The present invention relates to I/O modules for industrial control systems and in particular to a low heat dissipation I/O module that may flexibly accommodate the digital control of AC power with zero-crossing detection.
Industrial controllers are specialized computer systems used for the control of industrial processes or machinery, for example, in a factory environment. Industrial controllers differ from conventional computers in a number of ways. Physically, they are constructed to be substantially more robust against shock and damage and to better resist external contaminants and extreme environmental conditions. The processors and operating systems are optimized for real-time control and execute languages allowing ready customization of programs to comport with a variety of different controller applications. Typically, the controllers have a highly modular architecture that allows different numbers and types of input and output modules to be used to connect the controllers to the process or machinery to be controlled. This modularity is facilitated through the use of special “control networks” suitable for highly reliable and available real-time communication. Such control networks (for example, Ethernet IP) differ from standard communication networks (e.g. Ethernet) by guaranteeing maximum communication delays, for example, as obtained by pre-scheduling the bandwidth of the network, and/or providing redundant communication capabilities to high-availability.
As part of their enhanced modularity, industrial controllers may employ I/O modules dedicated to a particular type electrical signal and function, for example, detecting input AC or DC signals or controlling output AC or DC signals. Each of these I/O modules may have a connector system allowing them to be installed in different combinations in a housing or unit along with other selected I/O modules to match the demands of the particular application. Multiple units may be located at convenient control points near the controlled process or machine to communicate with a central industrial controller via a special control network such as Ethernet IP.
Normally different I/O modules are required for the control of AC and DC signals reflecting differences in the control devices used for the signals (e.g. triacs versus transistors). Different I/O DC modules may be required depending on whether the DC signal is “sourced” from the DC output module or “sinked” into the DC output module.
The circuitry of the I/O module communicating with the industrial controller (controller-side) is typically optically isolated from the circuitry communicating with the controlled process or machine (field-side) in order to prevent faults in the field-side from damaging the industrial control system or other I/O modules. For output circuits (such as AC output or DC output circuits) it can be necessary to provide a source of low voltage DC for operation of the field-side circuitry independent of the power available on the controller-side. Often this is done through the use of a voltage dropping resistance converting the high voltages controlled by the I/O module into a lower voltage suitable for powering the field-side circuitry. Such voltage dropping resistors are often physically large to handle the necessary power dissipation and must be spaced apart to aid in dissipating the incident heat.
The control of AC voltages is often provided by a triac or thyristor that may be switched at a zero-crossing of the AC signal (to reduce electrical switching noise and power dissipation) by a conventional trigger circuit relying on a phase shifted version of the AC signal being controlled.