Meeting the challenges of portable military devices with low-power design techniques
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.
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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.
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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.
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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.]
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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@example.com.
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