Simplifying design of industrial process-control systems with PLC evaluation boards (Part 1 of 2)

PLC evaluation board applications for industrial process-control systems are diverse, ranging from simple traffic control to complex electrical power grids, from environmental control systems to oil-refinery process control. The intelligence of these automated systems lies in their measurement and control units. The two most common computer-based systems to control machines and processes, dealing with the various analog and digital inputs and outputs, are programmable logic controllers (PLCs) and distributed control systems (DCSs). These systems comprise power supplies, central processor units (CPUs), and a variety of analog-input, analog-output, digital-input, and digital-output modules.

The standard communications protocols have existed for many years; the ranges of analog variables are dominated by 4 mA to 20 mA, 0 V to 5 V, 0 V to 10 V, +/-5 V, and +/-10 V. There has been much discussion about wireless solutions for next-generation systems, but designers still claim that 4 mA to 20 mA communications and control loops will continue to be used for many years. The criteria for the next generation of these systems will include higher performance, smaller size, better system diagnostics, higher levels of protection, and lower cost—all factors that will help manufacturers differentiate their equipment from that of their competitors.

This article examines the key performance requirements of process-control systems and the analog input/output modules they contain—and introduces an industrial process-control evaluation system that integrates these building blocks using the latest integrated-circuit technology. The article also looks at the challenges of designing a robust system that will withstand the electrical fast transients (EFTs), electrostatic discharges (ESDs), and voltage surges found in industrial environments—and present test data that verifies design robustness.

PLC Overview with Application Example
Figure 1 shows a basic process-control system building block. A process variable, such as flow rate or gas concentration, is monitored via the input module. The information is processed by the central control unit; and some action is taken by the output module, which, for example, drives an actuator.


Figure 1: Typical top-level PLC system

Figure 2 shows a typical industrial subsystem of this type. Here a CO2 gas sensor determines the concentration of gas accumulated in a protected area and transmits the information to a central control point. The control unit consists of an analog input module that conditions the 4 mA to 20 mA signal from the sensor, a central processing unit, and an analog output module that controls the required system variable. The current loop can handle large capacitive loads—often found on hundreds-of-meters long communications paths experienced in some industrial systems. The output of the sensor element, representing gas concentration levels, is transformed into a standard 4 mA to 20 mA signal, which is transmitted over the current loop. This simplified example shows a single 4 mA to 20 mA sensor output connected to a single-channel input module and a single 0 V to 10 V output. In practice, most modules have multiple channels and configurable ranges.

The resolution of input/output modules typically ranges from 12 to 16 bits, with 0.1% accuracy over the industrial temperature range. Input ranges can be as small as +/-10 mV for bridge transducers and as large as +/-10 V for actuator controllers—or 4 mA to 20 mA currents in process-control systems. Analog output voltage and current ranges typically include +/-5 V, +/-10 V, 0 V to 5 V, 0 V to 10 V, 4 mA to 20 mA, and 0 mA to 20 mA. Settling-time requirements for digital-to-analog converters (DACs) vary from 10 us to 10 ms, depending on the application and the circuit load.


Figure 2: Gas Sensor