slave axis processor algebraically sums the master position signal with the velocity commands that it issues to the servodrive on its axis. It is in this manner that each slave axis allows for position error in the master and, thus, maintains synchronization. Because each slave axis is controlled by its own processor, the speed of each slave axis can be programmed to maintain some relationship to the speed of the master. For example, the operator can set the speed of a slave axis to be some fraction or multiple of the master axis speed. The slave can also maintain some degree of phase offset to the master. In some applications, a slave axis might trace out some cam profile while synchronized to the speed of the master axis.

To obtain high accuracy, feed-forward systems update velocity commands anywhere from once every 500 ∝sec to once every millisecond, depending on the machine being controlled. High-speed conversions, for example, generally demand fast update speeds. Most controls for web processes employ absolute position encoders or resolvers to provide accurate position readings immediately at start-up. Tachometers are generally used for velocity feedback because they produce higher output at low axis speeds than filter networks.

These type controls can also be used with nearly any kind of motor and drive, including ac and dc brushless and dc brush motors and drives. The standard ±10-V outputs of such controllers are compatible with most motor drives including linear, PWM, and SCR. Though torque and horsepower requirements for differing applications frequently vary from less than 1 to 100 hp, feed-forward control systems are configured in the same manner for either extreme.

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