The Brainstorm: Wireless Power Sensor Supplies

In the Product Design & Development Brainstorm we talk with industry leaders to get their perspective on issues critical to the design engineering marketplace. In this issue, we ask:

What specifications/conditions should you consider when selecting wireless sensor power supplies?

Eric Eisenhut, VP Sales and Marketing, Kionix

Mobile consumer and healthcare electronic devices are relying on context awareness to enable more intuitive user experiences and application development. Sensors that can detect orientation, movement and tilt are often based on inertial sensing.

Key application considerations when determining the power budget for a wireless sensor product are dependent on the sampling rate, bus communication, mathematical analysis and supply voltage. Increasingly higher sampling rates of sensor data are needed in order to achieve accurate orientation and gesture performance.

These high sampling rates can cause unacceptable high levels of power consumption and the higher sampling rates will consume communication bus bandwidth if system processors are used. It has become evident that sensors are integrating more intelligence so that raw sensor data can be processed internal to the sensor and then passed to the system as an instruction or command. It is also valuable to have variable, selectable resolution when selecting a sensor.

Being able to select 8- or 12-bit resolution on the fly based on application needs is an advantage for power consumption.

The best power supply for a wireless sensor depends on the application and its requirements. A long-life, energy harvesting wireless sensor operating from a rechargeable battery or supercapacitor presents unique design challenges. Obviously, these types of wireless sensors need to have extremely low energy draw from the power supply.

One of the most important specifications is the operational lifetime of the power supply, which must meet or exceed the deployment lifetime of the application. The lifetime can be crudely estimated from the manufacturer’s shelf life specification. However, one must consider the self-discharge current, number of recharge cycles and the depth of discharge between recharges. All of these factors will affect the overall lifetime of the power supply and will vary widely with battery chemistry, supercapacitor construction and harvesting technology.

The electrical specifications of the storage element must also be considered. Voltage, capacity, ESR and peak current must be examined to ensure the specifications match the active and sleep requirements of the wireless sensor. Other important specifications include temperature range, energy density (size), weight and the always important, cost.

All of these specifications are not mutually exclusive. They must be examined as a whole to determine the affect one will have on another.

The ongoing developments in energy harvesting will have a significant impact on the remote sensors of the future. Many try to utilize the subtle energy sources throughout our environment. Although these are only able to produce small amounts of energy, they are usually large enough to sustain a remote sensor indefinitely. One such example is the use of a thermopile attached to hot pipes to produce energy for industrial pressure and temperature sensors. 

The voltage converter must be highly efficient so energy is not wasted. At first thought, a switching regulator may seem to be the most logical choice; however for many remote sensor applications a low dropout (LDO) linear regulator can provide a more efficient and appropriate power solution. Although LDO regulators are often labeled as having poor efficiency compared to switching supplies, comparable efficiencies can be obtained under optimal input and output voltages.

Batteries such as the primary LiMn02 battery have flat discharge curves that can ensure good LDO regulator efficiency across the entire battery lifetime. Most remote sensors spend relatively long periods of time in low power sleep modes compared to measurement and transmit operational modes. In sleep mode, the current consumption is very low and can approach the quiescent current of a voltage converter — becoming a major consumer of the sensor’s battery life.

The quiescent current of an LDO regulator is typically much less than a comparable switching regulator. During the measurement and transmitting period, relatively large peak currents can be demanded from the power supply. Switching regulators can be slow in reacting to these currents and often require additional bulk capacitors to meet the peak current demand. LDO regulators are able to respond much faster, reducing power lost during the transition period and reducing bulk capacitance requirements.

When selecting Wireless Sensor power supplies there are a number of conditions to consider. Conditions that must be factored in include sampling interval, temperature, network topology, RF environment, and whether or not the node behavior has been customized with the NI LabVIEW Wireless Sensor Network (WSN) Module Pioneer. With the LabVIEW WSN Module you can use graphical programming to customize node behavior by adding intelligence to extend battery life, embed local intelligence and interface with custom sensors.

In battery-powered systems, you must make important trade-offs because higher data rates and more frequent radio use consume more power. Today, battery and power management technologies are constantly evolving due to extensive research. Energy harvesting techniques are also becoming more prevalent in wireless sensor networks. With devices that use solar cells or collect heat from their environment, you can reduce or even eliminate the need for battery power.

National Instruments has found that when considering all of the above, NI WSNs deliver low-power measurement nodes that operate for up to three years on 4 AA batteries and can be deployed for long-term, remote operation.