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Instrumentation

Instrumentation. Prof. Phillips March 14, 2003. Electrical Instrumentation. Electrical instrumentation is the process of acquiring data about one or more physical quantities of interest using electrical sensors and instruments.

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Instrumentation

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  1. Instrumentation Prof. Phillips March 14, 2003 lecture16

  2. Electrical Instrumentation Electrical instrumentation is the process of acquiring data about one or more physical quantities of interest using electrical sensors and instruments. This data may be used for diagnostics, analysis, design, or to control a system. lecture16

  3. Instrumentation Examples • Every engineering discipline uses electrical instrumentation to collect and analyze data. • The following examples are illustrative of the different types of sensors and instrumentation that different engineering disciplines use. lecture16

  4. Strain Measurements Strain gauge lecture16

  5. Non-destructive Testing Ultrasound transducer lecture16

  6. Automotive Sensors Accelerometer Oxygen Sensor Airflow Sensor Oil Pressure CO Sensor Water Temperature lecture16

  7. Biomedical Ultrasound Transducer lecture16

  8. A/D Converter Sensor Computer Amplifier Typical Instrumentation System • Sensor - converts the measured value into an electrically useful value (a transducer) • Amplifier - “conditions” the signal from the sensor • A/D Converter - changes the signal into a digital format • Computer - processes, displays, and records the signal lecture16

  9. Sensor • The output of a sensor (transducer) is proportional to the quantity of interest. • The sensor output may be a • voltage or current (temperature, pressure) • resistance (strain gauge) • frequency (accelerometer) lecture16

  10. Amplifier • The output of the amplifier is (usually) a voltage. • The gain of the amplifier is set so that the voltage falls between lower and upper limits (for example, -10V to 10V). lecture16

  11. A/D Converter • Analog-to-digital (A/D) conversion consists of two operations: • Sampling: measuring the voltage signal at equally spaced points in time. • Quantization: approximating a voltage using 8, 12 or 16 bits. lecture16

  12. Instrumentation Issues • Noise • Signal bandwidth • Sampling • Amplifier characteristics • Feedback • Real-time processing • Control systems lecture16

  13. Noise Signal Signal + Noise lecture16

  14. Sources of Noise • Thermal noise caused by the random motion of charged particles in the sensor and the amplifier. • Electromagnetic noise from electrical equipment (e.g., computers) or communication devices. • Shot noise from quantum mechanical events. lecture16

  15. Effects of Noise • Reduces accuracy and repeatability of measurements. • Introduces distortion in sound signals. • Introduces errors in control systems. lecture16

  16. What to Do? How can we eliminate or reduce the undesirable effects of noise? • Grounding/shielding electrical connections • Filtering (smoothing) • Averaging several measurements lecture16

  17. Signal Bandwidth Conceptually, bandwidth (BW) is related to the rate at which a signal changes: Low BW High BW lecture16

  18. Bandwidth and Sampling A higher bandwidth requires more samples/second: Low BW High BW lecture16

  19. Bandwidth Limitations Every component in the instrumentation system has bandwidth limitations: • Sensors do not respond immediately to changes in the environment. • The amplifier output does not change immediately in response to changes in the input. • The A/D converter sampling rate is limited. lecture16

  20. Effects of BW Limitations Sensor Output Amplifier Output lecture16

  21. Amplifier Characteristics Amplifiers are characterized in terms of attributes such as: • Gain • Bandwidth and/or frequency response • Linearity • Harmonic distortion • Input and output impedance lecture16

  22. Op Amps • One commonly used type of amplifier is the Operational Amplifier (Op Amp). • Op Amps have differential inputs: output voltage is the amplified difference of two input voltages. • Op Amps have very large gains (>103). lecture16

  23. Op Amps (cont.) • Most op-amp circuits use negative feedback • Op-amp circuits can be designed to: • Provide voltage gain or attenuation. • Convert current to voltage. • Integrate or differentiate. • Filter out noise or interference. lecture16

  24. Feedback Often, sensors measure quantities associated with systems. The sensor output is used to control the system in a desired manner. System (Plant, Process) Control Feedback Path lecture16

  25. Example: Industrial Process Control • In many manufacturing processes (integrated circuits, for example) temperatures must be closely controlled. • Feedback can be used to maintain a constant temperature. lecture16

  26. Temperature Control The Control System sets the current supplied to the heating elements in the furnace to keep the material temperature at the desired (setpoint) value. Furnace and Material Control System Temperature Sensor Desired Temperature lecture16

  27. A car cruise control is a feedback system. How does it work? lecture16

  28. Benefits of Feedback • Provides stability with respect to changes in system parameter values. • Helps to obtain a (nearly) linear response from non-linear components. • Can be used to change the characteristics of a system under control. lecture16

  29. Class Example • Instrumentation Design Problem lecture16

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