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Instrumentation

Prof. Phillips

March 14, 2003

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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.

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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.

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Strain Measurements

Strain gauge

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Non-destructive Testing

Ultrasound transducer

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Automotive SensorsOxygen Sensor

Airflow Sensor

Water Temperature

Oil Pressure

Accelerometer

CO Sensor

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Biomedical

Ultrasound Transducer

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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

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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)

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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).

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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.

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Instrumentation Issues

• Noise

• Signal bandwidth

• Sampling

• Amplifier characteristics

• Feedback

• Real-time processing

• Control systems

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Noise

-2

-1

0

1

2

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

-2

-1

0

1

2

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1Signal

Signal + Noise

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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.

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Effects of Noise

• Reduces accuracy and repeatability of measurements.

• Introduces distortion in sound signals.

• Introduces errors in control systems.

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What to Do?

How can we eliminate or reduce the undesirable effects of noise?

• Grounding/shielding electrical connections

• Filtering (smoothing)

• Averaging several measurements

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Signal Bandwidth

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

High BW Low BW

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Bandwidth and Sampling

A higher bandwidth requires more samples/second:

High BW Low BW

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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.

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Effects of BW Limitations

-1

-0.5

0

0.5

1

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

-1

-0.5

0

0.5

1

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

Sensor Output

Amplifier Output

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Amplifier Characteristics

Amplifiers are characterized in terms of attributes such as:

• Gain

• Bandwidth and/or frequency response

• Linearity

• Harmonic distortion

• Input and output impedance

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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).

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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.

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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

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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.

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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

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

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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.

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Class Example

• Instrumentation Design Problem