Smart Microcontrollers in Multi-Sensor Field Transmitter Solutions



Integrated solutions ease the challenge of bringing together sensors and transmitters.

Industrial Transmitter Design

Many industrial automation and control applications require the precise sensing of multiple process or control variables such as temperature, pressure, voltage and flow. The vast majority of these applications still rely on line-powered solutions that provide both power to the system and the communication path between the sensor and the gateway. The wired solution consists of a voltage-controlled current source modulating the loop current in response to sensor signal outputs. The modulated current typically runs from 4mA to 20mA where the minimum range value of the sensor is represented by 4mA and the maximum range value of the sensor output is represented by 20mA. The linear region between these two end points represents intermediate values for the sensor output.

iSeries_BD
Figure 1. Schematic of a voltage-controlled current source

A typical configuration of the voltage-controlled current source is shown in Figure 1. The current through the loop is governed by the current divider circuit defined by R6 and R7 and the reference current through R5. Given the communication loop is based on a current value, the accuracy of the signal is not impacted by the voltage drop in the interconnecting wire. Thus, the distance between the transmitter and the receiver can be up to thousands of meters. In this case, the transmitter does not source the current. Current is drawn by an external voltage source connected to its output terminals. This translates to an additional advantage in that the communication loop provides power to the transmitter itself. Given that the minimum allowed measurement value is defined by the 4mA threshold, the entire system current for the transmitter solution needs to be below this cut-off point, typically below 3.5mA. Thus, margin is available for alarm low readings.

Current Loop Transmitter

A current loop transmitter comprises a sensor, sensor interface, microcontroller and a voltage-controlled current source. Figure 2 shows the block diagram of a typical transmitter. In the case of some microcontrollers, such as the MSP430i20xx family from Texas Instruments, the sensor signal can be sensed directly through the differential inputs of the 24-bit sigma-delta ADCs. Biasing of the current driver for the voltage-controlled current source can be achieved through a pulse-width modulated (PWM) signal with the appropriate duty cycle and filtering requirements. The 16-bit timer module can be used to make an adjustable PWM signal with a theoretical accuracy of up to 16 bits. This output signal of varying duty cycle can then be routed through a low pass filter designed to only allow a DC voltage source from its output. This filter is designed in a way so the cutoff frequency is lower than the PWM frequency, thus ensuring the monotonicity of the voltage output.

Figure 2
Figure 2: Block diagram of a current loop transmitter

As mentioned earlier, the transmitter can either be powered through the communication loop via a 2-wire implementation or through a separate power line not associated directly with the 4-20mA current loop, typically classified as a 3-wire or 4-wire solution. In both cases, a low-dropout (LDO) regulator steps down the current loop supply voltage to power the transmitter. Figure 3 offers a view of both a 2-wire and 3-wire solution. In the 2-wire solution it is important to maintain a current threshold below the 3.5mA in order to ensure the minimum transmitted measurement value of 4mA is achieved with margin for alarm low levels.

Figure 3
Figure 3: Block diagrams of 2-wire and 3-wire current loops

Sensor Interface

A number of advanced industrial market segments use high-performance acquisition systems with multiple channels to manage information on a near real-time basis from accurate industrial sensors. Typical examples include uninterruptible power supplies, industrial power meters and monitors, vibration and waveform analysis, instrumentation and control systems and also data acquisition systems that measure real-world parameters such as temperature, pressure, light, flow and force.

Developers should consider a solution where the differential input of an onboard PGA interfaces directly to a 24-bit sigma-delta ADC. This would enable direct, high-precision sensing of the sensor signal. Having additional independent 24-bit sigma-delta ADCs allows developers to add more sensor interfaces that enable simultaneous sampling of multiple industrial automation or process parameters. In a typical manufacturing example, the ability to simultaneously measure temperature across different regions of a process chamber enables industrial customers to receive an accurate measurement of the temperature gradient in the chamber. This enables a manufacturer to have more precise control of their development environment. Take a semiconductor integrated wafer process flow. Variations from a pre-defined temperature gradient in the process chamber can impact the yield, or at the very least, the performance of the integrated circuit. In these examples, each of the channels would consume an average of only 200µA during conversion. In comparison, other solutions are typically between 0.5mA to 1.0mA per channel. This low per-channel current flow would provide for simultaneous sensor sampling even under the stringent threshold requirements of a 2-wire current loop architecture.

A microcontroller with integrated smart analog acts as a bridge circuit between the sensor and transmitters. It not only performs linearization and control functions before giving it to the transmitter for feedback, but performs the high precision analog functions. This allows direct interface between sensors and transmitter without need for external components. This significantly reduces system power, area and cost with this solution.

Conclusion

A smart microcontroller can interface with various sensors, including temperature, pressure and other voltage output detectors simplifying designs for multi-channel field transmitter application. In industrial solutions where 2-wire implementations continue to be the primary interface, a smart microcontroller with low per-channel analog-to-digital-conversion current allows for simultaneous sampling of multiple sensor solutions to extend the capabilities of the overall solution while adhering to the strict loop current budgets.


Rafael MenaRafael Mena is a systems architect inside of TI’s Microcontroller business and has more than 20 years of experience in the design of electronic and system solutions. Rafael holds a B.S. in Electrical Engineering from the University of Texas at El Paso, an M.S. in Electrical Engineering from Arizona State University, a Ph.D. in Electrical Engineering from Ohio State University, and an MBA from University of Texas at Austin.

Tyler WittTyler Witt is MSP430 MCU Applications Engineer, Texas Instruments. Tyler holds both a B.S. and M.S. in Electrical Engineering from the University of Cincinnati. He is an alumni of the Point-of-Care Systems Design Laboratory (POCSDL), focusing in medical devices research. He has been with Texas Instruments for almost five years with experience as an MCU Automotive test engineer before transitioning into an MSP430 applications role in 2012. Tyler is currently focused on applications specific to the industrial end equipment market space, with a deep focus on sensor transmitter solutions.

Prajakta Desai

Prajakta Desai is a Product Marketing Engineer, Texas Instruments. She has been with TI—primarily focusing on embedded technologies—for more than eight years. She has supported TI’s Microcontroller business for the past three years in marketing low power and performance MCUs. Prajakta has a M.S. in Electrical Engineering from the University of Southern California where she studied VLSI and Microprocessors and an MBA from Southern Methodist University in Dallas.

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