Lecture 3 Basic Principles of ransducers By Hung Nguyen Maritime Engineering and Hydrodynamics Learning Outcomes: p. 3-3 Contents of : resistance transducers capacitance transducers inductance transducers and linear variable differential transformer (lvdt) piezo-electrical transducers - Definitions ransducer is a device that converts one form of energy or physical quantity to another. Often this energy or physical quantity is in form or the same. he energy or stimulus determines the quantity of the signal (R.G. Seippel, 1983). Sensor, Actuator, Detector, ransmitter: A sensor is a device that detects or measures a physical quantity a very simple measuring system An actuator (an opposite device of a sensor) is a device that converts a signal (usually electrical) to some action, usually mechanical (Sinclair 001). - Definitions A detector is a device used to sense the presence of a physical quantity or physical phenomenon (heat, radiation, etc.) A transmitter is a device used to convert an output from a primary element into a useable signal, which is then transmitted either to an indicating instrument or to a controller. he difference between sensor, detector, transmitter and transducer is very slight ransducer sensitivity: K t Δy = Example 3.1 Loaded spring (p. 3-5) Δu Δ y = x [m] Δu = F[kN] x 0.05[m] m K t = = = 0.005 F 10[kN] kn x 0.075 [m] F = 15 kn K = 0.005 [m / kn] = t [ ] Functions of transducer: measurement and control Characteristics of an ideal transducer: P3-5: P.3-5: fidelity, linear relationship Classification of transducers: electrical, mechanic, pneumatic Electrical transducers: resistance (potentiometer, resistance temperature transducer), capacitance, inductance (lvdt), piezo-electric 1
- Resistance type (Potentiometer) Linear Resistance type Potentiometer Principle v f x, R, x o = (,V) Carbon film Symbol Rotary Resistance type Potentiometer Unloaded R1 x i v o = V = V R x Maximum supply voltage: Example : R = 10kΩ, P = 40 mw, X = 4 cm, x = 1. cm. What is v o? V max = PR Resistance type Potentiometer: Loaded condition x R vo = V + x i R L x 1 x i 1 Example 3: p. 3-7 Use of potentiometer: o measure displacement (linear, angular) o measure level o measure force (as in strain gauges) o measure pressure o detect error in a control system (a comparator, or a comparison circuit) - Capacitance transducers Air Note: hermal resistance transducers (RDs, hermistors): Chapter 5 emperature Measurement Multi-Band Film Dielectric Symbol D/P Cell http://www.inotek.com/
Principle: x Capacitance C = ε ε o r A d (farads) Capacitance transducers: Characteristics dc dd ε ε = d 0 1 Material of relative permittivity ε r C Variable distance Variable effective area Examples of Capacitive Devices Example 6: how to calculate the sensitivity of a capacitance device dc ε0ε1 = dd d Use of capacitance transducers: o measure level, displacement o measure pressure, differential pressure, forces and moments o measure flow rate o measure acceleration Inductance transducers (variable inductor and l.v.d.t) Variable inductors Inductors and transformers Linear Variable Differential ransformers Inductance transducers: Principle μ N L 0 μ = r l A N L= S Symbol Inductance transducers: Example 7: how to determine the sensitivity of a single coil inductive transducer: (a) variation in relative permeability dl μ N A = 0 dμ l (b) variation in length of magnetic circuit dl dl μ = μ N A l 0 r 3
Inductance transducers Advantages: Large displacement can be measured Friction between plunger and body is insignificant, thus giving a long life Disadvantages: Not quite as linear as the potentiometer or the linear variable differential transformer (lvdt) Frequency range is limited to 0.1 of the excitation frequency of the a.c. bridge (not applicable to the FM system) Inductance transducers LVD = Linear Variable Differential ransformer Primary coil Secondary coils Movable core Principle http://www.rdpe.com/displacement/lvdt/lvdt-principles.htm Inductance transducers - LVD Advantages: here is no frictional contact between the core and the coils and therefore inductive device has long life than potentiometer Infinite resolution Disadvantages: Displacement frequencies of only up to 0.1 of the excitation frequency can be measured Complex electronic circuitry is required, including an oscillator for frequencies other than main frequency. Use of inductance transducers: o measure displacement o measure level o measure acceleration o pressure (differential pressure transmitter) o measure forces, moments and strain.5 Piezo-electric ransducers: Construction: using crystal materials: ceramic, quartz.5 Piezo-electric ransducers: Principle http://www.tky.3web.ne.jp/ Piezo-electric transducer Example 8 pp.3-13 4
Piezo-electric transducers: Applications: Pressure Forces and moment Acceleration Other types of transducer: Force-to-displacement transducers: Spring Cantilever Pressure-to-displacement t transducers: Diaphragms Bellows Displacement-to-pressure transducers: Nozzle-flapper system Nozzle-flapper with bellows Piezo-resistive transducers Summary of Four common types of transducer Resistance Capacitance Inductance Piezo-electric Other types of transducer: Self-study References http://www.rdpe.com/displacement/lvdt/lvdt -principles.htm http://www.analog.com/library/analogdialo gue/archives/4-08/lvdt 08/lvdt.html http://www.piezomaterials.com/ http://content.honeywell.com/sensing/sens otec/lvdt.asp Activities Activity 1 Capacitance displacement sensor Activities Activity LVD An LVD has a maximum core motion of 1.5 cm with a linearity of 0.3% over that range. he transfer function is 3.8 mv/mm. If used to track work-piece motion from 1. to + 1.4 cm, what is the expected output voltage? What is the uncertainty in position determination due to nonlinearity? +1.4cm SC -1.cm Work-piece motion mv Input F -1 mm to 14 mm 3.8 mv/mm Output 5
Activities Any Questions? Activity 3 Capacitance level sensor 6