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****************** Recommended Reading: Manufacturing Engineering and Technology Marks' Standard Handbook for Mechanical Engineers ******************
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The motors that drive the aforementioned ballscrews are generally either stepper motors or servomotors. When the pitch of the ballscrew is known it can be deduced that a number of revolutions, or parts of a revolution, translate to a known amount of linear motion of the table.
5.4.a) STEPPER MOTORS Stepper motors are motors capable of rotating in equal, and accurate, steps of angular motion every time they receive an electrical pulse. It is quite normal for a stepper motor to require 200 pulses to complete one revolution, thus enabling accurate linear positioning when coupled to a ballscrew and receiving an electrical pulse train from the machine controller. This simple form of control is known as open loop control and any errors in calibration or missed steps will result in compound errors forming during operation. Also any wear in the screw cannot be compensated for.
5.4.b) SERVO MOTORS Servomotors are more commonly used than stepper motors these days; they provide more power and smoothness than stepper motors but require feedback of their position by a position sensor. This feedback is known as closed loop control. Fig 5.4.1.
Click the image for full size picture Fig 5.4.1. shows a rotary encoder type position sensor keeping track of the ballscrew rotation and feeding back the signal to the machine controller so that any errors may be compensated for. Closed loop control provides better repeatability, but the use of a rotary encoder on the ballscrew does not directly feedback actual table position. To do this a linear encoder on the table itself is the most accurate form of control; the output of this encoder feeds back to the controller in a manner similar to the rotary encoder.
5.4.c) OPTICAL GRATINGS The most accurate type of linear encoder is the optical grating.
Click the image for full size picture Fig 5.4.2. shows an optical grating. The lines of the gratings are usually etched into glass (sometimes referred to as glass scales); one grating is fixed to the machine base, the other to the table. The etched lines are slightly inclined to each other, and produce an effect known as moiré fringes, which appear to move across the grating (this effect can be observed when a fence of closely spaced bars is observed through a similar fence, close to it – such as a footbridge over a motorway). The movement of these fringes is an exact mathematical relationship, dependent on the spacing of the lines and the angle of inclination. Using a photo-electric device to count the fringes, and their direction of travel, results in an extremely accurate method of measuring linear displacement. Resolution can be as good as 2-3 mm - optical gratings are frequently used on Co-ordinate Measuring Machines (C.M.M.s).
Suggested Reading: Manufacturing Engineering and Technology Kalpakjian, Schmid and Schmidt. Marks' Standard Handbook for Mechanical Engineers
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