Long range, battery powered, wireless environmental sensor interface devices

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 60/650,160 filed Feb. 4, 2005.

[0002] 1. Field of the Invention

[0003] The present invention relates generally to wireless sensing devices and sensor communication devices.

[0004] 2. Prior Art

[0005] In many industrial fields, it is important to continually measure and monitor environmental parameters. In agricultural industries, it is essential to monitor water content of soil and concentration of fertilizers. In chemical industries it is essential to monitor the air for possible hydrocarbon or toxic gas leaks that could lead to industrial accidents. At airports it is important to monitor the integrity of fencing surrounding the property. All of these applications can greatly benefit from a self-contained wireless data transmitter specifically designed to interface with sensors.

[0006] All of these example applications have similar requirements: that the device be low cost, easy to install, resistant to the weather, have long battery life, and can communicate large distances in order to easily cover a large area.

[0007] In the prior art, there exist three distinct classes of environmental sensors. The first class is of low-cost battery powered devices that are designed to operate around a home or building. They often take the form of wireless smoke detectors, motion detectors or weather stations. These devices are single purpose and short-range. There is no option to connect any external sensors or customize the sensor pack. Additionally, their short range design does not allow them to be used in large-scale industrial settings or large outdoor settings.

[0008] The second class of devices is the wireless sensor transmitters most similar to the device described in this document. They are designed to interface to a handful of different sensors and transmit the data back to a display terminal. Many of them also log data internally. While their range is relatively limited, they can transmit the data up to several thousand feet. These devices, however, are both large and expensive. They generally mount on poles or tripods in order to elevate the antenna. While they include batteries for nighttime operation, they heavily rely on solar cells to supply them with power. The mounting requirements and solar cell requirements add significant cost to the devices that impact their usability and installation time.

[0009] The third class of wireless sensor interface devices is a group of stand-alone sensor and communication devices that use wide area wireless communication technologies. These devices connect to a sensor and then communicate sensor readings via cell phone, pager, or satellite phone communication systems. These devices are designed for use in remote areas. These devices also required an above ground mounting and rely heavily on solar cells as a power source. While useful, these devices require that the operator pay for access to the wireless network. Cellular and satellite communication charges can add up quickly when transmitting data every few minutes. To avoid excessively high charges these devices often collect data for several hours or days and then transmit the data in a batch. The devices described herein use unlicensed band communication frequencies and do not require the operator to pay for service or licensing fees.

[0017] The preferred embodiments of the present invention long range, battery powered, wireless environmental sensor interface devices described herein are small, self-contained and can be placed on or even under the ground. For example, soil moisture sensors can be installed in seconds by making a small hole in the ground, inserting the node, and covering it back up. This is in sharp contrast to the existing wireless sensor interface devices that require both the purchase and installation of a mounting pole and large solar cell. With the low power design described in this document, small internal batteries provide all the power necessary to operate for several years. While battery replacement is possible, the low-cost nature of the device makes it practical to simply throw it away when the batteries die. These differences place this design is a different class than that of existing devices.

[0018] In addition to communicating with base-station type receiver devices, the present invention described herein is also capable of exploiting Mesh networking techniques. As each device is capable of bi-directional communications, individual sensor devices can communicate with each other. By using multi-hop networking techniques, data from distant nodes can be relayed toward collection points by intermediate sensor nodes. In this type of network, each sensor node can act as both a sensor and as a data relay point.

[0019] More information on Mesh networking that can be applied for use in embodiments of the present invention can be found in:

[0020] “System Architecture Directions for Network Sensors”, Jason Hill, Robert Szewczyk, Alec Woo, Seth Hollar, David Culler, Kristofer Pister, ASPLOS 2000, Cambridge, November 2000; and,

[0021] “Overview of Sensor Networks”, David Culler, Deborah Estrin, Mani Srivastava, IEEE Computer, Special Issue in Sensor Networks, August 2004; and, “Mica: A Wireless Platform for Deeply Embedded Networks”, Jason Hill and David Culler, IEEE Micro., vol 22(6), November/December 2002, pp 12-24.

[0022] The disclosures of the foregoing references are hereby incorporated herein by reference.

[0023] The ability to exploit Mesh communication techniques provides a significant advantage over other technologies. Mesh techniques makes network communication significantly more reliable. Additionally, it allows for automatic configuration and re-configuration to a dynamic environment. Node failure or movement can be handled automatically by autonomous reconfiguration of the Mesh network.

[0024] Thus the present invention relates to a device that can function as a member of a distributed set of environmental sensors and provide for the collection of data from them. The device can be connected to one or more environmental sensors and will wirelessly transmit the sensor data back to a collection device or Internet terminal for storage. Measuring just a few inches on a side, the device can be placed unobtrusively in the environment near the sensors.

[0025] The device has a sealed cavity that protects the communication electronics and battery source. One example of the device has a set of sensor interface wires emerging from the sealed cavity that can be connected to a plurality of sensors. Depending on range requirements an external antenna may also be used. For many applications an internal antenna will provide sufficient communication range. Use of an internal antenna will make the device more rugged. It is essential that all openings in the sealed cavity be weatherproof.

[0026] The preferred embodiments of the devices contain a 902-928 MHz narrow band, frequency hopping transceiver. This has been selected because it can be used highly efficiently for long-range communication. 900 MHz transmission propagates well in an outdoor environment. Additionally, FCC regulation allows the use of this band without licensing at transmission power up to 1 Watt. In some versions, the transmitter can transmit up to 1 Watt and uses frequency hopping to avoid interference.

[0027] In order to reduce the power consumption of the devices, precise time synchronization is used to allow the devices to remain powered off for long periods of time. After initial synchronization, a device will power itself off until data needs to be communicated. Through the use of time synchronization, the devices can power back on in sync with the receiver and transmit the data. This synchronization is maintained throughout the life of the devices. If synchronization is lost for any reason, a device can perform a resynchronization step as necessary. Generally, transmission is intermittent (short bursts), and the sensors will only be powered just before or during a transmission to greatly reduce sensor power requirements and provide a sample reading of the parameter being measured for communication.

[0028] Initial time synchronization is established when each device is powered on. During start-up, each device will transmit a time search message on sequential channels until it receives a reply. The reply message will inform the device of the current time and the current hopping channel. During operation, devices can transmit a time search message at any time and listen for a reply from surrounding nodes. Typically this is done on the current frequency hopping channel. However, it is also done on random hopping channels to check for alternate hop schedules. If an alternate hopping schedule is detected, the device can choose to switch to that schedule. This decision is generally made based on priority.

[0029] In one instance of the device, alkaline batteries are used. Alkaline batteries are selected because of their low cost and high energy density. One of the drawbacks of the alkaline batteries is that the voltage sags significantly as the battery discharges. To compensate for this, a preferred embodiment of the device uses a unique arrangement of three alkaline cells 20, as shown in the exploded view of FIG. 5 and the block 15, diagram of FIG. 6. The three Alkaline cells are connected in series (FIG. 6). Power is extracted from the first two batteries in the series connection of the three batteries. This allows them to initially supply a 3V source for the microcontroller MCU and transceiver (Radio and RX/TX switch) and a 4.5V source for the transmitter Power Amplifier and low noise amplifier LNA. As the batteries discharge, they will droop to 2V and 3V respectively. By connecting the batteries in this fashion, the microcontroller MCU contained in the device and the low power receiver can operate in the 3V to 2V range while the RF Power Amplifier and front end LNA can operate in the 4.5V to 3V range. If only one 3V source were used, then the device performance would degrade significantly when the supply drooped past 2.7V. Alternatively, if a single 4.5 V source were used, then a regulator would have to be provided to supply a maximum of 3.3 V to the transceiver and microcontroller. The power consumption of the regulator alone would significantly impact the device lifetime.

[0030] In the preferred embodiment of the devices, in addition to the unique battery connection, the devices also contain a power supply filter (The Super Cap.) between the power amplifier and the rest of the system. The filter is designed to isolate the microcontroller MCU and transceiver from the power drain of the Power Amplifier. If the Power Amplifier were to be directly connected to the 4.5V tap on the alkaline power source, a significant voltage spike would appear at both the 4.5V and the 3V tap when the amplifier was turned on. This would introduce a noise spike into both the radio and the microcontroller. To prevent this, a Current Limiting Resistor is placed between the 4.5V tap and a large buffer capacitor Super Cap. In a preferred embodiment, a 2 Farad capacitor is used in conjunction with a 47 Ohm current limiting resistor. This resistor prevents the Power Amplifier from drawing more than a few milliamps directly from the alkaline batteries 20. A majority of the current to run the Power Amplifier in short bursts is provided by the Super Cap. By limiting the surge current from the batteries, the 3V battery tap remains stable throughout a transmission. Without the Current Limiting Resistor, a voltage drop of up to 0.5 volts can disrupt proper operation of the low voltage components.

[0031] In addition to being able to connect external sensors to this device, it is also possible to include internal sensors in the package. A preferred embodiment of the invention is a device with integrated sensors built in to the device. By default, the device includes a battery voltage sensor and a temperature sensor. It can be extended to include internal accelerometers or even soil moisture and salinity sensors (see FIG. 6).

[0032] One preferred embodiment of the device has been designed so that it has three external stainless steel pins 22 that extend from the package to interface with the soil in order to determine the soil moisture content, conductivity levels and temperature, as shown in FIG. 1. By properly exciting the pins, the microcontroller MCU can measure the conductivity of the soil between the pins and the capacitance of the soil. The three pins that extend from the device are arranged in a line. The outer two pins are connected to ground. The inner pin is excited with a voltage and the response of the pin is measured. A higher capacitance corresponds to higher moisture content in the soil. Higher conductivity corresponds to higher salt content in the soil. A thermistor (temperature sensitive resistor) is placed at the base of one of the pins in order to measure the temperature of the pin, and therefore the temperature of the soil. To achieve accurate measurements, the moisture, salinity and temperature measurements are cross compensated. Soil conductivity naturally increases as soil moisture increases even if the salt content has not increased. Cross compensation allows salt content to be accurately measured across a wide range of moisture levels. The device of FIG. 1 may instead or also include wires for connection to one or more external sensors.

[0033] In a preferred embodiment, a single microcontroller MCU coordinates exciting the sensor pin, measuring the response and orchestrating the RF communication protocols. This creates a device that is both low cost and highly efficient.

[0034] In addition to sensing the soil moisture, temperature, and conductivity at a singe point, there is also a preferred embodiment that interfaces with the soil at multiple points. FIG. 2 shows a soil moisture sensor that interfaces with the soil at two depths using sensor pins 22 and 28.

[0035] A version of the environmental sensor device has been designed that contains an ultrasonic ranging sensor for detecting and reporting liquid levels and/or distances to objects as shown in FIG. 3. The device can be placed at fixed level above a holding tank and measure the distance down to the liquid inside. This can be used to monitor water levels in lakes and reservoirs. The microcontroller MCU excites the ultrasonic transducer 30 and measures the response in order to determine the range to the nearest object, and then communicates the result over the RF transceiver. In this design, the electronics are housed in a circular package 32 with an ultrasonic sensor 30 exposed at the bottom as shown in FIG. 3.

[0036] For external sensors, the external sensor interface is designed so a wide array of sensors can be connected. This includes analog interfaces, resistive interfaces and digital interfaces. Examples of each include analog accelerometers, resistive thermistors and digital soil moisture sensors. The resistive interface compares the external resistance to a set of internal known resistances. The device is able to auto-scale for external resistance ranging from mega-ohms to ohms.

[0037] A package has been designed for this device that is specifically for the use in soil monitoring for the golf industry and agricultural applications. Many golf superintendents want to place sensors underneath the surface of putting greens, fairways and teeing boxes. However, they don't want to damage the surface of the green by digging large holes. The present invention has a preferred embodiment that has been designed into a package with a cross-section of less than 4 inches. This form factor can fit inside the hole left by a cup-cutter. The cup-cutter is a tool that is used to extract a 4.25″ plug of dirt and grass from a putting green in order to place the golf hole and pin. This tool is used by all golf courses and creates a hole that can be easily filled by simply replacing the plug that was extracted. By designing the node so that it fits inside the hole left by a cup cutter, the node can be placed beneath the putting green very easily with tools that are already on-hand. A present invention device with an integrated soil condition sensor (or multiple ones) has been designed generally in accordance with FIG. 1 such that 3 metal pins exit the package parallel to the small cross-section so that the entire device with pins extending outward can be inserted into a hole in the turf created by a standard cup cutter as illustrated in the insertion process of FIG. 4. The device 34 is lowered into a hole in the turf so that the pins are parallel to the turf surface at the desired depth in the soil. The device and its protruding pins are then moved sideways to penetrate the undisturbed soil in the side of the hole. In this manner, the soil condition can be accurately measured at one or multiple depths (see for instance the device of FIG. 2) within one hole. After insertion the hole is repacked with soil and the turf cap plug is placed over the hole to restore the original turf surface.

[0038] The preferred embodiment of this device is constructed out of a rugged ABS plastic. The housing is molded ABS with a sealable lid so that it is robust against nearly all environmental conditions. The water tight lid must be robustly sealed to hermetically close the package so that no moisture is permitted to enter the package even under submerged conditions with large temperature swings over the day night cycles. Any protruding pins or wires must exit the package through sealed portals. The preferred embodiment uses O-rings held under pressure to seal between the package wall and the penetrating pins of the soil condition monitoring sensor.

[0039] Three D-cell batteries are placed the package and secured against a wall of the package to provide the source of Ground, 3V and 4.5V for the electronics. When fully sealed, there is a cavity above the batteries that is used to hold the electronics and circuit board necessary for the device operation. This cavity can also house an integrated antenna for the RF communication to and from the device. If external sensors are used, an Olflex Skintop connector part number s2109 with neoprene sealing ring may be used to form a seal with the wires exiting the device. This provides a seal capable of withstanding up to 70 PSI of pressure. The wires used to interface with external sensors may direct-burial CAT-5 cable. This type of cable is filled with silicon so that no air or moisture can travel down the cable and into the sensor interface device. Additionally, this connector provides strain relief to prevent the wires from being pulled out of the device. As mentioned before, FIG. 5 provides an exploded drawing of the assembly of the sensor node and FIG. 6 provides a block diagram of the electronics. In the case of nodes that are connected to external sensors, an external connection is made through a bundle of wires that exits the device. This is depicted in FIG. 7.

[0040] Of importance to the design of the wireless sensor interface node is the electronics that are used to control the device and perform the wireless communication. The preferred embodiment devices are managed by a MSP430F149 microcontroller from Texas Instruments. The transceiver used is a CC1020 from Chipcon. This transceiver provides low power operation and fine frequency control that is well suited for this communication environment. During transmission, the RF output is amplified by a Maxim MAX2235 RF amplifier chip designed to amplify 900 MHz transmission. When transmitting at full power, the combination of the CC1020 and the MAX2235 produce up to a 1 W output signal. In order to boost receive sensitivity, a RF. Micro Devices RF2361 Low Noise Amplifier (LNA) is used. The CC1020 and the MSP430F149 are capable of operating below 2.3 V while the Amplifier requires 2.7V-5V.

[0041] In order to current limit the drain on the battery from the RF amplifier operation, a 100 Ohm resistor is placed in series with the 4.5 V power supply leading to the Power Amplifier. To provide a current buffer, a 2 farad Aerogel ultra capacitor is used. This holds enough energy for a sustained three second transmission burst. Consequently the majority of the RF transmission energy comes from the capacitor, not directly from the batteries, with the batteries recharging the capacitor between transmissions.

Software

[0042] The hardware only represents a part of the total system. It must be accompanied by communication protocols and application software. The device runs an enhanced version of the TinyOS operating system. The device frequency hops at 150 hops per minute over 50 different channels evenly spaced across the 902-928 MHz band.

[0043] The device utilizes a unique protocol for establishing and maintaining precise hop synchronization. Upon initial boot-up, the device will transmit several search packets in order to quickly discover the current hop timing. A receiver node may pick up such packets and respond accordingly. Additionally, each receiver node will periodically send out sync announcements that can be received by a lost device. These sync announcements and replies to the sync requests contain information about the current channel and the current time value. Each device periodically checks the RF channel for the presence of a sync announcement. They check the channel for just a few milliseconds every second. If they detect that a sync announcement is eminent, they will leave their receiver on to receive the sync update. During a majority of the time, the transmitter node is powered down. Internally, it uses a low-power, 32 KHz, watch crystal to keep time. When it needs to transmit, it calculates what frequency the receiver has hopped to and transmits accordingly.

[0044] The software also handles interfacing with the sensors.

[0045] In a preferred embodiment, each supported sensor has a separate device driver that interacts with the sensor. The device driver creates a 16 bit sensor output value that is placed into one of the node's 4 sensor reading buffers for transmission to the receiver. The software can be configured to have alarm points where emergency messages are transmitted if the sensor value is too high or low. Additionally, the software can control how often data is sampled and transmitted.

[0046] The TinyOS operating is used in the devices of the preferred embodiment in order for compatibility with the Mesh networking technologies. TinyOS provides open-source public domain implementations of advanced Mesh networking protocols and techniques. It also provides a framework for proprietary network extensions.

[0047] Thus while certain preferred embodiments of the present invention have been disclosed and described herein for purposes of illustration and not for purposes of limitation, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.