Raspberry Pi vs CLICK PLC from AutomationDirect

[Music] is a Raspberry Pi suitable for industrial control applications and if so how does it compare to more traditional control hardware and methods such as commercially available programmable logic controllers known as PLC's we sought to answer those questions by creating a representative industrial project but to start what is a Raspberry Pi it's actually a family of small inexpensive single board computers with extensive capabilities and connectivity they're a mainstay of maker community projects of all sorts PLC's on the other hand are controllers with specific automation features built into industrial form factors the plc is generally more expensive than a Raspberry Pi but this is primarily because it is ready for industrial use right out of the box we designed a demonstration project using common industrial hardware it's a model-based control application where two independent measured variable inputs are evaluated by a controller which then commands an output the physical process is a small water heater with an electric heating element the monitored inputs are both the incoming water temperature and flow while the command that output turns the heating element on and off a calculation is performed in real time to decide how much heat must be added to achieve a specific temperature setpoint at the outlet this calculation is built on a mathematical model considering heat transfer and other factors rather than a simple feedback temperature loop giving it the ability to react proactively to changing input conditions rather than waiting for changing output conditions like a traditional temperature loop controller temperature is measured using a conventional RTD and the flow measurement comes from a differential pressure flow meter providing a 4 to 20 milliamp current loop both are common in any industrial environment the output commands the solid-state triac using zero voltage switching that can be turned on and offer each voltage cycle of the 60 Hertz power line resulting in adjustable heat for comparison the investigation tackles the same project with a Raspberry Pi 3 model B and an automation direct click PLC the Raspberry Pi has ample computing power for this relatively simple task so the challenge becomes interfacing with the external elements in creating appropriate code the Raspberry Pi has an input output connector but no native analog inputs accessory boards are available to provide analog inputs to the Raspberry Pi but using methods uncommon in industrial environments such as 0 to 3.3 volt inputs this required development of an intermediate signal processing circuit built from scratch to correct the inputs there are many programming options for the Raspberry Pi for this example we selected Python for the click PLC input output connections are a simple matter since it has modules designed specifically for these purposes also the native programming language offered all the needed commands the program in each system reads the flowmeter and RTD once each second program makes a calculation 60 times each second and based on conditions determines if it should turn on the heating element for the next cycle of AC power this means there are two loops running simultaneously the one second input reading loop and the sixty Hertz output controller in operation both controller configurations proved capable of performing the basic control functions they responded appropriately to changes in the flow and temperature inputs and commanded the heater output but there are differences noticed via the track indicator light the Raspberry Pi is able to maintain the desired temperature but the indicator light flickers irregularly because the Raspberry Pi 60 Hertz interrupt code uses variables created from the one-second loop but is not synchronized with it there is potential for the interrupt code to execute at a point where the variable is only partially updated causing an improper calculation the click PLC controls the water heater in a very predictable way most of the time the indicator light flickers on and off but sometimes stays off iran briefly these sustained instances are probably due to variations in flowing Inlet temperature all in all the click PLC is effective and stable since both platforms run the same algorithm the differences are likely caused by the raspberry pi being more susceptible to input signal noise and the previously mentioned timing issues both the noise filtering issue and the timing issue could be resolved with more extensive programming efforts which would be warranted for a production system so it was one better than the other the Raspberry Pi runs a variant of the Linux operating system which must perform multitasking operations to support the system io interfaces monitor keyboard mouse and more this adds uncertainty for executing faster near real-time industrial control tasks it is programmed using Python which is procedural but this application is interrupt driven creating an additional layer of programming complexity to avoid timing problems the Raspberry Pi also lacks built in housekeeping elements which are part and parcel of a PLC's operating system the click PLC was easier to program an easier to interface with the other elements the latter logic programming environment provides consistent behavior and avoids critical regions with asynchronous execution like most PLC's this one includes functionality like hardware and software watchdogs to warn users if something is not operating correctly and to bring the process to a safe State systems created from consumer electronics controllers may cost less than commercially available industrial products on a pure price and performance basis but they lack significant mission specific hardware and software benefits included in an industrial PLC do-it-yourself options can be cost effective but they require considerable engineering time and attention to integrate hardware and software