Page 740 - Mechatronics with Experiments

726 MECHATRONICS

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θ
Condition to start Condition to stop
jog motion
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θ jog

.. ..
θ a θ d

1 1

Time
FIGURE 10.9: Motion executed on STOP command.

parameters used to define the jog motion are jog speed, acceleration and decceleration
rates (Figure 10.9).

STOP: the rate at which the motion must be decelerated to a stop (zero speed) is defined
by some PMC configuration parameters. When the STOP command (or equivalent) is
executed, the motion is decelerated to zero speed from its current speed. The STOP motion
may be initiated by an operator command or a sensor trigger or under program control. The
only parameter needed to define the STOP motion is the rate of decceleration (Figure 10.9).

INDEX: a common name used to describe a move which causes a predefined positional
change in the axes. The described move in position can be either in absolute or incremental
units. Depending on the automation requirement, the typical automated cycle modes consist
of various forms of INDEXes synchronized to other axes’ motion and machine I/O status
(Figure 10.10). Parameters used to describe the INDEX motion are the distance, accelera-
tion and decceleration rates, top speed or total index time period. In some motion control
applications an index motion may have multiple speeds. Such moves are called compound
indexes. Compound indexes can be considered a combination of various trapezoidal and
triangular motion profiles. It is also very common to define an index motion with only
distance and total time period for the motion. In this case, the time period is used in three
equal portions: one third for the acceleration period, one third for the constant speed run
period, and one third for the decceleration period.

Example Consider a DC motor connected to a ball-screw (Figure 3.3). Let the ball-
screw lead be 0.2in∕rev (or equivalently it has pitch of 5 rev∕in). Assume that there are two
limit switches, one at each end of the ball-screw, to indicate the mechanical limits of travel.
In addition, there is a presence sensor at the mid point of the ball-screw’s travel range to
indicate a home position. Define a home motion sequence, a jog motion, a stop motion, and
an incremental motion of 1.0 in distance movements.
A possible home motion may be defined as follows: the speed at which the home
sensor trigger will be sought is 0.5in∕s. The acceleration rate is defined by the time allowed
for the motor to reach the home speed, which is t acc = 0.5s = 500 ms. Then when the home
sensor tigger turns ON, the motion will stop at the deceleration rate defined by the stop
motion. Then the axis will wait for one second, move back 0.2in in 1.0 s, wait for 1.0 s and