Design a Smaller PLC

Design a Smaller PLC

Kind of like a traffic cop directing the flow of cars from the middle of an intersection, programmable logic controllers (PLCs) occupy a similar role in an automated factory. PLCs make decisions for machines based on analog and digital inputs and outputs. They sense and control everything from simple lighting functions to environmental systems. When input stimuli, in the form of voltage of current, comes in from machines, sensors, or process events, the PLC has to accurately interpret and convert the stimuli for the CPU. The CPU then instructs the output systems that control actuators in the factory.

Designing PLCs calls for adhering to various criteria, given the harsh operating environments in which they must work. PLCs must be precise, while also flexible and configurable to support a variety of applications. They need to work without flaw for many years in environments where there could be a lot of electrostatic discharge (ESD), electromagnetic interference (EMI), radio frequency interference (RFI), and high-amplitude transient pulses. As the use of IO-Link and smart sensor technologies enable factories for distributed control, these technologies are driving factory automation equipment—including PLCs—to shrink. 

Reducing the PLC Footprint for Automated Factories

In recent years, Maxim engineers have done a lot of work to develop analog ICs that help reduce the PLC footprint, enabling these controllers to be placed closer to the edge of the manufacturing floor in an automated factory. As an example, consider a PLC analog input module (Figure 1). For these modules, small precision analog ICs can help meet the footprint goal. Let’s take a look at the considerations to keep in mind when evaluating circuits for these modules.

Figure 1. PLC 12-bit analog input module.

Input signals going into PLCs are varied. Large- and small-scale analog signals can range from millivolts to tens of volts or from milliamps to amps. A PLC analog input module accepts voltage and current-mode input signals. When choosing analog input circuits, your choice will likely depend on a variety of considerations:

  • The number and type of inputs the module will handle
  • Whether these inputs must be individually or group isolated
  • Sensor simulation and/or biasing needs
  • Multiple sensor sampling simultaneously
  • Your design’s targeted signal frequency range
  • Anti-alias filtering requirements
  • AD converter resolution
  • Sampling rate needed

Lowering Power in Key Components

In the signal chain component of Figure 1, we can see that input from the voltage sensors is routed to an analog-to-digital converter (ADC) for signal processing before the signal eventually gets to the microprocessor. In the signal chain, voltage references essentially regulate the voltage by producing a constant voltage, regardless of factors such as the loading on the device, temperature changes, or power supply variations. The MAX6069 precision shunt voltage reference comes in a 4-bump WLP and allows a wide 1µA to 2mA current range, meeting requirements for small size and low power. The IC supports industrial process control sensors and industrial IoT sensors, enabling the full mixed-signal chain at the tip of the sensor without requiring routing of sensitive analog data up a long rod. Being able to regulate a precision reference voltage with lower current frees up power for other functions, such as longer sensing runtime, more processing power and algorithms, and higher accuracy, lower noise, and wider bandwidth op amps.

Maxim offers small, low-power analog ICs for the other blocks in the diagram. Focusing on the rest of the signal chain, there are solutions like the MAX11105 compact, high-speed, low-power successive approximation ADCs, which consume only 5.2mW at 3Msps and 3.7mW at 2Msps. For the op amp, the MAX44260 is an example of an IC that provides high speed, precision, low noise, and low-voltage operation. These characteristics make this IC suitable for amplifying signals in industrial equipment.

After modern PLCs were introduced in the 1960s, they didn’t change too much over the years. But today's PLCs are a different breed, as process control now calls for higher levels of performance, smaller form factor, and more functional flexibility. By integrating multiple functions into small, low-power parts, Maxim provides analog ICs that support the demands of Industry 4.0