Best of both worlds – combining the advantages of decentralized Cam Switch I/O architecture with application centralized engineering

Requirements for industrial control systems widely differ depending on their respective task definitions. Accordingly different control concepts and architectures emerged for the diverse industries and applications. Due to this trend, vendors of control systems nowadays offer their customers differentiated product ranges. Starting from the elementary distinction between centralized and decentralized control architectures, the product ranges are diversified up to a very detailed level of granularity. Apart from purely technical characteristics of the control equipment (e.g. maximum reaction time), customers increasingly consider the efficiency of the control system’s programming, configuration and commissioning. An ideal control solution hence fulfils all the necessary technical requirements and additionally enables the user to perform an efficient and intuitive engineering according to the respective industry standards.

An important step in this regard is the usage of prefabricated and self-contained modules during the creation of control programs and configurations. This is evident in the increasing employment of function blocks in PLC programming languages according to IEC 61131-3. Another step to decrease engineering time is the application of vendor-independent standardized software modules for the implementation of frequently occurring tasks in control programs. Especially in the areas of manufacturing automation and motion control function blocks according to the PLCopen standard have proven to be useful. PLCOpen defines function blocks for single- and multi-axis movements as well as a state model for transitions between different motion types and error handling. The basic functionalities are extended by several function blocks for e.g. torque control and cam switches.

Due to the consistent structure and the flexibility of the PLCopen standard, users can create control programs in a very fast and concise manner. The widespread adaption of PLCOpen by vendors of industrial control systems allows users to use control systems of several vendors without prohibitive effort for programmer and commissioner training. Accordingly, the user’s dependency on single vendors decreases.

A frequently used application type in the area of manufacturing engineering is the electronic cam switch. Electronic cam switches consist of an arbitrary 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 are advantageous to mechanical solutions due to the possibility of very simple changes and extension to the cam configuration as well as significantly reduced maintenance effort. Common application examples of electronic cam switches can be found in all areas of machine building industries where actions have to be triggered high dynamically and synchronous to rotating mechanical components (e.g. a rotary disc). Such applications can e.g. be found in the packaging, printing, 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 (CI 511 und CI 512) for their PLC platform AC500. These novel bus modules are based on fast real-time capable Ethernet protocols and hence allow realizing extremely fast decentralized cam switches. EtherCAT was chosen as 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.

Applications that put their focus on the support of complex and flexible fieldbus topologies and do not depend on the bus couplers’ optimization for highly dynamic switching operations can gainfully employ equivalent bus couplers that support PROFINET IO communication (CI 501 and CI 502). These bus couplers additionally implement parallel standard Ethernet communication.

However, as already mentioned above, decentralized electronic cam switches rely on very high requirements concerning the bus-couplers reaction time. Accordingly, the EtherCAT bus modules CI 511 and CI 512 are the preferred options for implementing decentralized electronic cam switches. The topology of a typical decentralized electronic cam switch is depicted in figure 1.

Figure 1 : Topology of a decentralized electronic cam switch network

The EtherCAT bus master is implemented as a communication module of the AC500. The first node on the fieldbus is a rotary encoder with an EtherCAT interface that transmits the absolute angle value of a rotating machinery component. This node is succeeded by up to 254 bus modules. The bus modules comprise either only digital input or output terminals or alternatively also analog inputs and output terminals that can be configured for all prevailing analog signal types. Each bus module can employ up to 16 cam tracks and up to 32 cams.

Spatial constraints of traditional electronic (or even mechanical) cam switches are resolved by the decentralized system topology which allows distances of up to 100m between the individual bus modules. Hence spatially decentralized applications can be realized in straightforward manner. An additional advantage of the decentralized solution lies in the cost-effective setup that contains only one AC500 and multiple inexpensive bus modules. Nevertheless 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 ±200μs. This enables the cam switch’s usage in highly accurate and dynamic applications (e.g. the production of PET bottles).

Despite the possibility of a large number of bus modules, the parameterization 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. In the IEC 61131 conforming programming environment PLCOpen function blocks of type MC_CamSwitch are created and parameterized (c.f. figure 2). Hereby, 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 0.01°) as well as dead-time compensation.

Figure 2 : Cam switch engineering

The grouping of multiple cams to form a cam track is conducted in a graphical editor that is also used for the bus modules’ parameterization.
Accordingly, the decentralized cam switch network is configured in a central place and hence combines the high accuracy and dynamic of a decentralized system with a centralized system’s efficient engineering.

Figure 3:a highly dynamic rotating laser control system as exemplary application.