Machines using signal chains are everywhere around us. One could even argue that automation is entirely dependent on closed-loop signal chains; it’s what allows machines to cook our food, heat our air, and automate our vehicle transmissions or traction controls. Industrial factory and plant automation is equally reliant on signal chains, but on a more complex variant called the programmable logic controller (PLC). Anything and everything needing a control loop to monitor, manager, handle, regulate, limit, or organize something uses a signal chain.
How simple can a signal chain be?
Signal chains can greatly vary in complexity. For instance, household ovens sense temperature with a thermostat to achieve a desired temperature. If the temperature is too low, then the switch which completes the circuit is closed. This in turn activates the main burner. Similarly, once the thermostat detects the temperature is too high, the switch opens, closing the gas valve to the burner.
Modern vehicles use a more intricate signal chain called the electronic control units (ECU), and high-end vehicles can have up to 80 them. They too sense one or more physical parameters, apply logic, and produce a resulting action. Automotive safety and convenience systems such as seat belts, door locks, electric windows, and powersteering also depend on signal chains.
Signal chain vs PLC
Signal chains rely on hard wiring relays, timers, limit and position switches to automate every step in an operation. The system works reliably but is inflexible; every small change requires rewiring all the relays. As a result, it’s unsuitable for the intricate, changing operations of a factory floor. The introduction of a single new element in the assembly line would entail the rewiring of thousands upon thousands of relays as well as troubleshooting the whole thing. To put it bluntly, the system wasted valuable production time.
In 1968, the introduction of the PLC rectified this issue by creating the world’s first solid-state controller as an electronic replacement for hard-wired relay system. The PLC allowed machines to be reprogrammed using software or logic change, granting them the ability to quickly adapt to change. That saw mill capable of cutting a single type of widget suddenly become a multipurpose device. The implications of the PLC are so vast, that many items commonplace today would be otherwise unaffordable ― any manufacturing change would be extremely cumbersome or expensive.
PLC in an automated bakery
All automated operation subject to steady control and configuration can leverage a PLC, even a bakery. Consider the vast amount of subsystems collectively involved in producing Twinkies and other ungodly horrors: weight scales, valves, flow gauges, mixers, yeast-rising warming chambers, ovens, conveyer belts, fans, and packaging equipment. Their actions are time-critical and heavily based on the succession of the prior event. Incorporating a PLC into each of these subsystems applies the process-control needed for the entire assembly line to function.
Via Maxim Integrated
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