Basics of Motion Control

Synchronizing Motion Control Systems

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 µsec 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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Motion Control Systems: Introduction to Motion Control Systems

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