Browse Prior Art Database

Concurrent Fetching of Cartridges by Two Independent Library Pickers

IP.com Disclosure Number: IPCOM000112843D
Original Publication Date: 1994-Jun-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 4 page(s) / 137K

Publishing Venue

IBM

Related People

Baca, FA: AUTHOR [+9]

Abstract

Disclosed is a robotic library having two dual-gripper pickers which can act concurrently. Having two concurrent pickers reduces the average response time of the library to mount a cartridge, eqn. (1).

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 47% of the total text.

Concurrent Fetching of Cartridges by Two Independent Library Pickers

      Disclosed is a robotic library having two dual-gripper pickers
which can act concurrently.  Having two concurrent pickers reduces
the average response time of the library to mount a cartridge, eqn.
(1).

                              m

  Rt(m pickers) = St / ( 1 - U  )    (1)

  Where:  Rt   = average picker response time in seconds,
             St   = picker service (cycle) time in seconds,
             m    = number of pickers (1 or 2) acting concurrently in
                 one library box,
             U    = picker utilization, 0 < U < 1.

      In eqn.  (1), the picker utilization is the ratio of the
library workload and the maximum workload capable by the m pickers.
This utilization is shown in eqn.  (2).

  U = W/Wmax   (2)

  Where:  W     = customer workload in fetches per hour
             Wmax  = m*(3600/St) = maximum fetches per hour

      In eqn.  (2), we define the maximum workload of the m pickers
in a library box, Wmax, to be the number of pickers m times 3600/St.
The 3600 term comes from there being 3600 seconds per hour.  Thus,
two pickers each with picker cycle time of 10 seconds gives a Wmax of
720 fetches/hour, 2*3600/10.  If a two picker library (m=2) library
is processing a workload W of 300 fetches/hour, the average
utilization of each picker is 5/12, 300/(2*360).  The average
response time is equal to 10 seconds to service the request once a
picker is available plus 2.1 seconds spent waiting for one of the two
pickers to become available, or 12.1 seconds.

      If there was only one picker in the library (m=1) with the same
service time St of 10 seconds, the utilization of that picker
servicing the same workload W is twice as high at 5/6, (300/360).
Using eqn.  (1) with m=1, we now have an average response time of 60
seconds.

      This is of course much higher than the average response time of
12.1 seconds derived from eqn.  (1) for a two picker library.  It is
essential to note that for a given picker service time St in eqn.
(1), that the average response time Rt for a two picker library will
always be less than a single picker library.  The reasons for this
are as follows.  First of all, the utilization term U is higher for
the single picker library (m=1) than the two picker library (m=2)
because two pickers can be used concurrently instead of only one.
Secondly, the U term in eqn.  (1) is not squared for a single picker
library, as it is when two pickers are present in the same box.

      Thus, for identical picker service (cycle) times, a library
with two independent pickers always gives faster performance over a
single picker.  Also, the single-picker library saturates at half the
workload of the two-picker library.  It is for these reasons that we
described the following two dua...