An Engineer’s Guide to PACs and PLCs

PLCs are probably the oldest types of what we would recognize today as a logic controller, but the history of automation goes back much further than that. Primitive types of automatic control like the centrifugal governor even predate the steam engine. And the invention of the steam engine necessitated mechanical automation controls like temperature regulators, pressure regulators, and speed controllers. One example of primitive automation is the flyball governor.

The flyball governor is a weighted valve attached to the spinning drive shaft of a steam engine. When the engine speeds up, the governor spins faster and centrifugal force forces the weights away from the body, which in turn forces a piston downward that closes a valve and slows the engine. While hardly recognizable as “automation” today, these are nonetheless critical advances towards mechanical systems regulating some or all of their own processes.

Ex 1. Flyball Governor. A Primitive Form of Automation Control

The advent of control logic as we understand it today comes with the electrification of mills and factories in the early 20th century. Logic control begins with simple relay logic. In its earliest form, electro-mechanical relays were placed in sequence to respond to specific electrical signals. Some of the earliest applications of relay logic are in railway signaling and at coal-fired power plants. These early relays were simple gates with only two states: on or off. Using these relays, though, engineers were able to develop simple functions to automatically change states based on input. Relays were laid out by designers in line diagrams or in ladder diagrams where each rung specifies a function.

This ladder diagram, for instance, shows a simple pump control mechanism using feedback from a float sensor. The operator flips a switch to turn the power on, which then starts the pump. The pump fills the tank until the float sensor reaches its pre-set limit, at which point the float sensor opens the circuit and stops the operation. The operator then knows that the tank is full and that they can’t start the operation again until the float sensor goes below its upper limit.

Ex 2. Ladder Diagram for a Pump Control

This is a very simple application and a simple diagram, and you’ll notice that it doesn’t require a computer or any “smart” technology to achieve an important logic control function. The switches and electro-mechanical relays in each line act as logic gates for the system.

By the middle of the 20th century, these line and ladder diagrams had been codified into mechanical “ladder logic,” a type of logic programming we still use today. In the 1960s, though, the game changed forever. The late 1950s saw the invention of solid state electronics and the transistor. Transistors, put simply, are a type of relay or logic gate that can change state without mechanical help. The type of relays discussed above have a mechanical component that physically moves the relay into an open or closed position. A transistor, by contrast, is made of a semi-conductive material (usually silicon) that allows it to change state based on the charge applied, without mechanical help.

Transistors paved the way for modern computing, and one of their very first applications was in factory automation. Transistors are much smaller than mechanical relays and less power consuming than vacuum tubes. It became feasible in the 60s to build function blocks out of many transistors, and then to use transistors to make simple computers called Programmable Logic Controllers or PLCs.

PLCs saw widespread adoption in multiple industries by the mid to late seventies. PLCs are still manufactured today and used around the world. The main advantage that PLCs offered, when first adopted, was their ability to be re-programmed. Relay systems, once laid out, was next to impossible to program for any purpose other than that originally intended. PLCs could easily be programmed to perform a number of functions, and those functions could be expanded upon with external I/O modules.

Ex 3. Control Panel with PLC at Center

A PLC works by monitoring the state of inputs, executing the code or logic and turning on outputs based on the combination of the inputs and the logic. This allows the PLC to replace relays, timers, and counters with a single device that can be reprogrammed to handle a new task instead of rewiring. The PLCs were also much better suited to industrial environments and had a smaller footprint compared to relay systems.

As computing technology progressed from the ‘70s until now, the power and function of the PLC gradually expanded. Processors got smaller and more powerful, computing components became more compact and durable, and new programming languages were introduced to heavy industry. Today, the modern incarnation of the PLC is the Programmable Automation Controller or PAC. Thanks to the march of progress, it is difficult to draw a hard distinction between a PLC and a PAC. One of the key differences is programming language: PLCs tend to use ladder logic and PACs can use full programming languages like C and C++.

A PAC is a device that splits the difference between a PLC and a PC and offers some of the benefits of both. PACs combine the “always on” reliability and industrial hardiness of PLCS with the processing power and functionality of a PC.

On the one hand, PLCs are considered to be best suited for simple or high-speed machine control solutions. Typical PLCs have limited memory and focus on discrete IO with on/off control. PACs, on the other hand, are a better fit for more complicated system architectures. PACs excel when applied to applications like asset management, more advanced process control, and HMI functions. PACs, crucially, are also much more flexible from a programming perspective, offering ladder logic programming in addition to modern programming languages.

PACs also make system expansions easier than with a conventional PLC. A PAC can make it easier to add and remove sensors, and rack mounted PACs like those in ICP DAS USA’s X-PAC family can provide multiple channels of slot I/O. Slot I/O modules make it easy to adapt the control and automation system to a changing environment.

Another major advantage that PACs have over PLCs is the ability to multitask. Incorporating more powerful and/or multiple processors give PACs the ability to perform many functions at once. If your application requires a substantial number of I/O points, or if the application is one that is much larger in scale and requires extensive loop control, then a PAC is the best answer.

In the application diagram shown below, the XP-8341 PAC uses slot I/O modules to expand its DAQ and control capabilities extensively. It’s using an I-87068W relay output module for emergency start/stop functions and lighting control, while receiving Insulation detector signals using an I-8053W digital input module. These slot I/O modules give the PAC direct access to sensors and switches, but it’s also maintaining communication with remote I/O modules through its RS-232 port and communicating with the CANbus battery monitoring systems through an I-8120W Can bus communication module.

The BMU in the bottom layer is responsible for collecting the relevant information about the battery packs, such as the battery power, the temperature, the SOC, the SOH, the charging current, and the charging voltage, as well as providing this information to the battery information concentrators through the CAN Bus interfaces.

After collecting the information from the battery packs, the battery information concentrators regularly upload the information to the XP-8341 battery management platform through the CAN Bus interfaces. Two I-8120Ws devices are used as the CAN Bus communication interface expansion system in each battery information concentrator, where one of the I-8120W modules is responsible for communication with the BMU, while the other is responsible for communication with the battery management platform, whereby, in addition to achieving isolation between the two CAN Bus interfaces, the information from the device is also separated so as to reduce the bus load from the respective CAN Bus domains.

Ex 4. This Renewable Energy Storage Application Shows off a PAC's Versatility

The battery information concentrator not only collects the information from the BMU, but also constantly detects the current and voltage signals fed from the insulation detectors in the battery cabinet through the I-7012F and I-87017W modules, as well as regularly reports the data back to the battery cabinet management platform. As the maximum voltage for the battery cabinet can reach 700 VDC during the charging of the energy storage station, timely detection of the insulation status is required in order to prevent damage to the equipment or even the occurrence of work safety accidents caused by insulation failure.

It should go without saying that the role of a PAC goes beyond what a PLC can do. Scope of purpose is another key difference between PLCs and PACs. PLCs are typically used as simple controller that monitor one set of inputs and make another set of outputs in response. A PAC is a multi-purpose industrial controller that incorporates data acquisition and data logging functions in addition to its control functions. In the example, the XP-8341 PAC also provides an interface with the Ethernet control system and connection to the remote control room. PLCs typically offer more limited communication when compared to PACs. PACs offer all kinds of communication, natively and through I/O expansions, including: RS-232, RS-422, RS-485, RJ-45, Wi-Fi, Zigbee, Fiber Optics, and more. They also offer more robust protocol handling, with support for all major industrial protocols.

With continuing advances in technology, the differences between PLCs and PACs are getting narrower. There is, though, a difference in utility and functionality. Due to their more modular design and plethora of programming choices, it is easier to attach and remove components from a PAC.  PACs are capable of monitoring and controlling thousands of IO points, whereas PLCs face harder limitations to how many IO points they can manage.

PLCs work best in small-scale applications, like discrete machine control and small scale building automation. PLCs still offer the advantages of lower costs and decreased footprint, but come with a somewhat limited scope. PACs are geared for more complex automation solutions. They are well suited to advanced functions like: advanced process control, motion control, drive control, vision applications and HMI functions.  PACs offer multiple functions, running on one platform and can use PC-based software to program, monitor and collect data.

To sum up, both PLCs and PACs take advantage of modern technology. PLCs in particular are very different now than they were in 60s and 70s. Contemporary PLCs and PACs are even taking advantage of IIoT for live data collection and cloud storage. Both PLCs and PACs have a place in a modern manufacturing workflow, bringing their particular strengths to different applications.

Products Mentioned

XP-8341-CE6: Windows Compact Edition 6 based programmable automation controller with 520 Mhz CPU, 1GB RAM, and 4G onboard storage. Includes 3 Swappable I/O slots, 10/100 MB Ethernet, USB Port, and 4 Serial Ports.

I-87068W: Slot type I/O module. Features 4 channels of Form A Relay output and 4 channels of Form C Relay Output.

I-8053W: Slot type I/O module. 16 differential channels of sink/source digital input. It features intra-module and field to logic isolation.

I-8120W: 1 Port Intelligent CAN bus communication module. Slot type COM module.

I-7012F: Remote I/O RS-485 data acquisition module. This module features 1 voltage or current analog input channel, 1 PWM digital input, and 2 channels of sink type open collector digital output.

I-87017W: Slot type I/O module with 8 channels of analog input. This module accepts voltage and current values.