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Albaha University Faculty of Engineering Mechanical Engineering Department Lecture 3: Position, Displacement, and Level Ossama Abouelatta o_abouelatta@yahoo.

1 Albaha University Faculty of Engineering Mechanical Engineering Department Lecture 3: Position, Displacement, and Level Ossama Abouelatta Mechanical Engineering Department Faculty of Engineering Albaha University 2013 Aims This lecture aims: to identify position, displacement, and level sensors. to differentiate between potentiometric, capacitive, inductive, and magnetic sensors. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (2)

2 Outline Introduction Position, Displacement, and Level Potentiometric Sensors Capacitive Sensors Inductive and Magnetic Sensors LVDT and RVDT Eddy Current Sensors Transverse Inductive Sensor Hall Effect Sensors Magnetoresistive Sensors Magnetostrictive Detector Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (3) Introduction The measurement of position and displacement of physical objects is essential for many applications: process feedback control, performance evaluation, transportation traffic control, robotics, security systems, just to name the few. By position, we mean determination of the object s coordinates (linear or angular) with respect to a selected reference. Displacement means moving from one position to another for a specific distance or angle. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (4)

3 Selection of position and displacement detectors When designing or selecting position and displacement detectors, the following questions should be answered: 1. How big is the displacement and of what type (linear, circular)? 2. What resolution and accuracy are required? 3. What the measured (moving) object is made of (metal, plastic, fluid, ferromagnetic, etc.)? 4. How much space is available for mounting the detector? 5. What are the environmental conditions (humidity, temperature, sources of interference, vibration, corrosive materials, etc.)? 6. How much power is available for the sensor? 7. How much mechanical wear can be expected over the life time of the machine? 8. What is the production quantity of the sensing assembly (limited number, medium volume, mass production)? 9. What is the target cost of the detecting assembly? Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (5) Potentiometric Sensors A position or displacement transducer may be built with a linear or rotary potentiometer, or a pot for short. The operating principle of this sensor is based on R l a, where a is the crosssectional area and l is the length of the conductor. The ratio l a is called a geometry factor, for a wire resistance. From the formula, it follows that a resistance linearly relates to the wire length. Thus, by making an object to control the length of the wire, as it is done in a pot, a displacement measurement can be performed. Since a resistance measurement requires passage of electric current through the pot wire, the potentiometric transducer is an active type. That is, it requires an excitation signal, for instance, d.c. current. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (6)

4 Potentiometric Sensors A moving object is mechanically coupled to the pot wiper, whose movement causes the resistance change, Fig. (a). In most practical circuits, a resistance measurement is replaced by a measurement of voltage drop. The voltage across the wiper of a linear pot is proportional to the displacement d: v E d where D is the full-scale displacement and E is the voltage across the pot (excitation signal). D (a) Potentiometer as a position sensor; (b) Fluid level sensor with a float; (c) linear Potentiometers. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (7) Potentiometric Sensors For low power applications, high impedance pots are desirable, however, the loading effect must be always considered, thus a voltage follower may be required. The wiper of the pot is usually electrically isolated from the sensing shaft. To illustrate an application of a potentiometric sensor, Fig. (b) shows a liquid level sensor with a float being connected to the potentiometer wiper. Different applications require different potentiometer designs, some of which are illustrated in Fig. (c). (a) Potentiometer as a position sensor; (b) Fluid level sensor with a float; (c) linear Potentiometers. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (8)

Figure (a) shows one problem associated with a wire-wound potentiometer.

steps, Fig. (b), or a variable resolution.

5 Potentiometric Sensors Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (9) Potentiometric Sensors Figure (a) shows one problem associated with a wire-wound potentiometer. The wiper may, while moving across the winding, make contact with either one or two wires, thus resulting in uneven voltage steps, Fig. (b), or a variable resolution. Therefore, when a coil potentiometer with N turns is used, only the average resolution n should be considered: Uncertainty caused by wire-wound potentiometer A wiper may contact one or two wires at a time (a); uneven voltage steps (b). Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (10)

Potentiometric Sensors A concept of another implementation of a potentiometric position sensor with a continuous resolution is shown in the figure.

6 Potentiometric Sensors A concept of another implementation of a potentiometric position sensor with a continuous resolution is shown in the figure. The sensor consists of two strips the upper strip is made of flexible plastic sheet having a metalized surface. This is a contact or wiper strip. The bottom strip is rigid and coated with a resistive material of a total resistance ranging from several k to megohms. The upper conductive strip (wiper) and the bottom resistive strip are connected into an electric circuit. When a pusher (e.g. a finger) is pressed against the upper strip at a specific distance x from the end, the contact strip touches the bottom strip and makes en electric contact at the pressure point. That is, the contact strip works as a wiper in a pot. The contact between two strips changes the output voltage from E to ER x /R O, which is proportional to distance x from the left side of the sensor. The pusher (wiper) may slide along the sensor causing a variable output voltage. Principle of a pressure-sensitive potentiometric position sensor. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (11) Potentiometric Sensors While being quite useful in many applications, potentiometers with contact wipers have several drawbacks: 1. Noticeable mechanical load (friction). 2. Need for a physical coupling with the object. 3. Low speed. 4. Friction and excitation voltage cause heating of the potentiometer. 5. Low environmental stability (wear, susceptibility to dust). Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (12)

Capacitive Sensors The capacitive displacement sensors have very broad applications they are employed directly to gauge displacement and position and also as building

The ability of capacitive detectors to sense virtually all materials makes them an attractive choice for many applications.

The upper and lower plates are fed with the out-of-phase sinewave signals, that is, the signal phases are shifted by 180.

7 Capacitive Sensors The capacitive displacement sensors have very broad applications they are employed directly to gauge displacement and position and also as building blocks in other sensors where displacements is produced by force, pressure, temperature, etc. The ability of capacitive detectors to sense virtually all materials makes them an attractive choice for many applications. As an introductory example, consider three equally spaced plates where each has area A (Fig. a). The plates form two capacitors C 1 and C 2. The upper and lower plates are fed with the out-of-phase sinewave signals, that is, the signal phases are shifted by 180. Uncertainty caused by wire-wound potentiometer A wiper may contact one or two wires at a time (a); uneven voltage steps (b). Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (13) Capacitive Sensors Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (14)

Capacitive Sensors Both capacitors nearly equal one another and thus the central plate has almost no voltage with respect to ground the charges on C 1 and C 2 cancel each other.

This causes changes in the respective capacitance values: and the central plate signal increases in proportion to the displacement while the phase of that signal is the indication of the central

8 Capacitive Sensors Both capacitors nearly equal one another and thus the central plate has almost no voltage with respect to ground the charges on C 1 and C 2 cancel each other. Now, let us assume that the central plate moves downward by distance x (Fig. b). This causes changes in the respective capacitance values: and the central plate signal increases in proportion to the displacement while the phase of that signal is the indication of the central plate direction up or down. The amplitude of the output signals is: As long as x << x 0, the output voltage may be considered a linear function of displacement. The second summand represents an initial capacitance mismatch and is the prime cause for the output offset. The offset is also caused by the fringing effects at the peripheral portions of the plates and by the so-called electrostatic force. The force is a result of the charge attraction and repulsion which is applied to the plates of the sensor and the plates behave like springs. The instantaneous value of the force is: Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (15) Capacitive Sensors In another design, two separate plates are fabricated by using a Microelectromechanical system (MEMS) technology. The plates are macromachined of silicon. One plate serves for a displacement measurement, while the other is for reference. Both plates have nearly the same surface areas, however the measurement plate is supported by four flexible suspensions, while the reference plate is held by the stiff suspensions. This particular design is especially useful for accelerometers. A dual-plate capacitive displacement sensor. Micromachined sensing plate (a) and different suspensions for the sensing and reference plates (b) Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (16)

Capacitive Sensors In many practical applications, when measuring distances to an electrically conductive object, the object s surface itself may serve as a capacitor s plate.

(object). Note that the probe plate is surrounded by a grounded guard to minimize a fringing effect and improve linearity.

9 Capacitive Sensors In many practical applications, when measuring distances to an electrically conductive object, the object s surface itself may serve as a capacitor s plate. A design of a monopolar capacitive sensor is shown in the figure, where one plate of a capacitor is connected to the central conductor of a coaxial cable, while the other plate is formed by a target (object). Note that the probe plate is surrounded by a grounded guard to minimize a fringing effect and improve linearity. A typical capacitive probe operates at frequencies in the 3 MHz range and can detect very fast moving targets, since a frequency response of a probe with a built-in electronic interface is in the range of 40 khz. A capacitive proximity sensor can be highly efficient when used with electrically conductive objects. The sensor measures a capacitance between the electrode and the object. Nevertheless, even for the nonconductive objects, these sensors can be employed quite efficiently though with lesser accuracy. Any object, conductive or nonconductive, that is brought in the vicinity of the electrode, has its own dielectric properties that will alter the capacitance between the electrode and the sensor housing and, in turn, will produce a measurable response. Capacitive probe with a guard ring crosssectional view (a); outside view (b) Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (17) Capacitive Sensors Nowadays, a capacitive bridge becomes increasingly popular in designs of the displacement sensors. A linear bridge capacitive position sensor is shown in Fig. (a). The sensor comprises two planar electrode sets that are parallel and adjacent to each other with a constant separation distance, d. The increase in capacitance, and the spacing between the plate sets is relatively small. The stationary electrode set contains four rectangular elements while the moving electrode set contains two rectangular elements. All six elements are of about the same size (a side dimension is b). The size of each plate can be as large as is mechanically practical, when a large range of linearity is desired. The four electrodes of the stationary set are crossconnected electrically thus forming a bridge type capacitance network. Parallel-plate capacitive bridge sensor plate arrangement (a) and equivalent circuit diagram (b) Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (18)

(19) Thank You Ossama Abouelatta Mechanical Engineering Department Faculty of Engineering Albaha

10 Capacitive Sensors Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (19) Thank You Ossama Abouelatta Mechanical Engineering Department Faculty of Engineering Albaha University Albaha, KSA Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (20)

Advanced Measurements

Advanced Measurements Albaha University Faculty of Engineering Mechanical Engineering Department Lecture 5: Displacement measurement Ossama Abouelatta o_abouelatta@yahoo.com Mechanical Engineering Department Faculty of Engineering