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Industrial Automation

Stacker Cranes


PLC Program

Operator Interface


The “Movement Controller” coordinates the control of the heavy-unit-load conveyor system and stacker cranes, and usually interfaces with the client’s computer system.

It is best to give the “Movement Controller”full control over storage algorithms, rather than having the client’s computer making real-time decisions and imparting these to the “Movement Controller”.

This way the “Movement Controller” makes decisions, regarding the storage locations to be used, at the last possible moment thereby best optimising the storage and retrieval of loads.

The “Movement Controller” also provides batch and quarantine control.

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PLC Program

Woodgrove Digital Engineering P⁄L have developed a configurable PLC program for the control of stacker cranes, allowing the one program to control cranes with either single or double reach tines, with varying rack layouts (incorporating prohibited bays, and bays with varying heights), forward or backward facing cabins, varying tine position sensing methods, et cetera.

Some of the configuration is set explicitly, while much is drawn automatically from the rack layout. The configuration is saved and can be downloaded to a crane at any time.

This allows us to use the one program to control all stacker cranes not only in a single installation, but also throughout the country. Thus, as improvements are made, all sites can take advantage of them.

The configuration data is saved, for each crane, by the “Movement Controller” (typically after some tuning is performed, or before a fresh copy of the program is down-loaded). This configuration can be displayed to the operator or down-loaded to the PLC at a later time (typically after a fresh copy of the program is down-loaded). Refer to the Operator's Manual section Save and Restore Crane PLC Configuration

The configurable parameters also include functions which may need to change due to “weather” conditions, such as when there is excessive ice on the rails in cold storage warehouses.

Some of the features of this crane PLC program, compared to crane PLC programs from other suppliers are:

  • more elegant and effective screen counting and speed/positioning control,
  • quadrature tine position counting without the need for expensive PLC quadrature input modules - quadrature counting overcomes situations which can arise with single pulse counting causing the PLC to believe its tines are left of centre when they are, in fact, right of centre,
  • optional slowing of the hoist movement to complete in time with long travel to avoid damage to the mast which occurs when the hoist is stationary with the long travel running at full speed - parameters needed are learnt by the PLC and if the hoist arrives too early it will slowly rise and fall while waiting for the long travel to near completion,
  • finely tailorable speed control, even allowing for paired racking where the slots of each pair are closer to one another than to their neighbouring slots on the other side, due to the presence of the uprights (this can reduce cycle times by around three or four seconds), special provision is also made for hoist positions which are too close together to use the standard algorithms,
  • tine speed profiling, individually adjustable for left and right, with up to two special positions where alternate profiles can be used - these would generally be at the infeed and outfeed, which may or may not be at the same long travel and hoist positions, and a fourth profile for pallet return locations in the racking,
  • the program is also aware of locations which correspond to aisles which must be kept clear to aid access to the cranes or to provide exits in case of fire or ammonia leaks,
  • full automatic recovery of the crane when left between screens or with its forks extended or beyond end stops,
  • reporting of over sixty alarm conditions for maximum assistance to help correct faults,
  • providing extensive status information, indicating not only the position of the crane and its tines, and the states of its sensors, but also the speed and direction it is moving, along with an indication of any overshooting, providing continual indication of the effectiveness of the tuning and allowing you to further tune the crane motions from a comfortable position in front of the “Movement Controller”,
  • fully and consistently documented program, with data elements allocated in a logical fashion,
  • display of current hoist and long travel position, at the crane electrical cabinent and/or the crane cabin, to assist the crane operator when operating the crane manually for such tasks as stock-taking,
  • display of currently active alarms (in place of hoist and long travel position when the left and right tine buttons are both depressed), at the crane electrical cabinent and/or the crane cabin, to assist the crane technician when getting the crane back into service after correcting a fault,
  • display of current alarms, at the crane electrical cabinent and/or the crane cabin, to assist the crane operator when correcting a crane fault.

For more information on the reporting of status and alarms, please refer to the Operator Interface, or the Operator's Manual section Crane Status and Control.

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Operator Interface

The screen below (from the MS-DOS version of “Movement Controller”) displays the status of one of the stacker cranes.

Screen dump from DOS version of
Movement Controller, 
showing the status and alarms for a stacker crane, converted to a 
“gif” file, thereby losing some of the colour definition

Under the “status” column is currently shown the status of the long travel, hoist level, tines, and cabin. This status is always present, while other states, such as rack occupied photocell states, are added and removed below these as they come and go. Any crane alarms appear under the “alarms” column.

Below these columns is the graphical representation of the crane, showing that there is no alarm (the body of the crane is orange rather than red), showing that it is in service (its tines are red rather than yellow or grey), the tines are centred (otherwise they would be shown in their position relative to the crane), and none of its photocells are blocked.

The current position of the crane is shown at the bottom of the screen above which would be shown the half-cycle (pickup, or deposit, or, occasionally, travel), which it is currently executing, above which would be its next half cycle (this way there is no delay due to communications). To the left of these would be the overall command that it is executing, such as store or retrieve, and the stage at which it is with the command.

There are a number of commands the operator may issue to the crane, either to recover from difficulties, or to run tests on it. There is also the ability to control the crane manually.

Below is shown the manual control screen (also from the MS-DOS version):

Screen dump from DOS version of
Movement Controller, 
showing the manual control screen for a stacker crane, converted to a 
“gif” file, thereby losing some of the colour definition

The push buttons may be operated using the function keys, or moving the blue surround to a key and pressing the space bar.

Below the positioning buttons is shown the position status. It is currently not in position in long travel (as its light is off) but is in high position for the hoist.

Between the tines push buttons is shown the load off-centre status. The load is currently off centre to the left.

At the top of the screen is shown the status of the crane. Here the tines are yellow to indicate that it is in manual, and are shown to the left of the crane, fully extended to double deep, and raised to high position (indicated by the load being present).

The operator interface also provides for the:

  • initialising of the crane PLC configuration data files from information in its racking definition database,
  • saving of the crane PLC configuration data files to hard disc,
  • presenting the saved configurations for review purposes (the presentation of the saved data files is provided with explanatory text so that the operator can satisfy himself as to what exactly it is that he is about to restore to the crane), and
  • restoring the saved configurations to the crane PLC.

Please refer to the PLC Program section for details of the configurability of the PLC program.

For more information on the operator interface to the cranes, please refer to the Operator's Manual sections Crane Status and Control, and Save and Restore Crane PLC Configuration.

For information on configuring the racking and the stacker cranes please refer to Defining Racking and Stacker Cranes and the Operator's Manual section Define the Crane and Rack Arrangement.

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