One of the critical and often overlooked components of any signaling system is the power supply, especially for wired systems. However, as we know with wireless field devices that are normally operating from some form of energy storage device or the preferred term ‘power module,' typically a battery, a lot of effort goes into minimizing the demands made of the power source. Typical energy management techniques include putting the device in sleep mode for extended periods and only waking it up when it is time to make another process measurement, and then comparing that against the most recent measurement, and if it has not changed outside a predetermined dead band going, back to sleep for another cycle. The result is that we only wake up the entire device and transmit a signal when the process has "changed."
Despite these techniques, the need for a power source remains, and as we know from the purchase of "regular" batteries versus the "custom" batteries often found in digital cameras, having a defined interface certainly helps with overall system reliability, because we can interchange the power source as required. That's one of the reasons why the ubiquitous A, AA, C and D batteries are based on standards. That being said, it should come as no surprise that standards for wireless sensor nodes are also in development. The standard, which is close to completion, for defining similar requirements for industrial wireless systems is IEC 62952-1 Ed 1.0 "Power Sources for a Wireless Communication Device – Part 1: General Requirements of Power Sources."
When complete, the IEC 62952 series will have three parts: Part 1 – General Requirements of Power Sources; Part 2 – Battery Profile; and Part 3 – Energy Harvesting Specification. Here are some of the general requirements and definitions from Part 1 that will apply to all industrial wireless device power sources.
The power modules are classified into three types: a primary or secondary cell (Type A); contain primary or secondary cells (Type B); or consist of a generic energy harvesting adapter module (GEHAM) to connect to an external source of energy with a backup battery (Type C). Regardless of the type of power module used, the supply voltage must not exceed 20 V.
To insure interchangeability independent of supplier or manufacturer, the power module Type A or the cells in the power module Type B shall be functionally the same. For energy harvesting systems, the GEHAM adapter module shall match the power modules with respect to size and electrical power. In order to ensure reliable operation and mechanical interchangeability, outer dimensions form and weight of the power module Type A, Type B and Type C shall be the same and shall be within the defined tolerance, with the tolerances to be defined in Parts 2 and 3, The types of GEHAMs themselves will be specified in IEC 62952-3.
In addition, the electrical connector(s) shall be within defined tolerance to IEC 62952-2 or IEC 62952-3 (which is expected to pull heavily on the 90% completed, but never published ISA 100.18 document), in order to ensure required interchangeability. Special protections against environmental impacts such as corrosion, humidity and how to perform power source replacement in a hazardous environment will also be included in the connector specifications.
The interface specifications include measures to ensure the correct connection (i.e., polarity) of the power module. Connectors between the power module and the device are likely to be classified into the three categories as defined in the ISA100.18 document as ‘fixed and sealed,' terminal blocks (flying lead) or a pin connector, such as an M12 interface with defined connections, such as ground, power/positive, sense, etc., for each pin.
In addition to the above, IEC 62952-1 also includes minimum requirements for the power module with respect to the surrounding environment, including continuing to operate in an ambient temperature range between -40 °C to +85 °C with rate of temperature change of 0.5 °C / min. In addition, the power module shall be usable for temperature class T4 (<135 oC surface temperature) at ambient temperature of +60 °C and class T3 (<200 oC surface temperature) at ambient temperature of +80 °C.
Though not yet complete, and likely to take some time because many manufacturers have invested in the form factor they are presently using and our penchant for backwards compatibility, the good news remains that wireless power sources will someday be standardized. Technology will also help, since improvements in power cell technology will make it possible to develop smaller form factor power sources compliant with the specifications that will be able to fit, including the adapters, within the space being used by the present power sources.
As you can see we might consider power sourcing to be a simple process – and it is, once we use standards to make it that way.