CANopen in X-ray machines III
- CANopen manager software architecture on an X-ray control system
CAN Newsletter December 2008
Already in the early days of CAN, Philips Medical Systems noticed the advantages of CAN and decided to use this network protocol as communication network for interconnecting various components such as collimators, generators, and patient tables in their X-ray systems. To achieve a modular and open approach, a group within Philips Medical Systems, managed by Tom Suters, developed the first higher layer protocol for CAN, the CAN Message Specification (CMS), which was presented to the public in 1992. CMS provided the framework for the first official CAN based communication protocol specified by CAN in Automation, the CAN Application Layer (CAL). One of the reasons that may have lead to the limited market penetration was that CAL did not provide the functionality necessary for a straightforward description of generic device or application profiles.
As a consequence the CANopen application layer and communication profile was defined and published as official CAN in Automation standard in 1996. CANopen did not completely reinvent the wheel. Instead, it reused many elements and protocols of CAL and added missing but necessary features such as the object dictionary, which represents a simple way of describing and referencing application data. In fact, CANopen is now perceived as the enabling technology for the use of CAN in many different application areas.
Advantages of using CANopen in medical applications
Today, the necessity for using data communication systems in medical applications becomes more and more evident. Reasons for this are increasing requirements regarding:
- Interoperability, which is primarily required for the exchange of data between medical devices but also allows for the implementation of modular architectures of medical devices. At the same time interoperability enables central control of various medical devices, which is an important issue when looking for improved and more gentle respectively tolerable examination procedures.
- Improvement of autonomous operation of medical devices allowing for unsupervised examination procedures and improvement of patient safety.
- Cost reduction due to higher level of modularity in medical devices and faster execution of examination procedures.
Deploying Ethernet TCP/IP (transmission control protocol/internet protocol) or one of the emerging industrial respectively real-time Ethernet protocols as data communication system for control purposes like transmission of control data, commands and parameters is not adequate. Compared to CAN, the implementation of Ethernet requires more powerful and therefore more expensive CPUs (central processing unit) and thus more expensive hardware (this applies in particular to high-quality PHYs, transformers and connectors). A secure transmission of data without any loss of data is only possible with suitable protocols implementing acknowledgement mechanisms or with cyclic transmission of data. Also, for free or arbitrary topologies, wiring is typically less expensive for CAN installations compared to Ethernet networks where additional topology components such as switches or hubs may need to be deployed.
Integrating Ethernet technologies within medical systems is nonetheless suitable for the transmission of bulk data such as patient information data or diagnosis results, which may not be time-critical and is typically based on a physical peer-to-peer connection between sender and consumer.
Medical devices impose very specific requirements to communication systems in particular with respect to the control of the devices. These requirements are: Very high reliability and robustness for data transmission, short latency times for the transmission of important and high priority data, short error recovery times, data transmission as broadcast (producer/consumer principle) but also client/server data transmission, low device and implementation costs and simple manageability.
Analyzing the requirements of X-ray systems shows that CANopen is in fact perfectly well suited both as internal communication network used to interconnect all modules and functions inside the X-ray system as well as to connect external add-on devices to the X-ray system. Due to the nature of CAN, CANopen offers very high error reliability, short latency and error recovery times, a robust data transmission, various possibilities for the modularization of systems and networks, plug-and-play support and standardized system services. It allows building flexible network topologies and can be implemented at low costs. Furthermore, the technology of CAN and CANopen is already accepted by both the TÜV (Technical Monitoring Association) in Germany and the FDA (Food and Drug Administration) in the USA for the use in medical systems, as there are already a number of approved applications using this technology.
Within CAN in Automation, a special interest group (SIG) is working on the specification of device and application profiles for medical devices and applications (SIG medical devices) with focus on X-ray systems. An additional task force is working on a dedicated specification for the use of injectors as X-ray add-on devices.
