Page 158 - Embedded Microprocessor Systems Real World Design

ITEMS IN HOPPER. AWAITING FEED
OMRALL SPACING CONTROLLED TO MEET
MINIMUM THROUGHPUT REQUIREMENT

GAP BETWEEN ITEMS HAD TO BE WIDE ENOUGH
FOR DOWNSTREAM SORTING MECHANISMS

I
7
ITEMS ON TRACK
FEED MECHANISM
DIRECTION OF MOTION -0

Figure 4.4
Conveyor Belt System.

important to know when the processes should not be independent. A real-world
example will illustrate this: I worked on a motion control subsystem, developed by
a contractor, that feeds items into a moving transport system (see Figure 4.4). The
system has two requirements that are relevant here.
The first requirement is to meet a particular throughput in items per minute.
The second requirement is to maintain a minimum spacing between any two suc-
cessive items. Two independent processes run these two functions. The first process
controls a terminal motor speed to maintain throughput, and the second process
performs instantaneous adjustments to the motor speed to maintain minimum
spacing between individual items. These corrections are small or large, depending
on the predicted spacing error. The corrections reduce throughput but are
expected to occur infrequently. After a correction, the motor ramped back up to
the terminal speed.
The prototypes of the system worked well, but production units could not meet
the throughput requirement. It turned out that the spacing process was perform-
ing a few corrections to fix spacing and this reduced throughput. The throughput
process, not knowing about corrections, saw that throughput was too low and raised
the terminal speed. This required even more corrections, which lowered through-
put even more. The result was that the throughput process walked the speed up to
the maximum value, while the spacing process corrected more and more items,
until it was finally correcting (or overcorrecting) nearly every item. The reason this
showed up only in production was that the mechanical adjustments in the proto-
type allowed the system to “balance” and run without problems.
The solution for this problem was fairly simple: If too many corrections were
performed, terminal speed was lowered instead of raised. The amount of speed
reduction was weighted by the degree and number of corrections that had

Software Design 139