Wireless

Standardization coming for wireless sensor power sources

Documents are in development to define general requirements, batteries and energy harvesting that allow for use of C- and D-cells.

By Ian Verhappen, P.Eng.

With the abundance of wireless devices coming onto the market, one of the requirements they all need is power. Because wireless devices typically aren't connected to a permanent power supply, they also need the ability to store this power, hence some form of energy storage, in most cases a battery. We're all familiar with the standard sizes and connections for the batteries we use in battery-powered devices, which to no one’s surprise are standardized, in this case in the IEC 60086 series.

SC 65B WG16, under the leadership of Ludwig Winkel, who's also the convener for the fieldbus standards, is in the process of developing three standards for "Power sources for a wireless communication device." The three documents in development are IEC 62952-1 Ed 1.0, Power sources for a wireless communication device–Part 1, General requirements of power modules; Part 2, Profile for power modules with batteries; and Part 3, Energy harvesting. Parts 1 and 2 should be published in September this year, while Part 3 will be released by the end of 2017. All three documents are far enough along that manufacturers are able to incorporate the concepts in their devices today.

As with most multipart standards, the task for Part 1 is specifying the general requirements, and because IEC SC65B is concerned with process automation applications, the developed standards must also address the use and replacement of devices in explosive atmospheres. One aspect of this is the temperature classification, so Part 1 states the power module shall be usable for temperature class T4 (maximum surface temperature of 135 °C) at ambient temperature of 60 °C and optionally to 80 °C, while complying with temperature class T3 (maximum surface temperature of 200 °C) at an ambient temperature of 80 °C.

Other mechanical and physical requirements include continuing to operate under the following conditions: ambient temperature range -40 °C to 85 °C with a rate of temperature change of 0.5 °C per minute while the power module (and its connections) shall be able to withstand vibration at 2–9 Hz at 10 mm displacement and 9–200 Hz at 3 g (30 m/s2), as well as Type II shock with peak acceleration 25 g (250 m/s2) with no mechanical damage or interruption of the electrical connection.

Part 1 also defines three different formats that the power source (whose output shall not exceed 20 V) can take: primary or secondary batteries (Type A); a mechanical unit that contains primary or secondary batteries (Type B); or a generic energy-harvesting adapter module (GEHAM) with a backup battery (Type C).

Type A and Type B power modules shall use C-cell or D-cell batteries as specified in IEC 60086-1.

Part 2 builds on Part 1 by defining the battery profile, and provides selection criteria for a battery with a mechanical interface as given by the battery dimensions and electrical characteristics specified in IEC 60086-1 and IEC 60086-2 (primary batteries). Type A and Type B power modules shall use C-cell or D-cell batteries as specified in IEC 60086-1. The profile also specifies the lifecycle of a power module from storage, maintenance and operation, including disposal, as well as guidelines related to transport and exchange/replacement of the power source in an intrinsically safe environment.

The final document specifies requirements and a profile for a power source containing a generic energy harvesting adapter module (GEHAM), and is based on the lowest-common-denominator approach by specifying the minimum requirements to enable suppliers and purchasers to reliably acquire devices that work together from multiple vendors.

Electrically, the GEHAM shall accept, as a minimum, any energy harvester output up to 100 mW and 12 V DC (maximum), while the output voltage shall be DC, non-regulated and the ripple shall not exceed the stated maximum output voltage. The GEHAM output voltage shall be clearly labeled, identifying it as a nominal 5 VDC (for a nominal single, 3.6 V battery load) or 8 VDC (for a nominal dual, 7.2 V battery load).

In cases where a connector is used, IEC 61076-2-101 M12 A-Coded connectors shall be used with male connectors on both devices, meaning the cable ends shall be female on both ends. The connector can use from two to five conductors, and shall follow the following pin out and cable colors: Pin 1–ground (brown); Pin 2–communications (white); Pin 3–power (blue); Pin 4–sense (black); and Pin 5–reserved (gray).

The above represents a significant amount of work by the members of SC65E WG16 that, as you can see, will certainly help with the adoption of wireless field sensors by defining how each of these devices will be powered and the requirements for manufacturers of those power supplies to interface to the devices.