Introduction to Sensors and Transducers

Instrumentation plays a key role in the modern technological world. An essential component in mechatronic systems which is integrally linked to instrumentation is the sensor, whose function is to provide a mechanism for collecting different types of information about a particular process.

Sensors are used to inspect work, evaluate the conditions of work under progress, and facilitate the higher-level monitoring of the manufacturing operation by the main computing system. They can be used during pre-process, in-process and post-process operations. In some situations, sensors are used to translate a physical phenomenon into an acceptable signal that can be analyzed for decision making. Intelligent systems use sensors to monitor particular situations influenced by a changing environment and to control them with corrective actions.

In virtually every application, sensors transform real-world data into electrical signals. A sensor is defined as: A device that produces an output signal for the purpose of sensing of a physical phenomenon.

Sensors are also referred to as transducers. They cover a broader range of activities, which provide them with the ability to identify environmental inputs that can extend beyond the human senses. A transducer is defined as: A device that converts a signal from one physical form to a corresponding signal, which has a different physical form.

In a transducer, the quantities at the input level and the output level are different. A typical input signal could be electrical, mechanical, thermal, and optical. Signal detection is normally handled by electrical transducers in manufacturing industries involving certain process automation. A transducer is an element or device used to convert information from one form to another. The change in information is measured easily.

A spring is a simple example of a transducer. When a certain force is applied to a spring, it stretches, and the force information is translated to displacement information, as shown in Ill. 1. Different quantities of force produce differential movements, which are a measure of the force.

Displacement y is proportional to force F, which can be expressed as F = k # y where k is constant F _ applied force y _ deflection k _ constant

ILL. 1 PRIMARY TRANSDUCER

1 Introduction to Sensors and Transducers

The extent to which sensors and transducers are used is dependent upon the level of automation and the complexity of the control system. The modeling requirements of the complex control systems have introduced a need for fast, sensitive, and precise measuring devices. Due to these demands, sensors are being miniaturized and implemented in a microscale by combining several sensors and data-processing mechanisms. Many microsystems have been built on the "lab-on-a-chip" concept.

The entire unit can be contained in a silicon chip of the size less than 0.5 _ 0.5 mm.

Selection of a sensor or a transducer depends on

1. Variables measured and application.
2. The nature of precision and the sensitivity required for the measurement.
3. Dynamic range.
4. Level of automation.
5. Complexity of the control system and modeling requirements.
6. Cost, size, usage, and ease of maintenance.

Two important components in modern control systems (whether electrical, optical, mechanical, or fluid) are the system's sensors and transducers. The sensor elements detect measurands (variables to be measured) and convert them into acceptable form, generally as electrical signals. The maxi mum accuracy of the total system is controlled by the sensitivity of the individual sensors and the internally generated noise of the sensor itself. In a control system used for measurement and control, any parameter change either in measurands (variables to be measured) or in signal conditioning, has a direct effect on the sensitivity of the model.

Ill. 2 shows elements of a sensor-based measurement system. The function of the sensor is to sense the information of interest and to convert this information into an acceptable form by a signal conditioner. The function of the signal conditioner is to accept the signal from the detector and to modify in a way acceptable to the display unit. The function of the display-read out is to accept the signal from the signal conditioner and to present it in a displayable fashion.

The output can be in the form of an output display, or a printer, or it may be passed on to a controller. It also can be manipulated and fed back to the source from which the original signal was measured.

ILL. 2 A MECHATRONICS MEASUREMENT SYSTEM WITH AUXILIARY ENERGY SOURCE: Energy source, To controller, Display Source, Signal conditioner, Sensor detector, Feedback sensor ILL. 3 GENERAL INSTRUMENTATION SYSTEM and ITS COMPONENTS

Ill. 3 presents the components of an instrumentation system used for a general sensing application. A typical system consists of primary elements that sense and convert information into a more suitable form to be handled by the measurement system-signal conditioning stage for processing and modifying the information, an input/output stage for interface, and control with the external processes.

1 Sensor Classification

In the design of a mechatronics system, selection of a suitable sensor is very important. Table 3-1 summarizes some general sensor classifications.

Sensors are classified into two categories based on the output signal, power supply, operating mode and the variables being measured.

• Analog sensors: Analog is a term used to convey the meaning of a continuous, uninterrupted, and unbroken series of events. Analog sensors typically have an output, which is proportional to the variable being measured. The output changes in a continuous way, and this information is obtained on the basis of amplitude. The output is normally supplied to the computer using an analog-to-digital converter.

• Digital sensors: Digital refers to a sequence of discrete events. Each event is separate from the previous and next events. The sensors are digital if their logic-level outputs are of a dig ital nature. Digital sensors are known for their accuracy and precision, and don't require any converters when interfaced with a computer monitoring system.

TABLE 3-1 SENSOR CLASSIFICATION SCHEMES

= = =

Classification:

Signal Characteristics Power Supply Mode of Operation Subject of Measurement

Sensor Type:

Analog Digital Active Passive Null type Deflection type Acoustic Biological Chemical Electric Mechanical Optical Radiation Thermal Others

= = =

Another form of classification, active or passive, is based on the power supply.

• Active sensors: Active sensors require external power for their operation. The external signal is modified by the sensor to produce the output signal. Typical examples of devices requiring an auxiliary energy source are strain gauges and resistance thermometers.

• Passive sensors: In a passive sensor, the output is produced from the input parameters. The passive sensors (self generating) produce an electrical signal in response to an external stimulus.

Examples of passive types of sensors include piezoelectric, thermoelectric, and radioactive.

Based on the operating and display mode of an instrumentation system, sensors are classified as deflection type or null type.

• Deflection sensors: Deflection sensors are used in a physical setup where the output is proportional to the measured quantity that is displayed.

• Null sensors: In null-type sensing, any deflection due to the measured quantity is balanced by the opposing calibrated force so that any imbalance is detected.

A final classification of sensors is based on the subject of measurement. Such subjects include acoustic, biological, chemical, electric, magnetic, mechanical, optical, radiation, thermal, and others.

2 Parameter Measurement in Sensors and Transducers

Let us examine the instrumentation system model from the viewpoint of its functional elements in a generalized way. The elements contribute to the sensing and measurement of an instrumentation system and also influence the quality of the device.

Ill. 4 shows a block diagram of elements of a typical instrumentation system. The basic subsystems include the following modules.

• Sensing module

• Conversion module

• Variable manipulation module

• Data transmission

• Presentation module

ILL. 4 ELEMENTS OF AN INSTRUMENTATION SYSTEM: Measured Medium; Sensing module, Conversion module, Variable manipulation; Data transmission; Data display; Observer

The integrated effect of all the functional modules results in a useful measurement system. A description of each module is given here.

Sensing Module The first element to receive a signal from the measured medium and produces an output depending on the measured quantity. During the process of sensing, some energy gets extracted from the measured medium. In fact, the measured quantity gets disturbed by the act of measurement, making a perfect measurement theoretically impossible. Good instruments are normally designed to minimize the error of measurement.

Conversion Module Converts one physical variable to another. it's also known as a transducing element. In certain cases, the transduction of the input signal may take place progressively in stages, such as primary, secondary, and tertiary transduction.

Variable Manipulation Module Usually, this involves signal conditioning. Some examples of variable manipulation element are amplifiers, linkage mechanisms, gearboxes, magnifiers, etc. An electronic amplifier accepts a small voltage signal as an input signal and generates a signal that is many times larger than the input signal.

Data Transmission Module This sends a signal from one point to another point. e.g., the transmission element could be a simple device such as a shaft and bearing assembly or could be a complicated device, such as a telemetry system for transmitting signals from ground to satellites.

Data Display Module

Produces information about the measured quantity in a form that can be recognized by one of the human senses.

EXAMPLE 1 Home Heating System

The functional elements of a typical home heating system are shown in Ill.

ILL. 5 HOME HEATING SYSTEM EXAMPLE

Solution

The block diagram represents the six major system components and their interconnections. The interconnections completely define the inputs and outputs for each of the six major blocks. For instance, the thermostat block processes two input signals (a room temperature and a temperature set point,) to produce one output signal, which is sent to a mechanical relay switch. The thermostat acts as a primary sensor and transducer.

EXAMPLE 2 Pressure Sensor

An example of a pressure sensor in the form of a spring-loaded piston and a display mechanism is shown in Ill. This pressure sensing instrument can be broken down into functional elements. The source is connected to a pneumatic cylinder. The pressure acts on the piston and spring mechanism. The spring works as a primary sensor and variable conversion element. The deflection of the spring is transferred to the display as a movement of the dial indicator.

ILL. 6 SCHEMATIC OF A PRESSURE SENSOR

3 Quality Parameters

Sensors and transducers are often used under different environmental conditions. Like human beings, they are sensitive to environmental inputs such as pressure, motion, temperature, radiation, and magnetic fields.

Sensor characteristics are described in terms of seven properties discussed and illustrated in the following subsections.

1. Sensitivity
2. Resolution
3. Accuracy
4. Precision
5. Backlash
6. Repeatability
7. Linearity

Sensitivity

Sensitivity is the property of the measuring instrument to respond to changes in the measured quantity. It also can be expressed as the ratio of change of output to change of input as shown in Figures 3-7 and 3-8.

ILL. 7 BASIC TRANSDUCER MODEL

ILL. 8 INPUT-OUTPUT RELATIONSHIP: Sensitivity is measured by S = delta_O delta_I where S is the sensitivity, represents change in output, and represents the change in input.

e.g., in an electrical measuring instrument if a movement of 0.001 mm causes an output

delta_I delta_O voltage change of 0.02 V, the sensitivity of the measuring instrument is S = 0.02 / 0.001 = 20 V/mm

Resolution

Resolution is defined as the smallest increment in the measured value that can be detected. it's also known as the degree of fineness with which measurements can be made. e.g., if a micrometer with a minimum graduation of 1 mm is used to measure to the nearest 0.5 mm, then by interpolation, the resolution is estimated as 0.5 mm.

Accuracy

Accuracy is a measure of the difference between the measured value and actual value.

Accuracy depends on the inherent instrument limitations. An experiment is said to be accurate if it's unaffected by experimental error. An accuracy of _ 0.001 means that the measured value is within 0.001 units of actual value. In practice, the accuracy is defined as a percentage of the true value.

Percentage of true value = [measured value - true value / true value] (100)

If a precision balance reads 1 g with error of 0.001 g, then the accuracy of the instrument is specified as 0.1%. The difference between the measured value and true value is called bias (error).

Precision

Precision is the ability of an instrument to reproduce a certain set of readings within a given accuracy. Precision is dependent on the reliability of the instrument.

EXAMPLE 3 Target Shooting

Ill. 9 presents an illustration of degree of accuracy and precision in a typical target-shooting example.

Solution

The "high precision, poor accuracy" situation occurs when the person hits all the bullets on a target plate on the outer circle and misses the bull's eye. In the second case, "high accuracy, high precision", all the bullets hit the bull's eye and are spaced closely enough. In the third example, "good accuracy, poor precision", the bullet hits are placed symmetrically with respect to the bull's eye but are spaced apart. In the last case, "poor accuracy, poor precision", the bullets hit the target in a random order.

ILL. 9 TARGET SHOOTING EXAMPLE: Poor accuracy, High precision; High accuracy, High precision; Good average accuracy, Poor precision; Poor accuracy, Poor precision

Backlash

Backlash is defined as the maximum distance or angle through which any part of a mechanical system can be moved in one direction without causing any motion of the attached part.

Backlash is an undesirable phenomenon and is important in the precision design of gear trains.

Repeatability

Repeatability is the ability to reproduce the output signal exactly when the same measurand is applied repeatedly under the same environmental conditions.

Linearity The characteristics of precision instruments are that the output is a linear function of the input. However, linearity is never completely achieved, and the deviations from the ideal are termed linearity tolerances. The linearity is expressed as the percentage of departure from the linear value (i.e., maximum deviation of the output curve from the best-fit straight line during a calibration cycle). The nonlinearity is normally caused by nonlinear elements, such as mechanical hysteresis, viscous flow or creep, and electronic amplifiers.

4 Errors and Uncertainties in Mechatronic Modeling Parameters

Modern mechatronic technology relies heavily on the use of sensors and measurement technology. The control of industrial processes and automated systems would be very difficult without accurate sensors and measurement systems. The economical production of a mechatronic instrument requires the proper choice of sensors, material, and hardware and software design. To a large degree, the final choice of an instrument for any particular application depends upon the accuracy desired. If a low degree of accuracy is acceptable, it's not economical to use expensive sensors and precise sensing components. If, however, the instrument is used for high-precision applications, the design tolerances must be small.

Any system which relies on a measurement system will involve some amount of uncertainty. The uncertainty may be caused by the individual inaccuracy of sensors, random variations in measurands, or environmental conditions. The accuracy of the total system depends on the interaction of the components and their individual accuracy. This is true for measurement instruments as well as production systems, which depend on many subsystems and components. A typical instrument may consist of many components that have complex interrelations, and each component may contribute to the over all error. The errors and inaccuracies in each of these components can have a large cumulative effect.

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