Signal Conditioning. Overview

Transcription

1 Signal Conditioning Raul G. Longoria ME 244L Dynamic Systems and Controls Lab Overview Need for signal conditioning Types of circuits Amplifiers Filters Other types 1

2 Signal conditioning A signal conditioning system or element takes the output from a sensing element and converts it into a form that is more suitable for further processing. Sensor Sensing Element Examples: deflection bridges, amplifiers, oscillators, f/v converters, etc. Signal Conditioning Signal Processing Signal processing A signal processing element takes the output of a signal conditioning element and converts it into a form more suitable for presentation. Sensor Sensing Element Signal Conditioning Signal Processing Data Presentation 2

3 Potentiometric Ballast circuit Voltage divider x R p a Load Vs Lp R x p V l R l b Vs Vl R p R l L p x R p + output signal v o R g = supply voltage = load voltage = total pot resistance = load resistance = total length of resistance element = displacement to measure = fractional displacement = x /L p = resistance between slider and fixed contact R b V s R p (1- ) R p R Th V Th Finding the Thevenin resistance R Th V l R l Thevenin equivalent circuit Angular potentiometer Potentiometric Angular Potentiometer Terminals Calibration +θ "Mechanical input" Vout +Vout +5 V 5 V Vout θ 3

4 Potentiometric Voltage-balancing potentiometer circuit (used especially for thermocouples) Deflection Bridges Deflection bridges are used to convert the output of resistive, capacitive, or inductive sensors into a voltage signal IMPEDANCE VOLTAGE 4

5 Deflection Bridge: Thevenin Model E th = V = s Z Z 1 Z1 + Z Z 4 + Z Z th Z2 + Z3 Z Z 2 + Z Z Z4 Z 4 3 Provides a simplified model for analysis of the measurement system. E th Z th Z L Thevenin Equivalent Resistance Bridges Wheatstone (resistance) bridge the bridge is balanced when the ratio of resistances of any two adjacent arms is equal to the ratio of resistances of the remaining two arms (taken in the same sense) or, R 1 = R2 R 1 = R3 R R R R C D A B 5

6 Reactance or Impedance Bridges From Beckwith, Buck, and Marangoni, Mechanical Measurements, Addison-Wesley, 3rd ed, Bridge Circuits Null detector A I variable Null Detector 3 4 unknown I 1 +I 3 I 3 B C + V D 6

7 Amplifiers Amplifiers are needed to amplify low level signals and to improve the impedance matching between measurement components. We make use of linear scaling amplifiers to change the signal amplitude. Operational Amplifiers Most common; basic building block High gain Low output impedance High input impedance Inverting input +v s v o +v s G amp saturates Non-inverting input (v+ v-) offset null adjust -v s amp saturates -v s 7

8 741 Op Amp The 741 is a common op amp package. Op amps also come dual or quad packaged. Engineer s Mini-Notebook F.M. Mims III (Radio Shack) Basic descriptions Practical applications 8

9 Common Measurement Amplifier Circuits J.P. Bentley, Principles of Measurement Systems, Longman Scientific and Tech., 2nd ed. Filters Filters improve signals by removing unwanted components, typically by using a frequency-selective design. Examples: low pass, high pass, bandpass, and notch 9

10 Ideal Filter Types from Figliola and Beasley Passive Filter Implementations using R, C, or L components can be bulky not always able to achieve desired response Active using op amps or digital ICs more options; can get gain as well 10

11 Mims Low-Pass Active Filter This is a low-pass filter implemented using a 741 op amp. Special Purpose Circuits Analog voltage-comparator Sample and hold Charge amplifier essential for piezoelectric type sensors converts charge to voltage 4-20 ma current loop transmitter vs transducer 11

12 Summary Discussed need for signal conditioning function transforming circuits amplification of signals filtering of signals special purpose circuits 12