1.1 Introduction
A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated glass tube. A thermometer converts temperature to an output voltage which can be read by a voltmeter. For accuracy, all sensors need to be calibrated against known standards.
Sensors are used in everyday objects such as touch-sensitive elevator buttons and lamps which dim or brighten by touching the base. There are also innumerable applications of sensors of which most people are never aware. Applications include cars, machines, aerospace, medicine, manufacturing and robotics.
A sensor receives and responds to a signal or stimulus. Here, the term ‘'stimulus'' means a properly or a quantity that needs to be converted into electrical form. Hence sensor can be defined as a device which receives a signal and converts it into electrical form which can be further used for electronics devices.
A sensor differs from a transducer in the way that a transducer converts the received signal into electrical form only. A sensor sensitivity indicates how much the sensors output changes which the measured quantity changes. For instance, if the mercury in a thermometer moves 1cm when the temperature changes by 1cm, the sensitivity is 1cm/c.
1.2 Classification of Measurement Error
- A good sensor obeys the following rules.
- Is sensitive to be measured properly
- Is insensitive to any other properly
- Does not influence the measured properly.
Ideal sensors are designed to be linear. The output signal of such a sensor is linearly proportional to the value of the measured properly. The sensitivity is defined as the ratio between output signal and measure properly. For instance if a sensor measure temperature and has a voltage output. The sensitivity is a constant with the unit (v/k); this sensor is linear because the is constant at any points of measurement.
1.3 Types of Battery Voltage Sensors / Meters
A meter is a measuring instrument. An ammeter measures current, a voltmeter measures potential difference (voltage) between two points and an ohmmeter measures resistance. A millimetre combines these functions, and possibly some additional once as well into a single instrument.
In designing a battery voltage sensor, there are two types of meters that can be employed, which are:
a) Digital meters
b) Analogue meters
Digital meters usually known as digital multimeters are designed and mass produced for electronics engineer. Even the simplest and cheapest types may include features which you are not likely to use. Digital meters give an output in numbers usually on a liquid crystals display or 7-segment display.
A typical digital voltmeter has a central knob which has a lot of positions and you must choose which one is appropriate for the measurement you want to make. If the meter is switched to 20vdc, for example, then 20v is the maximum voltage which can be measured.
This is sometimes called 20v fsd, where fsd is short form of full scale deflection.
1.4 Analogue Meters
An analogue meter moves a needle along a scale. Switched analogue millimetres are very cheap but are difficult for beginners to read accurately, especially on resistance scales. The meter movement is delicate and dropping the meter is likely to damage it.
Each type of meter has its advantages used as a voltmeter, a digital meter is usually better because its resistance is much higher,1m or 10 m, compared to 200k for an analogue meter on a similar range. On the other hand, it is easier to follow a slowly changing voltage by watching the needle on a analogue display.
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