Accuracy and surface quality are the goals of superior production processes. This is why machining companies invest a great deal of time and expense in tools, machine tools, controls with special functions and options, measuring technology, and, of course, employee skills. Unfortunately, axis motors still receive very little attention in this arrangement, despite being a decisive factor in production quality. Article by Heidenhain.
Electric motors are used in a wide variety of applications and must meet a broad spectrum of requirements. When it comes to axis motors in machine tools, for example, torque ripple and the inertia ratio between the motor and the load are crucial factors, along with the maximum torque. These criteria have a direct effect on the quality of the workpiece to be manufactured. HEIDENHAIN axis motors for machine tools offer a balanced inertia ratio and very low torque ripple for exceptional machining results and dynamic motion control.
A system’s rigidity is a crucial factor in its resilience to vibrations or milling forces—the greater the rigidity, the better. Rigidity, in turn, is heavily influenced by the inertia ratio between the motor and the moving mass of the feed axis (load). This means that larger motors provide greater system rigidity against milling forces or vibrations. Figure 1 shows, as a function of the inertia ratio between the motor and the load, how high a sudden load reversal must be in order to temporarily cause a given position error in a drive.
By way of illustration, a lightweight trailer attached to a large, high-torque vehicle will introduce fewer disturbances into the entire system when exposed to wind gusts or road damage than will a heavy trailer pulled by a lightweight vehicle of the same torque. This is true even though the lighter trailer is obviously much more susceptible to these influences than the heavier one. For a machine tool, this means that the largest possible motor should be moving the lightest possible table in order to minimise the effect of disturbances on the entire system (such as milling forces or vibrations arising at the table).
Unfortunately, if there is a significant difference between the inertias of the motor and the load, it is also necessary to lower the loop gains. Lowering the loop gains leads to lower rigidity, meaning that the entire system reacts more strongly to load disturbances, e.g., from milling forces or vibrations.
For a machine tool, a balanced inertia ratio between the motor and the load should therefore be selected. A balanced ratio provides the amount of rigidity needed to make the entire system insusceptible to external influences exerted on the load, meaning that these influences have no effect on the machining result, and, at the same time, making it possible to work with high loop gains. In other words, the trailer and the vehicle pulling it should be correctly matched to each other.
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