Software that drives Hardware
The Software that drives the Hardware in a Distribution Center
By John T. Phelan, Jr., P.E.
Famous nineteenth century Irish author George Bernard Shaw once wrote “The single biggest problem with communication is the illusion that it has taken place”. Although the quote was probably not written in the context of data flow, information processing and machine controls in an automated warehouse operation, the lesson still applies in that environment. The reaction when someone steps into a distribution center for the first time and witnessing the harmonious orchestra of conveyor systems, order picking technology, print and apply units, in line scales, sortation and a mass of other material handling technology typically starts with a ‘Wow!’ and quickly goes to a ‘How?’. The answer to this simple, yet extremely open-ended and complex question is software. Software is what controls the information flow from the highest enterprise level system all the way down to the on/off “switch” for a shipping sorter divert mechanism and everywhere in between. And just as Mr. Shaw eluded, the absence of this effective communications within the orchestra comes problems, big problems. That is why it is important to understand the various information systems and their critical roles in ensuring a smooth warehouse operation.
If the lowest level of software in a warehouse is a controller that directs an on/off switch on any given device, then the highest level of software is the Enterprise Resource Planning (ERP) System. ERP Systems usually have modules that may include the finance and accounting system, customer relationship management (CRM), human resources, project management, warehouse management (WMS) and a host of others. In fact, the standard term for the ERP system including its WMS module is called the “host” system. Host systems, whether they are home-grown, AS400 driven, custom-coded legacy systems or top of the line packages with names like Manhattan and Associates (PkMS, WM), Red Prairie, SAP, or Oracle, or any software package in between, typically do not function as the primary control system for warehouses and distribution centers. Instead, the host system works best when it acts like the corner office boss and simply tells his minions that they have tasks to perform, and that he doesn’t care how the tasks get done, as long as they get done on time, accurately and report back when complete. Therefore, the communication flow in the warehouse usually starts and ends with the host system and the data that is sent to and from the next lower level are large batch files sent infrequently so as to not bog down the host system so it can do what it is meant to do instead of very frequent, short transactions.
Warehouse Control System (WCS)
For distribution centers that require multiple device input in order to make multiple decisions in near real time, such as carton or tote routing, pick directing, sortation, or automated weigh, print and applying shipping labels, a Warehouse Control System (WCS) is usually in place and is the next level lower than the host system. Using the previous comparison, if the host system is the corner office boss simply barking out “handle it!”, the WCS is the highly caffeinated ops boss on the receiving end of the bark, determining what needs to be done, how it needs to be done, when it is to be done, and who is going to do it.
Physically, the WCS is usually a licensed software package residing on a networked PC that is usually located at an operations supervisor’s workstation and/or near the main control panel for the material handling system. The WCS usually has a base component to it along with modules that are bolted on to the base component that would be specific to the functions that the material handling system is accomplishing. For example, TriFactor’s WCS, the C3 (Command, Control & Communicate), base system is called the Navigator and some of the modules include the Host Interface, MHE Interface, Order Manager, Route Director, Sort Director, QC Auditor, Inventory Manger and User Interface.
Not all systems require all of these components. Additionally, many technologies or other subsystems require the use of their own proprietary software for proper control. An example would be a specific manufacturer’s Automated Storage and Retrieval System (AS/RS) or a Pick to Light (PTL) solution within a pick module. The technology supplier would typically provide the appropriate communication protocol to the WCS so that it communicates and functions efficiently within the entire system operation. Therefore, the WCS PC serves as the primary connection to external interfaces, such as the upper level host system and the lower level material handling equipment controllers.
In addition to being the main communication broker between all systems and subsystems, the WCS also performs many of the data processing and decision making through the use of the stored procedures within the WCS database. For example, in a distribution center that has a split case picking to conveyor and automated sorting operation, The WCS directs the routing of cartons on the conveyor system based on information supplied by the host system, as well as provides user interface screens for monitoring, control, and diagnostics. The primary function of the WCS is to provide routing directives to the material handling equipment controller (PLC), based on information received from the host system. At designated locations throughout the system the conveyor PLC, utilizing fixed mount scanners on the conveyor, read each carton’s bar-coded ID. The PLC transmits the carton ID to the WCS for routing purposes and begins tracking the product along the conveyor path. The WCS interrogates the database to determine the appropriate routing for the carton, based on information received from the host, and sends the routing information on to the PLC. When the carton arrives at the divert location, the PLC checks the lane status and diverts the carton into the assigned lane, if possible. The PLC reports back to the WCS with a divert confirmation messages, indicating the status of the action taken. The WCS uses the divert confirmation messages from the PLC to update the location and disposition for the associated carton. Operations personnel can interrogate the status of any carton utilizing the WCS user interface screens.
These user interface screens are important tools because they allow operators to have the ability to view and analyze diagnostic and statistical data in order to make operational decisions and improvements within the distribution center. The WCS uses intuitive graphical style functions such as drop down menu lists, pop-up dialog boxes, check boxes, radio buttons, etc., allowing operators to utilize the system with little or no training.
Programmable Logic Controller (PLC)
The Programmable Logic Controller (PLC) is the hardware that houses the fastest, most real-time computing for discrete processes, most of which are simple on/off signals or messaging small strings of data. The PLC is typically located in the main control cabinet or the control cabinet of a sub-system and nowadays is Ethernet connected to the hardware input/output (I/O) devices such as motors, photo-eyes, proximity sensors, scanners, or any other electrical component. PLC code is a collection of relay logic developed in a graphical user interface software package on a PC that is called “ladder logic”. The term is derived from the lines and boxes that resemble the rungs and vertical rails of a ladder. Once the code is ready for use, the program is compiled and downloaded it into the PLC.
Since the PLC houses the software that either controls or takes the input from specific devices and also transmits the data taken from those devices, the PLC is considered the lowest level of software in the distribution center. As a result, once thoroughly tested, the PLC code rarely falters or incurs bugs that would impair the proper execution of the program. In fact, probably the most common issue surrounding the machine controls from the PLC is a device out of alignment, such as a photo-eye, so that it does not provide accurate input to the PLC for proper processing. That is why the first troubleshooting task when control devices such as motors, merges or diverts are not operating properly is not checking the PLC code, but verifying that all the inputs to the PLC are positioned and working properly.
Human Machine Interface (HMI)
One of the last licensed software tools that are common in a modern mechanized distribution center is a Human Machine Interface (HMI) that normally takes the form of a touchscreen either bolted into a cut out of the front door of the main control cabinet or on a floor mounted pedestal located near an operator station. The reason why this HMI is located either at the control panel or at a remote operator station is because it is used as a supervisory and manual or automatic control of the material handling equipment through a visual representation of the system, including alarms and start/stop pushbuttons. The HMI tool that TriFactor uses, which is an industry standard and therefore very common, is called Wonderware by Invensys. The program takes input from the PLC as well as provides input to the PLC which allows the touchscreen the ability to visually report the status of all devices connected to the PLC as well as control any devices that are also controlled by the PLC. As an example in a case conveyor system, our Wonderware terminal will display an overview of the system and even sub systems with status of each device running/stopped, blocked/unblocked and other general status.
In the event that an operator would like to start a device or the entire system of devices, instead of mashing a pushbutton on the front of the conveyor control panel, the operator would touch a soft key on the HMI touchscreen. Likewise, stopping all or part of the system is equally as simple. Additionally, since the software acquires and can store input from the PLC, which is basically the status of the system devices, troubleshooting and reporting becomes a little easier due to the visual display capabilities of the HMI. TriFactor typically provides two sets of displays; Maintenance and Alarm History. The Maintenance screens are the means to navigate to the various subsystems’ visual representations in order to view and control them. The Alarm History is just as the name implies, it is a visual representation of the faults and alarms that the system has incurred over the defined period of time. Although the HMI is not necessarily a required hardware and software package for an automated distribution center, it does serve an excellent purpose in visually reporting system status since the only other means would be to either walk the entire system to look for issues or connect a laptop to the PLC and decipher the ladder logic, which takes a unique skill set.
In summary, although the maze of machinery inside a distribution center seems daunting and complex, it can only work efficiently with the proper brains behind the brawn. Many times, there are multiple types of software that have to work together, communicate effectively and process information accurately so that the distribution center can fulfill the mission.
John T. Phelan, Jr., P.E. is Chief Operating Officer of TriFactor, LLC, a material handling systems integrator based in Lakeland, Fla. He can be contacted at 863-577-2243 or firstname.lastname@example.org. For more information visit www.trifactor.com.