A machine table driven by closed loop positioning system consists of a servo motor, lead screw and an optical encoder. The lead screw has a pitch of 0.200in and is coupled to the motor shaft with a gear ratio of 4:1 (4 turns of motor for 1 turn of lead screw). The optical encoder generates 150 pulses/rev of the lead screw. Table has been programmed to move a distance of 3.0in at a feed rate of 16.0in/minute. Determine:
a) How many pulses are received by the control system to verify that table has moved exactly 3 inches;
b) Pulse rate;
c) Motor speed that corresponds to the specified feed rate.
(Note that pitch is the axial distance traveled for one revolution of the screw)
What will be an ideal response?
This problem is similar to the worked example given in Chapter 4. Section (c) of problem 4.1 dealing with motor speed is the new addition. Lead screw pitch = 0.2 in/rev; Motor r.p.m = 4 * lead screw r.p.m. Lead screw generates 150 pulses/rev; Distance to be moved, S = 3in; Feed rate = 16in/min.
(a) Time required to travel 3 inches (t) = 0.188 min. If the lead screw pitch is 0.2 inches and the distance traveled is 3 inches, it will cause 15 revolutions of the screw. Each revolution of the screw generates 150 pulses.
(b)
(c) Lead screw r.p.m =12000pulses/min divided by 150 pulses per revolution
= 80 rev/min
Motor r.p.m = 4 times lead screw rpm;
Motor r.p.m = 320
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