Today's portable military devices have strict requirements when it comes to Size, Weight, and Power (SWaP), which can make designing a field-appropriate device very challenging. A successful low-power mil-spec design with extended runtime is possible; however, designers must focus on specific feature and component selection, utilize careful hardware management, and select power-efficient displays and software.

Today's portable military devices have strict requirements when it comes to Size, Weight, and Power (SWaP), which can make designing a field-appropriate device very challenging. A successful low-power mil-spec design with extended runtime is possible; however, designers must focus on specific feature and component selection, utilize careful hardware management, and select power-efficient displays and software.

Size, Weight, and Power (SWaP) are the three most important
elements in portable military device design. If the system is too bulky, it
cannot be carried, and if it runs out of power, it cannot be used. A focus on
power can help solve all three SWaP requirements, because the power needed
directly impacts the size of the required batteries, and larger batteries
compromise the size and weight constraints of portable devices. In order to
design a successful portable device by maximizing power and minimizing size,
electronics and applications must be designed for low power. The problem is
that selection of some feature sets and components or even displays can
significantly reduce battery life, as can running them on inefficient enabling
software.

Accordingly, extending battery life begins with strategic
selection of the feature set and components, followed by careful hardware management. Displays
must also be vigilantly researched and managed, as displays are often the most
power-hungry feature of a portable device. However, displays have become
ever-crucial to successful military device design as evidenced by the increased
popularity of touch screens in such devices. Effective software management
should also be utilized to minimize power consumption (and thus device size)
without compromising system performance.

Targeted feature and component selection

Careful selection of a feature set and components is required
to maximize battery life while maintaining optimum functionality. When choosing
a feature set or components, target only essential features, hence avoiding
spending energy on unused components. Power consumption of common features is
often underestimated, so it is important to understand the power impact of each
major feature. For example, power consumption
of some interfaces, such as UARTs and GPIOs, is negligible, while other
interfaces such as cellular modems are capable of using up to 3W. Unexpected
power consumption can also occur with wired interfaces, which are generally
assumed to be low power. For example, higher-speed wired interfaces, such as
USB or Ethernet, can consume as much power as a wireless interface (Table 1).
This power consumption is in addition to increased processor power to parse
data from these devices.

Table 1: Higher-speed wired interfaces, such as USB or Ethernet, can consume as much power as a wireless interface.

(Click graphic to zoom by 1.7x)

Managing the hardware

After establishing the appropriate feature set and selecting
the necessary low-power components, managing hardware is the necessary next
step in creating a military device that meets SWaP requirements. Power-saving
techniques must be implemented to reduce the power of hardware when not in use,
and thus reduce the need for a larger battery that could compromise the SWaP of
a portable military device. The most common technique is using switchable
voltage rails on unused hardware. The primary advantage of switching off
voltage rails is that the hardware will use no power. This technique will offer
best results when the device interconnection to the rest of the system is
simple such as a USB port or UART. But designers should be aware that when powered-down
hardware has I/O lines going to several powered system components, interference
with the remainder of the system might occur as a result of these lines
behaving unexpectedly. From this state, it is also quite possible that hardware
might not re-initialize smoothly after being switched on again. Proper power
sequencing and re-initialization of the disabled hardware must be executed to
ensure that the device is again working properly.

Another power-saving hardware management technique is the
proper use of low-power modes, including reduced functionality and sleep states.
Unlike switchable voltage rails, the data lines will go to a safer high-impedance
mode that will generally not interfere with overall system operation. Another
advantage is that the device will typically recover more quickly than a
switchable rail when resuming to full operation and will not require as much
re-initialization. A device does
still consume some power in sleep mode, but consumption is generally nominal.
Some components offer limited functionality states where not every feature is
available, or the capabilities are limited. For example, many processors have the
ability to scale core frequency and voltage, which is called Dynamic Voltage and Frequency Scaling
(DVFS). The processor is still running but at a reduced frequency and cannot
process data as quickly. A combination of these hardware management techniques
will substantially reduce the power consumption of a portable military device.

Selecting the right display

When it comes to balancing power budgets with performance,
displays provide a unique challenge – particularly in military devices,
where power and size requirements are among the strictest. Therefore, LCDs are
typically not the best choice because mil-spec devices often require extended
runtime, along with daylight readability as the devices are often used outdoors
in direct sunlight for prolonged periods of time. Although the trend has been
to increase display brightness, this greatly impacts devices' battery life,
leading to a larger battery and device or to shorter runtime. To manage the
balance between display usability and military requirements, a number of
power-savvy display options are available. The most interesting and promising
are new emerging technologies such as Active Matrix Organic Light-Emitting
Diodes (AMOLED) displays and Electrophoretic Displays (EPDs).

AMOLED displays (left, Figure 1) provide high-contrast, vivid
colors, are viewable in sunlight, and do not require a backlight, unlike its LCD
predecessor (right, Figure 1). Most of the power of an LCD is consumed driving
the backlight, which must be on in order to view the display. In contrast, the
organic material used in each cell of the AMOLED emits light when voltage is
applied, hence AMOLEDs do not require a backlight. This greatly reduces the
display’s portion of power consumption in the device when displaying bright
colors on only portions of the display. Another key aspect of AMOLEDs that makes
them well-suited for mobile devices is their high contrast ratio, which is
typically 10,000:1, whereas, LCD displays are typically on the order of 300:1
or 500:1. This high contrast ratio means that when comparing an AMOLED with an
LCD of the same brightness, the AMOLED will be more daylight readable.

Figure 1: AMOLED displays (left) provide high-contrast, vivid colors, are viewable in sunlight, and do not require a backlight, unlike its LCD predecessor (right).

(Click graphic to zoom by 1.9x)

Even more power efficient than AMOLED displays are EPDs, or
electronic paper. EPDs are a persistent, bi-stable display, meaning they only
require power to update the image, not to maintain the image. EPDs have characteristics
of paper and can be viewed in ambient lighting or with night-vision devices.
Commercially, EPDs can be found in products like Sony PRS readers or the Amazon
Kindle. Militarily, the EPD can be found in rugged mil-spec devices, such as
the Soldier Flex Personal Digital Assistant (SFPDA), which was created by
InHand Electronics in cooperation with Natick Soldier Research Development and
Engineering Center and Army Research Lab, Sensors Electronic Device
Directorate, and in collaboration with Artisent, Inc., E-Ink Corp., and the
Flexible Display Center (FDC) at Arizona State University (ASU).

The SFPDA (Figure 2), designed for longer missions in the
field, has a typical power consumption well under 1 W, including device display
and a standard peripheral set. The SFPDA combines both low-power military
handheld and flexible display technology. The SFPDA’s reduced power consumption
is due to EPD low-power characteristics, careful feature selection, and
InHand’s BatterySmart software, resulting in a ruggedized, mil-spec handheld
providing more than six hours of continuous runtime and weighing less than one
pound.

Figure 2: The SFPDA, designed for longer missions in the field, has a typical power consumption well under 1 W, including device display and a standard peripheral set.

(Click graphic to zoom)

Managing software

As mentioned in the SFPDA example, advanced runtime is
pivotal in military devices, and software plays a large part in managing power
consumption. Even with careful selection and management, it is possible to have
a device capable of consuming 3 to 5 W in interfaces and display alone.
Therefore, hardware usage must be carefully managed through software to ensure
sufficient battery life while maintaining performance. Decreasing power consumption through well-written
code is essential to increasing battery life and decreasing device size.

Software applications written for portable devices that make
effective use of processors’ Dynamic Voltage and Frequency Scaling (DVFS) will
dramatically reduce power consumption. Ideally, the system processor should be
fully powered only when needed, then turned back down when the system is idle.
Intense processing from video and communications do not allow processors’
voltage and frequency to be manipulated without affecting performance.
Strategies such as applications using interrupts instead of polling, using
thread suspend states, and attempts to minimize communications will permit the
processor to make the most of DVFS. When managing DVFS, a separate application
will likely be required to prevent specific unit applications from fighting
over the system processor’s state. The example in Table 2 shows how much power can be saved by utilizing a power management
suite. [The example in Table 2 uses InHand Electronics’ BatterySmart software
on a Fingertip4 (COTS PXA270 SBC) running WinCE, which shows more than 500mW of
saved power with just the use of the BatterySmart suite.]

Table 2: An analysis of InHand Electronics’ BatterySmart software on a Fingertip4 (COTS PXA270 SBC) running WinCE, showing more than 500mW in saved power.

(Click graphic to zoom by 1.9x)

Meeting today’s demand for low-power mil applications

Achieving the strict SWaP requirements of today’s portable
military devices truly centers around power consumption, as size and weight are
dependent thereon. Accordingly, achieving low power consumption begins with
feature set and component selections. To minimize power consumption feature
sets must be limited to only the necessities, while employing low-power
hardware management. Often the largest component, displays can compromise
system life and SWaP is not properly chosen. Thus, it is important to manage
all aspects of the design – including utilizing power-efficient software.
With proper software management, system power consumption can decrease up to 30
percent, which, in turn, will minimize size and weight. Ultimately, these
tactics can help engineers successfully design a portable military device that
meets all of its SWaP requirements.

Lee Brindel is a
systems engineer at InHand Electronics, supporting the sales and marketing
teams as the technical lead for embedded systems and managing
customer requirements through the design process. Prior to InHand, he
worked as a project engineer, focusing on utility power management. Lee holds a
BSEE from Purdue University. He can be contacted at [email protected].

InHand Electronics

240-558-2014, ext. 227

www.inhand.com