E1C01 09/14/2010
15:40:34 Page 5
General Template for a Measurement System
A general template for a measurement system is illustrated in Figure 1.5. Basically such a system
consists of part or all of four general stages: (1) sensor–transducer stage, (2) signal-conditioning
stage, (3) output stage, and (4) feedback-control stage. These stages form the bridge between the
input to the measurement system and the system output, a quantity that is used to infer the value of
the physical variable measured. We discuss later how the relationship between the input information, as acquired by the sensor, and the system output is established by a calibration. We have already
discussed the sensor–transducer stage, so let’s move on to the signal-conditioning stage.
The signal-conditioning stage takes the transducer signal and modifies it to a desired
magnitude. This optional intermediate stage might be used to perform tasks such as increasing
the magnitude of the signal by amplification, removing portions of the signal through some filtering
technique, or providing mechanical or optical linkage between the transducer and the output stage.
For example, the translational displacement of a mechanic’s caliper (sensor) is often converted into a
rotational displacement of a pointer. This stage can consist of one or more devices, which are often
connected in series. For example, the diameter of the thermometer capillary relative to the bulb
volume (see Fig. 1.4) determines how far up the stem the liquid moves with increasing temperature.
It ‘‘conditions’’ the signal by amplifying the liquid displacement.
In those measurement systems involved in process control, a fourth stage, the feedback-control
stage, contains a controller that interprets the measured signal and makes a decision regarding the
control of the process. This decision results in a signal that changes the process parameter that
affects the magnitude of the sensed variable. In simple controllers, this decision is based on the
magnitude of the signal of the sensed variable, usually whether it exceeds some high or low set point,
a value set by the system operator. For example, a simple measurement system with control stage is a
household furnace thermostat. The operator fixes the set point for temperature on the thermostat
display, and the furnace is activated as the local temperature at the thermostat, as determined by the
Sensor stage
Signal
path
Transducer
stage
Signal conditioning stage
Output
stage
Control
stage
Control signal
Process
Calibration
Figure 1.5 Components of a general measurement system.
1.2 General Measurement System 5
15:40:34 Page 5
General Template for a Measurement System
A general template for a measurement system is illustrated in Figure 1.5. Basically such a system
consists of part or all of four general stages: (1) sensor–transducer stage, (2) signal-conditioning
stage, (3) output stage, and (4) feedback-control stage. These stages form the bridge between the
input to the measurement system and the system output, a quantity that is used to infer the value of
the physical variable measured. We discuss later how the relationship between the input information, as acquired by the sensor, and the system output is established by a calibration. We have already
discussed the sensor–transducer stage, so let’s move on to the signal-conditioning stage.
The signal-conditioning stage takes the transducer signal and modifies it to a desired
magnitude. This optional intermediate stage might be used to perform tasks such as increasing
the magnitude of the signal by amplification, removing portions of the signal through some filtering
technique, or providing mechanical or optical linkage between the transducer and the output stage.
For example, the translational displacement of a mechanic’s caliper (sensor) is often converted into a
rotational displacement of a pointer. This stage can consist of one or more devices, which are often
connected in series. For example, the diameter of the thermometer capillary relative to the bulb
volume (see Fig. 1.4) determines how far up the stem the liquid moves with increasing temperature.
It ‘‘conditions’’ the signal by amplifying the liquid displacement.
In those measurement systems involved in process control, a fourth stage, the feedback-control
stage, contains a controller that interprets the measured signal and makes a decision regarding the
control of the process. This decision results in a signal that changes the process parameter that
affects the magnitude of the sensed variable. In simple controllers, this decision is based on the
magnitude of the signal of the sensed variable, usually whether it exceeds some high or low set point,
a value set by the system operator. For example, a simple measurement system with control stage is a
household furnace thermostat. The operator fixes the set point for temperature on the thermostat
display, and the furnace is activated as the local temperature at the thermostat, as determined by the
Sensor stage
Signal
path
Transducer
stage
Signal conditioning stage
Output
stage
Control
stage
Control signal
Process
Calibration
Figure 1.5 Components of a general measurement system.
1.2 General Measurement System 5