Kulite Wireless Sensors

1 123 Semiconductor Products IncPage 1 of 8 Kulite Wireless SensorsDavid KerrSr. Staff EngineerKulite Semiconductor Products, Willow Tree RoadLeonia, NJ 07605 ABSTRACTW ireless technology and the sensor industry are advancing in terms of capability,performance, size, and standardization. A number of potential new applications fortransducers can be considered when coupled with Wireless capability. Previous wirelesssensor designs have used narrowband radios and proprietary communications more than one of these can be a problem, however, since these devices caninterfere with each other and disrupt the flow of data. A Wireless sensor architecture isproposed that provides enough flexibility to use in many applications. Multiple sensorscan be used and work together in an integrated network. An example of a sensor usingthis design is ) Comparing Wired vs.

2 Wireless installationsA typical transducer installation can be described as follows:Figure 1 Typical Wired sensor InstallationA typical Kulite transducer consists of a silicon sensor mated to circuitry that cancompensate, filter, amplify, and/or linearize the analog output in an appropriate way forthe end user s application. Typically, the end user converts the analog signal into digitaldata for processing by a data acquisition USER123 Semiconductor Products IncPage 2 of 8A Wireless sensor adds functionality by including an analog-to-digital converter and dataprocessing to deliver data directly in a format for computer. A typical installation wouldlook like this:Figure 2 Wireless sensor InstallationIn the case of multiple Sensors , the reduced installation effort is even more and switching can be replaced with Wireless networking:Figure 3 Multiple Sensors in a Wired InstallationFigure 4 Multiple Transducers with Wireless NetworkingSensorSignalconditioningA-to-D XCVRTRANSDUCERXCVRDAC/PCEND USERS ensorSensorSensor.

3 XCVRDAC/PCEND USERS ensorSensorSensor..A/DA/DA/D..MUXDAC/PCE ND USER123 Semiconductor Products IncPage 3 of 82) Design GoalsThe Kulite Wireless transducer is designed to provide many benefits: May be built with any of Kulite s wide range of high-accuracy silicon Sensors . Maintain accuracy over a wide temperature range using advanced compensationtechniques. Networking capability Operate in noisy environments with other RF interference Provide security of data transmission. Maximize transmission range Reduced total installed costIn addition, the design needed to be based on a flexible platform, to allow rapidcustomization for unique customer ) System DesignA Block diagram of the Wireless Transducer follows:Figure 5 Wireless Transducer Block DiagramPressure sensor : Kulite manufactures piezoresistive bridges to meet many each bridge, some small compensation circuitry is used to bring the sensor outputinto a range suitable for processing.

4 The components for this circuit are unique for eachsensor, but the rest of the transducer design is independent of the sensor used. Thus, thewireless transducer can be made for virtually any pressure sensor : A temperature sensor is used for compensating the output of thepressure sensor , since silicon resistors change with temperature. As an added feature, thetemperature sensor output is also available from the Wireless A-to-D: This converts the analog voltage to a digital number. 16-bit A-to-D s areused for maximum accuracy: 216 = 65536, which corresponds to to DMCUA to DMACPHY123 Semiconductor Products IncPage 4 of 8 MCU processor: The MCU serves as the brains of the sensor . The processor servesseveral functions:1. Obtain digitally converted data from the sensors2. Compensate the pressure sensor output for temperature. The algorithm usespolynomial curve fitting to deliver extremely accurate Convert the sensor output into a data stream for the end Process requests from the end user5.

5 Communicate with the Media Access Controller (MAC)6. Store calibration data, serial number, address, and other data in set of programming commands is available to the user:CommandDescriptionValues$00 BbSet baud Rate to b (5,6,7or8) (b: 5:9600, 6:19200, 7:38400, 8:115000)$00 LaaSet device address(aa: 00 to FF address in Hex)$00 CccSet Config. Register(cc)$00C?Read Config. Register$nnWaabbWrite to EEPROM(aa: addr. in Hex; bb: one byte Hex )$nnVvSet Average mode(v: 0 7: 0 = No Avg. 7=High Avg.)The following command set is used to communicate with the transducers (nn is thedevice address) CommandDescriptionResponse$nnPRequest Pressure*#####.## (0 to )$nnTRequest Temperature*-/+###.# ( to )$nnURequest Units Code*pt (p: Press. units, t: Temp. units)$nnARequest X-ducer type* ( 10 char. String)$nnSRequest Serial No.*########$nnDRequest Last Cal Date*wwyyyy ( week, year )$nnMRequest Mfr.

6 Date*wwyyyy$nnBRequest Baud Rate Code *# ( 5, 6, 7, 8)$nnERequest EEPROM Dump * (512 bytes )$nnFRequest Full Scale Press *##### ( 0 to 10000 )Media Access Control (MAC): This is the processor associated with thecommunications standard, such as IEEE It performs many functions, including:1. Encrypting data2. Establishing and maintaining network connections3. Power management4. Executes the communication standard s protocolPhysical Layer (PHY): This circuit transmits and receives RF as defined by thecommunications standard. It includes the baseband subsystem, which processes theDSSS algorithm for interference reduction. It also includes the radio subsystem, andconnects to the Products IncPage 5 of 84) Preferred EmbodimentA Wireless sensor suitable for permanent installation in an industrial or transportationapplication shall be discussed.

7 The unit will be powered with local power so that it willalways be available. A voltage regulator inside the transducer will convert the powervoltage to a stable reference for the sensor and Vcc for the electronics. The diagram ofFigure 5 is used, with the MAC and PHY designed for use with IEEE Thepressure range is determined by the sensor Frequency :The GHz band is the only frequency band that can be used globally. Standards bodiesand government regulators have agreed to keep the band free for use by that operate in this band can be bought and used Standard IEEE of its global acceptance, the band is the focus for emergingcommunications standards. Of these, the IEEE ( WiFi ) standard was selectedas the most useful standard for these applications. The protocol offers:1. High data rates up to 11 Mbps2. Networks can be built up to 128 nodes3.

8 Easy integration into existing Ethernet Emerging as the dominant standard for Wireless PC communication5. Data security via WEP and WPAN etworking CapabilityUsing the standard allows a network of up to 128 nodes, or 127 transducers anda test computer, as in this exampleFigure 6 A Network of up to 127 Transducers can be controlled by 1 test computerSince is an extension of the Ethernet standard, this Wireless network of sensorscan be integrated into an existing local area network (LAN), and data can be monitoredremotely from anywhere on the #1#2#3 NPower in123 Semiconductor Products IncPage 6 of 8 Avoiding Interference with DSSSThe Wireless transducers will likely be installed near other equipment transmitting RFsignals. In order to reduce susceptibility to this interference, a technique called DirectSequence Spread Spectrum (DSSS) is used.

9 DSSS combines a data signal at the sendingstation with a higher data rate bit sequence, called a chipping code (also known asspreading code). Essentially, this sends a specific string of bits for each data bit receiver uses the same code and is able to reconstruct the data stream, even if it hasbeen corrupted by noise. The longer the spreading code, the better the resistance example, using a spreading code of 10110111000: A transmitted 1 becomes 10110111000 A transmitted 0 becomes 01001000111 Even with several corrupted bits, the receiver can still tell the differencebetween a transmitted 1 and 0 .As a result of the modulation, the information energy gets distributed into the side lobes: Figure 7 Spectrum Analyzer view of a Narrowband Signal vs. DSSS SignalDSSS is very effective in environments with wideband noise. Successful transmissionand reception can occur, even if the DSSS signal is below the noise Integrity and Security Data from the pressure may need to be secure, or the integrity needs to be verified.

10 , Wired Equivalent Privacy (WEP) is the standard protocol for encryption andauthentication. A 64-bit key is used to encrypt the data, and a CRC-32 check is used toverify data strengthSignal strengthNoisefloorNoisefloor123 Semiconductor Products IncPage 7 of 8 Transmission RangeFCC requirements limit the output of the radio to 1W. This has a reach of 100m in openspace. However, in an office or laboratory environment, the range is reduced to 50m orless. Fortunately, the flexibility of the networking structure allows for sub-networks to be built and interconnected so that a Wireless network can be extendedbeyond this limitation. A set of sub-networks could each monitor a group of wirelesstransducers in a limited physical area and then send that data over the network (wirelessor wired) to a central node or test computer for processing. A variety of hardwaresolutions to create these networks are readily available and Wireless TransducersAdding Kulite Wireless transducers to an existing test system is usually compatibility makes it easy to integrate into any PC-based test system.