INPUT TRANSDUCERS

 

 

 

 

Transducers play a vital role in converting data from the real physical world into a measurable physical quantity. A transducer describes a device that transforms one physical quantity into another. Transducers can be used at the input (a microphone) or the output (a speaker) of a system. With electronic-measuring systems, the input transducer converts a quantity to be measured (temperature, humidity, flow rate, weight) into an electrical parameter (voltage, current, resistance, capacitance) that can be processed by an electronic instrument or system.  The word sensor is used increasingly in preference to transducer. However, a sensor is the “front-end” element of a measuring instrument.  When describing a device that performs a sequence of conversions of one quantity to another, transducer is better. 

 

Electronic Transducer

A transducer that provides output as an electrical signal:

·        voltage

·        current

·        or a change in resistance, capacitance, or inductance.

 

 

 

 

Passive Transducer

A transducer that requires no energy to operate, for example a solar cell

 

Active Transducer

A transducer that requires energy to be added, for example a photo-resistor

 

Selection of transducers

Many factors must be considered when selecting which type of transducer to use for a task:

Types of Transducers

Position

Temperature

Some common metal combinations used in commercial thermocouples are chromel-alumel (chromel is a nickel-chromium alloy and alumel is a nickel-aluminum alloy), iron-constantan (constantan is copper-nickel alloy), chromel-aluminum, tungsten-rhunium alloys, and platinum-10% Rh/Pt. Each of these types of thermocouple has a different temperature range, coefficient, and voltage characteristic and is designated by the letters E, J, K, W, and S, respectively. The overall temperature range covered by thermocouples is from -250șC to 2000șC.

Light


Other


Resistive Transducers

Voltage divider

If Zout = then

Vout = Vrt = (V Rt ) / (R+Rt).

This is non-linear unless Rt << R when Vout << V. If Rt >> R, then current through Rt changes a lot and current sensitivity becomes a potential problem.

Bridge Circuits

Wheatstone bridges are used for strain measurements. The diagram above shows a Quarter Bridge Wheatstone bridge. It consist of 4 resistors arranged in a diamond orientation. The DC voltage is from the top and bottom of the diamond and the output voltage is measured across the middle. If the output voltage is zero, the bridge is said to be balanced. One or more of the bridge maybe a resistive transducer (ie. strain gauge). By changing the strain from a resistive strain gage, the balanced bridge will be unbalanced. This will create a voltage to appear across the middle of the bridge. To rebalance it the resistors in the opposite bridges can be adjusted. The change in resistance that caused the output voltage can be measured and converted to units of strain

 Vout = V{(dR / R) / (4 + 2 dR / R)}

if dR/R << 1 then

Vout = V/ (4 dR /R)

 

Ref:

 

http://www.wlu.ca/~wwwphys/Courses/PC300/lectures/input_lec.html

 

 

Worked example: Calibration of a thermistor