10/20/2011 Topic 6 - Sensors 6.2 Loading of the signal source Learning Outcomes At the end of this topic, you should be able to: • Distinguish between primary and secondary transducers and identify the primary detectortransducer elements for various operations • Identify and explain the working principles of variable-inductance transducers, capacitive transducers, piezoelectric sensors and semiconductor sensors • Determine the sensitivity of the various transducers Digital thermometer Flow sensors 6.3 The secondary transducer Secondary transducer Primary transducer Secondary transducer Strain resistance change 6.4 Classification of first-Stage devices Primary transducer http://www.transducertechni ques.com/HSW-LoadCell.cfm Load displacement/strain Primary detectortransducer elements Mechanical Contacting spindle, pin Electrical Resistive Inductive Elastic member Capacitive Class I. First-stage element used as detector only Class II. First-stage elements used as detector and single transducer Class III. First-stage elements used as detector with two transducer stages Mass Piezoelectric Thermal Hydro-pneumatic Semi-conductor junction Photoelectric Hall effect 1 10/20/2011 6.5 Variable-resistance transducer elements Advantages of electrical elements : 1. Amplification or attenuation can be easily obtained 2. Mass-inertia effects are minimized 3. The effects of friction are minimized 4. An output power of almost any magnitude can be provided 5. Remote indication or recording is feasible 6. The transducers can often be miniaturized • Resistance of an electrical conductor: 6.6 Sliding-contact devices • Convert mechanical displacement input into electrical output, either voltage or current 6.7 The resistance strain gage • Application of a strain to a resistance element changes its resistance basis for the resistance strain gage 6.8 Thermistors • Thermally sensitive variable resistors made of ceramic-like semiconducting materials http://en.wikipedia.org/ wiki/Strain_gauge 2 10/20/2011 6.10 Variable inductance transducers 6.9 The thermocouple Inductance n. Physics the property of an electric conductor or circuit that causes an electromotive force to be generated by a change in the current flowing. http://en.wikipedia.org/wiki/Inductor ‘Inductance is typified by the behavior of a coil of wire in resisting any change of electric current through the coil.’ http://hyperphysics.phyastr.gsu.edu/Hbase/magnetic/ indcur.html#c2 http://hyperphysics.phy-astr.gsu.edu/Hbase/magnetic/indcur.html#c2 Variable-Inductance Transducers Variable selfinductance Single coil Two coil Variable mutual inductance Two coil Three coil Number of turns Variable reluctance Moving iron Inductance, Size of coil Moving coil Moving magnet Permeability of magnetic flux path 3 10/20/2011 • Inductance of a straight, cylindrical air-core coil • When flux path includes both magnetic material and air gap, the inductance may be estimated as d l http://info.ee.surrey.ac.uk/Worksh op/advice/coils/air_coils.html • In many instances, the permeability of the magnetic material is sufficiently high that only the air gaps need to be considered, thus • When an a.c. excitation is used, Inductive reactance: Total impedance: Let’s try 6.1. LET’S RECALL Consider an inductive displacement probe having a diameter of 0.25 in. If the probe is set at a “standoff” distance of 0.050 in. relative to a shaft, determine the probe sensitivity (mV/0.001 in. displacement) when the probe is used as shown in the circuit shown in Figure 6.25. Assume Eq. (6.3c) is valid here with n = 100 and that the excitation frequency is 1000 Hz. 4 10/20/2011 LET’S RECALL 6.10.1 Simple Self-Inductance Arrangements • Flux path may be changed by a change in air gap ha2 • Form of two-coil self-inductance: ha1 6.10.2 Two-Coil Mutual-Inductance Arrangements Pickup 6.11 The Differential Transformer V1 – V2 V1 V2 5 10/20/2011 6.12 Variable-Reluctance Transducers 6.13 Capacitive Transducers A • Capacitance C is given by εo d Permittivity of free space Dielectric constant of medium between plates Area of overlap Separation of plates • Capacitance of a stack of n equally spaced plates in which alternate plates are connected to one another is: • Examples in transducer applications εo d . . . . . . 1 Changing Dielectric Constant n • Examples in transducer applications 2 Changing Dielectric Constant Changing Area 6 10/20/2011 d1 1 Changing Area d1 1 Changing Distance Let’s try 6.3. For a capacitive displacement transducer whose behavior can be represented by Eq. (6.6a), determine an expression for the sensitivity deo/d(d) for an excitation frequency f if the transducer is used as shown in Fig. 6.26. Let’s try 6.6. Consider the capacitive displacement transducer in Fig. 6.26 to be governed by the following relationship: where C = capacitance (pF), A = cross-sectional area of transducer tip (in.2), d = air-gap distance (in.) Determine the change in eo when the air gap changes from 0.010 in. to 0.015 in. 6.14 Piezoelectric Sensors • Equivalent circuit for a piezoelement: • In terms of the stress 7 10/20/2011 6.15 Semiconductor Sensors Let’s try 6.2. It is desired to construct a dynamic compression force cell capable of measuring forces in the range of ±1000 N. If a quartz disk 1.0 mm thick and 10 mm in diameter is used as the sensing element, determine the force cell sensitivity (mV/N). • Semiconductor technology has produced compact and inexpensive sensors: Piezoresistive pressure sensor Capactive pressure sensor 6.15.1 Electrical Behavior of Semiconductors Accelerometer • Semiconductors also respond to strain: • Number of charge carriers, nc, is a function of temperature, T : • Since the resistivity of material is proportional to 1/nc, a semiconductor’s resistance decreases rapidly with increasing temperature 8 10/20/2011 6.15.2 pn-Junctions • Most semiconductor devices involve a junction, at which n-type and p-type doping meet: • When a voltage is applied to the junction, the current through it varies as shown: 6.15.3 Photodiodes • Irradiating light produces an additional current, Iλ, at the junction: • Photocurrent, Iλ, is directly proportional to the intensity, H, of the incoming light (in W/m2): • Voltage-current characteristics of a photodiode: 6.16 Light-Detecting Transducers (Self-study) • Two-dimensional arrays of photodetectors allow the contruction of digital cameras Image sensor (CCD) 9 10/20/2011 6.17 Hall-Effect Sensors • Hall effect - appearance of transverse voltage difference on conductor carrying current perpendicular to magnetic field: • Because the electrons carry a charge -q, they experience a magnetic force FB in the z-direction: • Charge distribution creates an electric field, E, whose force in steady state is equal and opposite the magnetic force on the electrons: • The magnitude of the electric field is E = vdB • Voltage difference across a conductor of height l is 6.18 Some Design-Related Problems Self-study Let’s try 6.10. The circuit of Figure 6.28 may be used to operate a photodiode. The voltage Vr is a reverse-bias voltage large enough to make diode current, i, proportional to the incident light intensity, H. Under this condition, i/H = 1µA/(W/m2). (a) Show that the output voltage, Vout, varies linearly with H. (b) If H = 1000 W/m2, Vr = 5 V, and an output voltage of 1 V is desired, determine an appropriate value of Rload. LEARNING POINTS 1. Fill in the table below: Element Load cell (compression) Operation Force to linear displacement Bourdon tube Torsional spring Seismic mass Thermocouple Thermistor Venturi Variable inductance coil Changing area capacitor 10 10/20/2011 LEARNING POINTS 2. List FOUR factors that affect the inductance of a coil whose flux path includes both a magnetic material and an air gap 3. List THREE methods that can be used to change the capacitance of a capacitive transducer 4. What is meant by ‘Hall effect’? 11