1 sensor fundamentals

7/23/2019 1 Sensor Fundamentals

http://slidepdf.com/reader/full/1-sensor-fundamentals 1/34

Panca Mudji Rahardjo, ST.MT.

Electrical Engineering - UB



A sensor is a device that converts a physicalphenomenon into an electrical signal.

As such, sensors represent part of theinterface between the physical world and theworld of electrical devices, such ascomputers.

The other part of this interface is represented

by actuators, which convert electrical signalsinto physical phenomena.



Why do we care so much about this interface?◦ Capability for information processing



Transfer Function ◦ The transfer function shows the functional relationship

between physical input signal and electrical output signal.Usually, this relationship is represented as a graph showingthe relationship between the input and output signal, andthe details of this relationship may constitute a completedescription of the sensor characteristics.

exp. accelerometer, Analog Devices’s ADXL150



Sensitivity The sensitivity is defined in terms of the relationship between

input physical signal and output electrical signal. It isgenerally the ratio between a small change in electrical signalto a small change in physical signal.

For ADXL150 is 167 mV/g.



Span or Dynamic Range The range of input physical signals that may be converted to

electrical signals by the sensor is the dynamic range or span.Signals outside of this range are expected to causeunacceptably large inaccuracy.

The stated dynamic range for the ADXL322 is ±2g. Forsignals outside this range, the signal will continue to rise orfall, but the sensitivity is not guaranteed to match 167 mV/gby the manufacturer. The sensor can withstand up to 3500g.



Accuracy or Uncertainty Uncertainty is generally defined as the largest expected error

between actual and ideal output signals. Sometimes this isquoted as a fraction of the full-scale output or a fraction ofthe reading. For example, a thermometer might beguaranteed accurate to within 5% of FSO (Full Scale Output).

“Accuracy” is generally considered by metrologists to be aqualitative term, while “uncertainty” is quantitative. Forexample one sensor might have better accuracy than anotherif its uncertainty is 1% compared to the other with an

uncertainty of 3%.



Hysteresis Some sensors do not return to the same output value when

the input stimulus is cycled up or down. The width of theexpected error in terms of the measured quantity is definedas the hysteresis. Typical units are kelvin or percent of FSO.



Nonlinearity (often called Linearity) The maximum deviation from a linear transfer function over

the specified dynamic range. There are several measures ofthis error. The most common compares the actual transferfunction with the “best straight line,” which lies midwaybetween the two parallel lines that encompass the entiretransfer function over the specified dynamic range of thedevice.



Noise All sensors produce some output noise in addition to the

output signal. In some cases, the noise of the sensor is lessthan the noise of the next element in the electronics, or lessthan the fluctuations in the physical signal, in which case it isnot important. Noise is generally distributed across thefrequency spectrum.



Resolution The resolution of a sensor is defined as the minimum

detectable signal fluctuation. Since fluctuations are temporalphenomena, there is some relationship between the timescalefor the fluctuation and the minimum detectable amplitude.

Therefore, the definition of resolution must include someinformation about the nature of the measurement beingcarried out.



Bandwidth All sensors have finite response times to an instantaneous

change in physical signal. In addition, many sensors havedecay times, which would represent the time after a stepchange in physical signal for the sensor output to decay to itsoriginal value. The reciprocal of these times correspond tothe upper and lower cutoff frequencies, respectively. Thebandwidth of a sensor is the frequency range between thesetwo frequencies.



libr tion

If the sensor’s manufacturer’s tolerances and tolerances ofthe interface (signal conditioning) circuit are broader than therequired system accuracy, a calibration is required.

For example, we need to measure temperature with anaccuracy ±0,5oC however, an available sensor is rated ashaving an accuracy of ±1oC. Does it mean that the sensor cannot be used? No, it can, but that particular sensor needs to becalibrated; that is, its individual transfer function needs to befound during calibration.



Calibration means the determination ofspecific variables that describe the overalltransfer function. Overall means of theentire circuit, including the sensor, theinterface circuit, and the A/D converter.

The mathematical model of the transferfunction should be known before

calibration.



eli bility

◦ Reliability is the ability of a sensor to perform a requiredfunction under stated conditions for a stated period.

◦ It is expressed in statistical terms as a probability that thedevice will function without failure over a specified time ora number of uses.

◦ It should be noted that reliability is not a characteristic ofdrift or noise stability. It specifies a failure, eithertemporary or permanent, exceeding the limits of a sensor’sperformance under normal operating conditions.



The electronics that go along with the physical sensorelement are often very important to the overall device. Thesensor electronics can limit the performance, cost, and rangeof applicability.

If carried out properly, the design of the sensor electronics

canallow the optimal extraction of information from a noisysignal.

Most sensors do not directly produce voltages but rather actlike passive devices, such as resistors, whose values changein response to external stimuli. In order to produce voltages

suitable for input to microprocessors and their analog-to-digital converters, the resistor must be “biased” and theoutput signal needs to be “amplified.”



Resistive sensor circuits



Capacitance measuring circuits Many sensors respond to physical signals by

producing a change in capacitance. How iscapacitance measured? Essentially, allcapacitors have an impedance given by



Since most sensor capacitances are relatively small (100 pF istypical), and the measurement frequencies are in the 1–100kHz range, these capacitors have impedances that are large(> 1 megohm is common).

With these high impedances, it is easy for parasitic signals to

enter the circuit before the amplifiers and create problems forextracting the measured signal.

For capacitive measuring circuits, it is therefore important tominimize the physical separation between the capacitor andthe first amplifier. For microsensors made from silicon, this

problem can be solved by integrating the measuring circuitand the capacitance element on the same chip, as is done forthe ADXL311 mentioned above.



Inductance measurement circuits Inductances are also essentially resistive elements. The

“resistance” of an inductor is given by XL = 2πfL, and thisresistance may be compared with the resistance of any otherpassive element in a divider circuit or in a bridge circuit.

Inductive sensors generally require expensive techniques forthe fabrication of the sensor mechanical structure, soinexpensive circuits are not generally of much use.

In large part, this is because inductors are generally three-dimensional devices, consisting of a wire coiled around aform. As a result, inductive measuring circuits are most oftenof the traditional variety, relying on resistance dividerapproaches.



Limitations in resistance measurement ◦ Lead resistance◦ The wires leading from the resistive sensor element have a

resistance of their own. These resistances may be largeenough to add errors to the measurement, and they mayhave temperature dependencies that are large enough tomatter.



◦ One useful solution to the problem is the use of the so-called 4-wire resistance approach (Figure 1.1.3). In thiscase, current (from a current source as in Figure 1.1.1) ispassed through the leads and through the sensor element.

◦ A second pair of wires is independently attached to the

sensor leads, and a voltage reading is made across thesetwo wires alone.



Output impedanc◦ To minimize the output signal distortions, a current

generating sensor (B) should have an output impedance ashigh as possible and the circuit’s input impedance shouldbe low. For the voltage connection (A), a sensor is referable

with lower Zout and the circuit should have Zin as high aspractical.



Limitations to measurement of capacitance ◦ Stray capacitance – Any wire in a real-world

environment has a finite capacitance with respect toground. If we have a sensor with an output that

looks like a capacitor, we must be careful with thewires that run from the sensor to the rest of thecircuit. These stray capacitances appear asadditional capacitances in the measuring circuit,and can cause errors.



One source of error is the changes in capacitance thatresult from these wires moving about with respect toground, causing capacitance fluctuations which might beconfused with the signal.

Since these effects can be due to acoustic pressure-induced

vibrations in the positions of objects, they are oftenreferred to as microphonics.

An important way to minimize stray capacitances is tominimize the separation between the sensor element andthe rest of the circuit.

Another way to minimize the effects of stray capacitances ismentioned later—the virtual ground amplifier.



•There are two types of sensors: direct and complex.

• A direct sensor converts a stimulus into an electrical signal or

modifies an electrical signal by using an appropriate physical

effect, whereas

•a complex sensor in addition needs one or more transducers ofenergy before a direct sensor can be employed to generate an

electrical output.



John Wilson, “Sensor Technology Handbook”,Newness, 2005.

Jacob Fraden, “Handbook of ModernSensors”,Third Edition. Springer. 2004.

1 sensor fundamentals

Documents