lecture161 Instrumentation Prof. Phillips March 14, 2003

lecture162 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.

lecture163 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.

lecture164 Strain Measurements Strain gauge

lecture165 Non-destructive Testing Ultrasound transducer

lecture166 Automotive Sensors Oxygen Sensor Airflow Sensor Water Temperature Oil Pressure Accelerometer CO Sensor

lecture167 Biomedical Ultrasound Transducer

lecture168 Typical Instrumentation System Sensor Amplifier A/D Converter Computer 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

lecture169 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)

lecture1610 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).

lecture1611 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.

lecture1612 Instrumentation Issues Noise Signal bandwidth Sampling Amplifier characteristics Feedback Real-time processing Control systems

lecture1613 Noise Signal Signal + Noise

lecture1614 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.

lecture1615 Effects of Noise Reduces accuracy and repeatability of measurements. Introduces distortion in sound signals. Introduces errors in control systems.

lecture1616 What to Do? How can we eliminate or reduce the undesirable effects of noise? Grounding/shielding electrical connections Filtering (smoothing) Averaging several measurements

lecture1617 Signal Bandwidth Conceptually, bandwidth (BW) is related to the rate at which a signal changes: High BW Low BW

lecture1618 Bandwidth and Sampling A higher bandwidth requires more samples/second: High BW Low BW

lecture1619 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.

lecture1620 Effects of BW Limitations Sensor Output Amplifier Output

lecture1621 Amplifier Characteristics Amplifiers are characterized in terms of attributes such as: Gain Bandwidth and/or frequency response Linearity Harmonic distortion Input and output impedance

lecture1622 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 (>10 3 ).

lecture1623 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.

lecture1624 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

lecture1625 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.

lecture1626 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. Desired Temperature Control System Furnace and Material Temperature Sensor

lecture1627 A car cruise control is a feedback system. How does it work?

lecture1628 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.

lecture1629 Class Example Instrumentation Design Problem