TAILIEUCHUNG - Troubleshooting Techniques

The time is gone when machines, controls, and drives were ‘‘add-ons,’’ as shown in Figure 1. Early machines used add-on drives. These drives had one thing in common: There was no interaction among the control, drive, and machine. To a large extent these machines were open loop. Today’s numerically controlled machines are tightly integrated machine systems. There is a great deal of interaction between the machine, drive, and control. It is this interaction that results in problems with stability, surface finish on the work part, and accuracy. The actual block diagram for today’s machine systems looks more like the block. | 5 Troubleshooting Techniques The time is gone when machines controls and drives were add-ons as shown in Figure 1. Early machines used add-on drives. These drives had one thing in common There was no interaction among the control drive and machine. To a large extent these machines were open loop. Today s numerically controlled machines are tightly integrated machine systems. There is a great deal of interaction between the machine drive and control. It is this interaction that results in problems with stability surface finish on the work part and accuracy. The actual block diagram for today s machine systems looks more like the block diagram shown in Figure 2. TECHNIQUES BY DRIVE The primary difference between the machine drives of yesteryear and those of today lies in the number and type of feedback signals that occur with modern control systems. It is true that early drives had load forces fed back to the drive and these forces could cause a droop in the feed rate with load called load regulation . Nonlinearities stiction lost motion etc. would not affect the stability because they were outside the servo loop but they could cause inaccuracies. By and large drives were hooked onto a machine and in spite of resonances in the mechanical drive stick-slip lost motion etc. Copyright 2003 by Marcel Dekker Inc. All Rights Reserved Fig. 1 Machine control system block diagram. the machines performed with less than acceptable accuracy by today s standards. With the ever-increasing demand for more accurate machines more and more of the machine was included in the control drive feedback loops. Today the machine control and drive are an integrated machining system. The existence of resonances between the drive motor and machine slide can make the drive unstable. A stick-slip condition on the machine slide can cause a null hunt discussed in the next section in the drive. Poor surface finish can result where load forces are fed back to a drive that has poor servo stiffness.

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