Technological and industrial advancements have continuously propelled human progress to new heights. The greatest leaps (or milestones) in the history of technological development are known as the Industrial Revolution. Now, we have entered a new phase: Industry 4.0.
The First, Second, and Third Industrial Revolutions brought steam and water power production, large-scale electrification of production, and computer technology, respectively. The Fourth Revolution, however, focuses on digitally automated factories. Enterprises are widely adopting digital automation technologies to improve efficiency through automatic control. This not only enables predictive maintenance but also increases output and supply efficiency. Fully networked operations allow for better planning of supply routes, reduced warehousing costs, and ensured higher utilization rates. In an era of ever-increasing demand, machine downtime is extremely costly. Networked factory control facilitates planning, thereby helping to prevent production disruptions.
The focus of Industry 4.0 is not just on developing new facilities. How to continue to fully utilize and expand existing infrastructure to achieve rapid amortization of digital factory costs is another important factor from a cost perspective. For smart factory operators, acquisition costs (the lower the better) and the need for major structural changes (ideally none) are crucial criteria that must be evaluated in cost-benefit analyses. A key aspect of realizing a smart factory is the adoption of field instruments with intelligent processing capabilities (i.e., so-called smart transmitters) to support factory monitoring and diagnostics, as well as networking with other newly added field instruments. These smart transmitters can be distributed throughout the factory, with different sensors interconnected, and can monitor components that were not previously networked.
Field instruments are the essential intelligent units for realizing Industry 4.0. These units require more comprehensive consideration; for example, instruments can be used in conjunction with various sensors such as resistance thermometers, thermocouples, and pressure sensors. Intelligent transmitters are intelligent field instruments developed from commonly used field instruments today. They are fully loop-powered or supplemented by an auxiliary power supply. Besides other components, intelligent transmitters primarily utilize a microprocessor containing the software that makes the transmitter intelligent. However, the intelligent functions of field instruments are not limited to the microcontroller's software. Diagnostics and other safety functions can also be integrated into other semiconductor modules (e.g., analog-to-digital converters, ADCs), allowing the microcontroller to include more processing software. Intelligent transmitters typically use standard 4mA to 20mA current loops, which limits the transmitter's maximum power consumption. Therefore, the power consumption of individual components must be strictly limited. This limit is 3.2mA if an alarm current as low as 3.2mA is used. The development trends of smart transmitters include low power consumption, small size, more functions, better performance, safety considerations and predictive maintenance (Figure 1).
Figure 1. Development trend of smart transmitters for Industry 4.0
Figure 2. Block diagram of intelligent transmitter
A typical smart transmitter signal chain is shown in Figure 2. This instrument contains a sensor and an ADC, typically consisting of an analog front-end and an analog preprocessing unit. The digital signal is sent from the ADC through an isolation barrier to the microprocessor, and then to the interface. In today's factory automation, a 2-wire solution with a 4mA to 20mA interface is commonly used. This requires a digital-to-analog converter (DAC). The Addressable Remote Sensor High-Speed Channel (HART) protocol allows bidirectional use of this interface. If the control room is also HART compatible, more complex processes can be transmitted via the HART protocol, enabling more applications through field instruments.
The following section details the function of each component and demonstrates a particularly efficient and space-saving example circuit. Built using Analog Devices' (ADI) modular design, this circuit meets all Industry 4.0 requirements, featuring high precision and low power consumption. Figure 3 shows the circuit block diagram (top) and a photograph of the actual circuit (bottom).
The sensor is connected to an analog-to-digital converter; in this example, it's the AD7124 24-bit Σ-Δ ADC. This ADC is a highly integrated module. It integrates a space-saving analog front-end, eliminating the need for external instrumentation amplifiers and operational amplifiers. The AD7124 can be flexibly designed with 4 or 8 differential inputs for use with a wide variety of sensors. Furthermore, this ADC features a programmable power supply, which is crucial, as passive temperature sensors require three different power modes. This allows for highly flexible power consumption design. The accuracy and rate of the output data depend on the selected power mode. Therefore, field instruments can also operate below a 3.2mA power limit, connecting to more powerful microprocessors or other sensors for parallel measurements. The AD7124 also features a variety of diagnostic functions, including:
* Read/write all data to the valid register. * Read only valid data into the register. * Verify complete decoupling of the voltage regulator (LDO). * Verify that the ADC modulator and filter performance meets specifications. * Verify for overvoltage or undervoltage.
These preventative measures not only help easily meet safety design standards, but also allow for advance planning of field instrument maintenance through HART protocol information transmission. By improving availability and reducing maintenance requirements, Industry 4.0 signifies a significant increase in efficiency.
Insulation of field instruments is another critical factor. Inadequate insulation can create ground loops and overvoltages, which can damage not only the instruments but also the connected programmable logic controllers (PLCs) when transmitting over a two-wire connection. Good insulation typically outweighs the current limits of loop-powered field instruments; in this example, the ADuM1441 digital isolator is used. At low data rates, this device requires significantly less power than previous solutions, thus providing adequate insulation within given power consumption limits.
Besides the AD7124 and ADuM1441 units, another key component in field instrumentation is the microcontroller. ARM®-based microcontrollers, such as the ADuCM3027/ADuCM3029, are commonly used. With active power consumption of less than 38µA/MHz, they are ideal for smart transmitters. ARM microcontrollers are widely used in industrial applications and are also well-suited for safety-related applications. The ADuCM3027/ADuCM3029 also features AES-128/AES-256 encryption, enabling additional security features. These microprocessors also integrate intelligent software that can be programmed to perform diagnostics, such as calibrating the AD7124, to ensure accurate measurements from the field instrumentation.
The HART protocol allows for intelligent design of field instruments without requiring extensive infrastructure. It can be used for 4mA and 20mA current loops, requiring a HART master and a HART slave. Users can leverage HART to establish digital connections between field instruments and PLCs. This creates an intelligent connection between the control room and field instruments. To implement HART, a HART modem is needed to connect to a HART-compatible DAC. These devices must be highly integrated and have extremely low power consumption. These two factors—small size and low power consumption—are prerequisites for Industry 4.0.
HAR can enable digital communication on existing current loops, but requires a HART modem to modulate the signal into a clean current signal. The AD5700 ultra-low power HART modem was specifically developed for this purpose.
The final major component of modern field instrumentation is the digital-to-analog converter (DAC). In Industry 4.0 scenarios, this device must also feature low power consumption and high integration. The DAC is a critical component of the entire circuit, and peripheral components should be integrated into the DAC as much as possible, rather than occupying more PCB space. A linear regulator that powers the entire field instrumentation system is one example. It can also communicate with a PLC to perform instrument control and monitoring. The AD5421 is a DAC that works well with HART modems.
The signal chain described in this article is a possible design for loop-powered field instruments compatible with Industry 4.0, suitable for pressure or temperature measurement. This intelligent transmitter can be used for intelligent monitoring, control, and feedback, and is particularly well-suited for small size and low power consumption requirements. Analog Devices' selected modules can meet current and future challenges.
Figure 3. Schematic diagram of the reference circuit (top) and physical image (bottom).
