Low-Power Sensor Solutions

Power saving is the Holy Grail of the electronics world, and sensors are no exception. Low-power requirements and features have become de rigueur for a broad range of sensor-based apps, and suppliers are stepping up to meet industry demand.

In any article on low power, it is essential to first discuss what we mean by "energy” and "power." The former is the total amount of work performed, and the latter is the rate at which the work is performed (energy used per unit of time). The familiar electronics equations that are applicable here are: energy = power × time and power = voltage × current. Consequently, this article will focus on voltage, current, and time parameters in examining low-power sensors.

In addition, it should be noted that sensor networks have various additional elements that have to be included in any overall power budget. Sensor deployments are often wireless, and their energy resources need to be optimized to preserve battery life. As such, power requirements in wireless systems must take into account range, bit rate, and other RF communications factors. For the purposes of this article, we will focus on the various low-power features found on the sensors themselves. For a low-power sensor system approach, see such TechZone articles as “Low-Power Wireless Sensor Networks” and “Energy Harvesting for Low-Power Wireless Sensor Nodes”.

Power misers for mobile apps

With rechargeable batteries providing 1,500 to 5,000 mWh total capacity, like all things related to mobile phones, accelerometers destined for phone use have had to become quite the power misers. Some can be sampled at 50 Hz using less than 0.02 mWh of energy. Targeting handsets, the Freescale MMA7660FC ±1.5 g 3-axis accelerometer (Figure 1) with digital output (I²C) is a very-low power, low-profile capacitive MEMS sensor with a range of user-configurable power-saving features. The device’s samples per second can be set over a wide range from 1 to 120 samples a second; its operating current is directly proportional to the samples per second rate employed. Further, the analog supply (AVDD) can be powered down to put the MMA7660FC into Off Mode, which typically draws 0.4 µA (Standby Mode: 2 µA, Active Mode: 47 µA at 1 ODR (output data rate)).

Figure 1: Block diagram of the Freescale MMA7660FC MEMS accelerometer.

An Auto-Wake/Sleep feature can toggle the sampling rate from a higher user-selected samples per second to a lower user-selected samples per second, changing based on if motion is detected or not. With any of the above options, the user can configure the part to the optimal power consumption level for the desired application.

The Freescale accelerometer can be used for sensor data changes, product orientation, and gesture detection through an interrupt pin (INT). The device is housed in a small 3 x 3 x 0.9 mm DFN package.

Aimed at hearing aids and home healthcare devices, the ADXL362 three-axis, digital-output MEMS accelerometer from Analog Devices consumes less than 2 μA at a 100 Hz output data rate, and 270 nA when in motion-triggered wake-up mode. Unlike accelerometers that use power duty cycling to achieve low power consumption, the ADXL362 (Figure 2) does not alias input signals by under sampling; it samples the full bandwidth of the sensor at all data rates.

The ADXL362’s 12-bit output resolution, and 8-bit formatted data provide for efficient single-byte transfers when a lower resolution is sufficient. Measurement ranges of ±2, ±4, and ±8 g are available, with a resolution of 1 mg/LSB on the ±2 g range. For applications where a noise level lower than the normal 550 μg/√Hz of the ADXL362 is desired, either of two lower noise modes (down to 175 μg/√Hz typical) can be selected at minimal increase in supply current.

Power-saving features include:
  • Adjustable threshold sleep/wake modes for motion activation
  • Wide supply and I/O voltage ranges: 1.6 to 3.5 V
  • Operates off 1.8 to 3.3 V rails
  • High resolution: 1 mg/LSB
  • Power can be derived from a coin-cell battery
  • 1.8 μA at 100 Hz ODR, 2.0 V supply; 3.0 μA at 400 Hz ODR, 2.0 V supply
  • 270 nA motion-activated wake-up mode
  • 10 nA standby current

Figure 2: Functional diagram of the ADXL362 three-axis, ±2 g/±4 g/±8 g digital output MEMS accelerometer.

It includes a deep multi-mode output FIFO, a built-in micropower temperature sensor, and several activity-detection modes including adjustable threshold sleep and wake-up operation that can run as low as 270 nA at a 6 Hz (approximate) measurement rate. The ADXL362 operates on a wide 1.6 to 3.5 V supply range, and can interface to a host operating on a separate, lower supply voltage.

More examples

Instead of flipping switches or adjusting controls, occupancy sensors are used in a wide variety of applications, including turning on and off lighting and adjusting temperature in automotive applications. Not only does occupancy sensing save up to 50 percent of the cost of lighting in large offices, these devices also offer a variety of low-power modes.

Zilog’s ZMOTION™ detection module is used as an intrusion-detection solution and features a hypersense-detection mode for occupancy-sensing applications. The pyroelectric sensor and clip-on Fresnel lens combine to provide a compact solution without sacrificing performance. It also has the ability to change lenses, providing the flexibility to suit a variety of applications. At power-on, the pyroelectric sensor requires time to stabilize before motion detection can occur. The ZMOTION MCU monitors the direct signal from the pyroelectric sensor to determine when it has achieved stability and, as a result, the required power-on stabilization time is minimized. The module is only 25.5 x 16.7 mm (and only 11 mm thick), so it can easily fit into many size-constrained applications. The sensor module has a 2.7 to 3.6 V operating voltage with extended operating temperature range (–40° to +105°C), and Zilog also offers a ZMOTION development kit (Figure 3).

Figure 3: ZMOTION dev kit for occupancy sensing applications.

There are many commercial, medical, and industrial settings that depend on low-voltage sensors where the power supply is limited. Honeywell’s HIH-5030/5031 Series Low Voltage Humidity Sensors operate down to 2.7 VDC, often ideal in battery-powered systems where the supply is a nominal 3 VDC.

The HIH-5030/5031 series humidity sensors are designed specifically for high-volume OEM users. Direct input to a controller or other device is made possible by this sensor’s near-linear voltage output. With a typical current draw of only 200 A, the HIH-series is ideally suited for many low-drain, battery-operated systems.

Sometimes it is the size limitations of the application that dictate how low power a sensor must be. Within the image sensing arena, the OmniVision OV3640 ¼ inch, high-performance 3-Mpixel camera sensor with image stabilization and MIPI is a feature-rich, CMOS CameraChip sensor in a ¼ inch optical format. It is based on OmniVision's 1.75 μm OmniPixel3 architecture with ultra-stack height (ULSH) for excellent low-light sensitivity (500 mV/lux-s) and significantly improved noise and dynamic range (65 dB).

The highly-integrated OV3640 (Figure 4) incorporates an advanced Image Signal Processor (ISP) with new features, such as an image stabilization/anti-shake (AS) engine that requires no external components, also reducing power consumption. An embedded microcontroller supports the internal auto focus (AF) engine, and the programmable general-purpose I/O modules enable external auto-focus control.

Figure 4: The OV3640 functional block diagram.

The OV3640 also contains an integrated compression engine (JPEG), simplifying bandwidth-limited interfaces. It is small enough to fit standard 8 x 8 mm fixed-focus sockets as well as 8.5 x 8.5 mm AF sockets, making it ideal for drop-in upgrades of existing camera modules used in existing lower-megapixel camera phone designs.

Applications for the ultra-low-power OV3640 include cell phones, toys, PC multimedia, and digital still cameras.

Technology marches on

In collaboration with Imec, the Leuven research center in Belgium, Delft University of Technology, and the Eindhoven University of Technology, the R&D organization Holst Centre recently provided a preview of the next generation of power-saving sensors by showing a self-calibrating sensor device capable of harvesting RF energy at lower input power levels than current state-of-the-art solutions. Measurements in an anechoic chamber in the European 868 MHz ISM band showed 26.3 dBm sensitivity for 1 V output, and 25 m range for a 1.78 W RF source. The maximum end-to-end power conversion efficiency of the harvester is reported to be 31.5 percent.

Key building blocks of the RF energy harvester are a five-stage cross-connected bridge rectifier, a high-Q antenna, and a 7-bit capacitor bank. The rectifier is brought at resonance with the antenna by means of the capacitor bank. A control loop is added to compensate for any variation in the antenna-rectifier interface and passively boosts the antenna voltage to enhance the sensitivity. The capacitor bank and the rectifier have been implemented in standard 90 nm CMOS technology, and are ESD protected. The active die area occupies only 0.029 mm².

According to researchers at Holst, this design overcomes several limitations of existing RF energy harvesters. Today’s RF harvesters either have poor sensitivity, require calibration, a special technology process, or a large chip/antenna area. Compared to existing solutions, the new device consumes a smaller antenna area, favoring applications demanding a small form factor (such as sensors), while operating at a lower frequency. Thanks to its sensitivity and wireless range performance, an increased area can now be covered by the RF source. This further makes the device suitable for powering small sensor systems in applications where other energy sources such as light, vibrations. or thermal gradients are not available. It is also expected to pave the way towards harvesting RF energy for sensors from ambient Wi-Fi or GSM signals.

For more information on the parts mentioned in this article, use the links provided to access product information pages on the Hotenda website.