Page 758 - Mechatronics with Experiments

744 MECHATRONICS
t cycle necessary for L . If we calculate the L 3,max , it will determine the number of spacing
3
conveyors necessary following the queue conveyor. The result is same as
( )
A
N spacing = Int (10.24)
A
max
( )
L 3
= Int (10.25)
L 3max
If we choose to use only one spacing conveyor and meet the acceleration/deceleration
limits, then the t cycle requirement is calculated for the desired L spacing, which sets the
3
maximum line speed achievable with one spacing conveyor while meeting the accelera-
tion/deceleration limits.
In summary, the queue conveyor design is affected by the following parameters:
{A , L , t , N }.
max 3 transfer spacing
If three of these parameters are specified, the fourth one can be calculated. The maximum
allowed acceleration and deceleration rate and the desired spacing is given. Then, if we
choose to use N spacing conveyor (i.e., N spacing = 1 or 2), that determines the minimum
t transfer (hence t cycle ) time period in order not to exceed the acceleration limit. That sets the
maximum through-put rate for a conveyor with specifications: 1∕t cycle
{A max , L , N spacing }
3
Let us consider the case that the input conveyor part feedrate may fluctuate. If it is
desired that the conveyor system should be able to adjust to that condition, we need to
add additional sensors. Assume we know L , and the desired spacing L . First, we need to
p 3
measure the moving average part input rate, N . From that, we can calculate the nominal
1
speed of the queue conveyor and the output conveyor. In addition, we can add three ON/OFF
sensors on the queue conveyor to indicate the following conditions: queue high, queue low,
queue empty. If the queue is low, the conveyor speed V can be reduced by a percentage, if
2
the queue is high it can be increased by a percentage. If the queue is empty, it should stop and
go into a homing routine to fill the queue before continuing the normal cycle. As we modify
the motion of the queue conveyor, the output conveyor will track it with the appropriate
ratio since the output conveyor speed is gear ratioed to the queue conveyor as its master. A
possible homing sequence before the automatic cycle begins is shown in Figure 10.16.
Algorithm: Constant Gear Ratio Spacing Conveyors with a
Queue Conveyor

I/O Required:
Sensors (Inputs):
One presence sensor (Photo electric eye or proximity
sensor) and control logic to count the part rate input
Three presence sensors to detect queue conveyor part
level: high, low, empty
Actuators (Outputs):
Queue conveyor speed control,

Output conveyor speed control by electronically gearing
(slaved) to the speed of the queue conveyor

Control Algorithm Logic: