Wednesday, January 08, 2014: Purpose of this Document: The purpose of this document is to first get a basic understanding of conveyor based material handling systems and terminology and second to understand how several Rockwell Automation control products can be used to solve these applications.
Application Description: Figure 1 shows a tray conveyor system with 3 zones. Each zone typically consists of a powered roller connected by a belt to other non-powered rollers to form a motorized segment. The powered roller is normally driven by an electronic amplifier interface that is turned on/off via a single 2V dc output. Each zone is usually the same length and determined by the size of the product being conveyed.
Each zone also includes a sensor to determine if a tray is present in the zone. These sensors are typically an optical blockage detector, but may be of other types such as a mechanical switch. The sensor provides a signal when it is blocked by the presence of a tray. So in summary, the control of a single conveyor zone boils down to a discrete input from a sensor and a discrete output to the powered roller.
Control Challenges
Traditionally, a material handling application would have had a single logic controller with a large program and I/O wired across large distances to that controller. The Rockwell Automation solution distributes the zone control into the I/O devices themselves. This type of implementation reduces wiring costs and, if a logic controller is used, a smaller user program can be used, which allows for lower program execution times.
CONTROL SYSTEM DESCRIPTION:
consisting of:
- 1799-ZCIO Multi-Zone Controller
- 22ZC Single Zone Controller
- 44N Smart Sensors
Please indicate where in the control architecture this solution has been applied:
- From the HMI workstation to a higher-level computer system
- At the HMI level
- Between the HMI workstation and the controller(s)
- From controller to controller (peer to peer or peer to lesser controller)
- Controller code specific to the application or required for the application
- From controller(s) to field devices
- At the field device
- Other aspects of the application
Normal Transfer Operation
The transfer operation refers to the movement of items from the tail of the conveyor to the head. In Figure 1, trays move from Zone X, to Zone Y, to Zone Z. When there are no trays on the conveyor, the zone powered roller motors are stopped. In Figure 1, as a tray leaves the upstream Zone X, a signal informs the Zone Y to start its motor to accept the tray. As the tray enters Zone Y, it moves along and eventually triggers the zone’s sensor. This signals the downstream Zone Z to start its motor to accept the tray. After the tray has completely cleared the zone, the motor is turned off to conserve power, lower mechanical wear, and reduce noise.
Accumulate Operation
During the course of operation, it is sometimes desirable to have a particular zone stop and accumulate trays. The accumulate signal is triggered by a downstream zone and instructs the zone to turn off its motor as soon as a tray blocks its sensor. This zone then sets an accumulate signal for the upstream zone. As the trays come down the conveyor, they accumulate with one tray per zone. If an accumulate signal is removed on the most downstream zone, it will transfer its tray downstream and remove its accumulate signal for the upstream zones to cause the trays to move forward.
Singulate Operation
Singulate is an extension of Accumulate in that it automatically lets each tray be sent downstream from a zone with one zone length, plus one box length of operation. This is accomplished by the singulate zone holding the upstream accumulate signal until the tray has cleared its sensor.
Divert Operation
As the name implies, during the normal operation of a conveyor, it may become necessary to divert trays from a specific conveyor path onto another path. This diverting could be triggered by numerous means, but by far the most prevalent is due to the output of a bar code reader. The tray code has a bar code label attached to it that determines what path needs to be taken by that tray. Once read, a decision is made on how to divert the tray to its intended destination. This could be a specific storage location, a certain truck in shipping, or numerous other places the tray may need to go.
Merge Operation
Once the diverting of a tray has been done, it may become necessary to merge that tray into another stream of trays. An orderly merge is necessary so that no disruption of the stream occurs and so that the tray that is being merged does not touch other trays. At the merge junction, if a tray is already entering from the stream, then the merging box must wait. During this wait, the zone immediately upstream from the merge junction must accumulate trays until the merging tray enters the stream and reaches the next downstream zone. Conversely, if a merging box is already entering the merge junction and a stream box approaches, this box must wait at the upstream zone until the merge is complete.
Zone Interlocking Summary
On a typical conveyor system, where accumulation is done, we now understand that along with the sensor input and powered roller output, additional I/O is needed. This I/O would be a minimum of two additional outputs and 2 additional inputs per zone, for straight accumulation. One output would alert the downstream zone that a tray is coming and the other output would go to the upstream zone to tell it to accumulate trays. One input would come from the downstream zone to tell the zone to accumulate trays and the other input would come from the upstream zone to tell it that a tray is coming. Of course, doing diverting and merging functions would require even more inputs and outputs going to/from other adjacent zones.
Conventional Central Control Solution
Conveyor systems have traditionally used a central controller to handle zone control. This meant that the logic that would be handling a zone would be duplicated many times inside the controller user program. It also meant that it was easy to interlock the individual zones since the controller kept the status of all zones in its internal data table. However there has been a trend to distribute the zones into smaller groups of granularity. Once the zones of a conveyor system are split into different controllers, there needs to be a data connection between the controllers. The last zone in one controller needs an input and an output to the controller handling the next zone. Also, the first zone in the controller needs an input and output to the controller handling the upstream zone.
Multi-Zone Control (1799)
Zone Interlocking Parameters (ZIP) is a method of having I/O cards share data directly without hard wiring them together and without the use of a master DeviceNet scanner. One ZIP Capable I/O (1799-ZCIO) card can consume data from up to four other ZCIO cards over a DeviceNet network. A consumer of ZIP data can also produce ZIP data onto the DeviceNet network to other ZCIO cards. On the 1799 Embedded I/O card, each zone controlled by the card requires an input to connect to an external sensor and an output to connect to the actuator or powered roller amplifier.
Each ZCIO card produces data composed of its local inputs, local outputs and network outputs. Data produced on the network by a ZCIO card behaves as change of state (COS) data. That is, whenever the data changes inside a ZCIO card, it will be produced on the network. If the data does not change within a configurable time period, then the data will still be sent. This is called the heartbeat time. A production inhibit timer has also been provided so that ZIP data changing very often in a ZCIO card will not flood the network with traffic.
Devices that run the ZIP also support DevicLogix, which give the devices the ability to have their logic programmed dynamically by the user. ZIP boards with DevicLogix give the most versatile solution for conveyors because the conveyor logic is easily modified for individual system differences.
Single Zone Control (22ZC)
The 22ZC Zone Control system consists of a series of smart modules—one module per conveyor zone—connected via a flat cable, the 22ZC zone control architecture offers considerable wiring reduction when compared to the standard PLC model. It provides advanced methods of transporting, accumulating and releasing product as well as a variety of other features not available in ‘smart’ sensor architectures. A common platform for both pneumatic and powered roller systems, the 22ZC supports the two release modes (singulation and slug) and actuator types (pneumatic and powered roller), as well as more advanced functions, including a power conservation function, jam detection capabilities and selectable inverted output (normally open/normally closed).
The 22ZC modules accept either a mechanical or photoelectric sensor input; each module then provides an output to the actuator based on the desired zone control logic for the application. The on-board logic is easily configured without the need for programming tools and the configuration data can be broadcast to all upstream zone modules by pressing a single pushbutton. This plug-and-play operation plus system modularity and programmability make the 22ZC system an ideal solution for accumulation control.
Single Zone Control (44N)
The Allen-Bradley 44N smart sensor is a stand-alone solution that combines a photoelectric sensor and zone control logic, which eliminates the mechanical switches, the PLC and the interconnecting wiring of traditional approaches. Packaged in the Series 9000 enclosure, the 44N zone control sensor is tailored for use with pneumatic actuators or powered roller and supports basic methods of transporting, accumulating, and releasing product. Outfitted with three leads that connect to upstream and downstream 44Ns and the actuator, these sensors are able to communicate with each other; each 44N stops and starts its respective zone based on the status of the other zones as reported by the surrounding smart sensors. Interconnected 44Ns can control up to 25 zones and the on-board variable time delay and selectable release modes (singulation or slug release) allow system functionality to be adjusted simply and easily for optimal conveyor throughput. Plus, the quick connects on the 44N’s three leads provide the plug-and-play capability crucial to supporting the OEM’s desire for increasingly modular conveyor systems; the control portion of the connection of conveyor sticks on the plant floor is limited to three thread-on connections per zone as opposed to hard wiring hundreds of sensor and actuator leads back to a central controller.
Important User Information Solid state equipment has operational characteristics differing from those of electromechanical equipment. Safety Guidelines for the Application, Installation and Maintenance of Solid State Controls (Publication SGI-1.1 available from your local Rockwell Automation sales office or online at http://www.ab.com/manuals/gi) describes some important differences between solid state equipment and hard-wired electromechanical devices. Because of this difference, and also because of the wide variety of uses for solid state equipment, all persons responsible for applying this equipment must satisfy themselves that each intended application of this equipment is acceptable. In no event will Rockwell Automation, Inc. be responsible or liable for indirect or consequential damages resulting from the use or application of this equipment. The examples and diagrams in this document are included solely for illustrative purposes. Because of the many variables and requirements associated with any particular installation, Rockwell Automation, Inc. cannot assume responsibility or liability for actual use based on the examples and diagrams. No patent liability is assumed by Rockwell Automation, Inc. with respect to use of information, circuits, equipment, or software described in this document.
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Publication RA-AP005A-EN-P – August 2003
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