General concepts and terminology for Transducers, Sensors, and Actuators
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A transducer is a device that conve...
A transducer is a device that converts a signal from one physical form to a corresponding signal having a different physical form. Therefore it is an energy converter. This means that the input signal always has energy or power. But in measurement systems one of the two components of the measured signal, which is multiplied to yield power, is usually so small that it is negligible, and thus only the remaining component is measured.
When measuring a force, for example, we assume that the displacement in the transducer is insignificant. That is, that there is no ‘loading” effect. Otherwise, it might happen that the measured force is unable to deliver the needed energy to allow the movement. But there is always a certain power taken by the transducer, so we must ensure that the measured system is not perturbed by the measuring action.
Since there are six different kinds of signals—mechanical, thermal, magnetic, electric, optical, and chemical—any device converting signals of one kind to signals of a different kind is considered to be a transducer. The output signals can be of any useful physical form. In practice, however, only devices offering an electric output are called transducers. This is because electric signals are used in most measurement systems. Some of the advantages of electronic measurement systems are the following:
I. Electrical transducers can be designed for any nonelectric quantity, by selecting an appropriate material. Any variation in a nonelectric parameter yields a variation in an electric parameter because of the electronic structure of matter.
2. Energy is not drained from the process being measured because the transducer output signals can be amplified. With electronic amplifiers it is easy to obtain power gains exceeding 1010 in a single stage.
3. A large number of diverse integrated circuits are available for electric signal conditioning or modification. There are even transducers that incorporate these conditioners in a single package.
4. Many options exist for information display or recording by electronic means. These options permit us to handle not only numerical data but also text, graphics, and diagrams.
5. Signal transmission is more versatile for electric signals. Mechanical, hydraulic, or pneumatic signals may be more suitable in some circumstances, such as in environments where ionizing radiation or explosive atmospheres are present, but electric signals have replaced most non- electric signals. In fact in processing industries (chemicals, petroleum, gas, food, textile, paper, etc.) where automation was introduced some time ago, pneumatic systems are frequently found alongside more recent electric systems. However, in manufacturing industries (machinery, automotive, computing, and communication equipment, etc.) where automation is more recent and there are several discontinuous processes, we usually find only electronic systems used.
A transducer is a device that takes energy from the system that it measures to give an output signal that can be transduced to an electric signal and that corresponds to the measured quantity.
Sensor and transducer are sometimes used as synonymous terms. However, sensor suggests a broader meaning that includes the extension of our capacity to acquire information about physical quantities not perceived by human senses because of their subliminal nature or minuteness. Transducer suggests only that input and output quantities are not the same. The word modifier has been proposed for instances where input and output quantities are the same, but it has not been widely accepted.
The distinction between input-transducer (physical signal/electric signal) and output-transducer (electric signal/display or actuation) is seldom used today. The trend, particularly in robotics is toward using sensor to refer to the input transducer while actuator refers to the output transducer. Sensors are intended to acquire information. Actuators are designed for power conversion.
In this book we use sensor to refer to input transducers. Output transducers or actuators are beyond the scope of this work. Sometimes, particularly when mechanical quantities are being measured, an element called a primary sensor is introduced to convert the measured variable into a measuring signal. Then a sensor would convert that signal into an electric signal. For example, one would use a diaphragm as the primary sensor to measure a pressure difference hut measure deformation using a strain gage which serves as the sensor. In this book we will designate as sensor the whole device, including the package and leads.
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