可编程控制器评估板简化工业过程控制系统设计(上)

Introduction

The 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 controllers1 (PLCs) and distributed control systems2 (DCS’s). 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.

We will discuss the key performance requirements of process-control systems and the analog input/output modules they contain—and will introduce an industrial process-control evaluation system that integrates these building blocks using the latest integrated-circuit technology. We also look 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 µs to 10 ms, depending on the application and the circuit load.



Figure 2. Gas sensor.

The 4 mA to 20 mA range is mapped to represent the normal gas detection range; current values outside this range can be used to provide fault-diagnostic information, as shown in Table 1.
Table 1. Assigning currents outside the 4 mA to 20 mA output range.



PLC Evaluation System

The PLC evaluation system3 described here integrates all the stages needed to generate a complete input/output design. It contains four fully isolated ADC channels, an ARM7? microprocessor with RS-232 interface, and four fully isolated DAC output channels. The board is powered by a dc supply. Hardware-configurable input ranges include 0 V to 5 V, 0 V to 10 V, ±5 V, ±10 V, 4 mA to 20 mA, 0 mA to 20 mA, ±20 mA, as well as thermocouple and RTD. Software-programmable output ranges include 0 V to 5 V, 0 V to 10 V, ±5 V, ±10 V, 4 mA to 20 mA, 0 mA to 20 mA, and 0 mA to 24 mA.



Figure 3. Analog input/output module.

Output Module: Table 2 highlights some key specifications of PLC output modules. Since the true system accuracy lies within the measurement channel (ADC), the control mechanism (DAC) requires only enough resolution to tune the output. For high-end systems, 16-bit resolution is required. This requirement is actually quite easy to satisfy using standard digital-to-analog architectures. Accuracy is not crucial; 12-bit integral nonlinearity (INL) is generally adequate for high-end systems.

Calibrated accuracy of 0.05% at 25°C is easily achievable by overranging the output and trimming to achieve the desired value. Today’s 16-bit DACs, such as the AD5066,4 offer 0.05 mV typical offset error and 0.01% typical gain error at 25°C, eliminating the need for calibration in many cases. Total accuracy error of 0.15% sounds manageable but is actually quite aggressive when specified over temperature. A 30 ppm/°C output drift can add 0.18% error over the industrial temperature range.

Table 2. Output module specifications.



Output modules may have current outputs, voltage outputs, or a combination. A classical solution that uses discrete components to implement a 4 mA to 20 mA loop is shown in Figure 4. The AD5660 16-bit nanoDAC® converter provides a 0 V to 5 V output that sets the currrent through sense resistors, RS, and therefore, a through R1. This current is mirrored through R2.



Setting RS = 15 kΩ, R1 = 3 kΩ, R2 = 50 Ω and using a 5-V DAC will result in IR2 = 20 mA max.



Figure 4. Discrete 4 mA to 20 mA implementation.

This discrete design suffers from many drawbacks: Its high component count engenders significant system complexity, board size, and cost. Calculating total error is difficult, with multiple components adding varying degrees of error with coefficients that can be of differing polarities. The design does not provide short-circuit detection/protection or any level of fault diagnostics. It does not include a voltage output, which is required in many industrial control modules. Adding any of these features would increase the design complexity and the number of components. A better solution would be to integrate all of the above on a single IC, such as the AD5412/AD5422 low-cost, high-precision, 12-/16-bit digital-to-analog converters. They provide a solution that offers a fully integrated programmable current source and programmable voltage output designed to meet the requirements of industrial process-control applications.



Figure 5. AD5422 programmable voltage/current output.

The output current range is programmable to 4 mA to 20 mA, 0 mA to 20 mA, or 0 mA to 24 mA overrange function.

A voltage output, available on a separate pin, can be configured to provide 0 V to 5 V, 0 V to 10 V, ±5 V, or ±10 V ranges, with a 10% overrange available on all ranges. Analog outputs are short-circuit protected, a critical feature in the event of miswired outputs—for example, when the user connects the output to ground instead of to the load. The AD5422 also has an open-circuit detection feature that monitors the current-output channel to ensure that no fault has occurred between the output and the load. In the event of an open circuit, the FAULT pin will go active, alerting the system controller. The AD5750 programmable current/voltage output driver features both short-circuit detection and protection.

Figure 6 shows the output module used in the PLC evaluation system. While earlier systems typically needed 500 V to 1 kV of isolation, today >2 kV is generally required. The ADuM1401 digital isolator uses iCoupler®5 technology to provide the necessary isolation between the MCU and remote loads, or between the input/output module and the backplane. Three channels of the ADuM1401 communicate in one direction; the fourth channel communicates in the opposite direction, providing isolated data readback from the converters. For newer industrial designs, the ADuM3401 and other members of its family of digital isolators provide enhanced system-level ESD protection.



Figure 6. Output module block level.

The AD5422 generates its own logic supply (DVCC), which can be directly connected to the field side of the ADuM1401, eliminating the need to bring a logic supply across the isolation barrier. The AD5422 includes an internal sense resistor, but an external resistor (R1) can be used when lower drift is required. Because the sense resistor controls the output current, any drift of its resistance will affect the output. The typical temperature coefficient of the internal sense resistor is 10 ppm/°C to 20 ppm/°C, which could add 0.12% error over a 60°C temperature range. In high-performance system applications, an external 2-ppm/°C sense resistor could be used to keep drift to less than 0.016%.

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