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Ethernet is winning its way into ever-higher percentages of control systems for some very good reasons. It is popular and it can be found almost everywhere, so it has a commensurately low cost. Ethernet's
popular physical interface combined with standard protocols such as ICMP and TCP/IP ensure a high degree of compatibility.
With Ethernet, platform dependence is a thing of the past as PCs and other compute platforms can now easily replace expensive PLCs. Ethernet interfaces will continue to be available on virtually all platforms for many years to come, thus providing real protection from platform obsolescence. Ethernet is winning based on technical merits as well; its built-in electrical isolation eliminates the ground loops that plague most other field buses, and Ethernet data rates greatly surpass those of older field buses. Ethernet is the field bus of choice for modern designs.
All Things Considered
A great majority of control systems employ solid state relays (SSR) to control devices such as motors, solenoids and lights.
It should come as no surprise, then, that the ability to control these relays via Ethernet is highly desirable. Not so obvious, however, is the fact that there is much more to this than just turning relays on and off. System designers must consider a number of critical issues when relays will be controlled over an Ethernet:
Versatility
Ideally, a relay rack will accommodate industry standard relays such as the 0AC24.
This will ensure the high availability of low cost relays from multiple sources. The rack should provide a socket for each SSR so that different relay "flavors" (e.g., AC/DC, input/output types) can be installed on the rack as needed. Although higher densities can be achieved with multi-channel SSRs, there is a price to be paid for this in the form of reduced versatility and smaller load currents.
Flexibility
Many relay racks support only a single control supply voltage, typically 5 Volts DC.
Machine control applications, on the other hand, often employ devices which have differing supply voltage requirements (e.g., 5VDC, 12VDC, 24VDC). This disparity forces the system designer to compromise by either using sub-optimal devices or by providing a separate relay rack for each supply voltage (expensive!). A truly flexible relay rack will support multiple relay control voltages.
Diagnostics
Ethernet-based relay racks are complex, intelligent devices that include a CPU and communication interface. This complexity must not be allowed to burden field service personnel. Field service technicians
must be able to tell at a glance whether a rack is working.
If a rack is functioning normally it should give an appropriate visual indication. Similarly, a rack should indicate the presence of a malfunction and, ideally, it will indicate the nature of the problem as well. A visual indicator should be provided for each relay so that technicians can easily determine relay states.
Fail-safe Operation
Relays often control devices that must be turned off in a fail-safe manner for safety reasons (i.e., to prevent injuries to people or equipment). When such a requirement arises, the relay must turn off, even if
the relay rack has malfunctioned. In order to accomplish this, the relay rack must work in harmony with interlock and/or emergency stop contacts. These contacts, which supply the activation current to
specific combinations of relays, will deactivate the relays when opened. Unless the relay rack has built-in support for interlock contacts, it will be both difficult and expensive to implement fail-safe operation.
Embeddability
"Dumb" relay racks must be located near their controllers because of cable length limitations.
Smart relay racks, on the other hand, are not subject to this limitation. Smart relay racks can be located quite far away from their controllers and thus can often be found deeply embedded in their host systems. Deeply embedded electronic devices must always recover to a known, safe state when a fault occurs or upon loss of communication with the controller (due to unexpected controller shutdown or communication failure). Smart relay racks should always include a "watchdog" timer to protect against fault conditions, and a communication timer to detect communication loss.
Serviceability
System reconfiguration should not be necessary when a relay rack is replaced. In addition, look for other features that will simplify and speed up installation and serviceability. For example, all field
wiring connections to the rack should be implemented with quick-change connectors that require no tools. Also, consider how the relay rack is mounted to its host system. DIN rail mountable relay racks
can be installed or removed from an equipment cabinet in seconds; a feature that is appreciated by both system assemblers and field service personnel.
The Sensoray Solution
Sensoray’s model 2652/2653 I/O modules (8/16 SSR channels, respectively) include all of the features listed above. These modules can be used with other 2600 Family I/O modules to implement a robust, Ethernet-based measurement and control system that supports solid state relays, thermocouple and voltage measurement, analog output, general-purpose digital I/O, frequency and pulse-width measurement, pwm generators and incremental encoders.
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