Design Considerations for M2M Products

Machine-to-Machine (M2M) products are showing up everywhere, from electric utility meters to vending machines, and companies are quickly jumping into this fast growing market. Recent market research predicts an M2M market compound annual growth rate (CAGR) of 25%+ over the next 5 years¹ and the number of connected devices in the U.S. growing from 27 million to over 120 million in 2017². M2M is still a relatively new market that has finally taken hold as the major cellular carriers have embraced it and significantly lowered the cost barriers by providing low cost data plans targeted directly for M2M devices.

The type and number of M2M applications is rapidly expanding as companies realize the benefits and cost savings that can be realized with M2M devices. With the large number of devices being deployed, design engineers must be careful to consider all aspects of the system to make sure the hardware solution can properly support the application.

Basic Design Factors

The basic M2M product consists of a power source, an embedded processor board with network interface (cellular, Wi-Fi, Ethernet, etc.), sensors and an enclosure (Fig. 1).

1. This block diagram shows the main components of a basic M2M device. It consists of one or more sensors, an embedded controller, a communications link, typically wireless, and a power source, often a battery.

Design engineers new to M2M devices may be surprised by the complexity of design issues when combining a network, sensors, software and embedded processor board for an M2M product. Items that should be considered when designing and specifying an M2M product include the following:

Environmental considerations - Indoor, outdoor, cold, heat, rain, snow, humidity, salt, wind, shock/vibration.
Mechanical form factor - Size, weight, enclosure mounting options, DIN rail, pole and wall mount.
Power budget and source - AC, DC, battery, solar, operating power requirements, low power and sleep modes.
Network connection - Cellular GSM/CDMA, Wi-Fi 802.11abgn, Bluetooth, ZigBee, Ethernet.
Sensor interface requirements - RS232, RS422/RS485, Digital I/O, 1-Wire Interface, USB, ZigBee, Bluetooth.
Embedded processor board requirements - CPU type, processor speed, operating system, programming language, data storage.
Cost - Overall system and lifetime cost including power source, computing board, network interface, sensors and enclosure.

In the following sections, we’ll take a closer look at each of the above considerations and discuss the various tradeoffs and impact to the M2M product.

Environmental Considerations

The operating environment for the hardware will have a large determining impact on the type of hardware and software that can be used along with the overall product reliability and system cost. A majority of M2M applications are in outdoor or industrial environments and require operation over a wide temperature range. Off-the-shelf devices are typically specified as commercial temperature, 0 to 70°C, or industrial temperature, -40 to +85°C. The price difference for industrial temperature hardware varies widely, but as a rough metric one can add approximately 15-30% to the device cost. In addition, many hardware devices are not offered in industrial temperature versions so it is important to make sure this is specified early on when selecting the building blocks to use in a design.

Another factor to consider with operating environment is the type of enclosure required to protect the device from the elements (rain, snow, wind, humidity, etc.). If the unit is to be installed in an unprotected area, an IP or NEMA rated enclosure will be needed. In general, for outside environments an enclosure with a rating of IP65 or better should be used. See the Table for a quick summary and description of the IP ratings. (Note: IPxx ratings are given as “xx” combinations, where the first “x” is protection against solid objects and the second “x” is protection against liquids. Example IP67 = protection from dust and water immersion up to 1m.)

IP ratings for NEMA enclosures for protection from solid and liquid contaminants.

Enclosures with higher IP ratings are more expensive but one should always specify an enclosure that meets the environmental requirements. Moisture exposure is one of the top failure causes for outdoor installations. Other environmental factors such as salt spray, humidity and wind should be taken into account when specifying enclosure material, construction and mounting. Marine environments are especially harsh due to the salt spray and the designer should make sure to use powder-coated enclosures and stainless steel hardware for these types of installations. Other options for highly corrosive environments include conformal coating and/or potting the electronics in silicon or epoxy. Enclosures should also be adequately grounded for installation in locations that experience electrostatic events such as lightning or large motors starting.

Mobile environments, such as trucks, trains, busses, unmanned aerial vehicles (UAV’s) and heavy construction vehicles, pose a special set of challenges for the hardware due to the high levels of shock and vibration. The designer must make sure to consider this when choosing both the hardware and enclosure. The radio modules used for M2M applications typically come in different form factors. For cellular connectivity, there are USB dongles, PCIe Mini-card modules and manufacturer specific custom modules. USB dongles are typically used for laptop connectivity but are not well suited for embedded M2M applications. PCIe Mini-card modules have become a popular standard over the last several years with many different manufacturers offering products that support the different cellular standards (2G/3G/4G, GSM, CDMA, etc.). The PCIe Mini-card form factor also works well for mobile environments since the card is secured on one side by the connector and the other with screws to prevent it from shaking loose (Fig. 2). Manufacturer specific custom modules can also be used, but the designer must make sure the mounting scheme and connector system can handle the shock and vibration.

2. PCIe Mini-card is securely mounted on a Gateworks GW2380 embedded processor board.

Mechanical Form Factor

Many M2M applications require the hardware to be embedded into another device or sensor, so size and weight may be a concern. Smaller designs allow more flexibility in mounting. If the design uses off the shelf PCIe Mini-card radio modules then this will be the limiting factor. The PCIe Mini-card specification is 30 x 51 mm so a processor board will need to be at least 35 x 60 mm. PCIe Mini-cards in half sizes are now becoming available as well, which reduces the radio module down to 30 x 26.8 mm. As this form factor gains in popularity, it should pave the way for even smaller products in the future.

Enclosure mounting is also something that must be considered. Many industrial/factory based M2M applications use DIN rail mounting. For outdoor applications, pole or wall mount enclosures are typically used. For mobile environments, shock mounting with rubber bushings will help to reduce the device’s exposure to shock and vibration.

Power Budget and Source

After completion of environmental considerations, determination and selection of the enclosure, and understanding the mechanical form factor of the electronics hardware, the next step is selection of the electronic hardware.

Most embedded processor boards require a DC input voltage for power. This DC input is then converted using onboard DC/DC converters to create the other voltages needed by the integrated circuit (IC) devices on the board. Some boards will require a specific DC voltage and others a range of voltages. Boards with wider DC input operating ranges will give the designer more flexibility when choosing the power source. A typical DC input would be 9 to 18 VDC. If an AC voltage source is the only one available, then an AC/DC power supply will be needed.

Typical power requirements for the embedded processor board are in the 3 to 10 W range depending on the CPU speed, peripherals and board features. Wireless radios must also be factored into the power budget. Depending on the type of radio and the output transmit power, this can range from less than 1 W to over 20 W. Cellular radios typically require 1 to 1.5 W when idle and up to 4 W when transmitting. Wi-Fi radios can range considerably depending on output power. Typical Wi-Fi radios require 1 to 2 W when idle and then up to 4 to 6 W when transmitting. Some high power radios can require over 20 W when transmitting. A rough rule of thumb is the radio power requirement when transmitting is 10x the actual RF output power. For example, a 400 mW Wi-Fi radio in general will require 4 W of power when transmitting. The system designer must plan for these peak transmit power demands when designing the power system even though the device may only transmit the data once a day or week.

For remote locations, where no power sources are available, a solar cell with battery charger may be used or just a battery by itself depending on the application and how long the unit must operate. The M2M device in many cases can be powered directly from the battery. For battery powered applications, the power requirements of the embedded processor board and radio must be considered to ensure the system is sized correctly. Depending on the application, the designer may be able to put the embedded processor board into a low power or sleep mode between data reporting intervals to save battery life.

Network Connection

The network connection between machines can be through wired or wireless interfaces. Depending on the environment, application and availability of services, different network connection methods may be deployed. With the decline in pricing and the build out and expansion of cellular network coverage, GSM and CDMA are becoming popular for serving remote locations where the availability of Wi-Fi and internet are limited and only a small amount of data needs to be transferred. The price of a cellular data connection has come down dramatically in the last several years as the carriers have developed specific M2M data plans. Depending on number of devices and commitment term, monthly rates start near $3 per device.

Standard 802.11abgn Wi-Fi can be used to wirelessly network or mesh together multiple M2M devices and relay information back to a wired access point. Wi-Fi is also suitable for applications that require high bandwidth data rates such as video. Wi-Fi may also be used with directional antennas to allow long distance dedicated links between units. Ranges from several miles to over 30 miles can be achieved. Wi-Fi also supports various data encryption methods, which can be crucial for applications where sensitive data is being transferred. Other wireless technologies such as Bluetooth and ZigBee can also be used for connecting back to the network, however, they have limited range and bandwidth compared to Wi-Fi. Both Bluetooth and ZigBee have very low power requirements. Typically these protocols are used for communication to other sensors in the network.

Not all M2M devices need to communicate over a wireless network; in many cases, if a wired Ethernet network is available, this is the best choice. Wired connections have the advantage of better immunity to interference, better security and higher bandwidth. Ethernet is typically available on embedded processor boards so there is no additional cost impact.

Sensor Interface Requirements

It is difficult to categorize the wide variety of sensors available for M2M devices; however, there are some standard interfaces that are more commonly supported. Many manufacturers specialize in making sensors or general purpose interface boards, which can be used with embedded processor boards for gathering various types of sensor data. The following list summarizes the most common interfaces.

RS232 Serial - This serial standard has been around since 1962, but is still widely used today for low speed communication between embedded processor boards and sensors. This is a point-to-point protocol with a maximum distance of approximately 50 m. Longer distances can be achieved with special cables. At a minimum, three conductors are required. The hardware protocol is simple and widely supported by most embedded processor boards. Software support is also built into most embedded operating systems allowing for quick deployment.

RS422/RS485 Serial - This standard is similar to RS232, but uses differential signaling with twisted pair wires to allow longer distances between devices, up to 1200 m. Multiple devices can be connected to the same wires (multi-dropped nodes) allowing a single RS485 port to support up to 32 nodes. Communication is based upon a master-slave protocol so the software to support this protocol is more complex.

Digital I/O – This is typically used for a short distance, direct sensor connection. Typical embedded processor boards have several digital I/O available for interfacing to sensors. Most support standard 0 to 3.3 VDC logic levels and can only drive small amounts of current (8 to 16 mA). The designer must be careful when connecting to other devices that can create transients and potentially interfere with these signals or damage the electronics . External buffering or circuit protection may be required. Depending on the device, some custom software may need to be developed for monitoring and setting the digital I/O bits.

1-Wire Interface - This is a low speed system bus originally designed by Dallas Semiconductor Corp. that allows communication over a single wire (plus ground). There are a variety of devices available ranging from temperature monitors to high voltage digital I/O. Several open-source software drivers and examples exist with 1-Wire protocol support; however, some software integration may be required.

Universal Serial Bus (USB) - USB was developed in the mid 90’s and has quickly become a true universal serial bus in that the number of sensors/devices with USB interfaces has grown exponentially over the last several years. USB supports several data rates with transfer speeds up to 480 Mbps for USB 2.0 devices. The newer USB 3.0 specification supports 4.8 Gbps; however, it will take some time before current devices migrate to this newer specification. Software support for USB has also matured and drivers are generally available for most devices. The designer should, however, make sure drivers are available for the specific operating system that will be used. Power for USB devices is provided over the bus; so in many cases, the sensor/device can be directly powered over this connection. The USB 2.0 specification allows for cables of up to 5 m.

ZigBee and Bluetooth - Both of these wireless interface standards were developed in the late 90’s for short distance, low speed connections between devices. Bluetooth can support from 2 to 8 devices with what is referred to as a piconet topology. Only one master is allowed with multiple slave devices. Typically, most Bluetooth connections are point-to-point. ZigBee allows for more complex networks and supports peer-to-peer, star and mesh topologies. ZigBee is gaining momentum in the home automation market and many newer water, gas and electric meters support communication over ZigBee. Software support for ZigBee and Bluetooth typically requires specialized drivers so the designer should make sure drivers are available for the specific devices and operating system that will be used.

Embedded Processor Board Requirements

The previous sections have discussed different considerations that must be taken into account before selecting an embedded processor board. The number of embedded processor boards on the market is considerable, so the design engineer should first focus on the base set of requirements to help narrow down the candidates. Once a set of boards has been chosen, other factors to consider include CPU type, processor speed, operating system, on-board storage and hardware interfaces for supporting different network interfaces and sensor types.

The operating system and availability of software drivers plays an important role in the embedded processor choice. Different network protocols require dedicated drivers and protocol stacks. The two most common operating systems supported are Windows and Linux. For an embedded device that does not require a graphical interface, embedded Linux has become the clear choice. Using an embedded processor that can support Linux will greatly speed up development time and allow for future expansion and the ability to quickly support new technologies and sensor types. For high-volume applications, the designer may consider a dedicated microcontroller running a lightweight operating system; however, the development effort will be significantly more involved and future device support may be limited.

The minimum suggested requirements for an embedded Linux processor board are: 150 MHz or greater CPU speed, 32 MB DRAM memory and 4 MB Flash storage. There are many processor types available such as ARM, MIPS, PowerPC and x86. Each of these has advantages and disadvantages; however, in recent years the ARM architecture has taken a lead for embedded Linux devices. Many of the latest phones and tablet computers are based upon ARM processors. The ARM architecture offers low power consumption with excellent performance.

For network connectivity, most embedded processor boards support Ethernet out of the box. If the device requires a wireless interface, the Mini-PCI and PCIe Mini-card form factors, borrowed from the notebook market, have become popular in embedded designs. Many manufactures support these form factors, so there are a lot of choices for both Wi-Fi and cellular radios. The PCIe Mini-card is the primary form factor supported by cellular modem manufactures. It should be noted that embedded cellular modems use the optional USB sideband signals on the PCIe Mini-card connector for communication, which not all embedded computer boards support. The designer should check with the embedded computer board manufacture to make sure USB is supported on the PCIe Mini-card connector.

Data storage requirements must also be considered when choosing a board. In the past, hard disks (rotating media) would be used, but now with the declining cost and increased density, solid state Flash memory is the preferred media. Most embedded computer boards will contain some on-board Flash, which is used for the operating system, and then provide a separate Flash expansion connector for supporting Compact Flash, SD or microSD storage cards.

Long term product availability, consistency and reliability are additional factors that should be considered when choosing an embedded computer board. Embedded designs typically take from 12 to 18 months to go from concept to production, so the designer needs to make sure the board manufacture can provide consistent, revision controlled product and guarantee long term product availability over the lifetime of the project.

Cost

All of the design considerations discussed here have a direct impact on system cost. The designer must carefully balance all these considerations to come up with a cost effective solution. One trap many designers fall into is not looking at the product cost over the lifetime of the product. Product reliability impacts this greatly; one service call can easily wipe out any savings from using a less robust enclosure or lesser quality product. Paying a little more up front for better quality parts can save money in the long run.

Example Application

The following is an example energy monitoring application for reading the power and temperature of a transformer. This application had the following requirements:

Outdoor installation with exposure to the elements.
Needed to be compact and light weight. Mounted to power pole.
Powered from 240 VAC. If AC fails, the unit needs to send out a last gasp set of data using a backup battery.
GSM cellular for network connectivity with variable data reporting time ranging from every 5 minutes up to once every 24 hours.
Need to take data from a power meter that communicates over RS485 and temperature information from a 1-Wire temperature sensor. The data is to be posted to a remote SQL server on the internet.
Embedded processor board with PCIe Mini-card connector for cellular modem support. Requires extended temperature operation and Linux operating system.

After considering all the above, an off-the-shelf platform, consisting of an embedded processor board and sensor mezzanine board from Gateworks Corporation were chosen. The Gateworks GW2380 embedded processor board met all of the above requirements. It features a 300 MHz ARM9 CPU, 128 M DRAM and 16M Flash memory. The board operates over the industrial temperature range and contains a PCIe Mini-card connector with USB support allowing it to work with a variety of GSM/CDMA cellular modems. Gateworks also offers a companion sensor mezzanine board, GW16067, which adds RS232, RS485 and a 1-Wire interface. Additionally, the GW16067 can be configured with an onboard AC/DC power supply allowing the unit to be directly powered from a 120/240VAC source. The combination of the GW2380 and GW16067 make for a highly integrated, full featured package (Fig. 3).

3. This is a Gateworks GW2380 embedded processor board with GW16067 sensor interface board and AC power module.

For the cellular radio, an Option GTM671 PCIe Mini-card modem was chosen. This unit is highly integrated and can be used for both GSM and CDMA carriers. The modem contains a SIM carrier socket and microSD socket for external data storage. Additionally, the unit has Wi-Fi and GPS.

The GW2380 runs an open-source Linux distribution called OpenWRT. OpenWRT is optimized for highly embedded wired and wireless networking applications. Software for the unit consisted of driver software for the Option modem, a 1-Wire temperature sensor driver, RS485 serial driver and some shell scripts to push the meter and temperature data up to a remote web server running MySQL (http://dev.gateworks.com/powermeter/) using standard networking protocols. See Figure 4 for a screen shot showing a graph of some sample power/temperature data.

4. Screen shot of remote web server showing data of power and temperature variation over time.

For packaging, a temporary off-the-shelf enclosure was used with a cable gland for bringing out the interface cables. For mass production, a custom enclosure would be fabricated to reduce size and weight. See Figure 5 for the complete unit with enclosure.

5. Complete M2M unit showing sensors, main system board and enclosure.

Conclusion

This article has presented many of the design considerations that should be taken into account when designing M2M products. Looking at these considerations early in the design phase will help the designer to create a more robust and reliable product. As the M2M market segment grows, many companies are creating dedicated building blocks that can help simplify the design process and allow for quicker time to market.