(1)Bias error and basic countermeasures

・Positioning precision consists of two errors, namely "bias error", and "variation error".

・The "bias error" can be corrected toward the target value using positional measurement and control.Generally, the error can be corrected by measuring the result of positioning precision at the initial positioning, and adjusting positioning to correct the error for the target position, etc.

・Suppose, for example, the required precision is +/-0.1 mm for positioning, it is preferable to select a positioning measurement sensor whose resolution (minimum read scale) is more precise than the required precision by one digit (measurement precision of 0.01 mm or higher).

(2)Bias error and countermeasure for variation

・Median values of positioning for "bias errors" vary due to various factors.It is also an important requirement to minimize these variations of median values for positioning.

・Typical factors causing variations are errors caused by temperature changes (temperature drift), such as positioning errors due to temperature drift in measurement equipment, or thermal deformation in the mechanical components for automation devices.

・Thus, in order to minimize variations in "bias error" due to temperature drift, it is necessary to (a) stabilize temperature in measurement equipment, and to (b) have countermeasures to avoid effects from thermal deformations in the mechanical components for automation devices.
a)Stabilizing temperature in measurement equipment ⇒ Setting warm-up time
b)Avoiding effects of thermal deformation in automation devices ⇒
(1)Keep heat source away from positioning mechanism
(2)Countermeasure for blocking thermal conduction to positioning section
(3)Using materials with low thermal expansion rate
(4)Stabilizing ambient room temperature

・In the precision machine tools, techniques for minimizing effects on machining precision from thermal deformation are introduced to avoid skewed thermal deformations by adopting a symmetrical mechanical structure.Fig.1 is an example of a symmetrical structured drive mechanism for injection molding equipment.