“A PLC system consists of input modules, output modules and input/output modules. Because many of the inputs and outputs involve analog variables in the real world—and the controller is digital—the PLC system hardware design tasks will be primarily centered around the following: digital-to-analog converters (DACs), analog-to-digital converters (ADCs) , input and output signal conditioning, isolation between the electrical wiring of the input/output modules and the controller, and between modules.
A PLC system consists of input modules, output modules and input/output modules. Because many of the inputs and outputs involve analog variables in the real world—and the controller is digital—the PLC system hardware design tasks will be primarily centered around the following: digital-to-analog converters (DACs), analog-to-digital converters (ADCs) , input and output signal conditioning, isolation between the electrical wiring of the input/output modules and the controller, and between modules.
Figure 1 PLC system architecture showing various I/O module functions
The resolution of the I/O modules ranges from 12-bit to 16-bit, and the accuracy is 0.1% over the entire industrial temperature range. The analog output voltage range is usually ±5V, ±10V or 0V “5V, 0V” 10V, and the current range is 4″ 20mA or 0″ 20mA. Settling time requirements for the DAC range from 10ms to 100ms, depending on the actual requirements of the application. A wide range of analog inputs are available, ranging from ±10mV weak voltage signals output by bridge sensors; ±10V voltage signals from motor controllers, or 4″ 20mA currents for industrial process control systems. The conversion time depends on the required accuracy and the selected ADC architecture, from 10SPS to several hundred KSPS.
Digital isolators, opto-isolators, or electromagnetic isolators are used to isolate ADCs, DACs, and signal conditioning circuits on the system site from the controllers on the digital side. If the analog side of the system must also achieve sufficient isolation, converters must be used on each channel of input or output to maximize isolation between channels—power isolation is also required.
iCMOS technology is a new high-performance manufacturing process that integrates high-voltage integrated circuits with sub-micron CMOS and complementary bipolar processes, and is used in the input and output parts of PLC designs.
iCMOS technology enables a single-chip design to incorporate 5V CMOS and match it with higher voltage (16, 24 or 30V) CMOS circuits — so the same chip will have multiple power supplies of different voltages. With such flexibility in integrating a variety of components and operating voltages, submicron iCMOS devices offer higher performance, integrate more functions, and consume less power—while requiring significantly less board space Previous generations of high voltage products. The bipolar process in it provides an accurate reference source for ADCs, DACs and low offset amplifiers, with excellent matching and high stability.
The thin-film resistor has an initial matching characteristic of up to 12 bits, and can achieve 16-bit matching after trimming. Compared with traditional polysilicon resistors, the temperature and voltage coefficients are improved by 20 times. It is a high-accuracy, high-precision digital-to-analog Ideal for converters. On-chip thin-film fuses allow the integral nonlinearity, offset, and gain of a high-precision converter to be calibrated digitally.
PLC output module
The analog outputs of PLC systems—often used to control actuators, valves, and motors in industrial environments—use standard analog output ranges such as ±5, ±10V, 0V to 5V, 0V to 10V, 4 to 20mA, or 0～20mA. The signal chain for analog outputs often includes digital isolation – isolating the controller’s digital output from the DAC and analog signal conditioning. Converters used in digitally isolated systems primarily use 3-wire or 4-wire serial interfaces to minimize the number of digital isolators or opto-isolators required.
The analog output module of PLC system usually adopts two kinds of structure: The structure of one DAC per channel and the structure of one sample-and-hold of each channel. In the first architecture, each channel uses a dedicated DAC to generate the analog control voltage or current. There are many multi-channel DACs to choose from today, which take up less space and cost less per channel, but those that require channel isolation tend to use single-channel DAC architectures. Figure 2 is a typical configuration using one DAC per channel. This simplest DAC is a low-voltage single-supply type, powered by a 2.5V”5.5V power supply, and the output range is 0″VREF. The output signal can be conditioned to generate any desired voltage or current range. Bipolar output converters operate from dual power supplies and can be used in output modules that must output a bipolar voltage range.
Figure 2 Architecture of one DAC per channel
The quad D/A converter is an ideal choice for non-isolated multi-channel output designs. Up to 4 different output configurations can be realized by means of an external signal conditioning circuit. For example, Figure 3 shows how the 16bit 4-channel voltage output DACAD5664R provides an output range of 0 to 5V – it can also provide the output voltage range required by various standards through different connection methods, or through external four The op amp forms the sink current output. When configured as bipolar output, the external output of its internal reference source can provide the necessary tracking bias voltage.
Figure 3 Using multi-channel D/A converter to achieve ±5V, ±10V, 0V”10V, 0V”5V and other voltage and current sink outputs
Figure 4 shows a single-channel converter used in an isolated 4” 20mA current loop control circuit. The AD5662 is available in a SOT-23 package and is suitable for applications that require adequate isolation between the analog outputs.
Figure 4 A 4～20mA current control circuit
In Figure 4, the maximum output voltage swing of AD5662 is 5V, which is provided by the ADR02 voltage reference, which can regulate a precise power supply from the changing loop voltage. The 5V DAC output is converted into a 4-20mA current output through a hybrid circuit composed of an operational amplifier and a transistor.Because the input of the non-inverting end of the operational amplifier is at the virtual ground potential, the operational amplifier can adjust the current Is to maintain the relationship between the voltages on RS and R3 equal, so there is
The sum of the currents at N2 constitutes the loop current:
The currents are added at N1, so we have:
The 4mA offset component of the loop current is provided by the reference voltage:
The programmable 0″16mA current in the loop current is provided by the DAC:
Configure a sample-and-hold circuit per channel
Another alternative architecture is to use switched capacitors and buffers to form a sample-and-hold amplifier (HA) to store the output signal of a high-performance single DAC, as shown in Figure 5. These samples are switched between different capacitors by an analog multiplexer. Because the holdover accuracy of the system is determined by the rate of decline of the capacitance, frequent refreshes of these channels are required to maintain the required accuracy. Depending on the output requirements, a low-voltage single-supply DAC can be used, or a bipolar output DAC can be used. Buffers provide signal conditioning, presenting a high input impedance to the capacitor and low output impedance to drive the load.
Figure 5 Single DAC Architecture
Galvanic isolation of power supply and digital signals
In PLC, process control, data acquisition and control systems, digital signals generated by various sensors are transmitted to a central controller for processing and analysis. In order to ensure the safety of the voltage at the user interface and to prevent the transmission of transient spikes, galvanic isolation is required. The most commonly used isolation devices are optocouplers, transformer-based isolators, and capacitively coupled isolators.
Common optocouplers use light-emitting diodes (LEDs) to convert electrical signals into corresponding light intensities, and photodetectors to convert optical signals into electrical signals. Generally speaking, their LEDs generally suffer from low conversion efficiency, and photodetectors have a slow response speed; optocoupler isolators have a limited lifespan, which can occur with excessive temperature, operating speed, and power consumption. Performance fluctuates. They are generally limited to 1 or 2 channel configurations and require external components to achieve full functionality.
ADI has now developed a new isolation method that combines chip-scale transformer technology with integrated CMOS input and output circuitry. These iCoupler devices are lower than opto-isolators in size, cost, and power consumption, and are available in a variety of channel configurations and performance levels, with standard CMOS interfaces, and require no external components—and can operate in full Maintain its high performance and stability over temperature, power supply range and lifetime. iCoupler’s data rate and timing accuracy are 2-4 times higher than common high-speed optocouplers, and they use only 1/50 the power consumption of optocouplers, generate less heat, improve reliability, and cost less .
PLC input module
The architecture of the PLC system and the selection of input modules depend on the level of the input signal that needs to be monitored. Signals from various types of sensors and process control variables to be monitored, ranging from ±10mV all the way up to ±10V.
ADCs of many architectures can be used in industrial and PLC applications, including successive approximation (SAR), Flash/Parallel, integrating (including SD), and ramp/counting. When selecting an ADC for a particular application, the primary consideration is the input signal range, along with the required accuracy, signal frequency components, maximum signal level, and dynamic range. The most widely used are successive approximation ADCs and SD ADCs.
Successive approximation ADCs can provide 12bit to 18bit resolution with high throughput; they are ideal for multi-channel multiplexing applications that require monitoring of a large number of input channels at higher sampling rates.
The SD architecture ADC can provide a resolution of 16bit”24bit. They have a high oversampling rate and digital filtering capability to achieve high resolution and accuracy, but compared to SAR ADCs, the sampling rate is lower. SD architectures typically integrate a programmable gain amplifier (PGA) at the front end; in applications with converters per channel, this enables a direct interface between the sensor and the ADC without the need for external signal conditioning.
When measuring low-level signals from thermocouples, strain gauges, and bridge-type pressure sensors, a key requirement is the ability to perform differential measurements to reject common-mode interference and provide more stability in the presence of noise. reading. For example, differential inputs are often used in industrial applications to reject motors, AC power lines, or other common-mode noise that affects the ADC input.
The cost of single-ended input is lower, and the number of channels that can be provided can be doubled with the same number of pins, because they only need one analog input per channel, and these inputs are connected to the same ground point as the benchmark. They are primarily used in applications with high signal levels, low noise, and a stable common ground potential.
Figure 6 shows the various elements used in a discrete isolated PLC input module, including excitation, input signal conditioning, failsafe multiplexers to receive multiple input signals, a programmable gain amplifier, and an A/D converter. Traditionally, most of these were implemented with discrete ICs and passive components, but today they are integrated in ADCs and analog front ends.
Figure 6 What a typical discrete PLC input module can do
These ADCs can interface directly with sensors in a variety of applications, including PLCs, temperature measurement, weighing, pressure and flow measurement, and general-purpose measurement devices. Their refresh rate can be programmed from 4Hz to 500Hz, allowing simultaneous suppression of 50Hz and 60Hz signals at the selected refresh rate.
Industrial system designers of PLCs continue to strive for ever-increasing product performance and functionality at ever-shrinking budgets and board real estate. In order to provide integrated circuits that meet these stringent requirements and strive for every important position in the signal chain, Analog Devices has developed significant new manufacturing process flows. This process technology, known as iCMOS, combines high-voltage silicon integrated circuit technology with sub-micron CMOS and complementary bipolar technology to enable analog ICs capable of 30V operation (required in many industrial applications) while The flat size is smaller, the performance is higher and the cost is lower. The iCoupler isolation technology based on chip-scale transformers (rather than LEDs and photodiodes) can be combined with CMOS semiconductor functionality to provide low-cost isolation. The iPolar trench isolation process allows operation from supply voltages as high as ±18 V, which is far superior to conventional bipolar amplifiers while halving power consumption and reducing package size by 75%. These technologies are well suited to meet current needs and welcome a bright future.