Applications of Wireless Sensors
1. How do wireless sensors work?
The USB-ZigBee controller card is tied to the ZigBee Network Coordinator such as a PC to handle (command, process, display, communicate) all of incoming and outgoing ZigBee Devices or sensors. The wireless communication is based on IEEE 802.15.4 (for ZigBee) standard. Other standards for different type of communication maybe used. The typical STAR topology is:
Wireless Sensor Network STAR topology
2. The Sensor Module
Our sensor ZigBee module consists of the sensor daughter board (or an analog-to-digital converter (ADC) board), ZigBee chips, micro controller unit (MCU) and a flash memory as shown below. MCU and memory are integrated into the ZigBee chip. The entire sensor module will be battery operated with enough memory space to hold the firmware.
The sensor can be either resistance or capacitance based sensing mechanism, especially in MEMS form, and it detects small change in the object's resistance or capacitance.
ZigBee Strain Sensor Module Schematic
In general, signal amplification, sampling, Analog to Digital conversion, and the sensor and ZigBee chip interfacing are performed by the ADC chip. The ADC chip may include additional MCU or it may depend on external MCU. The firmware to operate the ZigBee chips will be stored in the EEPROM.
GreenVirtue intends to challenge the following issues in the future sensor module designs.
1) Controlling multiple MCUs and making the system unite as one
2) Adding some intelligence for the sensor to become fully autonomous for reducing unnecessary radio communications
3. The Sensor Module Firmware
The firmware has two roles. One is administrating and controlling the sensor circuit and the other is controlling ZigBee chips. Our firmware and driver structure is designed flexible for the system expandability so that they do not need to rewrite the entire driver/firmware later for different application.
4. ZigBee Stack and Driver Development
As it is described in the IEEE 802.15.4 standard, ZigBee wireless system provides economical and efficient data transfer method with full use of self-healing and self controlling mechanism. In addition, each sensor node will have unique ID assigned and data transmit and receive can be well organized to reduce the battery use so that the system will work long period without frequent replacement of its battery. Having an onboard micro controller (MCU) can reduce the data transmission time even further by pre-processing the data and keeping the data size smaller. The current module, however, keeps the stack very conventional form, as shown below.
Some users may require modifications to better fit to his/her application with using our development kit. For the detail of the development kit, see products section WN128-USB kit.
5. Separating the Application Layer
The current stack and the driver architecture include the basic handling of the sensor application layer but the power management algorithm is not included. We will separate the sensor interface layer independently so that any sensors can be integrated to the system easily.The new stack to accommodate the multiple sensors (and their applications) should look like:
6. Strain gauge STAR Topology Test
Clamp the plate’s one side so that this side does not move (see below). Then apply a known force to the sensor side. 2 sensors should read differently.
Applying a force to a sample strip
The data from the strain gauge was displayed on the coordinator PC or PDA. The wireless range is about 30 feet indoor.
8. Test Report
No force case
You can download and view the WN128 sensor test report from comparison test at University of California, Irvine.
View WN128 Sensor Test Report
9. Publications and Brochures
Development of a Wireless Strain Sensor System for Structural Health Monitoring (View)
Wireless-Zigbee Strain Gauge Sensor System For Structural Health Monitoring (View)
SPIE Presentation Material:
Wireless Strain Sensor System for Structural Health Monitoring (View)
Wireless Strain Sensor WN128 (View)
WN128 Development Kit (View)
WN128 Wireless Sensor Application (View)