How to Power Smart Multivariable Sensor Transmitters

Autor / Redakteur: Timothy Hegarty * / Dipl.-Ing. (FH) Thomas Kuther

Power supplies for smart transducers in industrial applications must meet the highest requirements. This article presents a transmitter design with BLE connectivity for a multivariable sensor.

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Practical implementation: A two-variable sensor transmitter with BLE connectivity
Practical implementation: A two-variable sensor transmitter with BLE connectivity
(Source: Texas Instruments)

Field-sensor transmitters used in applications for industrial automation, process control, actuator control, and home/building automation are used to measure temperature, pressure, displacement, proximity, and many other variables. The sensor electronics includes the sensor analog front end (AFE), a low-power microcontroller (MCU), high-precision data converters [both analog-to-digital converters (ADCs) and digital-to-analog converters (DACs)], input amplifiers, output drivers, and perhaps isolation. The sensor transmitter must communicate the sensed parameter data efficiently and reliably to a data aggregation point—for example, a host programmable logic controller (PLC) within a factory-automation environment.

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There are several options available for both wired and wireless connectivity that have enabled developers of intelligent-sensor designs to deploy advanced functionality and features such as multivariable sensing, [1-3] remote calibration, and advanced system-level diagnostic capabilities. Illustrated in Figure 1 is a block diagram of a multivariable sensor transmitter that measures relative humidity (RH) and temperature. [1] Specific applications include demand controlled ventilation (DCV) systems, smart thermostats and room monitors, fire-safety systems, refrigerators, printers, white goods, and medical devices. The system uses Bluetooth Low Energy (BLE) to broadcast to nearby Bluetooth-enabled peripherals. Optimized for low electromagnetic interference (EMI), a synchronous buck converter with wide input-voltage range (wide VIN) provides a low-noise 3.3-V supply rail for the sensor, MCU and DAC loop driver. [4]

Interfaces with Wired Communication

An example of a commonly-used interface with wired communication is the traditional 4- to 20-mA analog current loop that remains a very popular solution for long distance, one-way communication in noisy industrial environments. Illustrated in Figure 2 is the basic current-loop architecture, the convenience being that power is also derived from this two-wire connection as long as a minimum loop-current threshold is not exceeded. [4] Important considerations are involved to program and have bidirectional communication with remote sensor nodes, and have them operate reliably for long periods of time on low power. To exploit the full potential of digital field devices while retaining the traditional 4- to 20-mA loop circuit, the HART protocol offers a complementary mode of communication. [2] It not only delivers additional sensor data but also supplementary information in the form of remote diagnostics, system troubleshooting, or preemptive maintenance where it can be used to enhance the safety integrity level (SIL) rating of a system.

Aside from the 4- to 20-mA analog loop and other wired industrial protocols such as RS-232 and RS-485, IO-Link (standardized as IEC 61131-9) is an increasingly popular and cost-effective digital interface that uses a three-wire connection for linking sensors and actuators in industrial automation and control environments. [3] IO-Link lines, L+ and L–, designate the 24-V supply and GND lines, respectively, and C/Q is a bidirectional-data signal line. However, IO-Link’s point-to-point communication is limited to a maximum distance of 20 meters.

Interface Options with Wireless Sensors

Wireless-connectivity options can be demarcated by frequency band into sub-1 GHz for long-range and 2.4 GHz for short-range communications. For example, the utility grid developers of a smart-metering system might decide that the longer signaling range of a sub-1-GHz wireless protocol is best suited to their application. Meanwhile, intelligent-sensor applications with low-power and shorter range requirements may expand functionality with BLE or ZigBee implementations to provide features such as beaconing, over-the-air updates, smart commissioning, remote displays, and more.

Loop-Powered Sensor Trans­mitter with BLE Connectivity

Based on the system shown in Figure 1, Figure 3 shows a practical implementation of a temperature and relative humidity sensing, sensor transmitter with 4- to 20-mA wired and BLE wireless connectivity. [1] The solution size is 45 mm by 45 mm on a single-sided, 4-layer FR4 PCB. The essential circuit components are detailed in Table 1.

Optimized for micropower applications, the CC2650 MCU is uniquely flexible as it can be dynamically configured in both hardware and software to support one of several different 2.4-GHz radio standards, allowing communication with both ZigBee- and Bluetooth-based devices. BLE is the protocol of choice in this implementation, given its low power consumption, the availability of a full-featured Bluetooth-4.2-certified software stack, [5] and a wide ecosystem of devices.

Meanwhile, the HDC1080 humidity and temperature sensor uses an I2C interface to the MCU and is factory calibrated to provide measurement accuracies of ±2% and ±0.2°C for RH and temperature, respectively. The MCU in turn communicates over a SPI interface with a DAC161S997 loop driver to send humidity data with 16-bit resolution over a 4- to 20-mA loop. 0% and 100% RH correspond to loop currents of 4 mA and 20 mA, respectively.

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