Instrumentation

1
Instrumentation

• Prof. Phillips
• March 14, 2003

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.

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.

4
Strain Measurements
Strain gauge
5
Non-destructive Testing
Ultrasound transducer
6
Automotive Sensors
Oxygen Sensor
Accelerometer
Airflow Sensor
Oil Pressure
Water Temperature
CO Sensor
7
Biomedical
Ultrasound Transducer
8
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

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)

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).

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.

12
Instrumentation Issues

• Noise
• Signal bandwidth
• Sampling
• Amplifier characteristics
• Feedback
• Real-time processing
• Control systems

13
Noise
Signal
Signal Noise
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.

15
Effects of Noise

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

16
What to Do?

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

17
Signal Bandwidth

• Conceptually, bandwidth (BW) is related to the
  rate at which a signal changes

Low BW
High BW
18
Bandwidth and Sampling

• A higher bandwidth requires more samples/second

Low BW
High BW
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.

20
Effects of BW Limitations
Sensor Output
Amplifier Output
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

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 (gt103).

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.

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
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.

26
Temperature Control
Furnace and Material
Desired Temperature
Control System
Temperature Sensor

• The Control System sets the current supplied to
  the heating elements in the furnace to keep the
  material temperature at the desired (setpoint)
  value.

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

29
Class Example

• Instrumentation Design Problem