When endeavoring to reduce the energy usage of an existing environmental monitoring system, the first step toward determining the appropriate methods to apply is to define the extent of permissible modifications. The answers to the following questions can help focus energy reduction efforts where they are most appropriate for a specific system.
• Can printed circuit board layouts be altered, necessitating the construction of new monitors or the replacement of entire circuit boards in existing monitors? Or, must any hardware modifications be performed on existing circuit boards? • To what degree can the firmware be modified? Is it feasible to deploy
firmware updates on existing monitors?
• Is the monitor run-time configurable, and is it feasible to change and deploy a new configuration to reduce energy usage?
When answering the questions, it is important to keep in mind that all
modifications must be thoroughly tested, and the effectiveness of the modifications must be measured and evaluated before performing the modifications on a large number of monitors.
Next, it is essential to perform energy usage measurements in order to understand the energy usage characteristics of the subsystems and components in the monitor and to establish a baseline that can be used to evaluate the effectiveness of the modifications. Absolute accuracy of the estimates is not as important as consistency across the
measurements so that the relative contribution of each subsystem or component can be assessed. Where it is not feasible to take measurements, estimates based on component
data sheet information may suffice. In fact, a quick inspection of the supply current values appearing in module and major component data sheets can help focus
measurement and estimation efforts.
After the energy usage characteristics of the monitor are understood, the best candidates for reducing energy usage can then be identified. Initial energy usage
reduction efforts should target the areas of highest energy usage first since they are where the largest potential for energy savings can be found. However, small reductions that can add up should not be overlooked.
The following guidelines can help find the ways in which the energy reduction methods described previously can be applied.
• An analysis of possible energy savings from processor voltage and clock frequency changes or dynamic scaling would be worthwhile if it is feasible to make the related hardware and firmware changes.
• Look for opportunities to lower the duty cycle of high-power-use components. For example, a sensor module that uses 30% of the total system power and can be placed in a low-power idle mode when measurements aren’t being taken is a prime candidate. If extensive firmware modifications and testing can be performed, task scheduling combined with putting the processor in sleep mode between tasks might significantly reduce energy usage. • If the related hardware modifications are feasible, look for lower-current
alternatives for wireless communication and sensor modules, integrated circuits, and voltage references.
• Look for voltage dividers that could use higher resistances to reduce current flow while still providing adequately high stability and low output impedance. • Look for improvements in circuit design that could prevent power from being
wasted. For example, make sure that unused operational amplifier sections are properly biased to reduce energy usage and prevent oscillation. Or, better yet, replace the multi-op-amp device with one having fewer op-amp sections and lower power requirement.
• Look for components that are not needed during normal monitor operation and can be disabled or eliminated. For example, the driver for a serial port that is used only for debugging could be powered through a jumper that is disconnected when the monitor is deployed for normal use.
• Consider making circuit changes that could increase the amount of usable energy supplied by the battery. For example, if the monitor has a low-voltage shutdown feature, perhaps the low-voltage shutdown threshold could be reduced when operating from battery power in order to safely extract more energy from a primary battery or each charge of a secondary battery. Finally, the order in which the modifications are to be made should be
determined. The easiest modifications should be performed first, especially those that provide the greatest energy savings. After the easier modifications are made, it is important to measure the energy usage change in order to validate the original energy usage estimates as well as the reduction estimates. Also, it may be possible to deploy an intermediate version of the monitor in order to benefit sooner from the energy savings and increased operating time. The more difficult or time-consuming modifications
should be performed after the easier modifications are complete and their effects
evaluated. This approach helps ensure that the measurement, estimation, and evaluation methodologies are sound before investing additional time and effort.
CHAPTER FOUR: BATTERIES FOR ENVIRONMENTAL MONITORS Selecting the appropriate battery1 is essential to obtaining the desired operating time from an environmental monitor while still meeting other requirements of the
monitoring application, such as monitor size, cost, and reliability. This chapter discusses the battery features and characteristics that are pertinent to the selection of a battery for use in an environmental monitor. It begins by presenting the two fundamental classes of batteries—primary and secondary—in the context of their use in environmental monitors. Next, it lists and describes the battery characteristics that are most relevant to selecting a battery for use in an environmental monitor. It then expands the discussion of battery capacity by explaining the effect of discharge rate on the amount of energy a battery can actually deliver. Finally, the chapter concludes with guidelines for selecting the
appropriate battery for a monitor and application.