Moving up from hardwired relay logic

If you’re still trying to control machine functions with relays and discrete electromechanical devices, you should be looking at the small and inexpensive PLCs available these days. Their versatility and simplicity may surprise you.


In the early 1970s, programmable logic controllers (PLCs) went from a blueprint to one of the fastest growing segments of the control products market. This was mainly due to the ruggedness of their solid state components when compared with the moving parts in electromechanical relays. PLCs provide equipment, installation, troubleshooting, and labor cost savings by reducing wiring and associated wiring errors. They take up less space than the counters, timers, and other control components they typically replace. And their ability to be reprogrammed dramatically increases flexibility when changing control schemes.

Functional elements diagram. Source: Rockwell Automation

While the shift to PLCs isn’t new, some manufacturers and machine builders haven’t made the change and continue to evaluate their control systems to determine if the old new technology would benefit them. PLCs are ideal for control applications that require coordinated operation of electrical or electronic devices. Machines or processes that operate based on any of the following characteristics could be considered potential control applications:

  • Repetitive operations
  • Time-driven operations
  • Event-driven operations
  • High-speed control, and
  • Requirements for data acquisition/manipulation. 

Application examples include conveyors, form and fill operations, packaging, strapping machines, palletizing and wrapping machines, traffic light sequencing, gate control, cut-to-length lines, semi-automatic welding and painting, storage and retrieval systems, pump alternators, car washes, printing presses, vending machines, and many more.

These applications may be able to be controlled by relays, single board controllers (SBCs) or PLCs―all of which possess logic capabilities. It’s important to note that some machine builders and OEMs that switched from relays to SBCs are now considering making the switch from SBCs to PLCs. However, before selecting a control system, the applications’ requirements must be considered, as they help guide the selection process.

Application requirements

Regardless of the type of control system ultimately selected, the first step in approaching a control situation is to specify the following requirements for the application:

  • Input and output device requirements
  • The need for special operations in addition to discrete (on/off) logic, including timing, counting, sequencing, data acquisition, and calculations
  • Electrical requirements for inputs, outputs, and system power
  • How fast the control system must operate (speed of operation)
  • If the application requires sharing data outside the process, i.e., communication
  • If the system needs operator control or interaction, and
  • The physical environment in which the control system will be located.

To determine application requirements, designers need to begin by identifying all operations the control system needs to perform and the conditions that affect the system. As an example, imagine designing a control system for a parking garage with a 500-car capacity. The first step is to define and describe the car parking process. What is the desired operation for the parking garage?

  • The car approaches an automated ticket machine at a gate.
  • The driver pushes a button on the ticket machine to receive a ticket. If there is space left in the garage, the driver will receive a ticket. The machine should not provide a ticket if the garage is full or if the gate is already up.
  • Removing the ticket raises the gate and turns on a green “enter” light.
  • After the car clears the gate, the gate lowers and the green light shuts off.
  • The number of vehicles in the garage needs to be known at any time.
  • If maximum capacity is reached, a “garage full” sign is illuminated, the ticket machine will not provide a ticket, and the gate will remain down.
  • An alarm must sound when the gate is obstructed.                 

Input/output requirements

After defining the operation of the system, the next step is to determine what input and output devices the system requires. List the function required and identify a specific type of device. Also, group devices by whether they sense an event has occurred or is occurring (inputs), or whether they control something (outputs).

From the description of the parking garage control system, the following I/O requirements can be listed:

I/O requirements for parking garage control system. Source: Rockwell Automation

From the list of field devices, the parking garage control system requires seven inputs and six outputs.

Selecting a control method

Once application requirements have been defined, the next step is determining which type of control method can accomplish the task. To help determine which control method—relays, PLCs, or SBCs—is best suited for the task, develop a chart which integrates application requirements with control methods. The following chart (Figure 1) has been filled out for the parking garage example.

Figure 1: Comparison of application requirements and control options. Source: Rockwell Automation

As Figure 1 shows, all three control methods can accomplish the task, so selecting a control method cannot be based on application requirements alone. However, this does not mean that all three methods provide the optimum solution. To differentiate between control methods, evaluate the relative cost impact of each method using the following criteria (Figure 2).

Figure 2: Relative cost comparison of control methods. Source: Rockwell Automation

Space and cost

System designers usually consider physical space and cost for components, the two most important issues by far. Many applications, especially machinery, have a small, finite amount of space allocated for controls. If an assembled control system occupies more space than allotted, it often cannot be used because too many changes to the machinery would need to be made to accommodate it.

Once mounted on a panel, a relay-based control system typically occupies much more space than the equivalent control implemented with a micro PLC or SBC. With micro PLCs (Figure 3) available in the size of a brick and smaller, only the simplest relay-based system takes up less space. With the control system for the parking garage requiring 13 I/O connections and a counter, a micro PLC or SBC are the most space-efficient control solutions.

Figure 3: Machine builders need economical, convenient control solutions they can customize to suit their customers’ specific application needs. Small PLCs, such as this Rockwell Automation Micro810, offers the functionality and flexibility of a PLC for t

Several cost factors influence the selection of a control method, including control system design and development, costs for components, assembly, space, and logic implementation.

Control system design and development costs are incurred in the design of the system.

  • For a relay system, these costs are not applicable as the components have already been designed and produced.
  • For a micro PLC, these costs are not applicable because the PLC has already been designed and produced.
  • For an SBC, costs involve securing the services of an electronic engineer to design the board and test its viability (unlike relays and PLCs, SBCs are not typically available off-the-shelf).
  • Many installations require the control system to meet global industrial standards, such as UL, CE, or CSA. PLCs usually have been certified to meet those standards, while relay- and SBC-based systems typically have not. 

Component costs are for the control-related hardware. Costs also include receiving, inventory, and the quality control of the components.

  • For a relay system, this includes relays, mechanical timers, and counters.
  • For a micro PLC, all necessary hardware is packaged in the PLC.
  • For an SBC, this includes the board, its components, and circuitry. 

Assembly costs cover putting the components together so they are usable.

  • For a relay system, this includes mounting components on a panel and wiring the logic power.
  • For a micro PLC, the only assembly costs are for mounting the unit to a panel with screws or on a DIN rail.
  • For an SBC, this involves securing a manufacturing facility to produce it. For this reason, SBCs normally become economically viable only in high-volume or very unique applications.

Panel space costs include the size of the panel and the enclosure needed to house the control system. The larger the enclosure, the greater the material costs for it.

  • For a relay system with many components, size could be a prohibitive.
  • For a micro PLC, size is minimal.
  • For an SBC, size is usually minimal.

Logic implementation costs related to the installation of the logic into the control system (assuming costs for developing the logic are similar for all three control methods) include the following.

  • For a relay system, implementing logic involves wiring the components together. Each subsequent application requires the same amount of labor to assemble, debug, and adjust timer and counter presets.
  • For a micro PLC, costs include purchase of programming software or a hand-held programmer. Programming a subsequent application only requires downloading the program; there are no program debugging costs for duplicate applications. However, users still need to commission each control system.
  • For an SBC, costs involve retaining an electrical engineer to program a microprocessor. Programming each subsequent application typically requires copying a memory chip; there are no program debugging costs for duplicate applications. Commissioning is also required.

OEMs and machine builders that have already switched from relays to SBCs are now switching from SBCs to PLCs due to the price and performance of PLCs. In addition, advances in micro PLCs offer more flexibility, communications, and expanded memory. Plug-in modules for analog- and digital-I/O enable machine builders to personalize the controllers to increase functionality without expanding the product footprint or forcing users to buy more than “just enough” control.

Future costs

Total costs for a control system don’t end after implementation. After system start-up, it may be necessary to modify the control logic, document system changes, and troubleshoot the system.

With a relay-based system, rewiring costs associated with logic changes can be extraordinarily high. The labor involved with relays can be intensive and costly, especially if more than one machine needs rewiring. Further, documenting relay wiring logic changes requires drafting a new wiring diagram. Because this task is so tedious—and adds cost—system changes can go undocumented. In fact, short of tracing every wire, there is no way to ensure that the latest wiring diagram actually reflects the logic being executed by the system.

With an SBC-based control system, users typically cannot communicate with the microprocessor, and programming software is not available. Logic changes are not easy to implement, automated documenting capabilities do not usually exist, and users typically cannot upload or download programs. SBC-based systems are difficult to troubleshoot because they rarely have troubleshooting features built into their software. Users of these systems must go to the manufacturer for support because no one else understands the SBC operation.

PLCs offer considerably more flexibility. Programming software facilitates relatively quick logic changes and permits the new program to be easily downloaded to multiple machines. The program is always up-to-date, and documentation is accomplished with the push of a button. Troubleshooting help and diagnostic functions are a standard part of the software and can be conducted with the hand-held programmer, as well.

The data acquisition and communication capabilities of PLCs also deserve special mention, as they far exceed the capabilities of traditional relays. PLCs can gather information from the machine for production and status reports, out-of-spec or faulty parts count, total parts count, production rates, and machine run time, which is valuable for periodic maintenance operations. Further, PLCs can communicate this data to other control equipment or to operators in remote locations.

These capabilities help make PLCs the easiest control system to support. Assistance for programming and troubleshooting is available at reasonable costs from many sources. And, if a PLC fails, a replacement PLC can normally be purchased off-the-shelf from the nearest industrial electrical suppliers—there is no need to wait for a shipment from the factory. Furthermore, the ruggedness of PLCs compared to SBCs gives them a definite advantage in harsh environments or when durability is a primary consideration.

Should You Use a PLC?

During the 1970s and early ’80s, many engineers, manufacturing managers, and control system designers spent considerable time debating this issue, trying to evaluate cost effectiveness.

Today, one generally accepted rule is that PLCs become economically viable in control systems that require three to four or more relays. Given that micro PLCS cost only a few hundred dollars, coupled with the emphasis manufacturers place on productivity and quality, the cost debate becomes almost immaterial.

In addition to cost savings, PLCs provide many value-added benefits:

  • Reliability. Once a program has been written and debugged, it can be easily transferred and downloaded to other PLCs. This reduces programming time, minimizes debugging, and increases reliability. With all the logic existing in the PLC’s memory, there’s no chance of making a logic wiring error using relays. The only wiring required is for power, inputs, and outputs.
  • Flexibility. Program modifications can be made with just a few keystrokes. OEMs can easily implement system updates by sending out a new program instead of a serviceperson. End users can modify the program in the field, or, conversely, OEMs can prevent end users from tinkering with the program (an important security feature).
  • Advanced functions. PLCs can perform a wide variety of control tasks, from a single, repetitive action to complex data manipulation. Standardizing on PLCs opens many doors for designers and simplifies the job for maintenance personnel.
  • Communications. Communicating with operator interfaces, other PLCs, or computers facilitates data collection and information exchange.
  • Speed. Because some automated machines process thousands of items per minute—and objects spend only a fraction of a second in front of a sensor—many automation applications require the PLC’s quick response capability.
  • Diagnostics. The troubleshooting capability of programming devices and the diagnostics resident in the PLC allow users to easily trace and correct software and hardware problems.

PLCs followed a product development curve similar to that of the personal computer. Early PLCs were large, cost thousands of dollars, and had relatively few features. But with the evolution of microprocessors and other board-level components, PLCs grew in sophistication while size and cost shrank. For all major criteria by which control systems are evaluated—cost, size, flexibility, and supportability—PLCs provide the user with distinct advantages over other control options for many control applications. 

Sykora is a global product manager for Rockwell Automation. He has 27 years of experience with hardware and software technology in automation applications.

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