“As modern industrial networks gradually support advanced protocols, it is possible to remotely monitor and configure sensors on the factory floor in real time, reducing production downtime significantly. However, connecting the sensors and actuators to the wiring closet where the process controller is installed is still labor-intensive and tedious.
By Sean Long, Executive Director of Applications, Industrial and Healthcare, ADI; Konrad Scheuer, Senior Principal Engineer, ADI
As modern industrial networks gradually support advanced protocols, it is possible to remotely monitor and configure sensors on the factory floor in real time, reducing production downtime significantly. However, connecting the sensors and actuators to the wiring closet where the process controller is installed is still labor-intensive and tedious.
For example, to modify the production process, it is necessary to change the digital output (DO) voltage-driven valve to a valve that uses a 4-20mA analog output (AO) current, and the technician must change the wiring in the wiring cabinet. Move the valve connections from the DO channel to the AO channel by routing the wiring to a different IO module, or by replacing the IO card (if using a rack-mount module). A similar situation occurs if a digital input (DI) sensor has to be replaced with an analog input (AI).
While automation engineers will choose IO modules during the commissioning phase of a new process to have enough channels (allowing some redundancy), the increasing number of sensors and actuators over time means fewer and fewer spare channels are available, May result in not enough specific types of channels for subsequent upgrades. A spare DI isn’t of much use to a technician who needs an AI channel if there is no AI channel available. In a wiring closet, it may not even be possible to add a new (and costly) IO module. Manual intervention and associated production downtime increases significantly when different IO channel types require periodic recalibration.
Figure 1. Technician adjusts connections in wiring closet
Therefore, process automation engineers often desire to have a common IO channel that can be configured (and calibrated) remotely to easily perform any function (input or output, voltage or current) of any signal type (analog or digital) without the need for a technician to Operate in the wiring cabinet.
Next, ADI will review the main characteristics of using sensor and actuator signals in industrial environments, and introduce you to a new reference design that uses factory-calibrated, remotely configurable general-purpose IO modules to actually improve wiring cabinets. Work efficiency, accelerate the process of industrial automation.
The DI and DO signals are typically DC voltages in the 0-24V range. DIs are used for discrete liquid level detection, target detection, or to indicate the state of pushbutton switches. DOs are used to drive motors, actuators and solenoid valves. These products are available in a variety of configurations (high side, low side, and push-pull), depending on how the load is referenced, with drive current being the main specification, ranging from hundreds of milliamps to several amps.
The analog IO signal can be 4-20mA current, or a DC voltage typically between 0-10V (although bipolar options and wider voltage ranges are available). AI receives signals from sensors and is used to precisely measure quantities such as distance, pressure, light, etc., while AO is used to precisely control the movement and position of actuators.
In industrial settings, temperature measurement is primarily performed using one of two sensors, a thermocouple (TC) or a resistance-temperature detector (RTD) with 2-3-wire and 4-wire variants. Thermocouples are robust, have a wider operating temperature range, and are relatively less expensive than RTDs, but RTDs are more stable, more accurate, and have better linearity. The signal output level depends on the type of TC/RTD used and can be connected to the AI channel. Robustness (measured in compliance with the IEC-61000-4 transient immunity standard) is a key performance indicator for all types of industrial IO interfaces.
Generic IO Module Reference Design
Increased integration means that in the latest IO modules, a single channel can be configured as an input or output, but the analog and digital domains are still separate. However, Figure 2 shows a reference design functional diagram of a new IO module in which a single general-purpose UIO pin can be configured by software as AI, AO, DI, DO relative to a single ground pin (GND). Configurable modes include analog voltage input (0 to +10V), analog current input (0 to +20mA), analog voltage output (0 to +10V), and analog current output (0 to +20mA). It also includes a 0-24V digital voltage input compliant with IEC 61131-2 Type 1/2/3 and a push-pull/high-side digital output (capable of driving up to 1.3A). It also supports temperature measurements using resistance temperature detectors (RTDs) and provides built-in cold junction compensation for thermocouple measurements. Supports UIO mode and 2-, 3- or 4-wire temperature measurement using industry standard quad PCB terminals.
The AI and AO functions of this module are implemented using the MAX22000. The MAX22000 is a software-configurable analog input/output IC that can operate in voltage or current mode. The analog output signal is generated using its internal 18-bit DAC, while the integrated 24-bit ADC has a low noise PGA with high and low voltage input ranges to support RTD measurements. The DI and DO functions are implemented using the MAX14914A, which supports low-leakage process technology. The MAX14914A is a high-side/push-pull driver that can also be configured to operate as a DI. In addition to providing DIO functionality, the MAX14914A monitors output current in high-side and push-pull modes. The logic level corresponding to the DO state can be polled via the GPIOs on the MAX22000 GPIO, a necessary feature in safety-critical applications.
Figure 2. MAXREFDES185# Universal IO Module Reference Design Functional Diagram
This module uses the industry standard 12-way Pmod™ connector commonly found on many microcontroller and FPGA platforms. For ease of testing, the module can be configured via the software GUI using a USB to SPI adapter (such as USB2PMB2#) to provide a physical interface to the board. The GUI has two tabs – the General IO tab (Figure 3) has a drop-down menu to select analog or digital, input or output configuration. Depending on the selected mode, the GUI displays a simplified block diagram of the IC’s internal connections that enable the currently selected function.
Note: Pmod™ is a trademark of Diligent, Inc.
Figure 3. General IO tab of GUI
The analog input tab can be used for monitoring purposes to visually compare the voltage or current signal at the UIO pin with the signal measured by the 12-channel, 24-bit analog input device, the MAX22005. Hexadecimal values are also provided to easily correlate the two ADC cores.
A major advantage of this module is the ability to use the onboard MAX22005 for voltage and current calibration. The MAX22005 is a 12-channel, factory-calibrated analog input IC that can be used as a reference and can monitor the analog signal on the UIO pin. It is factory calibrated to 0.02% FSR at 25°C and 0.05% FSR at ±50°C. The calibration can be performed by clicking “Autocal” on the Universal IO tab of the GUI. Figure 4 shows the FSR accuracy of the UIO pin and the analog voltage signal on the MAX22005, both of which are much better than the 0.02% FSR expected by precision instruments and show a high degree of correlation.
Figure 4. Voltage Measurement Accuracy
Current measurements have similar accuracy, while Figure 5 shows the accuracy of temperature readings using a Fluke 724 calibrator to simulate a PT100 RTD sensor. Accuracy is within 1°C from -100°C to +300°C and within 0.02% FSR at room temperature. The total accuracy of the entire module reaches 0.1% FSR for a temperature change of ±50°C.
Figure 5. Temperature Measurement Accuracy
The power tracking feature limits the heat dissipation of the module. This is achieved through a combination of low quiescent current linear regulators and high efficiency buck converters. With only 8µA of quiescent current, the MAX17651 provides a regulated 24V supply from the DC input, while the MAX17532 and MAXM17552 step-down converters generate multiple analog output supply voltages, one of which can be programmed to five different presets between 4.2V and 24V. set value. This is done via the GPIO pins on the MAX22005, using an external FET to switch the feedback resistor. The power consumption of this module is usually 10mA under normal conditions, but if the current input or current output mode is selected, the current consumption will increase. A green LED indicates the presence of external power.
Although the module cannot be immediately translated into field applications in its current form, the module still exhibits a high degree of stability when testing the transient immunity of industrial equipment as specified in IEC 61131-2. It can withstand 1.2/50µs surges up to ±1.0kV with a total source impedance of 42Ω. Surge tested (line-to-line and line-to-ground) using 10 surge pulses and the module remained operational without damage. The data and control registers on the device IC are not corrupted, nor is the communication through the host adapter interrupted. The module was also found to be able to withstand up to ±4kV port-to-ground electrostatic discharge (ESD) for contact and air-gap discharges when tested on a field-connect terminal block. No damage was found, and the host communication remained normal after the test. The front view of the module (outside dimension is 75mm x 20mm) is shown in Figure 6.
Figure 6. MAXREFDES185# Reference Design
Figure 7 clearly demonstrates the flexibility and space saving benefits of choosing a single Universal IO module (UIO) instead of several standard modules. A single general-purpose IO module can perform four independent functions and be configured and calibrated remotely via software, while each standard module performs a single function and requires manual configuration and calibration.
Figure 7. One generic IO module replaces several standard modules
In the era of Industry 4.0, industrial equipment is required to be as adaptable and flexible as possible. However, as of now, manual rewiring and calibration of the IO interface has become an unavoidable limiting factor. Now, the ADI MAXREFDES185# remote configurable IO reference design can provide a clear roadmap for future IO modules, which can effectively improve flexibility and configurability. In addition to IO modules, this reference design and its component ICs are also suitable for applications in PLC and DCS systems, smart sensors and actuators.
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