Industrial Monitoring and Control Needs Special Interface and Communications Links

Nov. 12, 2012
Industrial solutions that include a fast easy way to interface RTD sensors to equipment and how dual mode HSPA EV-DO M2M devices make applications more flexible.

Industrial electronics is all about using sensors to monitor and measure physical characteristics for use in tracking performance or feedback control. Interfacing those sensors to the computers and communications equipment is the engineering challenge.

Once sensor data is available, the goal is to transmit it to some remote location for storage or measurement or even remote control. More and more, machine-to-machine (M2M) wireless over the cellular networks is coming into play for this job. Products from Maxim Integrated and Sierra Wireless feature unique and excellent solutions for these interfacing and communications challenges.

RTD-To-Digital Converter

Interfacing a resistance temperature detector (RTD) with an analog-to-digital converter (ADC) isn’t as easy as it looks. In addition to a high-resolution ADC to get the precision promised by an RTD, you need compensating circuitry for self heating and cable resistance. Maxim Integrated’s MAX31865 RTD-to-digital converter can handle the job (Fig. 1).

1. The Maxim Integrated MAX31865 interfaces a standard RTD sensor with a 15-bit ADC for high-precision temperature measurements. The RTD is interfaced to the chip by way of a standard four-wire connection that provides cable resistance compensation. An external reference resistor is needed. The input protection circuits are good for transients up to ±50 V.

RTDs are the most popular temperature sensors in industrial applications. They not only provide the highest precision in temperature measurement, they also can handle temperatures from –200°C to about 800°C. The resistance element is usually platinum, but nickel, copper, and other elements are sometimes used too. The platinum devices are known as PT-RTDs, and the most popular sizes are 100 Ω and 1 kΩ. The RTD is usually connected in series with a reference resistor to form a voltage divider, and the voltage across the RTD or the reference is measured and digitized.

The RTD resistance curve is linear except for a minor non-linearity caused by self heating. It can be ignored for some non-critical applications, but for greatest precision, some compensation is needed. This can lead to some tricky external circuitry to compensate for the second-order and third-order effects.

Compensation is usually needed for the wires connecting the RTD to the circuitry. Most RTDs are housed in a heavy industrial module designed to be mounted directly on the object whose temperature is to be measured. That usually means a long twisted-pair cable. This need causes the cable resistance to become a major factor in the accuracy of the temperature measurement. A short two-wire cable may need no compensation. Longer, four-wire cables are needed to provide the best compensation. A three-wire connection is an option.

The MAX31865 provides all of the needed compensation circuitry. Also, the delta-sigma ADC offers 15 bits of resolution with 0.03125°C nominal resolution. It provides a total accuracy and overall operating conditions of 0.5°C (0.05% of full scale) maximum. Conversion time is less than 21 ms.

The converter’s built-in ±50-V overvoltage protection eliminates the need for external protection circuitry, reducing board space and cost. It also offers automatic fault detection for an open or shorted RTD, cable faults, or an out of range voltage reading. A standard SPI bus is provided for passing the digital temperature reading to an external microcontroller or other circuits.

The MAX31865 comes in a 5- by 5-mm, 20-pin, thin quad flat no-lead (TQFN) package.

M2M Embedded Wireless Module

When designing a M2M product for different cellular carriers, you have to select the right radio technology to be compatible. M2M wireless modules designed to be embedded into industrial products are available for just about any wireless standard. But what do you do if the product must work on multiple yet typically incompatible networks? Or what if you would like one product that could handle any network?

The Sierra Wireless SL9090 embedded module covers almost anything except LTE (Fig. 2). As one of Sierra’s AirPrime SL series of compact, industrial-grade wireless modules, it enables M2M product manufacturers to operate on networks that use either HSPA+ or EV-DO networks. Designers also can address any network technology with a single product. Furthermore, it gives products operating options that formerly did not exist without considerable extra expense, power consumption, or size. The module can be used in fixed or mobile applications in the industrial, field service, energy, networking, smart energy, or transportation sectors.

2. The Sierra Wireless AirPrime SL9090 M2M wireless module covers multiple bands with either HSPA+ or EV-DO technologies. It includes GPS and dual noise-cancelling microphones. The 74-pin land-grid array measures 25 by 30 mm. The multiple heat transferring ground pads on the bottom help extend the operating temperature range.

The SL9090’s radio technology coverage includes two bands of cdma2000 EV-DO Rev. A (800/1900 MHz) and four bands of WCDMA HSPA+ at 14.4 Mbits/s including bands I (2100 MHz), II (1900 MHz), V (850 MHz), and VI (800 MHz). It also includes four bands of GSM/GPRS/EDGE coverage. The SL9090 switches seamlessly between the two different technologies.

In addition, the module includes a full GPS receiver. Antenna diversity is also provided. An internal ARM 1136JS 300-MHz processor is fully integrated. Interface I/O includes UART, USB 2.0, SPI, and GPIO. The module has dual microphones with a noise cancellation feature as well.

The 74-pin land grid array (LGA) package measures 25 by 30 mm in a JEDEC standard form factor. Large multiple ground pads provide very efficient heat dissipation. The extended temperature range is –40°C to 85°C.

If you’re designing an M2M product for the enterprise and need the confidence to ensure long product life in the field despite network changes and upgrades, this module may be your best choice. It also lets you launch services with one carrier but switch later if your business plans or strategies change.

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