DIGITAL CONTROLS MORE ACCURATE, REPEATABLE
THAN ANALOG NC

Improved numerical controls and servo drives are needed to meet the demands of today's faster, more productive NC machine tools.

By allowing for quicker response times, between the machine tool and the NC (numerical control), digital drives have many benefits over established analog drives. Quicker response time results in a more accurate cutter path relative to the programmed path. This is particularly evident in complex contouring and small incremental moves. The bottom line is more accurate, repeatable production at the NC machine tool.

Until recently, the majority of machine controls consisted of the CNC (computer numerical control), positional feedback devices (resolvers, or linear and rotary encoders), servo drives, and motors to control and position the machine tool's axes. The NC would be programmed by the operator at the machine or off-line at a programming station. This program would contain the axis moves required to position the machine tool while it cut the workpiece.

The servo drives feed the output signals from the control to the axis position motors. Machine movement would cause the resolvers or encoders to send a continuous signal back to the NC for position verification. Communicating position information (feedback) from the system to the NC is called the positional loop.

Speed control is derived from an interaction between the servo drive and the motor tachometer, referred to as the velocity loop. Although the velocity loop is related to action of the servo drive and the motor, the command signal is generated by the CNC and adjusted according to the positioned loop feedback via an analog sign.

Digital Control: Wave of the Future
With the development and increased application of digital drive systems, major changes have occurred that affect CNC and the machine tool. First, in digital drive systems the output from the CNC is strictly digital. When compared to analog, digital outputs are inherently more precise. This means that the signal coming from the CNC is much more accurate, especially when making speed control adjustments.

In addition, with CNC manufacturers now able to incorporate the digital speed controller into the CNC, both the position loop and the velocity loop are now controlled by the CNC. This allows both loops to be clocked by the CNC, resulting in more precise positioning of the machine tool. Positioning the machine itself is also improved significantly, which is particularly advantageous with the trend toward closer-tolerance cuts, more complex contouring and higher speeds.

Finally, digital speed-controlled CNC, integrated servo controllers and motor controllers, and the resulting improvement in product output and work efficiency, have created a number of other benefits for the machine operator:

	A larger operating range of the CNC with higher cutting resolution
	The ability to work with more complicated control algorithms
	Increased repeatability for high-production workpiece runs
	No CNC offset
	Integrated digital NC machine tool commissioning aids
	Shorter clock times from the CNC output, resulting in better reaction times from the CNC to the motors (improved positioning and velocity control)
	Smoother work surface/milling cuts on the workpiece.

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