Electronic Design of Mandragora's Wireless Sensor Networks Motes
Francisco G. Flores García, Víctor D. Velasco Martínez, María de J. Flores Medina Instituto Tecnológico de la Laguna
27200, Torreón, Coahuila, México [email protected], [email protected],
[email protected] Abstract
The hardware design of a Wireless Sensor Network mote, called Mandragora, is presented. The architecture was designed with modularity in mind. The primary modules functionality is described.
Measuring tests of differently timed signals by some motes are done. Results comparing star and mesh topologies, and results of delays of sensed data by different motes are shown. Results are analyzed and discussed to describe the characteristics of the devices as data acquisitors.
Introduction
Wireless Sensor Networks (WSN) are one of the top trends in research. As WSN are complex systems of many autonomous devices, for larger deployments, cheaper devices are better. Mandragora is a project created to monitor agricultural fields [1]. The first revision was used to monitor soil's moisture. When prototypes were manufactured, some design flaws were found, thus a redesign was needed.
Figure 1 – New version mote's units in 3D View Device Hardware and Description
Processing Uni t
The processing unit has a PIC18F4550 micro-controller, a 512Kb EEPROM, and micro USB circuitry.
The micro-controller takes care of controlling the protocol and the hardware in all modules. It also has the user program in charge of data sensing: if it should be timed, by event, or by request.
It also exposes other internal peripherals of the micro-controller, as two PWM outputs, and three 10 bit ADC, and many general purpose digital input/output pins.
Transceiver Unit
The transceiver unit uses a MRF24J40, an 802.15.4-2003 compliant transceiver. It has both PHY and MAC stack on board.
The module also has a CSMA-RP connector for an antenna, and LEDs for monitoring mote's status.
Power Unit
One of the crucial features of WSN motes are power supply duration. This power unit uses a CR232 button battery. By the use of an AS1320 power regulator and elevator, mote's voltage supply is set on 3.3V.
It sports a regulator turn-off function, so the micro-controller can turn off the 3.3V output, effectively turning off the transceiver. This helps in power saving.
Ibersensor 2012-October 16-19, Puerto Rico IB12-42
Acquisition Unit
This unit has one ADS8201 ADC. It is a 12 bit, 8 channel ADC. It has programmable gain, and the voltages references can be changed by two analog pins.
It has also an MCP4802, an 8 bit dual channel DAC. Each channel is an input of the voltage references pins of the ADS8201.
Both, programmabble gain, and the ability to control the voltage references through the MCP4802, enable sensing of small voltages much below the 12 bit resolution of the ADC.
This is done directly from the microcontroller.
ADC's input channels are grouped in three blocks of three, and each block's voltage can be turned on and off from the microcontroller by using an FP1206. It also has configurable jumpers to set the voltage supply of each bank.
Firmware & Protocol
The motes are running a custom made operating system, to make it multi-tasking. It has control of the hardware and the communication protocol. The user application is embedded in the operating system.
Two protocols are used. First the 802.15.4- 2003 star topology, done in hardware by the transceiver unit. Each mote communicates directly with the coordinator. Packets not directed to them by the coordinator are discarded.
The second protocol in use, is a simple flooding mesh protocol, used in the first version of Mandragora. All the motes can connect directly with the coordinator, but each mote has the promiscuous mode of the transceiver enabled, permiting them receiving data for other motes. If they find the data is not for them, they'll retransmit it in hope sometime they'll answer. This invalidates the hardware processing from the motes, and thus, times should be different.
Tests
A signal generator is put in sinusoidal wave mode, 3Vpp, +1.5V offset. The frequency of the generated signal is swept from 1Hz to 10KHz.
Three motes are prepared to sense the generated signal in the same channel. The coordinator issues requests every 100us to all motes to sense and transmit. This testbench is shown in Figure 2. The star topology is used.
The data received by the coordinator is sent to a computer through a serial port, and recorded in files. This test is repeated with the flooding mesh protocol.
Figure 2 – Testbench
Results
All signals obtained in the first test are presented vs time, and compared between them.
For the low frequency signals, the measurements recorded are almost the same between them, and similar to the original. When going up in frequency, a small offset is observed. In high frequencies the signal presents aliasing.
For the second protocol data, the same graphs are presented, showing a bigger offset, and aliasing at lower frequencies. Results of both protocols are contrasted.
Conclusions
The motes can be used to test signals in the sub-sonic frequency ranges. Heavy protocols can distort the instant when the mote starts sensing. Syncronization, and a tag indicating the time of the capture is good for signal reconstruction.
References
[1] M. de J. Flores M. et al. “Red Inalámbrica de Sensores para Monitoreo de Humedad Enterrada”. Proc. of the 7th Ibero-American Congress on Sensors, Lisbon, Portugal.
November 9-11. November 2010
