1 Summary Lecture: Part 1 Sensor Readout Electronics and Data Conversion Discovering Sensor Networks: Applications in Structural Health Monitoring

2 1. Electrical/Electronic Signals 2. Data (Analog-to-Digital) conversion 3. Resistance, Ohm’s Law, and the Wheatstone Bridge 4. Implementation of Wheatstone Bridge readout circuit for resistive sensors 5. Calculation of unknown resistor value from digital data 6. Design tradeoffs This lecture/lab module covered….

3 Wireless Sensor Systems… SENSORSSENSORS SIGNAL CONDITIONING (ELECTRONICS) MICROPROCESSORMICROPROCESSOR ACTUATORSACTUATORS SIGNAL CONDITIONING (ELECTRONICS) SENSED PARAMETERS CONTROL PARAMETERS Sensor Readout Power & Control AnalogDigital RF/Wireless Transceiver Communications with Wireless Network  In this four part lecture/lab sequence you will be exposed to wireless sensor systems – from a sensed parameter, through sensor readout and data conversion, and wireless transmission to a network.  In this lecture/lab we focused on the “Signal Conditioning” blocks (sensor readout and Analog-to-Digital converter) Transducers RF

4 Engineering Design Problem Statement  Given a particular type of sensor (transducer), which converts a physical parameter, such as mechanical strain, to an electrical signal, we needed to design a readout circuit to measure this electrical signal and convert it to a digital representation that can be subsequently communicated and/or processed  Design considerations include…  The range of variation of the sensor output signal  How much resolution and/or amplification is needed to detect the sensor signal output variation  How many digital bits are needed to represent the data with adequate precision  Power consumption

5 Analog vs. Digital Signals  Analog signals take on continuous amplitude values – EEs typically use charge, voltage, or current (current = charge flow per unit time).  Analog Signals represent the physical world. e.g. Electrocardiogram (ECG) signals are analog signals:  Digital signals take on discrete amplitude levels. Typically, we use binary signals which utilize only two levels. One level is referred to as logical 1, the other level is referred to as logical 0.  Modern computers and data communications systems process digital signals Digital signals themselves are really analog signals!

6 Analog-to-Digital Conversion  Analog signals are converted to digital signals through a process we call sampling – the amplitude of the analog signal is measured at specific instants in time (i.e. at a particular sampling rate) and stored.  After digitization, the continuous analog signal becomes a set of amplitude values separated by fixed time intervals (we call this a discrete time analog signal).  These discrete amplitude values are then converted to the closest digital (binary) representation

7 Data Conversion  A/D conversion: Analog-to-Digital Converter (ADC)  D/A conversion: Digital-to-Analog Converter (DAC) ADC DAC … Note that the output of the DAC is not a perfect representation of the original analog signal → we call this quantization error… …

8 Data Conversion (cont.)  N is typically used to represent the number of bits of resolution of a data converter, i.e. we have an “N-bit” data converter (audio CDs are 16-bit).  The least significant bit (LSB) of a data converter is the smallest difference in the analog quantity (e.g. voltage) that can be resolved from the digital representation.  Quantization error is the difference between the value recovered from the digital signal (i.e. using an ideal DAC) and the original analog signal value. V LSB = V FS /2 N  V quantization = ±V LSB /2

9 Wheatstone Bridge Resistance Readout Circuit  In the balanced bridge condition: R 1 /R 2 = R 3 /R X and  V = 0  Assuming the bridge starts off in the balanced condition, if Rx then varies due to an environmental condition (e.g. temperature, stress, etc.), a non-zero voltage will be detected across the bridge output nodes D and B, given by:  The instrumentation amplifier is used to isolate (buffer) the bridge output from the rest of the system and to amplify the signal if necessary. The instrumentation amplifier also has a reference voltage which shifts the output level. Therefore: V out = G∙  V + V ref  V out is then converted into a digital representation through A/D conversion for subsequent processing/communication  V - A/D V out V supply V ref G

10 Part 1 Lab: Sensor Readout and Data Conversion  In this lab, your team built up a working resistance bridge circuit on a protoboard and used it to read the value of an unknown resistance R x  In the next lab in the sequence R x will be replaced by a resistive strain gauge sensor  The bridge output was sensed by the instrumentation amplifier and passed to the on-board A/D converter.  The output of the ADC was be passed directly to your laptop via USB cable for recording and further analysis  The digital value was converted back to resistance by hand calculation in post-lab analysis, but it would be straightforward to automate this process in software Unknown Rx Freescale PBS12C32SLK Project board MCU ADC Instrumentation Amplifier USB

11 Sensor Readout Design Considerations  In this lab, the range of possible unknown resistance values was . The resulting Wheatstone Bridge output voltage range was directly digitized by the ADC (Instrumentation Amplifier G = 1) and transferred to the computer for analysis.  However, real-world resistive sensors may not present such significant resistance variations. In order to detect the smaller resistance variation, and hence smaller  V output from the bridge circuit, a higher ADC resolution (# of bits) will be required. For example, if the  R = ±0.5  from the nominal value in our bridge, the resulting  V ≈ ±0.005 V. This is the worst case quantization error for a 9 bit converter. However, if we want further resolution within that range, say 3 bits, then a 12 bit ADC would be required.  High bit depth ADCs will typically have higher power consumption, and may be more expensive.  Alternatively, we can increase the Gain of the Instrumentation Amplifier, such that the small range of variation is amplified up to the maximum range that can be handled by the amplifier. This is exactly the approach taken for the follow-on strain gauge sensor lab