|A Pinch of SERCOS, a Dash of Ethernet
A new control system led a packaging machine OEM to use three network protocols to achieve performance objectives.
*Reprinted Courtesy of Control Design/Putman
Manufacturers rely on packaging machine builders to provide a simple, unambiguous function. They need machines that reliably move products into containers without damaging the products or the containers.
To fill those packaging needs, many of these manufacturers turn to Triangle Package Machinery, a Chicago-based industrial OEM established in 1923. Triangle designs and builds packaging machinery and equipment to help food, industrial, medical, electronics, and other industries meet their demanding packaging requirements.
The Triangle/FlexCell cartoner (Figure 1) was designed to meet customer requirements for a bag-in-box cartoner with a small footprint that could run 90-100 cartons/min. with minimal changeover time. Before the introduction of the FlexCell, large continuous motion cartoners operating at speeds up to 250 cartons/min. were the only solution.
|Figure 1: Fills Containers, Fills a Niche
Before the introduction of the more compact, 90-100 carton/min. FlexCell, customers had to choose between large footprint, continuous-motion cartoners designed for speeds up to 250 cartons/min. or intermittent-motion units limited to 50-60 cartons/min.
However, meeting throughput requirements with these cartoners often was costly and consumed too much floor space. To deal with the floor space requirements, intermittent-motion cartoners topping out at 50-60 cartons/min. were employed.
Triangle created its Triangle/FlexCell cartoner to fill the gap between continuous and intermittent-motion cartoners. The sidebar details the performance improvements.
The FlexCell features a completely different way of advancing cartons and loading bags into the cartons. Instead of using gearboxes and shafts to transport the carton through the machine--a traditional lineshaft approach--the machine uses integrated motion control with digital servo drives and motors (Figure 2).
Control and Communications
Because the machine design featured many new components, it was necessary for us to choose a completely new control system. We were able to start with a blank slate and select the control system that best met our requirements for machine control, motion control, and communications.
As is the case with many industrial OEMs, our clients generally prefer one brand of control system components over another. Several of our major customers specify Rockwell Automation for machine controls. If Rockwell has a solution that fits our needs, we generally try to use their products to ease customer acceptance.
|Figure 2: Use of Vertical Space
Triangle reduced the machine footprint by using nested servo motors to make part of the carton handling and indexing function operate vertically.
We were familiar with many of the older Rockwell controllers, but none of them met our design criteria. We reviewed Rockwell's ControlLogix controller and found it had most of the features we needed, including a single control platform for the integration of logic and motion control.
The cartoner also makes extensive use of advanced networking services for integration of control system components including an HMI, the controller, variable-frequency drives (VFDs), servos, and sensors.
The ControlLogix platform also has third-party options, which give it flexibility that we plan to use for future designs. A multiaxis stepper card is available from AMCI (www.amci.com) and a Pentium PC-compatible coprocessor card from Online Development (www.oldi.com) with full backplane communications are just two that we are currently testing.
One of the most important features of the ControlLogix controller is its ability to simultaneously support multiple communication protocols. We needed digital networking for a variety of tasks including communication with client computing systems, between the controller and the HMI, between the controller and the servo drives, between the controller and the VFDs, and between the controller and the sensors (Figure 3).
Our evaluation revealed that different network tasks were best performed by entirely different types of digital communications protocols (Table I). We looked at the communication requirements, and ended up employing different networks to meet our machine control and communication requirements.
Servicing the Servos
Perhaps the most critical network in the FlexCell is the digital link among the controllers and the servo drives. We use six servo drives in our base model machine, and there are two common options that require servo drives, so our machine can employ up to eight servo drives.
The servo network had to be high-speed, noise-immune, and optimized for servo control and communications. SERCOS met all our communications requirements, and the protocol is fully supported by the ControlLogix
|Figure 3: It's All About the Architecture
Triangle needed digital networking for communication with client computing systems, between the controller and the HMI, between the controller and the servo drives, between the controller and the VFDs, and between the controller and the sensors.
We use an eight-axis motion control card with SERCOS communications to and among digital servo drives. The main processor has access to all the servo information because the motion controller is part of the PLC system.
SERCOS was designed for servo networks, so it has many built-in features that simplify servo networking including drive synchronization and high-speed status updates.
The basic SERCOS ring allows the drives to be synchronized more precisely in position control. For example, a drive can latch its position at a precise point in time based on a registration mark input. All drives in a SERCOS network use the same master clock, so any drive in the system can be interrogated to find its position at that point when the registration mark input was active.
In addition to control and coordination of the servos, the SERCOS network provides drive information such as following error drive current, fault type, drive heat sink temperature, motor temperature, instantaneous drive current, bus voltage, and other parameters. This information allows for better diagnostics and quicker problem resolution.
We use high-speed fiberoptic communications because of its noise immunity and connection simplicity. We have multiple axes that need to be cammed together, and SERCOS provided the best solution.
We also needed to distribute the drives in multiple enclosures, so the noise immunity is a plus for communication among multiple points. Controller-to-drive wiring is minimal--only an enable line and three-phase power is required.
Everyone Loves Ethernet
Another critical network task is communication between the FlexCell and our clients' higher-level computer systems. We anticipate that our clients will want to view machine operating parameters such as production data, uptime, and faults. In addition, we wanted to give customers the flexibility to view and adjust virtually all machine control parameters at their discretion.
Ethernet was the no-brainer choice for this network due to its low cost, high performance, and ubiquity within our clients' higher-level networks.
Although the decision to use Ethernet was simple, the implementation details were complicated. We decided to equip our machines with a four-port Ethernet switch. The switch and its integral modem are connected to the controller and to the HMI. Termination points are available for connection to two other Ethernet clients, most commonly a programming terminal and a customer's Ethernet network.
The controller, HMI, and programming terminal use the EtherNet/IP protocol, so communication among these devices is simple. Communication with other devices and computers that employ Ethernet but do not use the EtherNet/IP protocol is more complicated, but still much easier to implement that any non-Ethernet protocol.
Device-Level Networks Have Their Role
Ethernet is the most widely accepted protocol in both commercial and industrial networking. SERCOS is a very high-performance network optimized for servo drives. But neither of these networks was appropriate for communications with the FlexCell VFDs and sensors.
SERCOS was simply more than we needed for VFD control and communications. We found that DeviceNet was sufficient for VFD communications and very cost-effective.
DeviceNet is used to control between eight and 14 VFDs depending on the FlexCell application. We use DeviceNet for speed control as well as for on/off and direction control. DeviceNet gives us the flexibility to easily add drives, and the network is simple to connect and implement.
Because DeviceNet was originally a Rockwell Automation protocol it is seamlessly integrated with EtherNet/IP, the ControlLogix platform, and many other components. As an example of this seamless integration, EtherNet/IP and DeviceNet tag names are all common so an EtherNet/IP device can read the status of a DeviceNet device.
DeviceNet is now administered by an independent group not affiliated with Rockwell Automation, and this independence is fostering the adoption of this protocol by other vendors. This gives us the flexibility to use VFDs from other vendors, if needed, and it also allows us to use DeviceNet as our network for connection to sensors manufactured by a variety of vendors.
The FlexCell uses about 15 photoeyes to confirm whether or not bags are in the proper position and for other functions. Incorrectly positioned bags are tracked with motion control and rejected.
We could have used simple discrete inputs to connect these photoeyes, but we selected DeviceNet because of its diagnostic capabilities. A photoeye on DeviceNet allows the marginal excess gain to be read to determine if it is covered in dust. This can trigger a warning to clean the eye before it ceases operation, and can be done for all the eyes without additional inputs or wiring.
DeviceNet also simplifies wiring because sensors and other devices can be daisy-chained, so each point doesn't require a separate home-run connection.
This control and communication system relies on industry-wide standards and is truly open. This gives us the flexibility to use best-of-breed components from Rockwell Automation and from third-party vendors to accomplish our machine control and communication tasks.
Steve Bergholt is the chief engineer at Triangle Package Machinery. He can be reached at
|Table 1: The Need for Multiple Networks
||Why it is needed
||Widely used and accepted by our clients
Best price/performance ratio of any network
Easy integration with higher-level systems
||Optimized for servo control communications
Very high speed
Most servo drives support SERCOS
Many VFDs, sensors, other devices support it
Minimal wiring required
Lose the Lineshaft, Improve Performance
As a result of a new design incorporating servos, Triangle was able to deliver a cartoner with superior efficiency and versatility, including the following customer benefits:
|| Floor space: Decreased machine size from 240 sq. ft. to 50 sq. ft.
|| Product changeover: Reduced product changeover from 2-4 hours to 10 minutes
|| Maintenance costs: Reduced the number of moving parts and wear items by 70%
|| Delivery cycle: Shortened time-to-market from six months to three months
Triangle's first goal was to minimize the size of the machine, and the FlexCell now has the smallest footprint available for its speed range. Triangle achieved this by making part of the carton handling and indexing portion of the machine vertical rather than horizontal. With the FlexCell, flat cartons are elevated, set up, and indexed downward. Indexing on the short dimension of the carton increased the cycle rate and time for loading and decreased the machine length.
Delivering maximum machine flexibility was also a goal of the new design. End users often have to make frequent package size changes but must still deliver customer orders "just in time." Many conventional solutions require extensive changeover times and costly part changes, so the FlexCell was designed to enable quick and inexpensive product changeovers.
Traditional cartoners are mechanically driven machines equipped with a main drive shaft, multiple gearboxes and numerous timing chains. The FlexCell needed an alternate drive source that could not only fit in a small space but also accommodate fast product changeovers.
This is what led to using a virtual drive train and integrated control technology with ControlLogix. The result is a new multidrive motion solution that not only synchronizes but also simplifies the operation.