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#43

MAKING SENSE

OF SENSORS

1.1. Anyone that has seen the array (1) of electronic test equipment used by auto-repair shops may have wondered how a small gizmo (2) with wires can be put in the exhaust pipe(3) of the automobile to give a read-out of the air/fuel mixture so the carburetor can be perfectly adjusted. Maybe you've wondered how a digital readout of miles-per-gallon is obtained while you are driving your car down the highway. Even if you haven't wondered about such things, there are electronic circuits to measure temperature, moisture(4), air quality or flow, and small linear motions. For example, hospitals are filled with instruments that read pulse rates(5), blood pressure, and many other biological parameters.

1.2.  Every instrument, no matter how sophisticated its application, or how complicated its design, contains at least one sensing element. That sensing element (or elements) is the heart of the device and is always a surprisingly simple electronic component. The basic purpose of a sensor is to convert (or transduce) differences in some environmental condition into a change in a circuit parameter (voltage, current, resistance, capacitance, inductance, etc.) that can be processed to achieve some desired result.

1.3.  Electronic sensing can be used anytime there are two conditions that can be compared. For simplicity, we will call them the normal and abnormal conditions. Those conditions may be hot and cold, light or dark, wet or dry, loud or quiet, or any other two dissimilar conditions. By using the proper electronic element in the proper signal­conditioning circuit, those varying conditions can be converted into a current or voltage to be displayed by some meter, or digital data for digital -display purposes. The sensing elements used can be resistive, capacitive, inductive, optical, or any combination of two or more of those methods. The sensing schemes presented here are just a few of the methods used and by no means represent all of the ways various conditions can be sensed.

Using resistance.

2.1. - Resistance is used for many sensor applications. Carbon resistors, which date back to the earliest days of radio, have long been known to exhibit (6) a change in resistance with temperature. They decrease in resistance as they get hot.

2.2.That is undesirable when a resistor is used in radio circuits. However, in sensing applications it is a useful trait, as we will explain. The resistance change becomes quite pronounced with large temperature variations, such as going from room temperature to very cold cryogenic temperatures. The resistance changes can be detected as a current change or voltage drop across a load resistor in a circuit, see fig. 1. Enough current is supplied to the circuit to cause a small amount of self heating in the sensing resistor. External cooling of the resistor will cause variations in the resistance. That type of sensing element is often used in level indicators for cryogenic-liquid containers. Thermistors evolved by taking the unfortunate resistor-temperature instability and exploiting it further (7) to achieve larger resistance changes.

Capacitance Variations.

2.3. A method of sensing with capacitance changes is shown in fig. 3. Two identical tuned circuits are driven by a common oscillator. One of the circuits contains a sensing capacitor (denoted Cs) and the voltage across the other tuned circuit is monitored with a high impedance voltmeter. When both circuits are tuned, the voltage across the metered circuit will be at a maximum. As the capacitance is changed by the sensing element, a voltage change will be detected in the metered circuit. Although the range is limited, the sensitivity is quite high for small changes in Cs.



Fig. 3 Even small changes in the sensing capacitance will strongly vary the voltage sent to the VTVM.

 Inductance Changes.

2.4.  A simple method for measuring small linear movements is shown in fig. 4. A small movable core in the field between the two windings will increase their coupling as the core moves in between them, thereby producing an increasing AC output at the secondary. The windings can be wound together on a hollow (8) form that allows the core to slide (9) in and out of its center. With the proper circuit, that type of sensor can even measure microinch core movements.



Fig. 4 Inductance varies with the movements of the core.

Optics.

2.5.  Many sensing circuits have been designed around optical sensing devices. Among the more common of those are infrared perimeter monitors, burglar alarms, and door annunciators. They all use a focused light source directed at a photo cell and signal an event if the light beam is interrupted.

2.6. Very simple concepts are often used for very sophisticated purposes. Fig. 5 shows a standard LED light source and a photo cell. in that configuration, the sensor can be used for smoke detection, card reading, or any other interrupted-beam application.

2.7. When the elements are mounted parallel to each other, as shown in fig. 6, the unit can be used as a reflective surface indicator. It can measure either the distance or quality of a reflective surface.

3. - No matter what type it is, the ultimate use of a sensor is to transform the normal and abnormal conditions of something into a current, voltage, or resistance that can be interpolated by a readout circuit. Almost anything can be sensed electronically, and all sensing circuits can make sense if they are studied.

Fig. 7 When liquid wets the side ofthe prism, it allows the light to diffuse   instead of reflecting it.

Reprinted with permission from Popular  Electronics Magazine, May 1989 issue.

Copyright Gernsback Publications, Inc., 1989.

Vocabulary notes:

1 - Answer the questions

§ 1.1
- What are the two establishments /institutions mentioned in the paragraph?

- What  examples  of  garage devices are shown here? What is each of them for ? (Answer with : it is for + ING)

- What do hospitals use sensing devices for?
-  List what  these instruments are able to measure.

§1.2

- What is to be found at the basis of each application ?

- Are sensing elements are used only in sophisticated or complicated instruments ?

- Give a definition of a transducer. (If necessary go to #18 in the Data bank (Technical Definitions) before making  a defintion.

2 - Find the word of the text which means ...

lecture (d'un instrument  de  mesure),  indication  (fournie  par celui-ci) - traiter (des informations) - parvenir à un certain résultat (careful, the verb does not mean 'achever', 'terminer')

3 - Fill in with DURING or WHILE

·         At the garage, they can diagnose the exhaust gas of your car  ... you  are  waiting.  ...  your car is being repaired, the mechanic will also check the plugs.

·         ... an operation, the patient's  blood pressure  is constantly monitored. In fact ... he is lying on the operating  table,  many  other  biological  parameters  are  also checked.

·         Did you have any problem with your car ... the holiday ?

4 - No matter.

     Look at this sentence : no matter how sophisticated the application   is   ...   = it does not matter how sophisticated the application   is   ... (=   SI   sophistiquée   QUE   soit l'utilisation)   [   no   matter  how  +  adjective  /  past participle]

     NO  MATTER  can  also be used with WHERE - WHEN -WHO - HOW - WHAT - WHY ( = peu importe où / quand / qui / etc. )

e.g. No matter why resistance varies with temperature, ... = it does not matter why resistance varies with temperature, ...

5 - What do you think of this summary of paragraph 1.3 ?  Make it correct if necessary.

Different  conditions  may  be  compared  by  means of electronic sensors  /  which  transform  voltage  or  current  into  digital displays.  /  Various electronic components may be used, / either alone or in a  combination  in  order  to  provide  a  proper signal-conditioning circuit.

6 - Answer the questions.(§ 2.1 and 2.2)

- Are carbon resistors a relatively new invention?

-What are the main characteristic of carbon resistors ?

- In what case is this characteristic a  negative point? ( If you can, explain why it is undesirable)

-What  is the  positive consequence of such an ability ?

- Does such a resistor give good results with  small temperature variations?

-What is the best application of such a resistor ?

-Why is a thermistor more efficient than  a  conventional  carbon

 resistor ?

7 Fill in the description of the circuit shown in fig. 1  with  the following  words  : IN SERIES , SENSING , HEAT ,ACROSS , WHILE , AS , OF, THUS,  MOUNTED, METER

The circuit is composed .... two resistors ....   ...., and a voltmeter  mounted  ....  (=parallel  to) the load resistance. Current is used to .... the .... resistor, .... an air flow cools  it down.   ....   the   temperature  of  the  resistor  changes,  the resistance also varies, which shows up on the ..... . ....  one can measure temperature changes in the air flow.

8 - Now, consider Fig. 2

(It should be remembered  that  different  gases  have  different cooling abilities.)

a/ Describe the circuit

b/ Try and explain how it operates.

c/ What it might be used for.

   You may need the following words and  phrases  :

a/ rheostat / variable resistance / balanced bridge circuit / and words from the exercise above.

b/ to alter / if the resistors are exposed to ... / to tune /  to

achieve  resistance  changes / it'll make the circuit unbalance = it'll cause the circuit to unbalance

c/ it is for + ING = it used to + infinitive /flow rate / to detect the presence of a gas.

9 - Use the following words in sentences so as to make the meaning clear.

supply (do you know a synonym ?) - amount - to cool - further  - to achieve (can you give a synonym ? )

10- § 2.3. How well have you understood the text?

(note :VTVM = vacuum-tube voltmeter : voltmètre électronique)

-What is a capacitance ? What can it be used for ?

-Explain : i) the circuit are tuned ii)  the voltage across the metered circuit

-The text  mentions  a  'high  impedance  voltmeter'  :  What  is  impedance ? Why should a voltmeter have a high impedance ?

-Can  you  think of other applications using a capacitance as a  sensing device?

11 - How to express CAUSE :

a/Look at theses examples 

·         With a carbon resistor, a temperature change  causes voltage to vary

·         With a carbon resistor, a temperature change  makes voltage ø vary

b/ Use MAKE and then CAUSE in the next sentences :

·         A change in the sensing capacity --> the metered voltage varies.

·         Even  a  small variation in Cs --> the meter  detects a voltage change.

·         Tuning both circuits -->  voltage  is  at  a maximum.

12 - § 2.4. Answer these questions before reading the paragraph

-What is induction ? (When is a current induced ?)

-Can you think of applications using induction changes ?

13  -   Read paragraph § 2.4.

a/translate the following words :

b/ Right or wrong?

·         An object moves between two windings

·         As it does so,  the AC output varies

·         The core may slide between two windings or ...

·         it may slide inside the windings

·         Unfortunately, this kind of circuit is not extremely sensitive

14 Describe the circuit and how it operates (Fig.4).

You can use :

a  movement  of  the  core; to cause somebody (something)  to (do)/ to make somebody (something)  ø (do); to  result  in;  thus changing.

15 - Translate: 'A  simple method for measuring movements'. Which other structure(s) could have been used to express the same idea ?

16 - How to express CONSEQUENCE

What  does  this  sentence  mean  ?  The coupling between the two windings changes, THEREBY PRODUCING an increasing AC  output.  Do

you know other ways to express consequence. Use them to say what happens when the nature or temperature of the  gas  changes  (see

Fig. 2)

17 - Have you understood the text? (§ 2.5. to 2.7.)

a/Find the word of the text which means :

-concevoir :

-concentrer (la lumière sur un foyer) :

-un but :               synonym: ...

b/ What do the following acronyms mean : LED - LDR ?

18 Fill  in  with a preposition from the following list (you may use some of them several times)

AT - IN - INTO - OF - ON - OVER -TO - WITH

-A voltmeter is always mounted ...  parallel  ...  (or  ...)  a  circuit,  so that you can always read a voltage ... this circuit.

-Fig. 1 shows a circuit composed ... two resistors.

-The LED and:the LDR should be relatively close ... each other.

-The voltage is ... a maximum.

-The circuit in Fig. 1 is based ... a simple property : a resistor exhibits a resistance change ... a certain temperature range,  so  that,  when  current  is supplied ... the circuit, the readout on  the meter is proportional ... the variation to be sensed.

-Changes  ...  temperature  result  ...   varying   the   sensing  resistance.

19 Describing a diagram.

Now, consider Fig. 7 :  first  describe  it,  then  explain  what would happen  if there was no liquid in the bowl and why you now

have a different readout.

You may use the following phrases :

glass rods attached to a prism ; air interface;  be  wetted  by a liquid ; to cause ... to (do) = to make ... ø (do).

ADDITIONAL EXERCISES

1 - In theses exercises you have had to say how  some components are mounted. Here is an exercise to help you express the  location  of  a  component and the way it is installed or mounted. You will use : to BE + past participle + preposition

The ammeter IS connectED TO the lamp

Adapt the verbs and fill in the blanks with : to wind (a)round  , to connect  across , to mount between , to  wire to , to  connect to , to attach to , to  mount on

- In Fig. 7 the prism ...... two glass rods.

- In Fig. 3 or 4, indicates where the circuit ...... the earth.

-T he  passage  about  induction explains that a coil can ...... a  hollow form.

- In Fig. 1, the voltmeter ...... the load resistance.

- In a motor, the rotor  ...... a shaft.

- In a moving coil ammeter, the  moving  coil  ......  a  U-shaped

 permanent magnet.

- In Fig. 2, R1 and R2 ...... a variable resistance.

2 – You also have had to express what  a component is for. Here are more ways to say the thing:

 it is used to + base verbale

     It is designed for + ing (il est conçu pour)

     It is intended to + base verbale (il est prévu pour)

     It is aimed at + ing

     It serves to + base verbale

     It allows the user / the component / the unit to + base verbale

     It enables me/ the system/ operators to + base verbale

     The job / the basic purpose of a … is to + base verbale

     Its function is to + base verbale

     It serves as / it acts as a means of + ing

Use these phrases to talk about optical sensing devices (see § 2.5. to 2.7.)

3 - Ability to describe circuits and to explain how they work is essential. Fill in the following text with : ATTACHED TO, CHANGE, TO CHANGE, COLLECTOR, COMPRISES, CONNECTED ACROSS, CONNECTED TO, CORE, COUPLINGS, EMITTER, OUTPUT, RERSULTS IN, THEREBY, WINDINGS.

This  circuit  ...  (= is composed of) a float which is ... a ... , so that a change in the water (and the float) level ... moving the core up and down.     There are two secondary ... and two  capacitors  ...  one  of the  windings  (W1)  and a resistance. The base of the transistor is ... W1, the ... to the resistance and capacity; and  the  ... to the other secondary winding (W2). The movements of the core vary the ... between the primary and the secondary windings. This  causes  the  circuit  frequency ... (= it makes the circuit frequency ...), ... varying the  ... frequency.

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