Sensor Applications

1 Eddy Current Transducer

Eddy current transducers are used to detect the presence of nonmagnetic but conductive materials. They are also used in nondestructive testing applications, including flaw inspections and location of defects.

Defects may include changes in composition, structure, and hardness, as well as cracks and voids. In addition to detecting the presence or absence of an object, eddy current transducers can be used to determine material thickness and non-conductive coating thickness. Depending on the application, eddy current transducers can vary in diameter from 2 to 30 mm. Direct contact with the specimen is not required, which makes it ideal for unattended continuous process monitoring.

When a conducting material is placed in a changing magnetic field, an electromotive force (EMF) is induced in it. This EMF causes localized currents to flow, which are known as eddy currents. Eddy currents can be induced in any conductor but are most noticeable in solid conductors.

e.g., when the magnetic core of a transformer or rotating machine is subjected to a change in magnetization, eddy currents are produced. Ill. 82 shows the principle behind the eddy current transducer.

ILL. 82 EDDY CURRENT PRINCIPLE

A nonferrous plate moves in a direction perpendicular to the lines of flux of a magnet. Eddy currents generated in the plate are proportional to the velocity of the plate. The eddy currents set up a magnetic field in a direction that opposes the magnetic field that creates them. The output voltage is proportional to the rate of change of eddy currents in the plate.

The eddy current sensor, shown in Ill. 83, has two identical coils, one coil is used as a reference, and the second coil is used to sense the magnetic current in the conductive object.

ILL. 83 SENSING and REFERENCE COILS IN AN EDDY CURRENT TRANSDUCER

Eddy currents produce a magnetic field which opposes that of the sensing coil, resulting in a reduction of flux. When the plate is nearer to the coil, the eddy currents as well as the change in magnetic impedance are both larger. The coils form two arms of an impedance bridge. The bridge has a supply frequency usually 1 MHz or higher. In the absence of a target object, the output of the impedance bridge is zero. As a target moves closer to the sensor, eddy currents are generated in the conducting medium because of radio frequency (RF) magnetic flux from the active coil.

Inductance of the active coil increases, causing a voltage output in the bridge circuit.

Eddy current transducers are designed with shielded and unshielded configurations. The shielded transducer has a metal guard around the ferrite core and the coil assembly. This shielding focuses the electromagnetic field to the front of the transducer and allows the transducer to be installed in a metal structure without influencing the range of detection. The unshielded transducer can sense from its sides as well as its front.

The block representation of the signal processing in an eddy current transducer is shown in Ill. 84. Using sensitive eddy current transducers, differential motions of .001 mm are easily detected. Eddy current transducers are attractive because of their low cost, small size, high reliability, and their effectiveness while operating at elevated temperatures.

ILL. 84 SIGNAL PROCESSING IN EDDY CURRENT TRANSDUCERS

Sensor -> Impedance bridge -> Filter -> Calibration

SUMMARY Eddy Current Transducers

When a conducting material is placed in a changing magnetic field, an electromotive force (EMF) is induced in it. This EMF causes localized currents to flow are called eddy currents. A nonferrous plate moves in a direction perpendicular to the lines of flux of a magnet. Eddy currents are generated in the plate that are proportional to the velocity of the plate. The output voltage is proportional to the rate of change of eddy currents in the plate.

ILL. 85: Reference coil, Sensing coil; Object

Applications

• Eddy current transducers are used as proximity sensors.

• Used in non-destructive testing applications, including flaw inspections and defect location.

• Used to determine material thickness and non-conductive coating thickness.

Features

Direct contact with the specimen is not required which makes it ideal for unattended continuous process monitoring.

Hall Effect

The Hall Effect is the generation of a transverse voltage in a conductor or semiconductor carrying current in a magnetic field. The Hall effect results in the production of an electric field perpendicular to the directions of both the magnetic field and the current with a magnitude proportional to the product of the magnetic field strength, the current, and various properties of the conductor.

Position Sensing

As the magnet moves back and forth at that fixed gap (Ill. 86), the magnetic field induced by the element becomes negative as it approaches the north pole and positive as it approaches the south pole.

ILL. 86

Applications

• Hall effect sensors are used for proximity, level, and flow sensing applications.

• Devices based on the Hall Effect include Hall-effect vane switches, Hall-effect current sensors, and Hall-effect magnetic-field strength sensors.

Features

• Hall effect sensors provide liquid-level measurement without any electrical connections inside the tank.

• Tend to be more expensive than inductance proximity sensors, but have better signal-to-noise ratios and are suitable for low speed operation.

2 Hall Effect

Hall effect transducers are used to measure position, displacement, level, and flow. They can be used as an analog motion sensing device as well as a digital device. The Hall effect occurs when a strip of conducting material carries current in the presence of a transverse magnetic field, as shown in Ill. 87. The Hall effect results in the production of an electric field perpendicular to the directions of both the magnetic field and the current with a magnitude proportional to the product of the magnetic field strength, the current, and various properties of the conductor. An electron of charge, e, traveling in a magnetic field, B, with a velocity v, experiences a Lorenz force F, and it's represented by:

F = e(v * B)

ILL. 87 HALL EFFECT PRINCIPLE: Magnetic source; Hall element; thickness, t; Transverse magnetic field.

An electric field, known as Hall's field, counterbalances Lorenz's force and is represented by an electric potential. The voltage produced may be used to produce field strength or a current.

Ill. 87 shows the Hall effect principle. Current is passed through leads 1 and 2 of the element. The output leads are connected to the element faces 3 and 4. These output ends are at the same potential when there is no transverse magnetic field passing through the element. When there is a magnetic flux passing through the element, a voltage V appears between output leads. This voltage is proportional to the current and the field strength. The output voltage is represented in terms of element thickness, the flux density of the field, the current through the element, and the Hall coefficient as

V = H [IB/t]

where H = Hall coefficient, which can be defined as transverse electric potential gradient per unit magnetic field per unit current density. The units are V-m per A-Wb/m2 I = current through the element (A) B = flux density of the field (Wb/m^2) t = thickness of the element (m) The overall sensitivity of the transducer depends on the Hall coefficient. The Hall effect may be either negative or positive, depending on the material crystalline structure, and is present in metals and semiconductors in varying amounts based on the characteristics of the materials.

EXAMPLE 9 Flux Density Measurements Using a Hall Element

A Hall element with dimensions 4 x 4 x 2 mm is used to measure flux density. The Hall coefficient (H) is _0.8 V-m per A-Wb/m2. Find the voltage developed if the field strength is 0.012 Wb/m2 and the current density is 0.003 A/mm^2.

Solution

Current = Current density _ area

_ 0.003 _ 4 _ 4 _ 0.0048 A

The voltage generated is V = 0.23 V

ILL. 88 HALL ELEMENT FOR ANGULAR MEASUREMENT

The Hall sensor is suspended between the poles of a permanent magnet connected to the shaft, as shown in Ill. 89. The probe is stationary, and the permanent magnet connected to the shaft rotates. With a constant control current applied to the electrical contacts at the end of the probe, the Hall voltage generated across the probe is directly proportional to the sine of the angular displacement of the shaft. Small rotations up to six degrees can be measured precisely with such probes.

The main advantage of such devices is that they have no contact, small size, and good resolution.

ILL. 89 ROTATIONAL TRANSDUCER Control terminals; Output terminals; Magnetic field

Output voltage generated for a rotation of _ degrees is summarized as V = HIB [ sin a /t]

Here theta is the angle between the magnetic field and the Hall plate.

Constructional Details of a Hall Effect Sensor

The Hall element requires signal conditioning to make the output usable for most applications. The signal conditioning electronics needed are amplifier stage and temperature compensation. Voltage regulation is needed when operating from an unregulated supply. Ill. 90 illustrates a basic Hall effect sensor. If the Hall voltage is measured when no magnetic field is present, the output is zero (Ill. 87). However, if voltage at each output terminal is measured with respect to ground, a non-zero voltage will appear. This is the common mode voltage (CMV) and is the same at each output terminal. it's the potential difference that is zero. The amplifier shown in Ill. 90 must be a differential amplifier in order to amplify only the potential difference (i.e., the Hall voltage).

ILL. 90 BASIC ANALOG OUTPUT HALL EFFECT SENSOR

The Hall voltage is a low-level signal on the order of 30 _ V in the presence of a one gauss magnetic field. This low-level output requires an amplifier with low noise, high input impedance, and moderate gain. A differential amplifier with these characteristics can be readily integrated with the Hall element using standard bipolar transistor technology. Temperature compensation is also easily integrated. As was shown by Equation 3-91, the Hall voltage is a function of the input cur rent. The purpose of the regulator in Ill. 90 is to hold this current constant so that the output of the sensor only reflects the intensity of the magnetic field. As many systems have a regulated sup ply available, some Hall Effect sensors may not include an internal regulator.

Analog Output Sensors

The sensor described in Ill. 90 is a basic analog output device.

Analog sensors provide an output voltage that is proportional to the magnetic field to which it's exposed. The sensed magnetic field can be either positive or negative. As a result, the output of the amplifier will be driven either positive or negative. Hence, a fixed offset or bias is introduced into the differential amplifier which appears on the output when no magnetic field is present and is referred to as a null voltage. When a positive magnetic field is sensed, the output increases above the null voltage. Conversely, when a negative magnetic field is sensed, the output decreases below the null voltage, but remains positive. This concept is illustrated in Ill. 91.

ILL. 91 HALL EFFECT SENSOR'S CHARACTERISTIC CURVE -- Saturation, Output voltage (volts), Null voltage; Input magnetic field

ILL. 92 ANALOG OUTPUT HALL EFFECT SENSOR

Also, the output of the amplifier can't exceed the limits imposed by the power supply. In fact, the amplifier will begin to saturate before the limits of the power supply are reached. This saturation is illustrated in Ill. 91. it's important to note that this saturation takes place in the amplifier and not in the Hall element. Thus, large magnetic fields will not damage the Hall effect sensors, but rather drive them into saturation. To further increase the interface flexibility of the device, an open emitter, open collector, or push-pull transistor is added to the output of the differential amplifier. Ill. 92 shows a complete analog output Hall effect sensor incorporating all of the previously discussed circuit functions.

Digital Output Sensors

The digital Hall Effect sensor has an output that is just one of two states: ON or OFF. The basic analog output device illustrated in ill. 90 can be converted into a digital output sensor with the addition of a Schmitt trigger circuit. Ill. 93 illustrates a typical internally regulated digital output Hall effect sensor. The Schmitt trigger compares the output of the differential amplifier with a preset reference. When the amplifier output exceeds the reference, the Schmitt trigger turns on. Conversely, when the output of the amplifier falls below the reference point, the output of the Schmitt trigger turns off.

ILL. 93 DIGITAL OUTPUT HALL EFFECT SENSOR: Regulator; Hall element; Differential amplifier; Schmitt trigger; Vs; Digital output; Ground

Open-Collector Output and Pull-Up Resistor

(Ref. 2) A Hall effect encoder with open collector output either drives the output LOW or lets it float. Hence, to drive logic HIGH with an open-collector output, we should add an external resistor, called a pull-up resistor, as shown in Ill. 94(b).

Applications of Hall Effect Transducers

Hall effect transducers are widely used as proximity sensors, limit switches, liquid level measurement, and flow measurement. They are also used for sensing deflections in biomedical implants. Hall effect transducers are constructed in various con figurations depending on the application. Hall Effect principle is used to make devices such as, Hall effect vane switches, Hall-effect current sensors, and Hall-effect magnetic field strength sensors.

Hall Effect sensors tend to be more expensive than inductance proximity sensors but have better signal-to-noise ratios and are suitable for low-speed operation.

Position Sensing ll. 95(a) shows a schematic of a Hall Effect sensor used for sensing sliding motion. A tightly controlled gap is maintained between the magnet and the hall element. As the magnet moves back and forth at that fixed gap, the magnetic field induced by the element becomes negative as it approaches the North Pole and positive as it approaches the South Pole. This type of position sensor features mechanical simplicity, and when used with a large magnet, it can detect position over a long magnet travel.

ILL. 94 OPEN COLLECTOR OUTPUT WITH and WITHOUT PULL-UP RESISTOR

ILL. 95 (A) SLIDING SENSOR (B) OUTPUT CHARACTERISTICS

The output characteristic of the sensor has a fairly large linear range, as shown in Ill. 95(b).

It is necessary to maintain rigidity in linear motion and prevent any orthogonal movements of the magnet when the sensor is used for measuring sliding motion.

ILL. 96 HALL SENSORS and MAGNETIC WHEEL SETUP Rotor shaft; Counterclockwise direction when viewed from motor side; Magnetic multipole wheel ILL. 97 CONSTRUCTIONAL DETAILS OF A MOTOR WITH INBUILT HALL SENSOR (REF. 3)

Method for Measuring the Angular Position of a Motor Shaft

Ill. 96 shows the setup of using Hall Effect sensor along with a permanent magnet multi-pole wheel for measuring the position, and Ill. 97 shows the constructional details of a motor with one such inbuilt Hall Effect encoder (sensor). As seen in Ill. 96, there are two Hall sensors, A and B, which are required to measure the position and the direction of rotation of the rotor shaft.

We know that, when the South Pole comes in front of the Hall element, a positive voltage is developed and the trigger is turned ON. With the North Pole, a negative voltage (or zero volt age with the bias in the differential amplifier) is developed and the trigger is turned OFF. With the current position of the poles on the wheel and the sensors, as shown in the Ill. 96, if the rotor rotates by an angle [...] in counterclockwise direction when viewed from the motor side, the output signals from the digital output Hall sensors A and B will be of the form represented in Ill. 98.

ILL. 98 HALL SENSORS OUTPUT SIGNAL -- Counterclockwise motion; Clockwise motion

ILL. 99 HALL SENSORS OUTPUT STATES CHART --

As seen from Ill. 98 there is a 90° phase difference between the output signals; hence, these sensors are also known as quadrature encoders. The ON (1) and OFF (0) states of the output signals from A and B are used to create the logic for measuring the position as well as the direction of the motor. Ill. 99 shows the tabular representation of these states for 1 pulse (i.e., for the rotation of the rotor shaft in counterclockwise direction by an angle ) for the setup shown in Ill. 96. Also, it would be important to know here that if we have n-pole wheel, we get n/2 pulses for every revolution of the rotor shaft. With a quadrature encoder, we get 4 counts for every pulse. From Ill. 99, if we compare the state of A with the previous state of A and the state of B with the previous state of B, we find that if the state of A or state of B is changing, we have to increment the count by 1 if it's moving in the same direction or decrement it by 1 if it's moving in the opposite direction. The decision for incrementing or decrementing can be made if we com pare the state of A with previous state of B, as shown in Ill. 100 for both counterclockwise and clockwise movement of the rotor shaft.

Considering counterclockwise direction of the motor to be positive, we would need to increment the count by 1 if the state of A is different from the previous state of B and decrement the count by 1 if the state of A is same as the previous state of B. Based on the discussion, a logic was developed to count the rotation of the motor shaft which is discussed further in Section 7.

ILL. 100 COMPARISION CHART OF SENSOR A STATE WITH PREVIOUS STATE OF SENSOR B

ILL. 101 LIQUID LEVEL BY HALL EFFECT: Sensor - Float

Liquid Level Measurement

Determining the height of a float is one method of measuring the level of liquid in a tank. Ill. 101 illustrates an arrangement of a Hall element and a float in a tank made of non-ferrous material (e.g., aluminum).

As the liquid level goes down, the magnet moves closer to the sensor, causing an increase in output voltage. This system provides liquid level measurement without any electrical connections inside the tank.

Flow Measurement

Ill. 102 shows how a Hall element is used for flow measurement. The chamber has fluid-in and fluid-out provisions. As the flow rate through the chamber increases, a spring-loaded paddle turns a threaded shaft. As the shaft turns, it raises a magnetic assembly that energizes the transducer. When the flow rate decreases, the coil spring causes the assembly to go down which reduces the transducer output. The design of the magnetic assembly and sliding screw-nut assembly is calibrated to provide a linear relationship between the measured voltage and the flow rate. Ill. 103 presents a photograph of a typical Hall Effect flow sensor.

3 Pneumatic Transducers

Pneumatic transducers are non-electrical in nature and widely used in industrial instrumentation for measurement and gauging applications. Pneumatic systems use air as a medium for transmitting signal and power. They are sensitive, simple to design, and sensitive in operation. Pneumatic transducers used for displacement convert changes in length or surface displacement into changes pressure value. A schematic diagram of a pneumatic transducer is shown in Ill. 104.

ILL. 102 FLUID FLOW MEASUREMENT Hall magnets Hall element Sliding screw assly; Spring assly Paddle wheel; Fluid Out

ILL. 103 HALL EFFECT FLOW SENSOR

ILL. 104 PRINCIPLE OF PNEUMATIC BACK PRESSURE SENSORS

Typically, there are two chambers arranged in series and separated by an orifice. Air passes from the first to the second chamber-control orifice and to the atmosphere via the second orifice (the measuring orifice). The transducer shown has two orifices, Q1 and Q2. Orifice Q1 is called the control orifice. It has a diameter, d1, and effective area, Ac. The second orifice, Q2, is called the measuring orifice. It has a diameter, d2. Its effective area, Am, is variable and depends upon the distance x, which is the displacement of the workpiece.

[...]

Variation in the backpressure, Pb, can be caused by moving a resistive surface towards or away from the orifice Q2. Experimental results have shown that there exists a linear relationship between Pb and x over a limited range of x. Empirical results have shown that, for supply pressure between 15 kN/m2 and 500 kN/m2, the variation of Pb/Ps and Am/Ae is as shown in Ill. 104(b). The curve has a linear range Pb/Ps extending from 0.6 to 0.9. The extension to the linear part cuts the Pb/Ps axis at 1.1. The slope varies slightly, reducing with increasing supply pressure. For linear range, the relationship may be expressed as

[…]

Here b = 1.1 and K = slope of the curve. The backpressure Pb is measured by a pressure gauge.

Overall sensitivity is given by the rate of change of output with respect to the input. If the out put variable has a pressure gauge reading of _R, and the input variable has a surface displacement of _X.

The overall magnification is , and the overall sensitivity is dependent on the sensitivity of the measuring head, orifice size, and the supply pressure. The measuring head sensitivity is computed as ... Differentiating with respect to Am, reveals that the measuring head sensitivity increases with an increase in orifice size.

The overall sensitivity of the pneumatic transducer is a measure of the gauge displacement for any input change in displacement. This factor is sensitive to variations in the measuring orifice, changes in the backpressure, and also to the sensitivity of display gauges.

In addition to displacement measurement, pneumatic transducers are used in gauging applications where it's difficult to use electronic gauges because of the design limitations of high temperature, humidity, and contamination.

Ill. 105 shows a typical plug gauge, which inspects the internal diameter within the specified limits. Ill. 106 shows the ring gauge used for inspection of the external diameters.

Ill. 107 illustrates the principle of taper measurement and Ill. 108 shows the principle of measurement of the straightness of precision cylindrical bores.

ILL. 105 PNEUMATIC PLUG GAUGES ILL. 106 PNEUMATIC RING GAUGES: Back pressure sensing at the nozzle (Int. diam.) ILL. 107 PNEUMATIC TAPER GAUGES

ILL. 108 PNEUMATIC BORE GAUGES

4 Ultrasonic Sensors

Ultrasonic sensors are used mainly in the areas of inspection and testing, especially for non-destructive testing. Ultrasonic waves have frequencies higher than the audible frequency of 20 kHz. The penetrative quality of ultrasonic waves makes them useful for noninvasive measurements in environments (such as radioactive, explosive, and areas which are difficult to access). They are used for distance, level, speed sensing, medical imaging devices, dimensional gauging, and robotics applications.

The ultrasonic transducer emits a pulse of an ultrasonic wave and then receives the echo from the object targeted. The ultrasonic transducer consists of a transmitter, a receiver, and a processing unit. The transducer produces ultrasonic waves normally in the frequency range of 30 to 100 kHz.

Whenever an ultrasonic beam is incident on a surface, one portion of the incident beam is absorbed by the medium, another portion is reflected, and a third portion is transmitted through the medium.

In proximity sensing applications, the ultrasonic beam is projected on the target, and the time it takes for the beam to echo from the surface is measured. For non-contact distance measurements, an active sensor transmits a signal and receives the reflected signal.

If there is a relative movement between the source and the reflector, the Doppler effect, discussed earlier in this section , is employed. Using the Doppler method, it's also possible to precisely measure the position, velocity, and fluid flow.

Ultrasonic automotive vehicle detection systems are based on two techniques: pulse technique and Doppler shift technique. In the pulse technique, the detector measures the time, _t, spent between transmission and reception of an ultrasonic signal to determine the distance between transmit/receiver and the object. Using the Doppler technique, the frequency of the received ultrasonic signal changes in relation to the emitted frequency depending on the velocity, v, of the object. If the object is approaching the detector, then the frequency of the signal received increases in relation to the emitted frequency. it's reduced when the object is moving away from the detector.

Ultrasonic waves can be generated by the movement of a surface which creates compression and expansion of the medium. Transducers, such as piezoelectric transducers, are the excitation devices most commonly used for surface movement. As discussed in piezoelectric section, when an input voltage is applied to a piezoelectric element, it causes the element to flex and generate ultrasonic waves. This effect is reversible. Conversely, the element generates a voltage whenever it's subjected to vibrations such as the incoming ultrasonic waves. The typical operating frequency of the transmit ting ultrasonic element is close to 32 kHz. If the ultrasonic instrument operates in the pulsed mode, then the same piezoelectric crystals are utilized for transmitting and receiving purposes.

Ultrasonic Distance Sensing

The Ill. 109 presents a range sensing system. In this figure, d is the distance to the object, _ is the speed of the ultrasonic waves in the measured medium, is the incident angle, and t is the time for the ultrasonic waves to travel to the object and back to the receiver. Using these definitions the following equation is written,

Distance: d = t cos u / 2

ILL. 109 ULTRASONIC DISTANCE SENSING

The accuracy of the ultrasonic transducer is high and often in the order of one percent of the range measured. The sensors are used in robotics applications, where the robot manipulators need to avoid collisions and sense the distance of the object or obstruction in the vicinity of robot work space. Some robots are provided with an ultrasonic ranging system that assists the robot in positioning the gripper relatively close to the part. This system often functions in combination with another optical proximity sensor that assists in the precise positioning.

Ultrasonic Stress Sensing

Ultrasonic beams may be used for stress measurement. Ill. 110 presents a typical stress measurement system employing ultrasonic beams.

ILL. 110 ULTRASONIC STRESS SENSING: Applied stress to the specimen; Ultrasonic probe; Reference source; Control circuit

The system consists of an ultrasonic probe which is placed in contact with the specimen. The ultrasonic probe consists of an ultrasonic driver, receiver, and a control device to change the electrical signal to vibrations and vice versa. When in contact with the specimen, the ultrasonic transmitter causes waves to travel across the specimen. These waves are then received by the receiver and converted to an electrical signal.

The basic operating principle relies on changes in the propagation of sound in a specimen causing stress changes. The probe is moved around the specimen to map out the stress field distribution in various sections of the specimen. By rotating the probe, it's possible to determine the direction of the stress.

Ultrasonic Flow Sensing: The transducer that is based on this principle has been explained earlier.

5 Range Sensors

Range sensing techniques are of special importance in manufacturing automation applications.

Range sensors have been successfully employed in other areas as well, including the following.

• Automatic guidance systems for vehicles

• Robot navigation

• Collision avoidance

e.g., consider an industrial scanning and recognition operation in which a sensory robot must locate objects in a container, not knowing exactly where they are. The robot has to follow the sequence of operations which could consist of the following.

1. Scanning a bin containing objects and locating the object in a three-dimensional space.

2. Determining the relative position and orientation of the object.

3. Moving the robot manipulator to the object location.

4. Positioning and orienting the robot gripper according to the objects location and layout.

5. Picking up the object and placing the object at the required location.

In a stationary robot, the gripper must be oriented to the object position. In addition, it must also have the capability of sensing the distance. In automated guided vehicle applications, the vehicle must navigate its body to the object location and then move its work-holding device to grasp the object. Range sensors are typically located on the wrist of the robot manipulator. In some cases sensors also act as safety devices. Besides locating an object in a work cell, sensors are positioned to determine the human obstruction in the robot work cell.

Distance sensors are also used for three dimensional shape inspection. A specimen or a machine part can be inspected on the production floor using an inspection machine such as a coordinate measuring machine (cmc). By finding the distance of the object from a fixed location to various points on the object, it's possible to digitize the three dimensional shape of an object into discrete points.

Distance sensors used for workpiece inspection are also known as digitizers in the machine tool industry. Digitizers are normally used in machine tools, robots, and inspection devices to locate the position of objects and to identify the geometry of the objects in a three dimensional environment.

Some of these sensors also can be used as proximity devices. Proximity devices are used to give an indication of the closeness of one object to another object. A number of techniques are employed in range sensors including optical methods; acoustic, inductive, and electrical field techniques (e.g., eddy current, Hall effect, magnetic field); and others.

Range Sensing Principles

The following section explains various methodologies used for range measurement. Although the focus of this section is on optical techniques, the same principle is applicable to non-optical methods.

The basic triangulation principle is the method of triangulation which applies trigonometric principles to determine the distance of an object from two previously known positions. Ill. 111 illustrates the principle in a thickness-measuring application.

ILL. 111 TRIANGULATION PRINCIPLE TO MEASURE THICKNESS (R2 _ R1)

The source, typically a laser source, is focused on the surface of the object. A photodetector is used to determine the location of the spot. The distance, R2, and angle, , are known. Because the photo detector is located at a fixed distance in the work environment, the thickness of the part is calculated as

t = R2 - R1 = R2 - d tan u

Here d is found from the position of the light spot on the workpiece.

If two triangulation sensors are positioned a certain distance apart and both devices can align to a spot on an object, as shown in Ill. 112, then the two devices and the object form a triangle.

The distance, d, and two angles, 1 and 2, are known. The third angle is found by subtracting the two known angles from 180°.

ILL. 112 TRIANGULATION PRINCIPLE WITH TWO SENSORS

The distance from each device to the object can then be found by using the law of sines.

R1 = d sin u2 / sin [180 - (u1 + u2)]

Instrumental techniques using triangulation principles include the following six methods.

1. Spot sensing method

2. Light strip sensing method

3. Camera motion method

4. Time of flight technique

5. Binocular vision technique

6. Optical ranging using position sensitive detectors Range Sensing by Spot Projection Consider the situation in which a single imaging device is kept stationary and a projected light source scans the scene. If a single beam of light is projected onto an object, as shown in Ill. 113, the projected beam creates a light spot on the object that is reflected into sensing device, such as a camera, which is positioned at a known distance, d, from the spot projector. This produces a triangle between the projector, object, and camera. The range, R, is calculated using the triangle, which provides the distance of the object spot from the camera. The reflected light spot produces an image point, B, in the camera image. This image point is easily detected, as a bright "spot" in the image. The distance of the image point from the center of the camera image can be determined. Furthermore, the camera focal length f, is fixed. Since the focal length, f, and the image point distance, t, form the sides of a right triangle, the angle 2 can be calculated as

u2 = tan ^ -1 f t

ILL. 113 RANGE SENSING USING LASER SPOT PROJECTOR

From this, D, the distance between the projector and the image point can be calculated as D _ d t Where d is the distance between the projector and camera.

Depending on whether the image point is to the right ( ) or left (_) of the center of the cam era lens, t can be positive or negative. The angle the projector makes, 1, is known and from this information the range, R, can be calculated using the law of sines as shown in the Equation.

Range is the distance between the image point and the object point. To calculate the range from the camera lens, subtract the distance between the lens and the image point.

Digitization of an object is performed if the light spot can scan over the entire scene and the range calculation can be computed at each point in the scan. In three-dimensional digitizers, a light spot scans the scene from right to left and top to bottom, utilizing a rotating mirror, which can traverse the beam in a three-dimensional area.

Sensing by the Use of Light Stripe

The basic principle used in the light stripe method is an extension of spot sensing technique. Instead of projecting a spot of light, a stripe of light is projected on the scene. The imaging device creates a line of certain length. The image of the line is divided into individual image points, and the range is calculated for each point along the stripe. The range calculation is similar to that for spot sensing.

The light stripe can be formed by passing ambient or infrared light through a slit on the projector. The scene is scanned in a direction perpendicular to the stripe, resulting in a complete range mapping of the scene.

One limitation of light-stripe scanning is the poor depth resolution that is obtained for object surfaces that are parallel to the light stripe. It can be overcome by scanning the image in two directions, one perpendicular to the other.

The benefit of light striping is that it's relatively simple and fast, as opposed to spot sensing.

The object boundaries and regions can be determined by connecting the end points of the light stripe images. Thus, light striping aids in the image-segmentation process. This can be seen by examining the series of light stripe images.

Camera Motion Method

Another method used in the area of active triangulation is called the camera motion method. It involves moving the camera as illustrated in Ill. 114.

ILL. 114 ACTIVE TRIANGULATION USING MOVING CAMERA: Original position; Final position

Here a single camera is moved a given distance to produce two stereo images of the scene. In an effect analogous to a stereo system, a single moving camera replaces two stationary cameras.

Once the two images are obtained, the range calculations are made using the principle of disparity between the two images, as in stereovision.

Time-of-Flight Ranging Method

Time-of-flight, TOF, ranging involves calculating the time required for a signal to reach and return from an object. Since distance equals the product of velocity and time, the range of an object can be written as (3-99) Here, R is the range from the ranging device, _ is the velocity of the transmitted signal, and t is the time required for the signal to reach and return from the object.

Time-of-flight ranging has been used for optics, sound, and electromagnetic sources. The determination of range, using Equation 3-99, is the same for each type of signal; however, each type of signal has its own characteristics that affect the accuracy of the range data. Two significant features of the time of flight method are (1) beam width and (2) speed of the signal.

The width of the signal beam determines the amount of detail that can be recovered during the ranging process. A wide signal beam does not produce accurate range data for small object details, because it covers a larger area than a narrower beam. Narrow beams result in higher object resolution. The faster the signal reaches and returns from an object, the more difficult it's to deter mine its range.

Range Sensing By Binocular Vision

Binocular or stereovision, also known as passive triangulation, is analogous to human vision and sensing in terms of depth perception. Two imaging devices are placed a known distance apart. In a machine vision system, the imaging devices are usually diode-matrix or CCD cameras. Two parameters in the system are known: the distance between the cameras, d, and the focal length of the cameras. To calculate the range, R, from the cameras to a given point, P, on the object, both cameras scan the scene and generate a picture matrix. Given any point in the scene, such as point, P, there will be two pixels representing that point. One pixel is in the left camera image and the other is in the right camera image.

Each pixel is located at a given distance from the center of its image. Let t1 be the distance that the left-camera image pixel is located from the center of its image. Let t2 be the distance that the right-camera image pixel is located from the center of its image. If the two camera images are overlapped, the two image points, t1, and t2, will not coincide. There will be a certain distance between them.

This distance is calculated by taking the absolute value of their difference. The resulting difference is called the disparity between the two image points. The range, R, from the cameras to the object point is inversely proportional to the disparity between the values of t. As the disparity approaches zero, the range becomes infinite. Conversely, the range gets smaller as the disparity gets larger. As an example, consider the stereo system presented in Ill. 115.

The range of any point on the object can be approximated by:

ILL. 115 RANGE SENSING USING BINOCULAR VISION

where R = range from the left camera lens if the object point is in the right side of the scene

_ range from the right camera lens if the object point is in the left side of the scene

_ range from either camera if the object point is directly in the middle of the scene d = distance between camera lens centers f = focal length of cameras t1 = distance of the image pixel from the center of the left camera lens t2 = distance of the image pixel from the center of the right camera lens The range value, R, can be the distance from the left, right, or either camera, depending on where the object point is located in the scene. If the object point is located in the right half of the scene, R is defined as the range from the left camera lens. The left and right halves of the scene are divided by an imaginary line located exactly halfway between the two cameras. Also, the individual values of t1 and t2 can be positive or negative, depending on the location of a given image pixel relative to the center of its respective image.

e.g., if the image were between the two cameras, t1 would be negative and t2 positive.

Note, however, that the disparity is always the absolute value of the difference between the two image points and is used in the denominator of the range equation. Hence, the position of these two points must be precisely determined.

Ideally, it would be nice to find individual pixels in one camera image that matched those of a second camera image. However, in reality, one can't guarantee that two pixels with the same gray scale or color values were produced by the same object point. Stereo vision systems often search for similar edge or region features between two images to locate corresponding pixels. Edge-based stereo systems attempt to match stereo images by detecting intensity or color in edge mapping. Another matching technique is to take a pixel window from one image and pass it over the same general region of the second image until the best match is found. A displacement or disparity value is determined on the basis of how much the window must be displaced from the first image to match the second image. This value is then used to calculate the range.

Optical Ranging Using Position Sensitive Detectors

Optical principles are widely used for precision position measurement. Position sensitive detectors (PSD) based on optical sources have been effectively used in photographic devices. These devices consist of a small light source and position sensitive detector. The light emitting diode and collimating lens transmit a pulse in the form of a narrow beam. After striking the object, the beam is reflected back to the detector. The received intensity is focused on the position sensitive detector. e.g., let the beam be incident at a distance, t, from the center. The detector generates the output current I1 and I2, which is proportional to the distance t of the light spot on its surface from the center.

The sensor consists of a silicon device and provides position signals on a light spot traveling over its surface. The photoelectric current produced at each terminal is proportional to the resistance between the electrode and the point of incidence. If I is the total current produced by the light spot and I1 is the current at one of the output electrodes, the current produced at each terminal is proportional to the corresponding resistances and the distance between incidence and electrode. We replace the resistance's with distances as [...]

Ill. 116 shows the relationship between the focal length of the lens, f, the range, R, and various distances, R1 and D. R can be calculated as shown in Equation 116.

ILL. 116 TRIANGULATION PRINCIPLE APPLIED TO POSITION-SENSITIVE DETECTOR

Other Ranging Techniques The challenge in ultrasonic ranging is the difficulty in concentrating the sound energy into the narrow beam required to produce high object resolution for three-dimensional vision. Ultrasonic ranging is useful in robot navigation to detect the presence and range objects. Electromagnetic range sensing involves the use of radio frequency signals and is normally called radar. Radar has become useful in general, industrial, and military applications. The radio signal is transmitted into the atmosphere. The signal is reflected back from the object, and the distance or range to the object is determined using the time-of-flight relationship. Radar systems are efficient to measure the range of highly reflective metallic objects over relatively long distances but not useful for measuring relatively short distances of nonmetallic objects. Accurate depth measurement is difficult over short distances.

SUMMARYRange Sensing

The method of triangulation applies trigonometric principles to determine the distance of an object from two previously known positions.

Here d is found from the position of the light spot on the workpiece in Ill. 117.

t = R2 - R1 = R2 - d tan u

Optical Ranging Using Position Sensitive Detectors

The light emitting diode and collimating lens transmit a pulse in the form of a narrow beam. After striking the object, the beam is reflected back to the detector. The received intensity is focused on the PSD. The sensor consists of a silicon device and provides position signals on a light spot traveling over its surface. The range is calculated.

Laser Interferometer

Laser interferometer (Ill. 118) measures distance in terms of the wavelength of light by examining the phase relationship between a reference beam and a laser beam reflected from a target object.

Applications

• Range sensing techniques are used in manufacturing automation applications, such as automatic guidance systems, robot navigation, and collision avoidance.

• Optical principles are widely used for precision position measurement.

• Laser interferometers are also used for precision-motion measurement, checking of the linearity of precision-machine tool slides, and perpendicularity of machine-tool structures (mainly during installations of machine tools).

ILL. 118 Reference cube corner Source (a) Principle (b) Machine tool inspection; Beam splitter; Retro reflector; Target reflector; Laser source

Features

• One limitation of light-stripe scanning is the poor depth resolution that is obtained for object surfaces that are parallel to the light stripe. It can be overcome by scanning the image in two directions, one perpendicular to the other.

• Laser interferometers have extremely high order of accuracy and resolution in linear measurements from a few millimeters to a large distance

3.9.6 Laser Interferometric Transducer

A laser interferometer is an optoelectronic instrument that measures distance in terms of the wave length of light by examining the phase relationship between a reference beam and a laser beam reflected from a target object. It has extremely high order of accuracy and resolution in linear measurements from a few mms to a large distance. As shown in Ill. 118, the laser produces collimated light rays of single frequency present with phase coherence. The laser beam with an optical arrangement produces the reference beam. A part of the reference beam is transmitted to the target and a part of the reference beam is sent to the interferometer. The rays reflected from the target are recombined at the interferometer. The phase difference between the reference beam from the source, and the reflected beam from the target is equal to the extra length traversed by the beam. The digitized information from the difference between the two signals provide the distance information. As shown in the bottom Ill. 118(b), laser interferometers are also used for precision motion measurement, checking of the linearity of precision machine tool slides, and the perpendicularity of machine tool structures (mainly during installations of machine tools).

7 Fiber-Optic Devices in Mechatronics

Fiber-optic sensing is a new area in sensing and transmission that is expected to find wide spread use in Mechatronics applications. Main sensing applications using fiber optics are in the domain of temperature and pressure measurement. Since light can be modulated and transmit ted to large distances, even to normally inaccessible areas using fiber optic bundles, there had been a large increase in the fiber optic based sensors. Using fiber optic wave guides, light can be modulated along different paths as shown in Figures 3-119 and 3-120.

ILL. 119 OPTICAL FIBER

ILL. 120 INTERNAL REFLECTION

Optical fiber is basically a guidance system for light and is usually cylindrical in shape. If a light beam enters from one end face of the cylinder, a significant portion of energy of the beam is trapped within the cylinder and is guided through it and emerges from the other end. Guidance is achieved through multiple reflections at the cylinder walls. Internal reflection of a light ray is based on Snell's law in optics. If a light beam in a transparent medium strikes the surface of another transparent medium, a portion of the light will he reflected and the remainder may be transmitted (refracted) into the second medium. Light intensity, displacement (position), pressure, temperature, strain, flow, magnetic and electrical fields, chemical composition, and vibration are among the measurands for which fiber optic sensors have been developed.

Fiber bundles have highly internal reflective characteristics. The information can be transmitted either in the form of phase modulation or intensity modulation. Depending on the sensed property of light, fiber-optic sensors are also divided into phase-modulated sensors and intensity modulated sensors. Intensity modulated sensors are simpler, more economical, and widespread in application.

Two principles that are widely used in fiber-optic sensors are the reflective and the micro-bending principles. Both concepts sense displacement but can be used for other measurements, if the measurand can be made to produce a displacement. Ill. 122 shows the schematic of a displacement sensor, used in an intensity mode. The incident light is transmitted back from the object.

The analysis and comparison of transmitted and reflected intensities is done separately to give a measure of the distance. Any motion or displacement of the reflecting target can affect the reflected light that is being transmitted to the detector. The intensity of the reflected light captured depends on the distance of the reflecting target from the inspection probe. Disadvantages of this type of sensor are that they are sensitive to the orientation of the reflective surface and to the contamination.

ILL. 122 LIQUID LEVEL: Loss Liquid; To photodetectors

Figures 122 and 123 show examples of liquid level sensors. The level sensor in Ill. 122, consists of two sets of optical fibers and a prism. When the sensor is above the liquid, most of the light is received by the receiver. When the prism reaches the liquid level, the angle of the total internal reflection changes because of the difference in the refractive index liquid and air. There is a higher loss of intensity of light that is detected at the receiver. Ill. 123 shows another example of a level sensor. The U-shaped instrument modulates the intensity of passing light. The detector has two regions of sensitivity at the bent region of the U-shape. Sensitive liquid droplets covering the region move away from the region when the level sensor is lifted thereby providing a different out put than the former position. When the sensitive regions touch the liquid, the light propagated through the fiber drops.

ILL. 123 LIQUID LEVEL: Sensing region Initial (Not immersed in liquid), Droplets; Final (immersed in liquid)

Ill. 124 shows the schematic of micro-strain gauges. In this case, fiber-optic bundles are squeezed between two deformers. The external force influences the total internal reflection of the fibers. Instead of reflection, light beam moves orthogonally and refracts through the fiber wall. The modulated intensity of light by the applied pressure gives a measure of the applied force. Microbend fiber-optic strain gauges have application in the areas of tactile sensing and vibration monitoring. If a fiber is bent as shown in Ill. 124, a portion of the trapped light is lost through the wall of the fiber. The amount of the received light at the detector compared to the light source is a measure of the physical property influencing the bend.

ILL. 124 MICROBEND STRAIN GAUGES: Source Detector; Restoring spring; Applied force

ILL. 125 FIBER-OPTIC TEMPERATURE SENSOR: Interference Pulse generator Laser source; Detector; Amplifier and demodulator; Display

Ill. 125 shows the principle of fiber-optic temperature sensing. Such types of sensors are used in ships and large buildings where there is a need to transmit temperature data over large distances. The normal source of light is a pulse laser. The temperature is sensed by using the principle of back scattering of light. The delay occurring in the reflected laser pulses in comparison to the incident pulses is an indication of the measure of the temperature.

ILL. 126 FIBER-OPTIC LIQUID LEVEL SENSOR

Several fiber-optic sensing concepts have been used in measurement of temperature. These include reflective, microbending, and other intensity- and phase-modulated concepts. In reflective sensors, the displacement of a bimetallic element is used as an indication of temperature variation. Active sensing material (such as liquid crystals, semiconductor materials, materials that produce fluorescence, and other materials that can change spectral response) can be placed in the optical path of a temperature probe to enhance the sensing effect. The radiated light from a surface (which represents the surface temperature) can be collected and measured by a fiber-optic sensor called a blackbody fiber-optic sensor. Blackbody fiber optic sensors use silica or sapphire fibers, with the fiber tip coated with precious metal for light collection. These sensors can have a range of 500 to 2000°C. Fiber-optic temperature sensors have additional advantages of high resolution. Ill. 126 presents a photograph of a fiber-optic liquid level sensor. Several fiber-optic concepts are being used in design of fiber optic pressure sensors which have demonstrated high accuracy. Optical fibers have extensive application in telecommunication and computer networking, but their application as sensing devices is not that widespread. Optical sensing and signal transmission have several potential advantages over conventional electric output transducers and electric signal transmission.

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