Choosing a machine control architecture
30 November 2008
The best system may be like this one—it combines decentralised I/O with the advantages of centralised application engineering.
Topology of a decentralised electronic cam switch network, based on an EtherCAT network.
Depending on what they are needed for, machine control systems vary greatly from one application to the next. The most elementary consideration is whether the control system is centralised or decentralised. Some applications need the high speed of centralised control while others, especially extensive systems, may benefit from a decentralised approach. Next there are purely technical considerations, such as, how fast the reaction time needs to be, and how many I/O points are needed, and so forth.
Moving beyond these purely technical requirements, customers are increasingly asking about the efficiency of the control system’s programming system. How easy is it to configure and commission? Does it comply with industry standards, such as the IEC 6-1131 programming standard? These are not just vague desires; they are becoming important engineering considerations. An ideal machine control solution satisfies not only the technical specifications, but is also easy for the user to do his own engineering, and to do it efficiently and intuitively.
An important step in the direction of easier-to-engineer control systems is the usage of prefabricated, self-contained modules during the creation of control programs. To accomplish this, machine builders are using function blocks from IEC 61131-3 programming languages. Using these vendor-independent, standardised software modules for the implementation of frequently occurring tasks in control programs decreases engineering time.
This is especially true in the areas of manufacturing automation and motion control, where PLCOpen’s standard function blocks have proven to be useful. The organisation defines function blocks for single and multi-axis movements. It also defines state model function blocks for transitions between motion types and error handling. The basic functionality is extended by additional function blocks for torque control and cam switches.
The consistent structure and flexibility of the PLCOpen standard makes it easy for users to create control programs in a fast and concise manner. And the widespread adaption of this standard allows them to use control systems of several different vendors without a prohibitive amount of effort for training. Accordingly, the user’s dependency on single vendors decreases.
SOFTWARE CAM SWITCHES
A frequently used application in the area of manufacturing engineering is the electronic cam switch. Electronic cam switches consist of a number of electronic cams that activate or deactivate digital outputs according to a rotary encoder’s measurement value and their configured respective turn-on and turn-off angles. Cam tracks are created analogous to their mechanical counterparts by assigning several cams to a shared digital output.
Electronic cam switches have obvious advantages over mechanical solutions. They are easy to change, and their configuration can be extended with little effort. Maintenance is also significantly reduced.
Applications of electronic cam switches are common in all areas of machine building. They are used where certain highly dynamic actions have to be triggered synchronously with rotating mechanical components, such as rotary discs. These applications are found in the printing, packaging, manufacturing, and plastics processing industries.
In order to support their customers with these types of applications, ABB Stotz-Kontakt created a new generation of bus modules, the CI 511 and CI 512, for their PLC platform AC500. These novel bus modules are based on fast real-time capable Ethernet protocols and hence allow the realisation of extremely fast decentralised cam switches. EtherCAT was chosen as the bus protocol due to its high bandwidth, on-the-fly-processing, and slave-to-slave-communication. These capabilities offer the best prerequisites for highly dynamic cam switches.
ABB Stotz-Kontakt also has bus couplers that support PROFINET communication, the CI 501 and CI 502. These are for applications that put their focus on the support of complex and flexible fieldbus topologies and do not depend on the bus couplers’ optimisation for highly dynamic switching operations. They implement standard Ethernet communication.
Decentralised electronic cam switches rely on very high requirements concerning the bus-couplers’ reaction time, so the EtherCAT bus modules CI 511 and CI 512 are the preferred options for implementing these types of switches. A typical topology is shown in Figure 1.
The EtherCAT bus master is implemented as a communication module of the AC500 PLC. The first node on the fieldbus is a rotary encoder with an EtherCAT interface. It transmits the absolute angle value of the rotating machinery component. This node is succeeded by up to 254 bus modules. The bus modules comprise either digital-only or digital/analogue I/O terminals that can be configured for all analogue signal types. Each bus module can employ up to 16 cam tracks and up to 32 cams.
Traditional electronic (or even mechanical) cam switches have spatial constraints—they must be located relatively close to one another. These constraints are resolved by the decentralised system topology which allows distances of up to 100m between the individual bus modules. With software-based electronic cam switches, spatially decentralised applications can be realised in straightforward manner.
An additional advantage of the decentralised solution lies in the cost-effective setup that requires only one AC500 PLC and a number of EtherCAT bus modules, which are inexpensive.
The cam switch network achieves very low reaction times. For instance, a cam switch network consisting of 40 bus modules can still switch its cams with an accuracy of +/-200 microseconds. This enables the cam switch’s usage in highly accurate and dynamic applications, such as in the production of plastic PET bottles for the food and beverage industry.
Despite the possibility of a large number of bus modules, the parameterisation of the cams can be conducted centrally in a simple and concise manner. The engineering of the entire cam switch network occurs in a single place—the AC500 PLC’s programming environment.
The AC500 uses the IEC 61131 based CoDeSys® programming software. With this software, the PLCOpen function blocks ‘MC_CamSwitch’ are created and parameterised, as shown in the screen shot of Figure 2. The cams are assigned to specific bus modules and provided with all relevant switching information. This information comprises the cam’s switch-on and switch-off angles (in increments of 0.01 degrees) as well as dead-time compensation.
The grouping of multiple cams to form a cam track is conducted in a graphical editor that is also used for the bus modules’ parameterisation.
Accordingly, the decentralised cam switch network is configured in a central place and hence combines the high accuracy and dynamic of a decentralised system with the confines of a centralised system’s efficient engineering.
The decentralised electronic cam switch network was presented at ABB’s stand at Hannover Messe 2008, using a highly dynamic rotating laser control system as the application.
Diagrams and captions
Figure 1: Topology of a decentralised electronic cam switch network, based on an EtherCAT network.
Figure 2: Cam switch engineering on a central computer, the AC500 PLC programming environment.
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