Asensor is often defined as a device that receives and responds to a signal or stimulus. This definition is broad. In fact, it is so broad that it covers almost everything from a human eye to a trigger in a pistol. Consider the level-control system shown in Fig. 1.1 [1]. The operator adjusts the level of fluid in the tank by manipulating its valve.
Variations in the inlet flow rate, temperature changes (these would alter the fluid’s viscosity and, consequently, the flow rate through the valve), and similar disturbances must be compensated for by the operator.Without control, the tank is likely to flood, or run dry. To act appropriately, the operator must obtain information about the level of fluid in the tank on a timely basis. In this example, the information is perceived by the sensor, which consists of two main parts: the sight tube on the tank and the operator’s eye, which generates an electric response in the optic nerve. The sight tube by itself is not a sensor, and in this particular control system, the eye is not a sensor either. Only the combination of these two components makes a narrow-purpose sensor (detector),
which is selectively sensitive to the fluid level. If a sight tube is designed properly, it will very quickly reflect variations in the level, and it is said that the sensor has a fast speed response. If the internal diameter of the tube is too small for a given fluid viscosity, the level in the tube may lag behind the level in the tank. Then, we have to consider a phase characteristic of such a sensor. In some cases, the lag may be quite acceptable, whereas in other cases, a better sight tube design would be required. Hence, the sensor’s performance must be assessed only as a part of a data acquisition system. This world is divided into natural and man-made objects. The natural sensors, like those found in living organisms, usually respond with signals, having an electrochemical character; that is, their physical nature is based on ion transport, like in the nerve fibers (such as an optic nerve in the fluid tank operator). In man-made devices,
Variations in the inlet flow rate, temperature changes (these would alter the fluid’s viscosity and, consequently, the flow rate through the valve), and similar disturbances must be compensated for by the operator.Without control, the tank is likely to flood, or run dry. To act appropriately, the operator must obtain information about the level of fluid in the tank on a timely basis. In this example, the information is perceived by the sensor, which consists of two main parts: the sight tube on the tank and the operator’s eye, which generates an electric response in the optic nerve. The sight tube by itself is not a sensor, and in this particular control system, the eye is not a sensor either. Only the combination of these two components makes a narrow-purpose sensor (detector),
which is selectively sensitive to the fluid level. If a sight tube is designed properly, it will very quickly reflect variations in the level, and it is said that the sensor has a fast speed response. If the internal diameter of the tube is too small for a given fluid viscosity, the level in the tube may lag behind the level in the tank. Then, we have to consider a phase characteristic of such a sensor. In some cases, the lag may be quite acceptable, whereas in other cases, a better sight tube design would be required. Hence, the sensor’s performance must be assessed only as a part of a data acquisition system. This world is divided into natural and man-made objects. The natural sensors, like those found in living organisms, usually respond with signals, having an electrochemical character; that is, their physical nature is based on ion transport, like in the nerve fibers (such as an optic nerve in the fluid tank operator). In man-made devices,
Fig. 1.1. Level-control system. A sight tube and operator’s eye form a sensor (a device which converts information into electrical signal).
information is also transmitted and processed in electrical form—however, through the transport of electrons. Sensors that are used in artificial systems must speak the same language as the devices with which they are interfaced. This language is electrical in its nature and a man-made sensor should be capable of responding with signals where information is carried by displacement of electrons, rather than ions. Thus, it should be possible to connect a sensor to an electronic system through electrical wires, rather than through an electrochemical solution or a nerve fiber. Hence, in this book, we use a somewhat narrower definition of sensors, which may be phrased as A sensor is a device that receives a stimulus and responds with an electrical signal.
The term stimulus is used throughout this book and needs to be clearly understood. The stimulus is the quantity, property, or condition that is sensed and converted into electrical signal. Some texts (for instance, Ref. [2]) use a different term, measurand, which has the same meaning, however with the stress on quantitative characteristic of sensing.
The purpose of a sensor is to respond to some kind of an input physical property (stimulus) and to convert it into an electrical signal which is compatible with electronic circuits. We may say that a sensor is a translator of a generally nonelectrical value into an electrical value. When we say “electrical,” we mean a signal which can be channeled, amplified, and modified by electronic devices. The sensor’s output signal may be in the form of voltage, current, or charge. These may be further described in terms of amplitude, frequency, phase, or digital code. This set of characteristics is called the output signal format. Therefore, a sensor has input properties (of any kind) and electrical output properties.
The term stimulus is used throughout this book and needs to be clearly understood. The stimulus is the quantity, property, or condition that is sensed and converted into electrical signal. Some texts (for instance, Ref. [2]) use a different term, measurand, which has the same meaning, however with the stress on quantitative characteristic of sensing.
The purpose of a sensor is to respond to some kind of an input physical property (stimulus) and to convert it into an electrical signal which is compatible with electronic circuits. We may say that a sensor is a translator of a generally nonelectrical value into an electrical value. When we say “electrical,” we mean a signal which can be channeled, amplified, and modified by electronic devices. The sensor’s output signal may be in the form of voltage, current, or charge. These may be further described in terms of amplitude, frequency, phase, or digital code. This set of characteristics is called the output signal format. Therefore, a sensor has input properties (of any kind) and electrical output properties.
Any sensor is an energy converter. No matter what you try to measure, you always deal with energy transfer from the object of measurement to the sensor. The process of sensing is a particular case of information transfer, and any transmission of information requires transmission of energy. Of course, one should not be confused by an obvious fact that transmission of energy can flow both ways—it may be with a positive sign as well as with a negative sign; that is, energy can flow either from an object to the sensor or from the sensor to the object. A special case is when the energy is zero, and it also carries information about existence of that particular case.
For example, a thermopile infrared radiation sensor will produce a positive voltage when the object is warmer than the sensor (infrared flux is flowing to the sensor) or the voltage is negative when the object is cooler than the sensor (infrared flux flows from the sensor to the object). When both the sensor and the object are at the same temperature, the flux is zero and the output voltage is zero. This carries a message that the temperatures are the same.
The term sensor should be distinguished from transducer. The latter is a converter of one type of energy into another, whereas the former converts any type of energy into electrical. An example of a transducer is a loudspeaker which converts an electrical signal into a variable magnetic field and, subsequently, into acoustic waves. This is nothing to do with perception or sensing. Transducers may be used as actuators in various systems. An actuator may be described as opposite to a sensor—it converts electrical signal into generally nonelectrical energy. For example, an electric motor is an actuator—it converts electric energy into mechanical action.
Transducers may be parts of complex sensors (Fig. 1.2). For example, a chemical sensor may have a part which converts the energy of a chemical reaction into heat (transducer) and another part, a thermopile, which converts heat into an electrical signal. The combination of the two makes a chemical sensor—a device which produces an electrical signal in response to a chemical reaction. Note that in the above example, a chemical sensor is a complex sensor; it is comprised of a transducer and another sensor (heat). This suggests that many sensors incorporate at least one direct-type sensor and a number of transducers. The direct sensors are those that employ such physical effects that make a direct energy conversion into electrical signal generation or modification. Examples of such physical effects are photoeffect and Seebeck effect.
In summary, there are two types of sensors: direct and complex. A direct sensor converts a stimulus into an electrical signal or modifies an electrical signal by using an appropriate physical effect, whereas a complex sensor in addition needs one or more transducers of energy before a direct sensor can be employed to generate an electrical output.
A sensor does not function by itself; it is always a part of a larger system that may incorporate many other detectors, signal conditioners, signal processors, memory devices, data recorders, and actuators. The sensor’s place in a device is either intrinsic or extrinsic. It may be positioned at the input of a device to perceive the outside effects and to signal the system about variations in the outside stimuli. Also, it may be an
internal part of a device that monitors the devices’ own state to cause the appropriate performance. A sensor is always a part of some kind of a data acquisition system. Often, such a system may be a part of a larger control system that includes various feedback mechanisms.
internal part of a device that monitors the devices’ own state to cause the appropriate performance. A sensor is always a part of some kind of a data acquisition system. Often, such a system may be a part of a larger control system that includes various feedback mechanisms.
To illustrate the place of sensors in a larger system, Fig. 1.3 shows a block diagram of a data acquisition and control device. An object can be anything: a car, space ship, animal or human, liquid, or gas. Any material object may become a subject of some kind of a measurement. Data are collected from an object by a number of sensors. Some of them (2, 3, and 4) are positioned directly on or inside the object. Sensor 1 perceives the object without a physical contact and, therefore, is called a noncontact sensor. Examples of such a sensor is a radiation detector and a TV camera. Even if
we say “noncontact”, we remember that energy transfer always occurs between any sensor and an object. Sensor 5 serves a different purpose. It monitors internal conditions of a data acquisition system itself. Some sensors (1 and 3) cannot be directly connected to standard electronic circuits because of inappropriate output signal formats. They require the use of interface devices (signal conditioners). Sensors 1, 2, 3, and 5 are passive. They generate electric signals without energy consumption from the electronic circuits. Sensor 4 is active. It requires an operating signal, which is provided by an excitation circuit. This signal is modified by the sensor in accordance with the converted information. An example of an active sensor is a thermistor, which is a temperature-sensitive resistor. It may operate with a constant-current source, which is an excitation circuit. Depending on the complexity of the system, the total number of sensors may vary from as little as one (a home thermostat) to many thousands (a space shuttle).
Electrical signals from the sensors are fed into a multiplexer (MUX), which is a switch or a gate. Its function is to connect sensors one at a time to an analog-to-digital (A/D) converter if a sensor produces an analog signal, or directly to a computer if a sensor produces signals in a digital format. The computer controls a multiplexer and an A/D converter for the appropriate timing. Also, it may send control signals to the actuator, which acts on the object. Examples of actuators are an electric motor, a solenoid, a relay, and a pneumatic valve. The system contains some peripheral devices (for instance, a data recorder, a display, an alarm, etc.) and a number of components, which are not shown in the block diagram. These may be filters, sample-and-hold circuits, amplifiers, and so forth.
To illustrate how such a system works, let us consider a simple car-door monitoring arrangement. Every door in a car is supplied with a sensor which detects the door position (open or closed). In most cars, the sensor is a simple electric switch. Signals from all door sensors go to the car’s internal microprocessor (no need for an A/D converter as all door signals are in a digital format: ones or zeros). The microprocessor identifies which door is open and sends an indicating signal to the peripheral devices (a dashboard display and an audible alarm).Acar driver (the actuator) gets the message and acts on the object (closes the door).
An example of a more complex device is an anesthetic vapor delivery system. It is intended for controlling the level of anesthetic drugs delivered to a patient by means of inhalation during surgical procedures. The system employs several active and passive sensors. The vapor concentration of anesthetic agents (such as halothane, isoflurane, or enflurane) is selectively monitored by an active piezoelectric sensor, installed into a ventilation tube. Molecules of anesthetic vapors add mass to the oscillating crystal in the sensor and change its natural frequency, which is a measure of vapor concentration. Several other sensors monitor the concentration of CO2, to distinguish exhale from inhale, and temperature and pressure, to compensate for additional variables. All of these data are multiplexed, digitized, and fed into the microprocessor, which calculates the actual vapor concentration. An anesthesiologist presets a desired delivery level and the processor adjusts the actuator (the valves) to maintain anesthetics at the correct concentration.
Electrical signals from the sensors are fed into a multiplexer (MUX), which is a switch or a gate. Its function is to connect sensors one at a time to an analog-to-digital (A/D) converter if a sensor produces an analog signal, or directly to a computer if a sensor produces signals in a digital format. The computer controls a multiplexer and an A/D converter for the appropriate timing. Also, it may send control signals to the actuator, which acts on the object. Examples of actuators are an electric motor, a solenoid, a relay, and a pneumatic valve. The system contains some peripheral devices (for instance, a data recorder, a display, an alarm, etc.) and a number of components, which are not shown in the block diagram. These may be filters, sample-and-hold circuits, amplifiers, and so forth.
To illustrate how such a system works, let us consider a simple car-door monitoring arrangement. Every door in a car is supplied with a sensor which detects the door position (open or closed). In most cars, the sensor is a simple electric switch. Signals from all door sensors go to the car’s internal microprocessor (no need for an A/D converter as all door signals are in a digital format: ones or zeros). The microprocessor identifies which door is open and sends an indicating signal to the peripheral devices (a dashboard display and an audible alarm).Acar driver (the actuator) gets the message and acts on the object (closes the door).
An example of a more complex device is an anesthetic vapor delivery system. It is intended for controlling the level of anesthetic drugs delivered to a patient by means of inhalation during surgical procedures. The system employs several active and passive sensors. The vapor concentration of anesthetic agents (such as halothane, isoflurane, or enflurane) is selectively monitored by an active piezoelectric sensor, installed into a ventilation tube. Molecules of anesthetic vapors add mass to the oscillating crystal in the sensor and change its natural frequency, which is a measure of vapor concentration. Several other sensors monitor the concentration of CO2, to distinguish exhale from inhale, and temperature and pressure, to compensate for additional variables. All of these data are multiplexed, digitized, and fed into the microprocessor, which calculates the actual vapor concentration. An anesthesiologist presets a desired delivery level and the processor adjusts the actuator (the valves) to maintain anesthetics at the correct concentration.
Vehicle. (Courtesy of Nissan Motor Company.)
Another example of a complex combination of various sensors, actuators, and indicating signals is shown in Fig. 1.4. It is an Advanced Safety Vehicle (ASV) that is being developed by Nissan. The system is aimed at increasing safety of a car. Among many others, it includes a drowsiness warning system and drowsiness relieving system.
This may include the eyeball movement sensor and the driver head inclination detector. The microwave, ultrasonic, and infrared range measuring sensors are incorporated into the emergency braking advanced advisory system to illuminate the break lamps even before the driver brakes hard in an emergency, thus advising the driver of a following vehicle to take evasive action. The obstacle warning system includes both the radar and infrared (IR) detectors. The adaptive cruise control system works if the driver approaches too closely to a preceding vehicle: The speed is automatically reduced to maintain a suitable safety distance. The pedestrian monitoring system detects and alerts the driver to the presence of pedestrians at night as well as in vehicle blind spots. The lane control system helps in the event that the system detects and determines that incipient lane deviation is not the driver’s intention. It issues a warning and automatically steers the vehicle, if necessary, to prevent it from leaving its lane.
In the following chapters, we concentrate on methods of sensing, physical principles of sensors operations, practical designs, and interface electronic circuits. Other essential parts of the control and monitoring systems, such as actuators, displays, data recorders, data transmitters, and others, are beyond the scope of this book and mentioned only briefly.
Generally, the sensor’s input signals (stimuli) may have almost any conceivable physical or chemical nature (e.g., light flux, temperature, pressure, vibration, displacement, position, velocity, ion concentration, . . .). The sensor’s design may be of a general purpose. A special packaging and housing should be built to adapt it for a particular application. For instance, a micromachined piezoresistive pressure sensor may be housed into a watertight enclosure for the invasive measurement of aortic blood pressure through a catheter. The same sensor will be given an entirely different enclosure when it is intended for measuring blood pressure by a noninvasive oscillometric method with an inflatable cuff. Some sensors are specifically designed to be very selective in a particular range of input stimulus and be quite immune to signals outside of the desirable limits. For instance, a motion detector for a security system should be sensitive to movement of humans and not responsive to movement of smaller animals, like dogs and cats.
This may include the eyeball movement sensor and the driver head inclination detector. The microwave, ultrasonic, and infrared range measuring sensors are incorporated into the emergency braking advanced advisory system to illuminate the break lamps even before the driver brakes hard in an emergency, thus advising the driver of a following vehicle to take evasive action. The obstacle warning system includes both the radar and infrared (IR) detectors. The adaptive cruise control system works if the driver approaches too closely to a preceding vehicle: The speed is automatically reduced to maintain a suitable safety distance. The pedestrian monitoring system detects and alerts the driver to the presence of pedestrians at night as well as in vehicle blind spots. The lane control system helps in the event that the system detects and determines that incipient lane deviation is not the driver’s intention. It issues a warning and automatically steers the vehicle, if necessary, to prevent it from leaving its lane.
In the following chapters, we concentrate on methods of sensing, physical principles of sensors operations, practical designs, and interface electronic circuits. Other essential parts of the control and monitoring systems, such as actuators, displays, data recorders, data transmitters, and others, are beyond the scope of this book and mentioned only briefly.
Generally, the sensor’s input signals (stimuli) may have almost any conceivable physical or chemical nature (e.g., light flux, temperature, pressure, vibration, displacement, position, velocity, ion concentration, . . .). The sensor’s design may be of a general purpose. A special packaging and housing should be built to adapt it for a particular application. For instance, a micromachined piezoresistive pressure sensor may be housed into a watertight enclosure for the invasive measurement of aortic blood pressure through a catheter. The same sensor will be given an entirely different enclosure when it is intended for measuring blood pressure by a noninvasive oscillometric method with an inflatable cuff. Some sensors are specifically designed to be very selective in a particular range of input stimulus and be quite immune to signals outside of the desirable limits. For instance, a motion detector for a security system should be sensitive to movement of humans and not responsive to movement of smaller animals, like dogs and cats.
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