Advantages of using distributed logic systems with nano PLCs

July 11, 2022
Using one controller may seem simple, less expensive and an overall better choice, but there are many cases where using a distributed control system has clear advantages, especially when used with nano PLCs.

Many controls engineers get into a habit over the years of using the same solution over and over again, even with advancements in technology and new product releases. One area in particular that seems to become routine is using a centralized control system that consists of a single programmable logic controller (PLC) unless the application requires the use of more components. Using one controller may seem simple, less expensive and an overall better choice, but there are many cases where using a distributed control system (DCS) has clear advantages, especially when used with nano PLCs.

By shifting to using a DCS or using a hybrid of distributed and centralized control systems, one can gain many benefits, the first being faster reaction times. Using multiple PLCs allows an engineer to move the controllers closer to the inputs and outputs. This is important for large machinery that uses a single controller because it cuts down on the distance of communication cables. In serial or bus type communication protocols such as Modbus RTU or CANOpen, the longer the cables, the baud rate, or the speed of the data, is reduced. In turn, that slows the reaction time of the system. Therefore, by using multiple nano PLCs that can control hundreds of inputs and outputs as well as execute control logic, reaction times of systems can be faster.

Another way multiple PLCs can improve reaction time is by freeing up valuable central processing unit utilization. Large applications with hundreds or even thousands of inputs and outputs, as well as complex math algorithms being used, such as PID loops or pulse width modulation, require a lot of computing power. Splitting all of these processing tasks between several controllers ensures that large programs can fully execute within the required reaction time. Using several nano PLCs to split up a task may be a good decision instead of using a more complex and expensive micro or small category controller that might be close to full CPU utilization.

One unique application where fast reaction time is critical is in motion control. In motion control applications, like those that use servo drives, the system needs to respond in a matter of milliseconds. By using a DCS, the PLC sends course motion commands to a fine-tuned motion controller that can react and make adjustments in single milliseconds. The controller that sends the course commands still needs to do so in a reasonable time frame of tens of milliseconds. The simplest way to accomplish this is with a nano PLC that can act as a Modbus TCP another industrial Ethernet protocol.

Multiple controllers can also be an advantage by helping reduce installation costs. Many times when engineers, especially ones that are a little green, design a control system, they often overlook installation costs. Not only does the purchaser of the system need to pay an electrician to pull the wires and cables, they need to pay for the material itself. Right now costs of materials are high so installation costs are a big concern for the end user. Projects that consume a large area will need to have long runs for communication cables if there is only one PLC. By purchasing several controllers, these communication cable runs can be greatly reduced which can save tons of money and installation time.

An additional reason to consider using a DCS is to reduce component costs by purchasing several nano PLCs versus a single and more expensive PLC. The number of features provided is increasing with time in all categories of PLCs, and this includes the nano PLC. Nano controllers are now able to do everything from acting as a Modbus TCP master and subordinate, connect to Internet of Things platforms and perform complex mathematical functions like PID loops. In many cases,  purchasing several of these low-cost components and having them communicate with one another to pass necessary data can save the purchaser a lot of money compared to a more expensive category PLC.

Moreover, modern PLCs are commonly connected to many other devices such as drives, sensors or other controllers via an industrial Ethernet protocol like Modbus TCP, Ethernet IP or ProfiNet. Every controller has a maximum number of connections that it can handle. A nano controller might only be able to handle four connections; however a controller in the small category of controllers might be able to handle thirty connections. If a nano controller costs $100 and the small controller costs$1,000, that makes the price per connection $25 versus $33 dollars a connection. Therefore, the cost advantage for connecting a device is in the favor of the nano controller.

A third benefit of using a DCS is that the modularity of the system allows the system to be easily expanded or modified. In a centralized control system, if changes need to be made to the system or any maintenance is needed, the single PLC is stopped which in turn takes down the whole machine or process. By having several controllers that run their own individual processes, it allows parts of the system to stay up and running. Also, any program mistakes that come along with modifications such as a new process in a centralized control system can have unforeseen effects on the entire application. By using separate controllers, bugs can be isolated to the new process, and even better yet the bugs can be worked out prior to installation.

In summary, using a DCS has many obvious advantages. A DCS can improve reaction times, reduce installation costs and provide productivity advantages for any maintenance or additions to machines. Moreover, using nano PLCs within the DCS can help save money on components and installation costs. The above reasoning makes it hard for engineers, new and experienced, to ignore when building new control systems.

Travis Quinn graduated with a degree in Electrical Engineering from the University of Wisconsin. Afterwards, he spent time as a controls engineer in the material handling field before starting his position at Eaton as an application engineer. He has been in his current position for three years where his focus is on automation and sensors.

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