Page 366 - The Mechatronics Handbook

FIGURE 18.1 Analog and digital sensor outputs.

FIGURE 18.2 Sensor sensitivity.

18.2 Resolution

The resolution of a sensor is the smallest increment of input that can be reliably detected. Resolution is
also frequently known as the least count of the sensor. Resolution of digital sensors is easily determined.
A 1024 ppr (pulse per revolution) incremental encoder would have a resolution of
1 revolution 360 degrees degrees
------------------------------ × ------------------------------ = 0.3516 -----------------
1024 pulses 1 revolution pulse
The resolution of analog sensors is usually limited only by low-level electrical noise and is often much
better than equivalent digital sensors.

18.3 Sensitivity

Sensor sensitivity is defined as the change in output per change in input. The sensitivity of digital sensors
is closely related to the resolution. The sensitivity of an analog sensor is the slope of the output versus
input line. A sensor exhibiting truly linear behavior has a constant sensitivity over the entire input range.
Other sensors exhibit nonlinear behavior where the sensitivity either increases or decreases as the input
is changed, as shown in Fig. 18.2.

18.4 Error

Error is the difference between a measured value and the true input value. Two classifications of errors
are bias (or systematic) errors and precision (or random) errors. Bias errors are present in all measure-
ments made with a given sensor, and cannot be detected or removed by statistical means. These bias
errors can be further subdivided into

• calibration errors (a zero or null point error is a common type of bias error created by a nonzero
output value when the input is zero),
• loading errors (adding the sensor to the measured system changes the system), and
• errors due to sensor sensitivity to variables other than the desired one (e.g., temperature effects
on strain gages).

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