Mechanical Eng. Dept.: Sensors & Signal Conditioning
Mechanical Eng. Dept.: Sensors & Signal Conditioning
Mechanical Eng. Dept.: Sensors & Signal Conditioning
• Figure 1. Thermocouples are inexpensive and can operate over a wide range of temperatures.
• Thermocouples are the most commonly used temperature sensors because they are relatively inexpensive yet
accurate sensors that can operate over a wide range of temperatures. A thermocouple is created when two
dissimilar metals touch and the contact point produces a small open-circuit voltage as a function of temperature.
You can use this thermoelectric voltage, known as See beck voltage, to calculate temperature. For small changes
in temperature, the voltage is approximately linear.
• You can choose from different types of thermocouples designated by capital letters that indicate their
compositions according to American National Standards Institute (ANSI) conventions. The most common types of
thermocouples include B, E, K, N, R, S, and T.
3 • 2.Risistence Temperature Detector (RTD)
• Figure 2. RTDs are made of metal coils and can measure temperatures up to
850 °C.
• A platinum RTD is a device made of coils or films of metal (usually platinum).
When heated, the resistance of the metal increases; when cooled, the resistance
decreases. Passing current through an RTD generates a voltage across the RTD.
By measuring this voltage, you can determine its resistance and, thus, its
temperature. The relationship between resistance and temperature is relatively
linear. Typically, RTDs have a resistance of 100 Ω at 0 °C and can measure
temperatures up to 850 °C.
4 • 3.Thermistor
• Figure 3. The most common method for measuring load, pressure, and torque is to employ a full-bridge strain
gage-based transducer.
• The most common method for measuring load, pressure, and torque is to employ a full-bridge strain gage-based
transducer. A load cell, used to measure load and force, consists of an array of strain gages, which measure the
deformation of a structural member and converts it into an electrical signal. Pressure transducers operate under
the same principle as load cells. Strain gages, mounted on a diaphragm where pressure is applied, measure the
deformation of the diaphragm that is proportional to the pressure. Torque sensors are composed of strain gages
that are affixed to a torsion bar. As the bar turns, the gages respond to the bar's shear stress, which is proportional
to the torque.
• Load, pressure, and torque sensors can output low- or high-level voltages, depending on its excitation
requirements. Low-level sensors are typically powered by a measurement device and output mV signals. High-level
sensors (or amplified sensors) require higher external power sources to operate, and output ±5 V, ±10 V, or 4-20 mA.
7 Displacement and Position Sensors
• Types of Displacement and Position Sensors
• Proximity probes, linear variable differential transformers (LVDTs), and rotary
variable differential transformers (RVDTs) are popular sensors for measuring
position. However, there are many different ways to measure position, including
encoders and string potentiometers.
• Figure 1. Eddy current proximity probes are sensors that measure relative
proximity.
8 • Eddy current proximity probes are sensors that measure relative proximity. They use changes in
voltage to measure shaft surfaces that rotate or reciprocate. Because they are no contacting
transducers, proximity probes are mounted on a reasonably stationary mechanical structure, such
as a bearing housing. From the mounting point, they measure the static and dynamic
displacement behavior of the moving machinery. Use the proximity probe measurement type
when you want to measure a dynamic position, such as an air gap between parts of moving
machinery.
• Figure 2. LVDTs operate on the principle of a transformer and consist of a stationary coil
assembly and a moveable core.
• LVDTs operate on the principle of a transformer and consist of a stationary coil assembly and a
moveable core. An LVDT measures displacement by associating a specific signal value for any
given position of the core. LVDT signal conditioners generate a sine wave for the primary output
signal and synchronously demodulate the secondary output signal. The demodulated output is
passed through a low pass filter to remove high-frequency ripple. The resulting output is a DC
voltage proportional to core displacement. The sign of the DC voltage indicates whether the
displacement is to the left or right. The main advantage of the LVDT transducer over other types
of displacement transducer is the high degree of power.
• RVDTs are the rotational version of LVDTs and generally operate over an angular range of ±30°–
70°.
9
Vibration Sensors
• For example, thermocouple signals have very small voltage levels that must be
amplified before they can be digitized. Other sensors, such as resistance
temperature detectors (RTDs), thermistors, strain gages, and accelerometers,
require excitation to operate. All of these preparation technologies are forms of
signal conditioning.